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DEVELOPING A LIGHTWEIGHT SIMULANT AND A MINIATURE

TRIAXIAL DEVICE FOR LUNAR AND MARTIAN

by

YURU LI ZIMMERMAN

Submitted in partial fulfillment of the requirements

For the degree of Doctor of Philosophy

Dissertation Advisor:

Professor Xiangwu Zeng

Department of

CASE WESTERV RESERVE UNIVERSITY

August 2016 CASE WESTERN RESERVE UNIVERSITY

SCHOOL OF GRADUATE STUDIES

We hereby approve the thesis/dissertation of

Yuru Li Zimmerman

candidate for the degree of Ph.D.*.

Committee Chair

Xiangwu Zeng

Committee Member

Adel Saada

Committee Member

Xiong Yu

Committee Member

Weihong Guo

Date of Dense

7/12/2016

*We also certify that written approval has been obtained for any proprietary material contained therein. TABLE OF CONTENTS

LIST OF TABLES ...... VI

LIST OF FIGURES ...... VIII

ACKOWLEDGMENTS ...... XI

ABSTRACT ...... XII

1. INTRODUCTION...... 1

1.1 History of ...... 1

1.2 Future of Space Exploration ...... 11

1.2.1 Why the ...... 11

1.2.2 Why ...... 14

1.3 Motivation for research ...... 15

1.4 Scope of Work ...... 16

1.5 Outline of the Dissertation ...... 18

2. LITERATURE REVIEW ...... 19

2.1 Introduction ...... 19

2.2 Lunar environment ...... 19

2.3 Lunar ...... 24

2.3.1 Formation of the lunar regolith ...... 26

2.4 Lunar investigation ...... 28

2.4.1 Soil investigation on the Moon ...... 28

2.4.1.1 Introduction ...... 28

2.4.1.2 Interaction ...... 30

I

2.4.1.3 Interaction ...... 35

2.4.1.4 Analysis ...... 37

2.4.1.5 Trenching Tests and Boulder Tracks ...... 38

2.4.1.6 Penetrometer Tests ...... 41

2.4.2 Investigation on returned ...... 47

2.4.2.1 Soil Samples...... 48

2.4.3 Engineering properties of lunar soil ...... 52

2.4.3.1 Introduction ...... 52

2.4.3.2 Particle size and shape ...... 53

2.4.3.3 Specific Gravity ...... 59

2.4.3.4 Density ...... 61

2.4.3.5 Compressibility ...... 68

2.5 Strength Properties Investigation ...... 73

2.5.1 Introduction ...... 73

2.5.2 Techniques for Measuring Soil Strength Properties ...... 75

2.5.2.1 ...... 76

2.5.2.2 Torsional Ring Shear Test ...... 79

2.5.2.3 Triaxial Test ...... 80

2.5.3 Strength Properties of Lunar Soil ...... 84

2.5.3.1 Laboratory Measurements of ...... 85

2.6 Review of lunar soil simulants ...... 93

2.6.1 MLS-1...... 94

2.6.2 JSC-1 ...... 98

II

2.6.3 JSC-1A ...... 102

2.6.4 GRC-3 ...... 106

2.6.5 Other Lunar Soil Simulants ...... 108

3. DEVELOPMENT OF A LIGHT WEIGHT MARTIAN SIMULANT ..110

3.1 Introduction ...... 110

3.2 Mechanical Properties of ...... 113

3.2.1 Particle Size Distribution ...... 113

3.2.2 ...... 114

3.2.3 Shear Strength ...... 115

3.3 Mechanical Properties of Martian Soil Simulant ...... 115

3.3.1 Particle Size Distribution ...... 116

3.3.2 Bulk Density ...... 117

3.3.3 Shear Strength ...... 117

3.4 Method for Creating the CWRU-1 Simulant ...... 118

3.5 Geotechnical Properties of CWRU-1 ...... 120

3.5.1 Specific Gravity ...... 120

3.5.2 Particle Size Distribution ...... 121

3.5.3 Maximum and Minimum Density ...... 122

3.5.4 Compressibility ...... 123

3.5.5 Shear Strength ...... 125

3.6 Conclusion ...... 128

4. DEVELOPMENT OF A MINIATURE TRIAXIAL TESTING

APPARATUS ...... 130

III

4.1 Introduction ...... 130

4.2 Apparatus Description...... 132

4.2.2 Loading Unit ...... 138

4.2.3 and Confining System ...... 139

4.2.3.1 Vacuum Unit...... 139

4.2.3.2 Confining Unit ...... 139

4.2.4 Measurement System ...... 140

4.2.4.1 Displacement Transducer and Load Cell ...... 140

4.2.4.2 Data Acquisition Instrument ...... 141

4.2.5 Others ...... 143

4.3 Types of Testing for the Miniature Triaxial Apparatus ...... 144

4.4 Testing Procedures...... 144

4.5 Testing Results ...... 147

4.5.1 Triaxial Strength Tests for JSC-1A...... 147

4.5.2 Triaxial Strength Tests for Standard ...... 152

4.6 Calibration Tests ...... 154

4.7 Conclusions ...... 155

5. CONCLUSIONS AND RECOMMENDATIONS ...... 157

5.1 Introduction ...... 157

5.2 Summary of Conclusions ...... 158

5.2.1 Geotechnical Properties of CWRU-1 ...... 158

5.2.2 Miniature Triaxial Testing Apparatus...... 159

5.3 Recommendations for Future Study ...... 160

IV

REFERENCES ...... 162

V

LIST OF TABLES

Table 1-1 Exploration Timeline (Vaniman 1991, Anonymous 2015h) ...... 6

Table 1-2 Martian Exploration Timeline (Anonymous, 2015j) ...... 9

Table 1-3 Lunar Exploration Theme (Wilson 2007) ...... 12

Table 1-4 Subcategorized Objectives for lunar exploration (Wilson 2007) ...... 12

Table 2-1 Physical comparison of the Moon and (Vaniman 1991b) ...... 21

Table 2-2 Summarized results of specific gravity test ...... Error! Bookmark not defined.

Table 2-3 geotechnical properties of lunar soil from Lunokhod observations

(Gromov 1998)...... 34

Table 2-4 Average gradient for lunar soil near site (Mitchell et al. 1972) ...... 47

Table 2-5 Average particle shapes of lunar soil (Carrier et al., 1991) ...... 56

Table 2-6 Average particle shapes of lunar soil (Carrier et al., 1991) ...... 60

Table 2-7 Estimates of lunar soil in situ bulk density (Carrier et al., 1991)...... 63

Table 2-8 Best estimates of average bulk density (Mitchell et al., 1974) ...... 65

Table 2-9 Relative density of the lunar soil (Carrier et al. 1991) ...... 66

Table 2-10 and of the lunar soil (Carrier et al. 1991)...... 67

Table 2-11 Bulk density and void ratio of returned lunar soils (Gromov 1998) ...... 68

Table 2-12 Compressibility parameters of lunar soil (Carrier et al., 1991) ...... 71

Table 2-13 Compressibility index of lunar soils (Carrier et al., 1991) ...... 71

Table 2-14 Table Measured minimum and maximum densities of lunar soil (Carrier et al.,

1991)...... 72

VI

Table 2-15 typical values of lunar soil and angle for the inter-crater areas

(Carrier et al. 1991)...... 85

Table 2-16 Summary of lunar soil cohesion and friction angle (Carrier et al., 1991) ...... 90

Table 2-17 Comparison of friction angle and cohesion of lunar soil and MLS-1 (Perkins

1992, Batiste and Sture 2005) ...... 97

Table 2-18 Summary of shear strength parameters of JSC-1...... 101

Table 2-19 Results of triaxial tests performed on JSC-1A (Zeng et al., 2009) ...... 105

Table 2-20 Results of triaxial tests performed on GRC-3 (Zeng et al., 2007)...... 108

Table 3-1 Physical properties of material found at different landing sites,

the MMS simulants, and JSC Mars-1 simulants (Arvidson et al.1989; Moore et al.1999;

Shaw et al.2009; Sullivan et al.2011; Peters et al.2008) ...... 118

Table 3-2 Test Results of Specific Gravity of CWRU-1 ...... 120

Table 3-3 Test Results of Maximum and Minimum Densities of CWRU-1 ...... 123

Table 3-4 Triaxial test results of CWRU-1 ...... 127

Table 4-1 Results of miniature triaxial tests performed on JSC-1A ...... 151

Table 4-2 Results of triaxial tests performed on JSC-1A (Zeng et al., 2007) ...... 152

VII

LIST OF FIGURES

Figure 1-1 Stonehenge (Delso, 2014) ...... 1

Figure 1-2 's sketches of the moon. (Galileo, 1610) ...... 2

Figure 1-3 Sites. (Credit Wikipedia) ...... 5

Figure 1-4 Martian Landing Sites. (Credit Wikipedia) ...... 6

Figure 2-1 Lunar highlands and maria (credit NASA) ...... 22

Figure 2-2 (credit NASA) ...... 24

Figure 2-3 Typical lunar soil porfile (Horz et al. 1991)) ...... 27

Figure 2-4 (Credit: Courtesy Association) ...... 32

Figure 2-5 Track of in 1973. On the left (arrow) is the mark made by the rover’s 9th wheel. (Credit: Russian Academy of Sciences & Roskosmos) ...... 33

Figure 2-6 First footprint on the Moon by Neil (Credit NASA) ...... 38

Figure 2-7 Location of (Anonymous, 2015) ...... 43

Figure 2-8 Lunar surface is exposed to solar and constantly pounded by . (Credit: Larry Taylor, Univ. of Tennessee) ...... 53

Figure 2-9 Grain size distribution of lunar soil. (Carrier et al. 2003) ...... 55

Figure 2-10 Irregular lunar soil particles (credit: NASA) ...... 56

Figure 2-11 wall from Apollo Mission 17 (credit: NASA) ...... 58

Figure 2-12 Overview of direct shear test ( 2002) ...... 78

Figure 2-13 Typical direct shear test setup (Bardet 1997) ...... 78

Figure 2-14 Diagram of triaxial test equipment (Das 2002) ...... 83

Figure 2-15 Mohr circles and developed strength envelope...... 84

VIII

Figure 2-16 Backscattered image of lunar simulant MLS-1 (Sibille et al., 2006)

...... 95

Figure 2-17 Particle size distributions of MLS-1 and JSC-1 (Perkins and Madson, 1996)

...... 96

Figure 2-18 Backscattered electron image of lunar simulant JSC-1 (Sibille et al., 2006) 99

Figure 2-19 Particle size distribution of lunar regolith and JSC-1 (Mckay et al., 1994) 100

Figure 2-20 Particle size distribution of lunar regolith and JSC-1 (Zeng, 2009) ...... 104

Figure 2-21 Particle size distribution of lunar regolith and GRC-3 (He, 2011) ...... 107

Figure 3-1 Flight Rover “” in the JPL Spacecraft Assembly Facility (Lindemann et

al., 2006) ...... 110

Figure 3-2 Particle size distribution curve of light weight simulant, GRC-3 lunar simulant,

MMS sand, and JSC Mars-1 (Peters et al. 2008) ...... 117

Figure 3-3 Image of light weight simulant ...... 119

Figure 3-4 Results of 1D compression test for light weight simulant ...... 125

Figure 3-5 Deviator stress versus axial strain recorded in traxial tests on light weight

simulant (density = 0.64 g/cm3) ...... 127

Figure 3-6 Mohr stress circles for light weight simulant (density = 0.64 g/cm3) ...... 128

Figure 4-1 Small and large samples after failure (Scott, 1987) ...... 131

Figure 4-2 Miniature triaxial testing system ...... 133

Figure 4-3 Cell base for the miniature triaxial testing system ...... 135

Figure 4-4 Cross section of cell base for the miniature triaxial testing system ...... 136

Figure 4-5 Plane view of cell base for the miniature triaxial testing system ...... 136

Figure 4-6 Cell cover for the miniature triaxial testing system ...... 137

IX

Figure 4-7 Setup of a soil sample ...... 138

Figure 4-8 Vacuum unit for the miniature triaxial testing system ...... 139

Figure 4-9 Displacement transducer and load cell for the miniature triaxial testing system

...... 140

Figure 4-10 Displacement transducer for the miniature triaxial testing system ...... 142

Figure 4-11 Squeeze bottle for sample preparation ...... 143

Figure 4-12 Sample mold for sample preparation ...... 143

Figure 4-13 Sample top cap ...... 144

Figure 4-14 JSC-1A soil sample before and after failure ...... 148

Figure 4-15 Deviator stress versus axial strain recorded in triaxial tests on JSC-1A (Density

= 1791 kg/m3) ...... 150

Figure 4-16 Mohr stress circles for JSC-1A (Density = 1791 kg/m3) ...... 151

Figure 4-17 Deviator stress versus axial strain recorded in triaxial tests on standard sand

(Density = 1771 kg/m3) ...... 153

Figure 4-18 Mohr stress circles for standard sand (Density = 1771 kg/m3) ...... 154

Figure 4-19 Cotton sample for calibration tests before loading ...... 155

X

ACKOWLEDGMENTS

First, I would like to extend my deep gratitude to my advisor, Professor Xiangwu

Zeng, for his instruction, guidance and support during the past years of my study. He has

given me valuable opportunities to be involved in many interesting research projects. He

is also a great mentor who encourages me to keep a proactive attitude in my study and a

positive faith in my life.

I would like to also recognize the generous assistance of Professor Adel S. Saada

and Professor Xiong Yu for their valuable guidance and encouragement for both my

research and personal life. In particular, I would like to thank the support of Saada Family

Fellowship which provided financial support throughout my graduate study here.

I also want to sincerely thank Professor Weihong Guo for her extraordinary

teaching and for being a member of the graduate committee.

I am also grateful to the assistance and support from Jim Berilla for experiment preparation and adjustments.

I sincerely appreciate to all faculties, staffs, and my fellow graduate students in the

Department of Civil Engineering for their consistent support over the years.

Last, but most importantly, I would likt to thank my husband Grant Zimmerman

for his , support and encouragement throughout this portion of my academic career.

My appreciation is also extened to my parents, parents-in-law, and all the members in my big family for their support and love.

XI

Developing a Lightweight Martian Simulant and a Miniature Triaxial Device for Lunar and Martian Soils

ABSTRACT

By

YURU LI ZIMMERMAN

A major frontier of space exploration is to construct permanent bases and test beds

for future lunar and Martian exploration. Two essential components are in situ resource

utilization and surface mobility. In situ resource utilization describes the usage of native

lunar and Martian materials to reduce construction costs on site. Surface mobility is study

of the interaction between soil and vehicle traction systems to develop effective lunar or

Martian vehicles. Both of these entities call for a thorough understanding of the mechanical and engineering properties of lunar and Martian soils. However, only limited information has been gained so far with regard to the physical and mechanical properties of lunar and

Martian soil. This dissertation discusses the development of a new miniature triaxial testing device to investigate the shear strength of dry cohesionless soil (lunar soil and lunar simulants) of less than 10 g, and comparisons are made between the testing results and that achieved through conventional triaxial tests to confirm the validity of the device. In addition, a light weight Martian soil simulant, CWRU-1, was also developed for large quantity application for the study of high shrinkage mobility tests. CWRU-1 was mechanically characterized via laboratory experiments and compared to characterizations of Martian soil and previously developed Martian soil simulants.

XII

1. INTRODUCTION

1.1 History of Space Exploration

“Two things awe me most, the starry sky above me and the moral law within me.”

______Immanuel Kent

Throughout human history, people have always been amazed by the sky above us. The beautiful starry sky has always been one of man’s greatest interests. For centuries, the , the

Moon, and the position and motion of the stars have always fascinated us, serving as a source of inspiration for art, literature, music and ancient mythology. One of the most famous and ancient civilizational structures is Stonehenge. Stonehenge (Figure 1-1) is the remains of a ring of standing stones set within , built around 3000 BC to 2000 BC, showing evidence of astronomical knowledge serving their astrological system (Anonymous 2015a).

Figure 1-1 Stonehenge (Delso, 2014)

Galileo Galilei made some of man’s first observations of celestial objects with his early telescope (refer to Figure 1-2). His recorded observations of the craters and mountains of the Moon

1 was perhaps man’s first step closer to the Moon. He defied contemporary Aristotelian principles in showing the Moon to be uneven, rough, and filled with depressions and bulges rather than a perfect smooth sphere (Anonymous 2015b).

Moving forward to the , man developed technologies to travel where we had only before gazed. On October 3, 1942, a team of German engineers and scientists successfully launched the first man-made object, the V-2 rocket, into sub-. In the following years, the first image of Earth was taken from space in 1946 (Anonymous 2015c).

Figure 1-2 Galileo's sketches of the moon. (Galileo, 1610)

On October 3, 1957, scientists and engineers in the launched , the world's first artificial into space, which successfully remained in for 3 .

The launch of Sputnik 1 indicated a new era of scientific and technological development later known as the (Anonymous 2015d).

2

The Space Age was characterized by a close race in mostly between the

USA and the Soviet Union with a focus on the Moon. This “Moon Race” began in 1959 with the successful launch of the first man-made satellite, the unmanned Soviet probe 1, into . In the same year, the unmanned Soviet probe reached the Moon surface (impact). In the following , the first photographs showing the was taken by the

Soviet probe . In 1966, the Soviet probe was the first to achieve a soft landing and first pictures from the Moon surface. Additionally, the Soviet Union completed the first human space flight, and cosmonaut became the first person to travel into orbit on April 12,

1961. (Anonymous 2015c and 2015e)

In order to compete with these Soviet successes, U. S. President John F. proposed a national goal of “landing a man on the moon and returning him safely to Earth” within a decade

(1960). "First, I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the Moon and returning him safely to the earth. No single space project in this period will be more impressive to mankind, or more important for the long-range exploration of space." (Stenger 2001)

That goal was achieved on , 1969, when and Buzz made a safe landing on the moon's Sea of Tranquility. The Space Age reached its peak when Neil

Armstrong stepped off the lunar and proceeded to make the first human walk on the moon surface. The landing of Apollo 11 was one of the most significant moments of the 20th century, as Neil Armstrong said “One small step for man, one giant leap for mankind”.

In summary, a total of 65 Moon landing were made in the mid- to the mid-.

Only nine of them completed a successful round trip and returned with lunar soil and samples

(Anonymous 2015f). Besides Apollo 11, five more Apollo crews completed lunar surface landing.

3

However, after in 1976, lunar exploration stopped, the Soviet Union started to focus on

Venus and the USA on Mars.

The moon did not return to the spotlight until the 1990s, when more and more countries

engaged in lunar exploration. In 1990 Japan launched spacecraft, becoming the third country

to orbit around the Moon. A small, low-cost lunar orbital probe, SMART 1 was launched by the

European Space Agency on September 27, 2003. China followed and successfully launched the

Chang'e 1 robotic lunar orbiter on October 24, 2007. After that, India launched an unmanned lunar orbiter, Chandrayaan-1, on October 22, 2008. The commercial lunar mission has also begun; the first commercial moon was accomplished by the Manfred Memorial Moon Mission on

October 28 2014. (Anonymous 2015f)

Table 1-1 provides a complete summary of past lunar exploration missions and Figure 1-3 provides a map of the location of landing sites on the Moon from the past missions.

The US and Soviet Union have shown interest in Mars since the 1960s. Starting in 1960, the Soviets made the first attempt to reach Mars by launching a series of probes including the intended first flybys and hard (impact) landing (Mars 1962B). However, the first successful fly- by of Mars was achieved by NASA's 4 on July 14–15, 1965, and became the first to orbit around Mars on November 14, 1971. Later, the Soviet Union contacted

Mars’ surface on December 2, 1971, and lander completed the first Martian soft landing.

(Anonymous 2015g)

In 1970s, NASA started the to observe meteorologic, seismic and

magnetic properties on Mars as as to search for . Two orbiters were successfully

launched on 1975, and two landers, and , remained operational for six and three

years, respectively.

4

In 1988, the Soviet Union launched probes 1 and 2 to study Mars and its two ,

with a focus on the moon Phobos. failed on the way while successfully took

photographs but failed before it released the landers.

In 1997, NASA Mars Global successfully achieved Mars orbit, finishing its

primary mapping mission in early 2001. At the same year, the NASA also

successfully landed in the on Mars and returned many images. On May 25, 2008,

Phoenix successfully landed on Mars and confirmed the presence of frozen .

To summarize, there have been a total of 55 missions targeting Mars as of 2011, including

11 flybys, 23 orbiters, 15 landers and six rovers (Anonymous 2015g). However, only 27 completed their mission and two achieved partial success. Table 1-2 provides a complete summary of past

Martian exploration missions and Figure 1-4 provides a map of the location of Mars landers and rovers.

Figure 1-3 Moon Landing Sites. (Credit Wikipedia)

5

Figure 1-4 Martian Landing Sites. (Credit Wikipedia)

Table 1-1 Exploration Timeline (Vaniman 1991, Anonymous 2015h)

Mission Date Country Accomplishment 2-Jan-59 USSR Flyby 3-Mar-59 USA Flyby Luna 2 12-Sep-59 USSR Impact Luna 3 4-Oct-59 USSR Probe 23-Aug-61 USA Attempted Test Flight 18-Nov-61 USA Attempted Test Flight 26-Jan-62 USA Attempted Impact 23-Apr-62 USA Impact 18-Oct-62 USA Attempted Impact 2-Apr-63 USSR Flyby 30-Jan-64 USA Impact 28-Jul-64 USA Impact 17-Feb-65 USA Impact 21-Mar-65 USA Impact 9-May-65 USSR Impact 8-Jun-65 USSR Attempted Lander 18-Jul-65 USSR Flyby 4-Oct-65 USSR Impact

6

Mission Date Country Accomplishment 3-Dec-65 USSR Impact Luna 9 31-Jan-66 USSR Lander 31-Mar-66 USSR Orbiter 30-May-66 USA Lander 10-Aug-66 USA Orbiter 24-Aug-66 USSR Orbiter 20-Sep-66 USA Attempted Lander 22-Oct-66 USSR Orbiter 6-Nov-66 USA Orbiter 21-Dec-66 USSR Lander 4-Feb-67 USA Orbiter 17-Apr-67 USA Lander 8-May-67 USA Orbiter 14-Jul-67 USA Attempted Lander 1-Aug-67 USA Orbiter 8-Sep-67 USA Lander 7-Nov-67 USA Lander 7-Jan-68 USA Lander 7-Apr-68 USSR Orbiter 15-Sep-68 USSR Return Probe Zond 6 10-Nov-68 USSR Return Probe 21-Dec-68 USA Crewed Orbiter 18-May-69 USA Orbiter 13-Jul-69 USSR Orbiter Apollo 11 16-Jul-69 USA Crewed Lander Zond 7 7-Aug-69 USSR Return Probe 14-Nov-69 USA Crewed Lander 11-Apr-70 USA Crewed Lander (aborted) 12-Sep-70 USSR Sample Return Zond 8 20-Oct-70 USSR Return Probe /Lunokhod 1 10-Nov-70 USSR Rover 31-Jan-71 USA Crewed Lander Apollo 15 26-Jul-71 USA Crewed Lander 2-Sep-71 USSR Impact 28-Sep-71 USSR Orbiter 14-Feb-72 USSR Sample Return 16-Apr-72 USA Crewed Landing 7-Dec-72 USA Crewed Landing /Lunokhod 2 8-Jan-73 USSR Rover 2-Jun-74 USSR Orbiter 28-Oct-74 USSR Lander

7

Mission Date Country Accomplishment Luna 24 14-Aug-76 USSR Sample Return Hiten 24-Jan-90 JAPAN Flyby, Orbiter, and Impact 25-Jan-94 USA Orbiter AsiaSat 3/HGS-1 24-Dec-97 HKG Lunar Flyby Lunar 7-Jan-98 USA Orbiter and Impact SMART 1 27-Sep-03 ESA Orbiter Kaguya (SELENE) 14-Sep-07 JAPAN Orbiter Chang'e 1 24-Oct-07 PRC Orbiter Chandrayaan-1 22-Oct-08 INDIA Orbiter Lunar-A Cancelled USA Orbiter and Penetrators LRO 18-Jun-09 USSR Orbiter LCROSS 18-Jun-09 USSR Impact Chang'e 2 1-Oct-10 PRC Orbiter GRAIL 8-Sep-11 USA Orbiter LADEE 2-May-13 USA Orbiter Chang'e 2 1-Dec-13 PRC Orbiter Delta IV- (EFT-1) 5-Dec-14 USA Orbiter

8

Table 1-2 Martian Exploration Timeline (Anonymous, 2015j)

Date Mission Country Result Reason USSR 1960 Korabl 4 Failure Didn't reach Earth orbit (flyby) USSR 1960 Korabl 5 Failure Didn't reach Earth orbit (flyby) USSR 1962 Korabl 11 Failure Earth orbit only; spacecraft broke apart (flyby) USSR 1962 Failure Failed (flyby) USSR 1962 Korabl 13 Failure Earth orbit only; spacecraft broke apart (flyby) 1964 US (flyby) Failure Shroud failed to jettison 1964 US (flyby) Success Returned 21 images USSR 1964 Failure Radio failed (flyby) 1969 Mars 1969A USSR Failure failure 1969 Mars 1969B USSR Failure Launch vehicle failure 1969 Mariner 6 US (flyby) Success Returned 75 images 1969 Mariner 7 US (flyby) Success Returned 126 images 1971 US Failure Launch failure 1971 419 USSR Failure Achieved Earth orbit only Orbiter arrived, but no useful data and Lander 1971 Orbiter/Land USSR Failure destroyed er Mars 3 Orbiter obtained approximately 8 months of data 1971 Orbiter/Land USSR Success and lander landed safely, but only 20 seconds of er data 1971 Mariner 9 US Success Returned 7,329 images 1973 USSR Failure Flew past Mars 1973 USSR Success Returned 60 images; only lasted 9 days Success/ Occultation experiment produced data and Lander 1973 Orbiter/Land USSR Failure failure on descent er 1973 USSR Failure Missed planet; now in solar orbit. Lander Viking 1 Located landing site for Lander and first 1975 Orbiter/Land US Success successful landing on Mars er Viking 2 Returned 16,000 images and extensive 1975 Orbiter/Land US Success atmospheric data and soil experiments er Phobos 1 1988 USSR Failure Lost en route to Mars Orbiter

9

Date Mission Country Result Reason Phobos 2 1988 Orbiter/Land USSR Failure Lost near Phobos er Mars 1992 US Failure Lost prior to Mars arrival Observer Mars Global 1996 US Success More images than all Mars Missions Surveyor 1996 Failure Launch vehicle failure Mars Technology experiment lasting 5 times longer 1996 US Success Pathfinder than warranty 1998 Japan Failure No orbit insertion; fuel problems Mars Climate 1998 US Failure Lost on arrival Orbiter Mars Polar 1999 US Failure Lost on arrival Lander 1999 US Failure Lost on arrival (carried on ) Probes (2) Mars 2001 US Success High resolution images of Mars Success/ Orbiter imaging Mars in detail and lander lost on 2003 Orbiter/Beagl ESA Failure arrival e 2 Lander Mars Operating lifetime of more than 15 times original 2003 Exploration US Success warranty Rover - Spirit Mars Exploration Operating lifetime of more than 15 times original 2003 US Success Rover - warranty Mars Returned more than 26 terabits of data (more than 2005 Reconnaissan US Success all other Mars missions combined) ce Orbiter 2007 US Success Returned more than 25 gigabits of data Mars Lander Mars Science 2011 US Success Exploring Mars' habitability Laboratory Phobos- Russia/ 2011 Grunt/Yingh Failure Stranded in Earth orbit China uo-1 Mars 2013 US Success Studying the Martian atmosphere and Volatile Evolution Mars Orbiter Develop interplanetary technologies and explore 2013 Mission India Success Mars' surface features, mineralogy and (MOM) atmosphere.

10

1.2 Future of Space Exploration

Following the footsteps of President John F. Kennedy, President George W. Bush

announced a new vision for America’s civil space program On January 14, 2004, setting forth goals of not only returning human expedition to the Moon, but also establishing permanent bases on the Moon and building test beds for future missions to Mars and other destination.

This new program, referred to as “Moon, Mars, and Beyond”, is to advance U.S. scientific, security, and economic interests through a robust space exploration program. The commitment of the program is to extend human presence across the . The first step is to send manned missions to the Moon by the year 2020, in preparation for human exploration of

Mars and other destinations. Different from the during , President Bush invited all nations to join this effort for the benefit of mankind.

1.2.1 Why the Moon

Why should we return to the moon?

NASA worked with 13 of the world's space agencies to develop a Global Exploration

Strategy from April 2006 through December 2006. This strategy explains several most often asked questions: why should we explore space? How can space exploration benefit life on Earth? And how can the moon play a critical role in our exploration of the solar system? Two of the primary questions have been answered "Why should we return to the moon?" and "What do we hope to accomplish through lunar exploration?"

Six lunar exploration themes and over 200 subcategorized objectives evolved from the discussions to provide justifiable ground for supporting returned lunar exploration and as goals to achieve during the mission. These themes and objectives are presented in Table 1-3 and 1-4.

11

Table 1-3 Lunar Exploration Theme (Wilson 2007)

Topic Details

Human Extend human presence to the Moon to enable settlement. Civilization Scientific Pursue scientific activities that address fundamental questions about the history of Knowledge Earth, the solar system and the universe - and about our place in them. Exploration Test technologies, systems, flight operations and exploration techniques to reduce Preparation the risks and increase the productivity of future missions to Mars and beyond. Global Provide a challenging, shared and peaceful activity that unites nations in pursuit of Partnerships common objectives. Economic Expand Earth's economic sphere, and conduct lunar activities with benefits to life Expansion on the home planet. Use a vibrant space exploration program to engage the public, encourage students Public and help develop the high-tech workforce that will be required to address the Engagement challenges of tomorrow.

Table 1-4 Subcategorized Objectives for lunar exploration (Wilson 2007)

Topics:

Objectives: Public Public Global Global Human Human Scientific Economic Expansion Knowledge Civilization Exploration Preparation Engagement Partnerships

Astronomy & Astrophysics X X X Heliophysics X X X X

Earth Observation X X X X X X

Materials Science X X X X

Human Health X X X X X Environmental Characterization X X X X

Environmental Hazard Mitigation X X X

Operational Environmental Monitoring X X X X

Life Support & Habitat X X X X

12

Topics:

Objectives: Public Public Global Global Human Human Scientific Economic Expansion Knowledge Civilization Exploration Preparation Engagement Partnerships

General Infrastructure X X X X

Operations, Testing & Verification X X X X X

Power X X X Communication X X X X X X Position, Navigation & Timing X X X X

Transportation X X X Surface Mobility X X X Crew Activity Support X X X X Lunar Resource Utilization X X X X Historic Preservation X X X X Development of Lunar Commerce X X X X

Commercial Opportunities X X X X X Global Partnerships X X X X X Public Engagement and Inspiration X X X X X X

13

1.2.2 Why Mars

Mars exploration was identified as a long-term goal in the vision for space exploration announced in 2004 by then USA President George W. Bush. On April 15, 2010, President Barak

Obama also stated on space exploration in the 21st century, "By the mid-2030s, I believe we can

send humans to orbit Mars and return them safely to Earth," he said. "And a landing on Mars will

follow."

Mars is the next goal for space exploration; it is the frontier for expanding human presence.

Recent research shows that valuable resources exist on Mars for sustaining life, such as water beneath the planet’s surface. Since Mars’ geological condition and climate cycles were comparable

to Earth’s (Anonymous 2016), it is suggested that Mars once had conditions suitable for life.

Studying about Mars will help us to learn our Earth’s past and future, it may also help answer the

question as to whether life exists beyond our home planet. More importantly, Mars is an achievable

goal. Following success of robotic exploration, the development of deep space technologies, and

studying the effects of long duration space missions on the human body, Mars is the next goal to

expand human presence in deep space and enable exploration of new destinations in the solar

system.

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1.3 Motivation for research

Of the aforementioned objectives to be accomplished via space exploration (returned lunar

exploration and Mars exploration), there are two imperative aspects: Resource Utilization (ISRU)

and surface mobility, which are also two essential components of the program “Moon, Mars and

Beyond”. ISRU is referred to as "the collection, processing, storing and use of materials

encountered in the course of human or robotic space exploration that replace materials that would

otherwise be brought from Earth." Surface mobility refers to study of the interaction between soil and vehicle traction systems. In order to develop effective lunar or Martian vehicles, it is necessary to build validated mobility models using prototypes tested under simulated conditions. Both

design of robotic roving vehicles and manned exploration vehicles depend on measurement of the

behavior of lunar soil under different loading conditions. In both cases, a thorough understanding

of the mechanical and engineering properties of lunar and Martian soils is needed. This research

focuses on the development of a new miniature triaxial testing device to investigate the shear

strength of lunar regolith and Martian soil in small quantity and development of Martian soil

simulant for large quantity application.

While knowledge has been gained about of lunar soils through lunar samples returned by the Apollo and Luna missions, the quantities brought to earth are too small for application in technology development. The proposed research will fill the gap to develop a

miniature triaxial device for evaluating the mechanical behavior of lunar regolith in small quantity.

Another objective of this study is to develop a Martian soil simulant for high sinkage

mobility tests (HSMT). A few Martian soil stimulants have been developed for Earth-based

Martian soil studies. However, they were usually made from exotic materials using complex

procedures and are only available in small quantities. Those simulants are not economically

15 suitable for large-scale HSMT planned for ISRU. Thus it is imperative to create a Martian soil simulant from readily-available soils which emulate the known mechanical properties of the

Martian soil. Soil mechanical properties, to the extent that we know for Martian soil, can be matched even if the mineral and chemical properties of a simulant are not the same as the Martian soils. Therefore, the purpose of the study is to create a mixture of readily-available soils for ISRU

HSMT.

1.4 Scope of Work

The focus of this research is the development of a miniature triaxial testing device to investigate the shear strength of lunar regolith and lunar simulants in small quantity and development of Martian soil simulant High Sinkage Mobility Test in large quantity application.

In this study, a comprehensive investigation was performed on physical and mechanical properties of lunar regolith, with an emphasis on the strength. A thorough literature review has been performed in the following sections, taking into account the results of past lunar missions including, but not limited to, data obtained by ’s observations, in situ lunar soil tests, returned lunar soil samples, and laboratory lunar soil tests. In addition, the composition and mechanical properties of past lunar soil simulants were also investigated, including MLS-1, JSC-

1, JSC-1A, and GRC-1. Besides, the research will also describe and compare the results of past

Martian missions by remote observations and in situ Martian soil tests. This research will also take into consideration the composition and mechanical properties of past Martian soil stimulants, including JSC-Martian-1 and MMS.

The objectives of this study include:

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• To review the properties of the actual lunar soil and understand the conditions under which its properties were determined.

• To review the properties of the current and past lunar soil simulants and understand the purpose and intent of creation and use.

• To better understand the mechanism of soil failure under triaxial condition, including the strength factor friction angle and cohesions.

• To develop a miniature triaxial soil testing device and determine its reliability by testing regular sand and lunar simulant as well as compare these properties to those of the lunar simulants obtained by standardized testing.

• To provide engineering recommendations for the use of this device on lunar soil and other extraterrestrial soil.

• To review the properties of Martian soil and past Martian soil simulants and understand the purpose and intent of creation and use.

• To develop a miniature light weight Martian soil simulant, and determine its geotechnical properties as well as compare these properties to those of the Martian soil and other

Martian soil simulants.

• To provide recommendations for the use of this material as a Martian soil simulant for High Sinkage Mobility Testing at the NASA . More specifically, to provide recommendations on the soil preparation of simulants on the Earth.

• To critically review the results of this study and develop suggestions for future work in this subject area.

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1.5 Outline of the Dissertation

Chapter 1 introduces past and future space exploration, including both lunar and Martian exploration and presents the motivation and scope of this research.

Chapter 2 provides a thorough literature review on physical and mechanical properties of lunar soil and lunar soil simulants as well as a review of in situ and laboratory test mechanisms and procedures used in the determination of the properties.

Chapter 3 provides an overview of Martian soil and Martian soil simulants study. It also includes a detailed description of the development of a light weight Martian soil simulant, as well as an analysis of the differing properties between the simulant presented and those of the previously described Martian simulants and recorded in situ properties on Mars. Also described is the comparison of geotechnical properties between CWRU-1 and those of Martian soil and Martian soil stimulant.

Chapter 4 provides the design and implementation of a miniature triaxial testing device. It describes the design, function, and calibration of the miniature triaxial device. It also outlines the experimental testing and resulting shear strength properties as determined by using the miniature triaxial device. Both regular sand and lunar simulant JSC-1A were tested using this device.

Comparison will be provided between the testing results using a miniature triaxial device and a traditional triaxial device, as reported by Chunmei and others.

Chapter 5 provides general conclusions and recommendation for the use of the miniature triaxial device as well as outlining suggestions for future work in this subject area.

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2. LITERATURE REVIEW

2.1 Introduction

As Gromov (1998) said, “the study of the physical and mechanical properties of the lunar soil started even before the first flights to the Moon were realized.” The early study of lunar soil was mainly based on radio-telescope investigations of the lunar surface. Photographs were later applied to lunar research after Ranger and Orbiter missions (Scott 1969). Further study of lunar soil was made by the soft of the Surveyor and Luna spacecrafts. Luna and

Apollo missions successfully returned with lunar soil and rock samples providing estimates of the lunar soil properties based on laboratory tests. However, there remain large gaps in our knowledge of lunar soils. This is due in part to the fact that the investigation on the Moon only represents a small fraction of the Moon surface and the returned lunar samples were too small to be used in destructive testing for measurement of the mechanical properties of the lunar soil. Moreover, the relationship between the lunar environment and the behavior of lunar soil is not well understood.

This literature review includes a general discussion of lunar environment, the formation of lunar regolith, and a brief summary of the engineering properties of lunar soil based on past investigations.

2.2 Lunar environment

The formation of lunar soil is a direct result of the unique environment of the Moon. In order to evaluate the engineering properties of the lunar soil, it is important to understand the lunar environment and its effect on the .

Unlike earth soil, which is usually referred to as a multi-phase system consisting of solid particles and pore fluids (air and water) (Halajian, 2007), lunar soil is generally considered as a

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mono-phase system dominated by solid particles only. This is due to the fact that the Moon has

negligible atmosphere and lacks water on its surface.

According to Vaniman (1991b), the most obvious environment differences are extreme

fluctuations, low gravity, and the virtual absence of an atmosphere. Absence of any

atmosphere is a distinct lunar environment factor. The lunar atmosphere is on the order of 10-12

Torr, 10-14 times less than that of earth. Different from the high concentration of nitrogen (78.09%)

and (20.95%) on earth, the lunar atmosphere consists mainly of neon, , ,

and argon.

Because no significant atmosphere exists on the moon, the lunar surface is virtually without

insulation. Therefore, surface on the moon are extreme, with an increase of as much

as 280 K during 24 hours, ranging from -387 (-233 ) to 253 Fahrenheit (123

Celsius), greatly depending upon whether a is in sunlight or in shadow. Due to the difference of distance to the Sun, the noon temperature fluctuates 6 K between aphelion and perihelion

(Langseth 1973). Because the sunlight is always horizontal at the lunar poles, permanent shadow exists at the bottoms of many polar craters, where the temperature may be as low as 30 K and relatively constant.

Low gravity is another distinct environment factor of the moon. Gravity on the moon is about one sixth of that on earth. In addition, other environmental variations, such as ionizing, solar, and comic radiation also have a significant influence on the lunar surface materials. The Table 2-

1 below summarizes the variations in physical properties of the Moon and the Earth-Moon system.

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Table 2-1 Physical comparison of the Moon and Earth (Vaniman 1991b)

Property Moon Earth Mean distance from Earth (orbital radius) 384,400 km -- Eccentricity of Orbit around Earth 0.0549 -- Recession rate from Earth 3.8 cm/yr -- 7.353 × 1022 kg 5.976 × 1024 kg Radius (spherical) 1738 km 6371 km 510.1 × 106 km2 Surface area 37.9 × 106 km2 (land = 149.8 × 106 km2) Flattening 0.0005 0.0034 Mean density 3.34 g/cm3 5.517 g/cm3 Gravity at 1.62 m/sec2 9.81 m/sec2 at equator 2.38 km/sec 11.2 km/sec Sidereal rotation time 27.322 days 23.9345 hr Inclination of equator/orbit 6°41’ 23°28’ Mean surface temperature 107°C day; -153°C night 22°C Temperature extremes -233°C to 123°C -89°C to 58°C Atmosphere ~104 molecules/cm3 day 2 × 105 molecules/cm3 night 2.5 × 1019 molecules/cm3 (STP) Moment of inertia (1/MR2) 0.395 0.3315 Heat flow (average) ~29 mW/m2 63 mW/m2 Seismic energy 2 × 1010 (or 1014?) J/yr 1017-1018J/yr 0 (small paleofield) 24-56 A/m

High velocity impacts and volcanism dominate the formation of lunar soil, creating two distinct landscape on the moon: the lunar highlands and the maria.

From this photographs taken from Earth (Figure 2-1), we can clearly see the distinctive aspects of the Moon, that is, the contrast between the bright and dark regions. The lighter areas are the lunar highlands, also known as terrae. The darker plains are the maria. The maria is also commonly known as smooth lowlands. Some lowlands are not covered by maria, such as within the Pole- basin. There is a higher density of impact craters on the terrae than in the maria.

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Figure 2-1 Lunar highlands and maria (credit NASA)

Aside from the dominant highlands and maria, the lunar landscape also includes other

characteristic landscape types, such as , domes and impact craters.

Rilles generally refers to the long and narrow channels resulting from the formation of localized channels. These can be categorized by their shapes: sinuous, arcuate, and linear.

One of the most renowned sinuous rilles is the , which is located on the rim of the

Imbrium Basin, at the Apollo 15 landing site. This rille is approximately 125 kilometers long and about 400 meters deep.

Domes on the moon are described as wide, rounded, circular features with a gentle slope rising in elevation a few hundred meters to the midpoint. It is a type of shield volcano, formed by lava erupting from localized vents. Lunar domes are typically 8 to 12 km in diameter, but can be as big as 20 km across. A small pit can also be found at some of the lunar domes’ peaks.

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Wrinkle ridges within the lunar maria region are features created by compressive tectonic

forces within the maria. These features represent buckling of the surface and form long ridges

across parts of the maria. Some of these ridges may outline buried craters or other features beneath

the maria.

Another typical lunar landscape is the (See Figure 2-2), which exists on the

moon with various shapes and sizes, from slight concavities to massive sharp rimmed basins filled

with boulders and rocks (Horz et al. 1991). The so called “impact cratering” phenomenon is the most notable geological process on the Moon. Generally speaking, the craters are created by the space process when solid bodies, whose size ranging from microscopic dust particles to or , impact the lunar surface at a high velocity (mean impact velocities for the

Moon are about 17 km per second). According to Horz et al. (1991), the impacting objects range

from 10-15 to 1020 grams in mass. The generated from the impact creates a compression shock wave that propagates away from the point of impact. It is then followed by a

rarefaction wave, which causes most of the ejecta including debris and other geological matter out

of the impact site. Finally a hydrodynamic rebound of the floor follows resulting in the formation

of craters.

As stated before, these craters in the Moon come from various sizes, which can be as small

as those appearing on individual particles of soil to the immense craters of large magnitude, such

as South Pole–Aitken Basin with a diameter of nearly 2,500 km and a depth of 13 km. In general,

a lunar crater is roughly formed in a circular shape. The smaller craters usually appeared in a bowl- like shape, whereas the larger crater typically has a central peak with flat floors. The large craters generally consist of smooth floors, a downward sloping wall, and a raised rim which comes from the ejecta, presenting slumping features along the inner walls that can form terraces and ledges.

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Some of the largest impact basins even display secondary concentric rings of raised material. The lunar history of impact cratering follows a trend of decreasing crater size with time. It is important to note that the largest impact basins were formed in the early stage, and then smaller craters came successively lying on top.

The age of lunar surface can be determined by counting the number of craters per unit area.

This is due to the fact that the size frequency distribution (SFD) of crater diameters on a given surface (that is, the number of craters as a function of diameter) tends to follow a power law with increasing number of craters with decreasing crater size.

Some of the most renowned lunar craters include Copernicus, Galileai, Maginus, Littro, and the South Pole Aitken basin, the largest crater on the lunar surface.

Figure 2-2 Lunar Craters (credit NASA)

2.3 Lunar regolith

Regolith is generally defined as “the layer or mantle of fragmental and unconsolidated rock material, whether residual or transported and of highly varied character, that nearly everywhere forms the surface of the land overlies or covers .” (Bates and Jackson, 1980).

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The entire lunar surface, except for a few crater walls and lava channels, is covered with several meters of dark light grey highly fractured and pulverized material, composing the lunar regolith and lunar soil. Although “lunar soil’ and “lunar regolith” are synonymously used, a physical difference exists between the two based on its particle size. The lunar regolith can be categorized into five types according to different particle size: mineral fragments, crystalline rock fragments, fragments, glasses of various kinds, and agglutinates. Lunar soil is defined as the finer- grained fraction of the lunar regolith, more specifically, “the sub centimeter fraction of the lunar regolith” (McKay et al, 1991).

According to Mckay (1991), the lunar regolith acts as the actual boundary layer between the solid part of the Moon and the matter and energy that fill the solar system. Therefore, the lunar regolith is of great importance for understanding the Moon and the around it, given the fact that it contains critical information about both of these regions. All the information about the moon is sourced from the lunar regolith, including the physical and chemical properties of lunar material directly measured on samples, both rocks and soils, which are collected from the regolith; and the information gained through trough experiments, either conducted by on the Moon or remotely monitored from the Earth. Given the surficial, unconsolidated, and fine- grained nature of the regolith, it will likely be utilized as raw material for permanent lunar base construction, mining, building and resource extraction for oxygen, , , aluminum, and , and other minerals.

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2.3.1 Formation of the lunar regolith

The negligible atmosphere and lack of water creates a unique environment for the formation of lunar regolith. Unlike the terrestrial regolith, the lunar regolith does not go through familiar terrestrial geologic processes, such as chemical weathering, running water, wind, and glaciations, but is developed through the process dominated by the continuous impacts by and micrometeorites and the steady bombardment by charged atomic particles from the Sun and stars. The lunar regolith is therefore continuously and gradually being fragmented, mixed, altered and evolved. According to Mckay (1991), the regolith is formatted by two stages. During the early stage, at locations of any exposed bedrock the impacts penetrate the relatively thin regolith that exists on the surface and exhume the fresh exposed bedrock. As more meteors and meteorites impact the surface, the regolith layer quickly accumulates. With a thicker layer of regolith, only the larger impacts penetrate through the regolith layer and new bedrock. In the later stage, most impacts are no longer able to penetrate to the bedrock, but to cause the regolith to be churned and pulverized into smaller particles. At this stage, the regolith accumulates much slower and requires larger impacts to penetrate through the existing regolith layer to yield new bedrock. When the lunar surface is being impacted, a combination of destructive and constructive process occurs, which determines the nature and history of the regolith. The destructive process refers to the fracture or pulverization of regolith and the excavation of existing regolith through impact energy. The constructive process refers to the agglutination, melting and fusing of soil particles caused by heat resulting from the high impact energy. This process yields impact-produced breccias and impact melt rocks containing minerals, rocks, and glass (Mckay et al, 1991). Other space weathering process also exists on the lunar surface, including particle irradiation, via , of various degrees, electrostatic particle

26 transportation, and volcanic activity. Compared to the meteorites impacts, these weathering processes play a less influential role in the formation of the lunar regolith (Oravec, 2009). In general, a thicker regolith and smaller particle size suggests an older lunar surface. Accordingly, it is agreed that the regolith varies in thickness from three to five meters in the lunar maria regions and from about 10 to 20 meters in the highland regions. Beneath the surficial finely fractured lunar regolith layer exists a much thicker layer reffered to as the megaregolith. This layer generally consists of fracutted bedrock and large scale-ejecta and is typically more than one meter. The properties, characteristics, and behavior of the megaregolith layer are not well understood. Figure

2-3 below presents the upper crust of the moon.

Figure 2-3 Typical lunar soil porfile (Horz et al. 1991))

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2.4 Lunar soil investigation

As Gromov (1998) stated, “the study of the physical and mechanical properties of the lunar soil had been started even before the first flights to the Moon were realized”. Early lunar surface investigation was mainly earth-based studies using radio and telescope as well. Later photographs were sent back by the United States Ranger and Orbiter missions (Scott 1969). The following successful soft landings of the Surveyor and Luna spacecrafts provided the first crude estimates of the physical and mechanical properties of the lunar soil. After several successful missions to the

Moon with returned lunar soil samples, more information and knowledge has been obtained regarding the physical and mechanical properties of lunar soil. This section below presents a general review of the past investigations that have been performed to provide a better understanding of the engineering properties of lunar soil. This review is grouped as follows:

(1) Study of the physical and mechanical properties of lunar soil under in situ conditions:

(2) Study of the physical and mechanical properties by testing of the returned lunar soil samples;

(3) Summary of the physical and mechanical properties of lunar soil.

2.4.1 Soil investigation on the Moon

2.4.1.1 Introduction

Successful lunar landing made it possible to conduct in situ measurement of lunar soil samples. The in situ lunar regolith properties were obtained via spacecraft (Luna 9 and Luna 13), rovers (Lunokkhod 1 and Lunokkhod 2), and observations by astronauts (Apollo missions)

(Gromov, 1998). The first successful soft landing of Surveyor I was made in June of 1966,

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followed by Surveyors III, V, VI, and VII in succession (Johnson et al. 1995). With the success of

the Surveyor missions, the initial estimates of the mechanical properties of lunar regolith were

characterized through in situ experiments performed on the lunar surface samples, including

, impact and trenching tests. It is concluded from the test results that there is a

layer of incompressible, slightly cohesive, silty to fine-grained sand material at the lunar surface near the landing site of the Surveyor missions. It is worth noting that a small cohesion of lunar regolith was observed (cohesion was estimated in the range of 0.35 to 0.7 kPa, Scott 1969, Johnson et al. 1995), which contributes greatly to the strength properties of the soil, due to the low gravity on the lunar surface. Based on the results from the Surveyor mission, it was calculated that an astronaut’s boots would sink approximately one to two inches into the lunar surface soil (Scott

1969). It was also concluded that the surface mobility is generally in good condition except in traversing crater slopes with angles greater than 15 degrees.

The Apollo missions later greatly expanded knowledge about lunar soil. In addition to the similar soil experiments performed during Surveyor missions, more advanced experiments were carried out during Apollo missions 11, 12, 14 15, and 16. As Scott (1969) stated:

“The main sources from which soil mechanics data could be extracted were as follows:

1. Real-time astronaut observations, descriptions, and comments.

2. coverage of the astronaut activities on the lunar surface.

3. Sequence camera, still camera, and close-up camera photography.

4. Spacecraft flight mechanics telemetry data.

5. Interactions between various objects of known geometry and weight and the lunar

surface, such as 1) the Lunar Module; 2) the astronauts, 3) the Early Apollo Scientific

Experiments Package (EASEP) instrument units.

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6. The Apollo Lunar Hand Tools.

7. Various poles and shafts which were inserted into the lunar surface in the course of

the extravehicular activities, including a contingency sampler handle, the Solar Wind

Composition Experiment Staff, a flagpole, and core tubes.

8. Astronaut debriefings.

9. Preliminary examination of earth-returned lunar soil and rock samples at the Lunar

Receiving Laboratory.”

Among all the Apollo missions, the data acquired during the Apollo 15 mission offered by far the best estimate of the in situ physical and mechanical properties of lunar soil due to the extended stay time, the , the improved soil testing and data collecting equipment, such as the Self-Recording Penetrometer (Costes et al. 1972). However, the major data

obtained during Apollo mission focused on the mare regions and locations in close proximity to

the highland terrain. Little investigation was performed on the highland materials. The detailed

lunar soil in situ experiments are categorized in the following subsections. A complete summary of in situ soil experiments and measured geotechnical properties are summarized in the next

section.

2.4.1.2 Robot Interaction

The in situ properties of the lunar soil as well as its structure were estimated by observing

the distortion of the soil and the interaction process caused by the Lunokhod chassis (Gromov

1998). Soil destruction mode cracks, shear planes, and steep trench-like walls were observed under

the movement of the self-propelled Lunokhod chassis, which indicated the lunar soil sample is

likely a fine-grained soil with notable cohesion (Lenovich et al., 1976). According to Leonovich

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et al. (1976), “The structure of the soil was determined from the visual assessment of the character

of the deformation of soil beneath Lunokhod’s mover and ninth wheel while analyzing television

images and panoramas of the lunar surface.” As he stated, “Under the effect of the ninth wheel a

clear- trail is formed having a lighter tone than un-deformed surface which attests to small-

grained structure of soil.” A large number of studies of the physical and mechanical properties of

the lunar soil were performed by this means, utilized by both unmanned vehicles Lunokhod 1 and

2. Effective comparisons of soil investigations can be made between the two missions. (Refer to

Figure 2-4 and Figure 2-5.)

The estimate of the physical and mechanical properties of lunar soil was determined by visual evaluation, analysis, and conclusion. Studies of the depth of the ninth wheel’s track indicated that the top layer of lunar soil to depths of 2 cm was characterized to be loose with a high bearing strength of 2 to 4 kPa (Leonovich et al. 1976). A more detailed summary of the in situ

geotechnical properties of lunar soil based on the void ratio is shown below in Table 2-3. More specifically, as described in Gromov (1998), “the void ratio for soil in-situ (sic) was determined on the basis of experimental measurements of bearing capacity versus void ratio made on simulants

(sic) chosen according to results of studying physical and mechanical properties of lunar soil samples delivered to Earth.” According to the Table 2-3, a void ratio of 0.8 to 1.0 is most frequently encountered in situ, especially at locations with a relatively even surface and uniform relief; the looser soil (void ratio 1.3 to 1.0) can be found in locations of crater forming processes or other forms of relief (eg, fresh crater edges with small dimensions or steep slopes), while the extremely loose soil is not typically found in situ except for some isolated bumps and small beds. Existence of the dense soil (void ratio less than 0.8) at various locations of lunar surface indicated that diverse processes formed and packed the upper layer of lunar soil.

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Figure 2-4 Lunokhod 1 (Credit: Courtesy Lavochkin Association)

According to Lenovich et al. (1971), on the lunar surface a bearing strength in the range of

0.2 to 1.0 kg/cm2 (equal to 19.6 kPa to 98.1 kPa) with a mean value of 0.34 kg/cm2 (equal to 33.3 kPa) is most frequently encountered. These values are slightly higher than the bearing capacity provided by Gromov (1998). Based on the data from Lunokhod 1, it can be concluded that both the in situ density and the in situ mechanical strength parameters of the lunar soil generally increase

with increasing depth. (Costes et al.; 1971 and Johnson and Carrier, 1971).

According to Mitchell et al. (1972), during the Apollo 14 mission, the in situ physical and

mechanical properties of lunar soil was investigated by analyzing tracks formed by the Modular

Equipment Transport via various methods including bearing capacity theory. From the MET track

testing result, it can be concluded that the friction angles of the lunar soil were estimated in the

range between 37 and 47 degrees. It was also noted that in the crater areas, the soil on the crater

rims and slopes are generally stronger than the soil in the intercrater areas.

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Figure 2-5 Track of Lunokhod 2 in 1973. On the left (arrow) is the mark made by the rover’s 9th wheel. (Credit: Russian Academy of Sciences & Roskosmos)

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Table 2-2 In situ geotechnical properties of lunar soil from Lunokhod observations (Gromov 1998)

Void Ratio Soil Parameters > 1.3 1.3 – 1.0 1.0 - 0.9 0.9 – 0.8 < 0.8

Bearing Capacity < 7 7 – 25 25 – 36 36 – 55  55 (kPa) Cohesion < 1.3 1.3 – 2.2 2.2 – 2.7 2.7 – 3.4 > 3.4 (kPa) Friction Angle < 1.3 1.3 – 2.2 2.2 – 2.7 2.7 – 3.4 > 3.4 (degree) Relative frequency of 0.005 0.25 0.3 0.3 0.15 occurrence (%) Areas of Isolated shallow depth Fresh crater Typical bumps and On elements or re-worked edges with Locations of small beds of very Inter-crater soils; stone small the Lunar of fine- eroded areas like dimensions; Surface grained craters formations. steep slopes material Isolated stones

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2.4.1.3 Spacecraft Interaction

More knowledge and about the characteristics of lunar soil with respect to the various locations on the moon has been obtained through the Apollo missions from the interaction between the and the lunar surface soil. It was noted that during all Apollo missions, as well as Surveyor missions, that the color of the lunar surface undisturbed areas appeared lighter than the disturbed areas on the lunar surface (Mitchell et al. 1973). The reason behind this is not yet well understood. Some have reasoned that disturbance alters the texture, affecting the light reflected off the soil

During Apollo 11 mission, Astronauts Neil Armstrong and noted that when the module first started to land the visibility is good initially until reaching 30.5 m (100 ft.) above the lunar surface, when the surface dust was stirred up by the exhaust gas and the visibility was impaired at 73 m (240 ft.) above ground surface. The astronauts observed that the first surface erosion occurred at altitudes between 70 and 27 m (230 to 90 ft.) above ground surface (Scott,

1969). According to Scott (1969), the material being eroded was a regolith layer on the lunar surface with lower strength than the underlying layer of lunar soil. It was also worth noting that since the erosion and soil disturbances were observed at a relatively low altitude the surface layer of lunar soil therefore possesses some amount of cohesion within. Although was not directly measured on landing, it is estimated to be between four and six inches of erosion. The footpads on the lunar module penetrated only 8 cm (3 in) of the surface soil and the astronauts’ boot impressions became deeper as they ventured further from the module.

During the Apollo 12 mission, the astronauts experienced similar visibility decrease due to erosion of the soil, except that this time they noted no visibility during the final stages of decent.

According to Scott (1969), it was due to several factors including a lower thrust of the Apollo 11

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Lunar Module (approximately 5%) and a shorter lateral descent distance of Apollo 12. Besides, the footpads of the Apollo 12 Lunar Module penetrated approximately 2.5 cm (1 in) into the lunar surface.

Astronauts from Apollo 14 also spotted dust at the altitude of 30.5 m (100 ft) above the ground surface. According to Scott (1969), the dust seemed to be less visually disturbing than that encountered during Apollo 11 and 12 missions. He also pointed out that since the sun angle was different for each of the Apollo missions, the visibility of the dust therefore cannot serve as a reliable indicator of the quantity of surface material stirred up from the lunar surface during spacecraft decent. Similar behavior of the soil in response to the force of the footpads of the lunar module on the soil indicates that the soil of the Apollo 14 landing sites presented similar geotechnical and mechanical properties as that of the lunar soil from previous landing sites.

Dust erosion was first observed by the Apollo 15 astronauts at an altitude of 42.5 m (140 ft) above the ground surface. In addition, they reported a complete loss of visibility during the last

16.5 m (54 ft.) of descent. According to Scott (1969), the visibility condition was even worse than that of the Apollo 12 landing. However, as stated previously it is more likely to be caused by different sun angles than different quantities of soil erosion during landing. Penetration of the footpads of the Lunar Module of Apollo 15 into the lunar soil was again only a couple of centimeters into the surface.

The astronauts of the Apollo 16 commented that they first observed lunar surface dust erosion at altitudes ranging from 24 m (78 ft.) to 14.5 m (48 ft.) above the lunar ground surface and stated that there were no visibility issues throughout the entire landing process. This is the first report of such phenomenon out of all the previous Apollo missions. It was also noted that the

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footpads of the Luna Module of Apollo 16 penetrated only a few centimeters into the soil which

is similar as that of the previous mission.

Similar to Apollo 16, the first visible signs of dust erosion were report by the Apollo 17

astronauts at altitudes of 18 m (60 ft.) above the ground surface, but no visibility issues were

reported throughout the landing process. Mitchell (1973) stated that the surrounding area to a

distance of approximately 50 m from the Luna Module was likely to be influenced by the exhaust

from the descent. It was also reported that there were fewer small rock fragments (less than 10 cm

in diameter) and soil clumps were seen within the Lunar Module lading site. According to the crew

members’ observations and comments, the mechanical properties of the lunar soil in the area of

this landing site presents similar geotechnical behavior as the lunar soils in areas of previous

Apollo landings. Based on the records from all six Apollo missions, Scott (1969) concluded that

the erosion and visibility issues caused by dust expelled by the descent engines are affected by the

descent trajectory, descent rate, sun angle and torque.

2.4.1.4 Footprint Analysis

Astronauts’ footprint (refer to Figure 2-6) serves as an important tool to estimate the

porosity and relative density of the surface layer of lunar soil by analyzing the footprint depth on

the lunar soil (Mitchell et al., 1973). A total of 144 were investigated and analyzed based

on the Apollo 17 missions. It was estimated that the contact stress of the lunar footprint is 7 kPa,

and the relative density was estimated based on the assumptions that the minimum and maximum

porosity of the lunar soil is 58.3 and 31.0 percent, which was previously determined based on a crushed lunar soil simulant (Mitchell et al. 1973). Therefore, the relative density can be calculated by the following equations,

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( ) ( ) = × (2-1) ( ) ( ) 1−𝑛𝑛𝑚𝑚𝑚𝑚𝑚𝑚 𝑛𝑛𝑚𝑚𝑚𝑚𝑚𝑚−𝑛𝑛 𝐷𝐷𝑟𝑟 1−𝑛𝑛 𝑛𝑛𝑚𝑚𝑚𝑚𝑚𝑚−𝑛𝑛𝑚𝑚𝑚𝑚𝑚𝑚 Where the n is the in situ porosity of the lunar soil, and nmax and nmin are the maximum and minimum porosity of the lunar soil, respectively. It should be noted that some variability may exist in the in situ estimates of porosity with respect to the different landing sites on the moon, based on the estimation of the maximum and minimum porosity.

Figure 2-6 First footprint on the Moon by Neil Armstrong (Credit NASA)

2.4.1.5 Trenching Tests and Boulder Tracks

Surveyor III and VII were the first to dig into the lunar surface by using the surface sampler. Later, the astronauts of the Apollo 12 missions reinvestigated and photographed the trenches dug during the Surveyor III mission. It was noted that these trenches had vertical depths as great as 17.5 cm and were still in very good condition after the lapse of 31 months between missions (Johnson et al. 1995). This suggested that the cohesion which enables the trenches to stay vertical is not affected over time due to the unique lunar environment and the space weathering

38 processes of the Moon. It was also observed that there was a 2 to 15 centimeter soil layer on top of a rock in the Surveyor VII trench (Johnson et al., 1995).

The trench excavated during the Apollo 12 mission also stayed vertical in nature with a vertical depth of 20 cm. Three different soil strata were observed along the trench wall including a dark surface layer, an intermediate agglutinate or glassy layer, and a lighter colored layer. A trench was dug into the lunar soil near the rim of a crater during the Apollo 14 mission. The trench wall was very weak and stayed for only a short period of time. According to Johnson, the cohesion of the soil in the trench area was estimated to be only 10 percent that of the soil in the surrounding areas on the Moon. It was indicated from the trench analysis that the soil on the crater slopes is likely less dense and weaker than the soil in the overlying flat and level areas. However, Mitchell

(1973) noted that the trench constructed by Apollo 17 which was excavated near the rim of

Crater at Station 4 revealed a very high cohesion in the material. He defined the cohesion as, “a tendency of the material to break into chunks.” Based on the investigation of the coring sample from the location, it was observed that there is a large portion of glass or agglutinate particles existing in the upper orange layer of soil which consisted mostly of an orange soil. The cohesion observed in the lunar soil comes from the elongated and angular shape of the soil particles. The unusually high cohesion here is likely an indication of the extremely irregular shape and interlocking of the solid particles.

Based on investigation of the all the Apollo and Surveyor trench testing results, it can be concluded that regional variations and variations with respect to the depth exist in the soil properties on the Moon. During Apollo 17 mission, astronauts also investigated the tracks caused by lunar boulders rolling or skidding down lunar slopes, in addition to the trench investigation performed during the previous lunar missions. By analyzing the test results, Michell et al. (1973)

39 found out that, “in a qualitative sense, boulder tracks serve as exploratory trenches and can provide information about regolith thickness and history, and the relative sharpness of track features provides some indication of soil movement after track formation.” The information collected from the boulder tracks, including slope angle and bearing pressure can also be used to analyze the strength and density of the lunar soil in the region of the boulder track (Hovland and Mitchell,

1971). The friction angle values were estimated based on the bearing capacity theory for footing on slopes (Meyerhof 1951). By assuming the soil density of 1.6 g/cc and a cohesion of 1 kPa representative of the upper values for cohesion of lunar soil, the friction angle was determined

(Mitchell et al. 1973). It is worth note that in the friction angle analysis, an error in the pre- estimated cohesion of up to 10 kPa would result in an insignificant error of only one to two degrees

(Mitchell et al., 1973). The frequency distribution of the friction angle values was estimated from the boulder track analysis in the Taurus area from the Apollo mission. An average friction angle of 37.3 degrees with a standard deviation was deduced from the analysis for the Taurus

Littrow region on the Moon. Theoretically speaking, the flow of coal refuse would take place under static loading only when the is above the liquid limit. Therefore, one of the most important hypotheses in this research considered the liquid limit as the critical water content at which the coal refuse flow will be triggered. Based on such hypothesis, the samples were prepared with initial water content higher than liquid limit for static loading tests. However, under the impact loading, excess might be generated to increase the flowability of coal refuse even if the initial water content is below liquid limit. Therefore, in tests on impact loading, some samples were prepared with initial water content lower than liquid limit.

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2.4.1.6 Penetrometer Tests

Luna 17 mission performed the first penetrometer test on the moon by using the Lunokhod

1 penetrometer in 1970. The Soviet Union Lunokhod 1 penetrometer has a conevane end effecter

with cone base area of 5 cm2, a cone height of 4.4 cm, and an apex angle of 60 degrees. Four cone vanes are arrayed in the penetrometer by an angle of 90 degrees. These vanes spanned approximately 7 cm in diameter.

The Lunokhod 1 penetrometer was mounted on the chassis of the Lunokhod 1 .

It measured the strength of the lunar soil by being driven into the soil and rotated when the vehicle was stopped. The data of the depth of penetration and the accompanying force was obtained by a set of different sensors. The force required to rotate the vanes and the rotation angle were also recorded. The maximum penetration depth of Lunokhod 1 penetrometer was 10 cm and the applied force was greater than 196 N (44 lb). During the Luna 17 mission, the penetration tests were performed regularly every 15 to 30 meters of the traverse of the lunar surface. Johnson and Carrier

(1971) provided the penetration data from four different locations on the lunar surface in their report. It is important to note that the depth zero represents when the base of the cone is flushed with the ground surface, and all the negative values therefore are indications of the initial insertion of the cone up to the point where the base of the cone is flush with the soil surface. Lenovich et al.

(1976) also investigated and reported the cone vane penetrometer results of Lunokhod 2.

According to Lenovich, 7.5 percent of all tests accounted for the loose soil overlying a hard base,

3 percent accounted for the cases of hard stony soil, and 4 percent accounted for the case in which hard soil overlaid loose soil. It was also concluded from the test results that the strength of lunar soil in inter-crater locations was generally much lower than that of the crater rims. It was also noted that the soil strength of the rim of the crater decrease as the size of the crater decrease (with

41 respect to the diameter). This corresponds to the facts that the lunar soil relocated from the interior of the crater to the rim of the crater following the ballistic flight paths, and the rate of settlement of soil in the rim of the crater increases with flight distance and thus increases with crater size.

However, there is no specific records to show the locations on the lunar surface where these data were obtained. According to Johnson and Carrier (1971), the locations were generally described as: horizontal section of the lunar surface, crater slope, crater rim, and surface section covered by small stones. Since the route of motion of Lunokhod 1 traversed over 10,000 meters of the near the Sea of Rains, it can be inferred that the four penetrometer tests were recorded at locations within this vicinity. Lenonovich et al. (1976) also noted that the Lunokhod 2 traversed the region of the Lemonnier crate, which is located in the at the east of the Mare Imbrium, that is, in the transitional zone from the sea or mare region to the highland region. Figure 2-7 shows the location of the Mare Imbrium on the lunar surface.

A testing device which is referenced as the Apollo Simple Penetrometer or ASP was uitilized during the Apollo 14 mission. The ASP has a cone tip of 30 degree attached to a shaft approximately 0.95 cm in diameter and 68 cm in length. This device is used to determine the variation of penetration resistance with respect to different lunar locations. According to Johnson et al. (1995), this device was manually pushed further into the lunar ground surface as far as possible with one hand. When it reached the maximum penetration depth, it was furthered pushed into the ground with the use of both hands. The one and only operator of this device is astronaut

Astronaut . The penetration resistance of the lunar soil was estimated by applying the forces that Mitchell could exert with single and double-hands as well as the depth of the penetration for each run. The maximum penetration depth obtained was 68 cm below the lunar ground surface. The results from the tests were interpreted to relate to the soil strength parameter

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of cohesion and internal friction angle. Comparison with the data from the Surveyor mission found that the values of cohesion and friction angle were a bit higher than that of the Surveyor mission.

This is due partly to the fact that the applied penetration forces were not precisely measured.

Figure 2-7 Location of Mare Imbrium (Anonymous, 2015)

A more advance type of lunar cone penetrometer was utilized during the Apollo 15 and 16 missions, which is called the Self-Recording Penetrometer (SRP). The SRP was developed at the

Geotechnical Research Laboratory at the Marshall Space Flight Center (MSFC). The purpose of the device is to provide a graphical representation of the in situ penetration resistance of the lunar soil with respect to the depth of the soil profile. The SRP consisted of a detachable component with a handle, a data recording unit and probe components.

Flethcer et al. (1973) stated in his report, “The handle connects to the upper end of the data recording unit, and the probe connects to the lower end thereof. The data recording component has

(sic) metal recording drum on which a stylus scribes a permanent record. A pad assembly is

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slidably (sic) mounted on the probe to serve as a reference plane by resting against the soil surface

before the probe is force (sic) into the soil. The pad assembly is connected by a cable to the data

recording component so that the movement of the pad assembly relative to the probe as it enters

the soil will actuate the scribe to indicate the depth of penetration. Also, the data recording

component includes a mechanism to rotate the drum proportionately to the amount of force exerted

on the handle to cause penetration of the probe into the soil. motions of the drum

and stylus combined to produce a continuous force penetration diagram on the surface of the

recording drum.”

According to Flethcer et al. (1973), when the device was pushed into the lunar terrain by

an applied downward force, a plated cylindrical drum rotated correspondingly. Scratches

were therefore simultaneously made along the drum height according to the penetration depth.

Interchangeable end effectors were also attached with the device in three different sizes with cone

base areas of 129, 323, and 645 square mm, respectively, an apex angle of 30 degrees, and a 2.54

by 12.7 cm flat bearing plate. A SRP system had a total weight of 23 N (5 lb.) on Earth. The force

was applied through a spring-loading mechanism for this penetration device. Its maximum load is

111 N (25 lbs) and the maximum penetration depth is 75 cm (Johnson et al., 1995)

A total of six SRP tests were performed during the Apollo 15 missions. 323 mm2 cone tips were used in four of the tests and 2.54 by 12.7 cm2 flat bearing plates were used in two tests. The

test results which were scribed on the recording drum was returned to Earth for analysis. The tests

were performed during the end of the second extra-vehicular activity (EVA) on August 1, 1971.

And all the data acquired through the tests were taken near the Lunar Module at station 8, which

also located at the site of the Apollo Lunar Surface Experiments Package (ALSEP). ALSEP

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consists of a set of different scientific equipment to run autonomously for yielding long-term

environmental information about the lunar surface.

The SRP tests were operated and indexed by Lunar Module pilot . According

to Costes et al. (1972), “two of the cone penetration measurements were made within and adjacent

to an LRV track, and the other two were made adjacent to and at the bottom of a 30 cm deep trench

with a vertical sidewall.” The raw data from the penetration tests can be obtained from the National

Space Science Data Center (NSSDC) as part of its Lunar Data project. The data were documented

in the handwritten plots, which showed the stress applied at the penetrometer tip as a function of

depth of penetration.

It was noted that the surface-reference pad tended to raise the shaft of the device when it

was vibrated during the SRP test. It was likely caused by the lack of friction between the reference-

pad busing and the shaft, which was originally anticipated much higher. Therefore, even though

the weight of the reference pad was virtually balanced, it was still not enough to keep it from

slipping when the force of the spring was actuated to retract the pad. This resulted in a less accurate

recording of the penetration depth during the tests. Another possible cause of error was noted when

placing the SRP onto the surface when holding it by the housing, which showed records of vertical

penetration scribes without any accompanying recorded force. The errors may also be caused when

the operator leaned on the penetrometer to provide a relatively steady push for the penetrometer to

into the ground and lost balance. This required some weight to be remove from the

penetrometer to allow the spring to go back, which would lead to a spike in the reading.

Based on the SRP results, Carrier et al. (1991) estimated the encountered lunar soil to have an internal friction angle ranging from 46.5 to 51.5 degrees and cohesion from 0.25 to 1.0 kPa.

Mitchell et al. (1972) provided a summary of the cone penetration resistance test in terms of

45 average penetration resistance gradient with respect to the various depths (refer to Table 2-4). This data was obtained from the Apollo 15 landing site by using the cone tip with base area of 323 mm2.

Based on the penetration test results, a comparison with the terrestrial simulation data indicated that that the density of the in situ lunar soil near the trench at Station 8 ranged between 1.92 and

2.01 g/cc (Costes et al. 1972). The void ration of the in situ lunar soil from this area was accordingly found to range from 0.54 to 0.61 by assuming a specific gravity of 3.1.

A total of ten tests were performed during the Apollo 16 mission. 129 mm2 cone tips were used in six of the tests, 323 mm2 cone tips were used in two, and 2.54 by 12.7 cm2 flat bearing plates were used in two tests. The tests were performed during the second extra-vehicular activity

(EVA) on April 22, 1972. Lunar Module pilot was the tests operator and indexed each of the runs. The first four SRP tests were taken at Station 4, which was located at the side of

Stone Mountain and the rest of the tests were done at station 10, that is, the ALSEP site. It was noted that index number 9 and 14 were not included in the data set due to the fact that the index number 9 was skipped and index number 14 showed error in recording. The raw data from the tests can also be obtained from NSSDC, and they are shown in the same formats as that of the

Apollo 15 data. It is of great importance to note that there is error in recording the penetration depth for the SRP tests of Apollo 16 due to the facts that an additional length of the cone was attached to the bottom of the penetrometer.

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Table 2-3 Average cone index gradient for lunar soil near Apollo 15 landing site (Mitchell et al. 1972) Average Penetration Location Penetration Depth (cm) Resistance Gradient (kPa/mm) Adjacent to trench 8.25 4.06 Bottom of trench < 10.25 > 3.25 5.97 (Upper 2 cm) In LRV Track 5.25 (Upper 2 cm) 4.36 (Lower 4 cm) Adjacent to LRV Track < 11.25

2.4.2 Investigation on returned lunar soil

There are three major soil sample sources for lunar soil investigation on the Earth, including, the returned lunar soil from the Soviet Luna missions and Apollo missions, as well as the returned lunar soil from the lunar meteorites discovered on Earth. According to Vaniman et al.

(1991), a total of 10 meteorites of lunar origin have been recovered on the Earth from the Antarctic cap. These meteorites were well preserved in the ice cap after they made impacts on Earth.

Although study confirmed their lunar origin, however, these samples show variation in composition of the lunar materials from the lunar geologic samples recovered and returned to the

Earth. A total of 321 grams of lunar surface material were collected and returned to the USSR from the Luna missions 16, 20, and 24. These samples were collected from the lunar maria area, and are representative of the lunar mare material.

Scott (1969) stated, “The Apollo lunar landing missions provided the first opportunity for direct collection of data relating to the physical characteristics and mechanical behavior of the surface materials of an extraterrestrial body by other than remote means.” The six successful

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Apollo landings, Apollo 11, 12, 14, 15, 16 and 17 provided a total of 381.7 kilograms (841.5 lbs) of lunar soil and rock fragments (Vaniman et al. 1991a). It is important to note that the majority of these soil samples represent the dark basaltic lunar mare material while very little were sampled from the lunar highland area to present the lighter-colored -rich anorthositic rocks, which were mainly collected during the Apollo 16 missions.

All the Apollo samples were intended to be well sealed and transported to the Earth in their original vacuum environment. However, the lunar samples were exposed to the Earth’s atmosphere due to contamination of the lunar dusts which caused the seals to fail. All the geological lunar samples are currently preserved at the (JSC) Lunar Reviving Laboratory

(LRL) in , .

2.4.2.1 Apollo Soil Samples

There were six major type of lunar samples collected from the Apollo missions, including contingency samples, bulk samples, documented samples, selected samples, raked samples, and core samples (Vaniman et al., 1991a). The contingency samples were collected by the contingency sampler device which consists of a small rake and scoop. During the Apollo missions 11, 12, 14 and 15, after the first astronaut exited the lunar module, he would rake, scoop, and bag a small amount of the surface lunar material, which is referred to as the contingency samples. The majority of the collected samples are the fine-grained fraction of the lunar regolith due to the limitation of the sampling method. It was noted that a few small rock samples were obtained by the sampling astronauts. The contingency samples were collected in the very beginning before any further lunar investigation to make sure that some of the lunar surface materials were obtained in case the

48 mission had to be terminated early. However, as stated before, those samples were collected in a way that is only representative of a very small fraction of the lunar surface material.

Bulk soil sample was collected during Apollo mission 11 by astronaut Neil Armstrong.

Taken from 23 different locations near the lunar module, they represent a larger sample area than the contingency samples. A total of 38.3 kilograms (84.5 lbs) of lunar soil and rock material was collected to return to the Earth for laboratory testing. However, these samples were considered to be highly disturbed and cannot yield a reliable estimate of the bulk density of in situ lunar soil

(Johnson et al., 1995).

The lunar samples documented with photographs and written observation both before and after sample collection are generally referred to as documented samples (Vaniman et al., 1991a).

These documented photographs and descriptions were intended to help determine the “relationship of a sample and its surroundings before astronaut activity disturbed the surface” as well as “the effects of collection activity on the surface characteristics” after the astronaut activities and sample collection (Vaniman et al., 1991a). These documented material helped greatly in material identification and classification by providing data of color, reflectivity, description of surface features near the sample, and orientation as well as depth of burial (for rock samples). Besides, some samples were documented only with written description of astronauts’ observation from both before and after collection. This documentation method allowed the astronauts to collect the samples in a quicker fashion and thus obtain a larger collection of lunar surface samples.

In order to obtain a good number of rock materials, rake samples were collected during the

Apollo missions 15, 16, and 17. According to Vaniman et al., the astronauts used the sampling device to rake the surface materials on the moon to collect only the rocks larger than 1 centimeter, and left all the finer materials on the surface. After the samples are placed in the individual bag,

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one or more scoops of the fine-grained surface regolith will be added into the rock sampling bag

from the corresponding rake sampling location.

Core samples as described by Vaniman et al. (1991a) occupied a small fraction of 5.2

percent by mass of the total returned lunar samples collected during the Apollo mission. However,

they are extremely valuable given that it is the only returned sample type to provide dependable

information about the near surface texture and stratigraphy of the lunar regolith. According to

Vaniman et al., (1991a), “A returned core is the only type of Apollo sample that permits the detailed study of variations in the physical and chemical properties of the lunar regolith with depth”.

These samples also recorded the solar activity and flux, and structure of the lunar

regolith, and can be used to evaluate the lunar cratering history.

A total of 21 hammer core tubes were driven into the lunar surface near the landing sites

and collected lunar regolith samples during the Apollo 11, 12, 14, 15, 16, and 17 missions

(Johnoson et al., 1995). According to Mckay et al., (1991), the sample diameter ranged from 2 cm

(Apollo Mission 11 to 14) to 4 cm (Apollo Mission 15 to 17), depending on the type of driven tube

used, and the sample length ranged from 10 cm (Apollo Mission 11) to 298.6 cm (Apollo Mission

17). Ten of the samples were single length and 11 were double length. The coring samples were

first obtained by hammering the hollow drive tubes into the soil. However, these samples were

generally short in length (about 30 cm) and secured incomplete sections. The first lunar sample

with clear indication that the lunar regolith is layered was collected near the Apollo 12 landing site

on the rim of a 10 m diameter crater, via a double core driven sampling tube (Mckay et al., 1991).

During the earlier Apollo Missions (Apollo 11, 12, and 14), thick-walled coring tubes were utilized according to the terrestrial standards. According to Carrier et al. (1971) and Johnson et al. (1995), this type of coring tube produced significant disturbance of soil during sampling. A new thin-

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walled aluminum core tube was first applied into the lunar regolith sampling during the Apollo

Mission 15. This new type of driven tube was developed to reduce the disturbance of soil during

sampling, while increasing the size and quantity of sample, and thus create an easier and better

sampling method (Costes et al., 1972). This thin-walled hollow aluminum driven tube has a length

of 37.5 cm, inner diameter of 4.13 cm and outside diameter of 4.38 cm. This core tube can be

driven individually or used in combination. The ones that are connected together are called “double

core tube”. The sampling device were also equipped a flat disk-like device, “keeper”, which had

the same diameter of the tube to be pushed inside downward to the top of the sample to keep it in

place. When the sampling tube was removed from the ground, a Teflon cap was placed at the bottom of the tube.

Comparison of astronaut observation showed that the experience of driving the core tubes into the lunar soil varied for each different location and mission. It was reported that the cored tubes could be easily driven into the ground during the Apollo 11 and 12 missions. In comparison, observation from the Apollo 14 mission showed that the core tubes could only be driven to shallow depth (Johnson et al. 1995). According to Mitchell et al (1973), the core samples collected by double core tubes during the Apollo 17 mission revealed that higher density samples were found in the lower tube than in the upper tube. Similar observation was reported with regard to the Apollo

15 and 16 core tube samples. It can be inferred that the lunar soil density generally increases with

increasing depth. Comparison between the core tubes also indicated that the soil samples collected

with larger diameter and thinner walled tubes were generally less disturbed and served as a more

reliable source for the in situ bulk density study.

During Apollo Mission 15, 16, and 17, a battery powered rotary drill, the Apollo lunar

surface drill (ALSD), was used to collect additional core samples. More than 4 kg of rotary drill

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core tube samples have been collected, with a diameter of 2 cm and depth of 221 to 292 cm

(Vaniman et al., 1991a).

2.4.3 Engineering properties of lunar soil

2.4.3.1 Introduction

As Gromov stated (1998), the knowledge of the physical and mechanical properties of the

soil is of great importance because it is fundamental and basic for any engineering activities aimed

at construction of lunar bases and for mineral resource explorations. According to Mckay (1991),

the space weathering process occurred on the lunar surface in combination to yield “a regolith

whose structure, stratigraphy, and history may vary widely, even between locations only a few

meters apart.” However, compared to the range of soils encountered on Earth, geotechnical properties of lunar soils are relatively narrow (Carrier et al., 1991). The lunar soils formed by space weathering processes such as impacts tends to yield a heterogeneous and poorly-sorted

(well-graded) soil (refer to Figure 2-8). This can be contrasted to the variety of environmental processes effecting terrestrial soil types. Also, the lack of water, , minerals and organic

materials on the Moon reduces lunar soil variability. A third factor distinguishing lunar soils from terrestrial soils is the smaller variety of minerals composing the lunar soil (Carrier et al., 1991).

The geotechnical properties of lunar soil therefore fall into a relatively narrow range.

This section provides a general summary of the engineering properties of the lunar soil as

determined by investigation performed in situ by and astronauts on the lunar soil, in the

laboratory on the returned lunar soil samples, and by from the Earth’s surface or

lunar orbit. A tabulated summary of all recommended geotechnical parameters of lunar soil is

provided in Appendix A. The chemical and mineral composition of the lunar soil will not be

discussed here since they are not pertinent to the engineering or mechanical studies. An in-depth 52

discussion on the chemical properties of lunar soil can be found in Haskin and Warren (1991)

while the mineral properties of lunar soil can be found in Papike et al. (1991).

Figure 2-8 Lunar surface is exposed to solar wind and constantly pounded by micrometeorites. (Credit: Larry Taylor, Univ. of Tennessee)

2.4.3.2 Particle size and shape

The particle size distribution is one of the most important geotechnical properties for a such as lunar soil, affecting the strength and compressibility of the material, as well as its optical, thermal, and seismic properties (Carrier, 1983). According to Carrier (1973,

1991), the particle size distribution is directly related to the origin and mode of deposition of a soil.

For lunar soil, the particle size distribution is dominated by the meteoroid impact process, that is, the comminution, agglutination, and continuous mixing process as previously described.

The particle size distribution of the lunar soil samples was determined primarily by a sieving method, which is generally effective for particles sizes greater than about 10 µm (Carrier et al., 1991). Due to the limitation of sample size, various sieving techniques have been developed and utilized including dry, dry with brushing, dry with sonic sifter, wet (Freon) with vibration, wet

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(methanol) with sonic sifter and wet (acetone) with vibration (Carrier, 1973). Other investigation

methods have also been utilized including dispersion, a conduct metric method, and

microphotography methods.

Carrier et al. (2003) conducted a thorough investigation on the particle size distribution of

lunar soil based on nearly 350 lunar samples, taken in the vicinity of seven different landing sites

on the Moon: Apollo Mission 11, 12, 14, 15, 17, and Luna Mission 24. The study results were

shown as in Figure 2-9. There are approximately 4500 points in Figure 2-9, about 90% of the data

are obtained from compilation by Graf (1993), the remaining ones are acquired by other sources:

Duke et al. (1970 a), Frondal et al. (1970), Hapke et al. (1970), Frondel et al. (1971), Heywood

(1971), et al. (1971), Linday (19710, LSPET (1971), Quaide et al. (1971), Sellers et al. (1971),

Carusi et al. (1972), Clanton et al. (1972), King et al. (1972), LSPET (1972), Mckay et al. (1972),

Basu et al. (2001), as well as Carrier (2003). As shown in the Figure 2-9, the lunar soil particle size distribution is nearly perfectly log-normal. It is also important to be noted that the proportions of the particle types which compose the lunar soil generally vary from location to location as well as with depth. Nonetheless, the majority of these lunar soil particles fall in a fairly narrow range of particle size distribution as shown in Figure 2-9. (Carrier et al. 2003).

It was also reported by Carrier et al. (1991), that the mean grain size of the lunar soil ranges from 40 to 800 µm, while the majority of the particles fall into the size range between 45 to 100

µm. The median particle size of the lunar soil ranges from 40 to 130 µm with an average of 70 µm.

According to Sibille et al. (2006), roughly half of the lunar soil consisted of very fine particles, which are much smaller than typical beach sand found on the Earth. More specifically, approximately 10% to 20% of the lunar soil is found to be smaller than 20 µm and a thin layer of

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dust exists on the lunar surface which adheres electro-statically to everything that comes in contact with the soil: spacesuits, tools, equipment and lenses (Carrier et al., 1991).

Gromov (1998) found that the overall uniformity of the particle size distribution was not significantly affected by the spatial variation of lunar soil during sample collection. However, the average particle size generally increased with increasing sample depth, based on the particle size distribution study of the lunar soil from Soviet investigations Luna 16 and Luna 20.

Figure 2-9 Grain size distribution of lunar soil. (Carrier et al. 2003)

In general, the investigation of the particle size distribution of lunar soil found that the lunar soil is a well-graded, sandy material, with approximately half of the soil particle by weight irresolvable with the unaided eye, otherwise described as SW-SM to ML as determined by the Unified System (USCS).

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Particle Shape

The lunar soil particles tend to be irregular in shape, ranging from round to extremely

angular, as shown in Figure 2-10. As described by Carrier et al., (1991), the lunar soil particles tend to be fairly elongated and are generally sub-angular to angular. The particle shape parameters of average lunar soils are presented in Table 2-5.

Figure 2-10 Irregular lunar soil particles (credit: NASA)

Table 2-4 Average particle shapes of lunar soil (Carrier et al., 1991)

Parameter Average Value Description

Elongation 1.35 Somewhat elongated Aspect Ratio 0.55 Slight to medium elongated

Roundness (Silhouette) 0.21 Sub-angular Roundness (Direct light) 0.22 Angular

Volume Coefficient 0.3 Elongated Specific Surface Area 0.5 m2/g Irregular, re-entarnt

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According to Carrier et al. (1991), the particles tend to pack together with a preferred

orientation of the long axes. This phenomenon has been observed in the past lunar core tubes as

well as the laboratory simulations. The in situ physical properties of lunar soil is therefore

anisotropic due to the preferred orientation.

Specific surface area is defined as the surface area of a particle divided by its mass. As

shown in Table 2-5, the average specific surface area of a lunar particle is approximately 0.5 m2/g,

which indicated that the surface area is nearly eight times that of equivalent-sized sphere for a

typical lunar soil. It can also be inferred that the lunar soil particles have extremely irregular

surfaces given this relatively large value, which will cause the particles to interlock with one

another and tends to be highly abrasive. The interlocking of the irregular particles can account for

why the cohesion exists in the dry granular lunar soil. According to Carrier et al. (2005), “the

cohesion (and the frictional shear strength) allowed the astronauts to dig trenches in the lunar

surface with smooth, nearly vertical walls” (as shown in the Figure 2-11). As reported by Carrier et al. (1991), the vertical trenches can be excavated and stayed open in the lunar surface for up to

3 m deep, with a safety factor of 1.5, due to the relatively low shear strength and low gravity. With the special properties of the individual particles, lunar soil can best be compared with a cobble- bearing silty sand, fine grained slag, or terrestrial volcanic ash (Carrier et al. 1991).

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Figure 2-11 Trench wall from Apollo Mission 17 (credit: NASA)

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2.4.3.3 Specific Gravity

The specific gravity, by definition, is referred to as the ratio of mass density of soil particles

to the mass density of pure water at 4º Celsius. It is a function of the proportions of rock and

minerals making up the soil (Sibille et al. 2006). As one of the most important geotechnical

properties, the specific gravity, however, did not receive as much attention as that of other

geotechnical parameters in the lunar samples program. This is due to the fact that the quantity of

lunar soil is insufficient for the standard specific gravity tests which required at least 30 g soil

samples for air comparison pycnometer and 50 g for conventional 500-cm3 water pycnometer tests.

Various techniques therefore have been developed to study the specific gravity of lunar soil by use of much smaller sample size and small volumetric apparatus.

Carrier summarized the specific gravity values of lunar soils and rock fragments based on past studies as shown in Table 2-6. In general, the specific gravity of lunar soil ranges from 2.3 to greater than 3.2. Carrier et al. (1991) suggests a value of 3.1 be applied for all general engineering analysis pertaining to the lunar soil, higher than the typical value of 2.7 for most terrestrial soils.

This is due to the sub-granular porosity of lunar soil. Generally speaking, the specific gravity tests measure the volume of the voids of samples by means of immersing the soil particles into a fluid

(water, air, or helium) to measure the volume that it displaces. However, the fluid cannot fill these sub-granular voids, which resulted in an underestimate of the void volume. The average specific gravity of the lunar soil was therefore thought to be underestimated. Based on the fact that sub- granular voids exist in the lunar soil, Carrier et al. (1991) suggest that the specific gravity of lunar soils would better be estimated in a range between 2.9 to 3.5.

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Table 2-5 Average particle shapes of lunar soil (Carrier et al., 1991)

Sample Specific Sample No. Weight Test Technique References Gravity, Gs (g) Costes et al. 10004 and 10005 49.1 3.1* Nitrogen pycnometry (1970a) Horai and Winkler 10020, 44 5.94 3.25** Water Pycnometry (1980) Horai and Winkler 10065, 23 4.48 3.12*** Water Pycnometry (1980) Suspension in density Dukke et al. 10084 1.5 3.01 gradient (1970b) Unnumbered (from Carrier 56.9 3.1* Air Pycnometry Apollo 12) (1970) Horai and Winkler 12002, 85 2.32 3.31*** Water Pycnometry (1980) R. F. Scott 12029, 8 1.10 2.9 Nitrogen Pycnometry (1988) Heywood 12057.72 2.9 Unknown (1971) Cadenhead et al. 14163,111 0.65 2.9 ± 0.1 Helium Pycnometry (1972) Carrier et al. 14163, 148 0.97 2.90 ± 0.05 Water Pycnometry (1973a, b) Carrier et al. 14259, 3 1.26 2.93 ± 0.1 Water Pycnometry (1973a, b) 3.2 ± Cadenhead et al. 14321, 74 Helium Pycnometry 0.1*** (1972) 14321, 156 3.2 Cadenhead et al. Helium Pycnometry ± 0.1*** (1972) Cadenhead et al. 3.0 ± (1974) 15015, 29 Helium Pycnometry 0.1*** Cadenhead and Stetter (1975) Cadenhead et al. 15101, 68 0.96 3.1 ± 0.1 Helium Pycnometry (1972) Carrier et al. 15601, 82 0.96 3.24 ± 0.05 Water Pycnometry (1973a, b) Horai and Winkler 70017, 77 2.55 3.51** Water Pycnometry (1976) Horai and Winkler 70215, 18 4.84 3.44** Water Pycnometry (1976) Horai and Winkler 72395, 14 3.66 3.07*** Water Pycnometry (1976) Horai and Winkler 77035, 44 3.68 3.05*** Water Pycnometry (1976) * Total soil sample; others were performed on submillimeter fraction;

** Single basalt fragment;

*** Single breccias fragment.

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2.4.3.4 Density

The bulk density of soil is defined as the mass of materials contained within a given volume.

The in situ bulk density is one of the major factors that influence the physical and geotechnical properties of soil, including the soil strength, compressibility, and permeability. In addition, the soil density also influences the bearing capacity, slope stability, seismic wave velocity, thermal conductivity, electrical resistivity, and the depth of penetration of ionizing radiation (Carrier et al.,

1991).

Even prior to the first lunar landing, the in situ bulk density of lunar soil was estimated to

be 0.3 g/cm3 based on data from remote sensing (Jaffe, 1965a and 1965b). A value of 0.4 g/cm3

was also estimated via the same method by Halajian (1965). During the Surveyor I Mission, a

much higher value of 1.5 g/cm3 was estimated, based on the records of interaction between the

lunar soil and the spacecraft footpads, as well as the television image analysis. The first in situ

bulk density of lunar soil was measured during the Luna 13 landing and a value of 0.8 g/cm3 was

obtained (Cherkasov et al., 1968). The most important data for the bulk density studies were

obtained through measurement of the returned core samples.

As stated before, core samples are the most valuable returned sample as they are the only

sample type that provide dependable information about the near surface texture and stratigraphy

of the lunar regolith. A total of 16 kg of lunar soil samples was obtained via driven core tubes,

from depths to 70 cm into the lunar surface.

During Apollo Mission 1, two core soil samples were collected via single drive core tubes

and measured bulk densities of 1.59 and 1.71 g/cm3. During Apollo Mission 12, in addition to the

two single core tubes, a double (two tubes connected end to end) core tube was also utilized. Based on the returned lunar samples, the measured bulk densities ranged from 1.74 to 1.98 g/cm3. The

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Apollo 14 mission utilized the same core tubes as Apollo 12. Four core samples were thus

recovered from the two single core tubes and one double core tube. The measured bulk density

from the single tube was 1.6 g/cm3, while that of the double tube was 1.73 and 1.75 g/cm3 for the upper and lower halves, respectively. In spite of the use of a double core tube in Apollo 12 and

14, the samples obtained from Apollo 11, 12, and 14 are generally considered as significantly disturbed due to the fact that the wall thickness was relatively large compare to the internal diameter of the core tube. A new thin wall drive core tube was used during the Apollo 15, 16, and

17 missions. The new core tubes increased the soil sample size and the amount of sample obtained with a reduced wall thickness, resulting in much less soil disturbance during sampling. It is noted that the sample recovery is nearly 100% by use of the new core tube. Thus, an accurate measurement of the in situ bulk density can be obtained based on samples collected by the new core tubes. During Apollo Mission 15, five samples, one single and two doubles, were collected by use of the new core tubes, with calculated bulk densities ranged from 1.36 to 1.85 g/cm3 (Carrier et al., 1992). According to Carrier et al. (1992), those values are close enough to be used as the in situ densities. Based on the returned nine core tube samples (one single and four doubles) from the

Apollo 16 mission, Mitchell et al. (1972b) calculated the in situ bulk densities to vary from 1.40

to 1.80 g/cm3. Eight core tube samples were collected during Apollo Mission 17: two singles and

three doubles. According to Mitchell et al. (1973), the in situ bulk density was estimated to range

from 1.40 to 1.80 g/cm3.

In addition, a rotary drill core tube was also utilized during the Apollo 15, 16, and 17

missions. More than 4 kg of was obtained by methods of rotary drill core tube, from depths of up

to 3 m. Similar to the thin-wall core tubes, the rotary drill core consists of a very thin wall, which

causes less soil disturbance during sample collection. The measured bulk densities based on the

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core samples are therefore regarded to be close enough to represent the in situ conditions (Carrier

et al., 1991). Both Apollo 15 and 16 drill stems consist of six sections, with a total assembled

length of 242 cm, while the Apollo 17 drill stem has eight sections, with a longer assembled length

of 321.8 cm. The measured bulk densities of the returned samples from Apollo 15 drill cores

ranged from 1.62 to 1.93 g/cm3. According to Carrier et al. (1974), the in situ densities are probably

±2% of the measured bulk densities in the stem. The in situ bulk densities of Apollo 16 drill cores were found to vary from 1.47 to 1.75 g/cm3, while the bulk densities of Apollo 17 drill cores vary

from 1.74 to 1.99 g/cm3. The in situ bulk densities of lunar soil based on various estimating

methods and measuring techniques are summarized in the Table 2-7 (Carrier et al., 1991):

Table 2-6 Estimates of lunar soil in situ bulk density (Carrier et al., 1991).

Source Bulk Density (g/cm3) Investigator

Remote Sensing 0.3 Jaffe (1965a, 1965b)

0.4 Halajian (1964) Robotic Measurements on

Surface Surveyor I 1.5 Christensen et al. (1967) Luna 13 0.8 Cherkasov et al. (1968) Scot and Roberson (1967, 1968 Surveyor I, III, and VII 1.5 a, b); Scot (1968) 1.1 (surface); 1.6 (depth of 5 Surveyor I Jaffe (1969) cm) Leonovich et al. (1971 and Lunokhod 1 / Luna 16 1.5 to 1.7 1972) Surveyor III 1.7 Jaffe (1969) Leonovich et al. (1975 and Lunokhod 1; 2 / Luna 16; 20 1.5 1977) Correlations with Simulated

Lunar Soil Astronaut Bootprints Inter-crater area 1.45 to 1.59 Mitchell et al. (1974) Crater rims (depth of 0 - 15 cm) 1.34 to 1.57 Mitchell et al. (1974) Vehicle Tracks

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MET and LRV 1.40 to 1.56 Mitchell et al. (1974) (Depth of 0 - 15 cm) Boulder Tracks 1.38 to 1.68 Mitchell et al. (1974) (Depth 0 – 400 cm) Penetration Resistance Apollo 11 < 1.81 to 1.92 Costes et al. (1971) Apollo 12 < 1.80 to 1.84 Costes et al. (1971) Lunokhod 1/ Apollo 14 to 16 1.58 to 1.76 Mitchell et al. (1974) (Depth 0 – 60 cm) Returned Core Samples Apollo 11 1.54 to 1.75 Coste and Mitchell (1970) 0.75 to > 1.75 Scott et al. (1971) .6 to 2.0 Scott et al. (1971) 1.55 to 1.90 Houston and Mitchell (1972) 1.7 to 1.9 Carrier et al. (1971) Luna 16 1.2 (1971) Apollo 14 1.45 to 1.6 Carrier et al. (1972a) Apollo 15 Carrier et al. (1972a) Core Tubes 1.36 to 1.85 Mitchell et al. (1972a) Carrier et al. (1974) Drill Tubes 1.62 to 1.93 Mitchell et al. (1972a) Luna 20 1.1 to 1.8 Vinogradov (1972) Apollo 16 Core Tubes 1.57 to 2.29 Mitchell et al. (1973) Drill Tubes 1.74 to 1.99 Carrier (1974) et al. (1977); Luna 24 1.6 to 2.1 (1977)

Based on results from the Apollo soil mechanics experiment S-200, Mitchell et al. (1974) provides the best approximations of the lunar soil bulk density with respect to depth, which are shown in Table 2-8. According to Carrier et al. (1991)., best estimates of bulk density values are deduced from the statistical average of Apollo 15 to 17 core tube densities. It can be concluded

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from the study that the bulk density of the lunar soil generally increases with increasing depth. It

is important to note that these values are representative of the bulk density of inter-crater areas.

Table 2-7 Best estimates of average bulk density (Mitchell et al., 1974)

Average Bulk Density (g/cm3) Depth (cm)

1.50 ± 0.05 0 to 15

1.58 ± 0.05 0 to 30 1.74 ± 0.05 30 to 60

1.66 ± 0.05 0 to 60

A hyperbolic relationship between density and depth was proposed by Carrier et al. (1991):

= 1.92( + 12.2)/( + 18) (2-2)

Where z is the lunar surface𝜌𝜌 depth in terms𝑧𝑧 of centimeters𝑧𝑧 (cm). Based on this relationship,

the bulk density ρ is estimated to be approximately 1.30 g/cm3 at the surface, increase to 1.52 g/cm3 at a depth of 10 cm, then 1.83 g/cm3 at a depth of 100 cm; and finally, it asymptotically

approaches a value of 1.92 g/cm3. This density relationship falls within the bounds established by

Mitchell et al. (1974) within a depth of 60 cm. According to Carrier et al. (1991), the maximum

density is probably fairly reasonable to a depth of 3 m.

Relative Density

The relative density is likewise one of the most influential geotechnical properties of lunar

soil, dependent on the sizes and shapes of the individual soil grains. The relative density is

determined based on the following relationship:

= × × 100% (2-3) 𝜌𝜌𝑚𝑚𝑚𝑚𝑚𝑚 𝜌𝜌−𝜌𝜌𝑚𝑚𝑚𝑚𝑚𝑚 𝐷𝐷𝑅𝑅 𝜌𝜌 𝜌𝜌𝑚𝑚𝑚𝑚𝑚𝑚−𝜌𝜌𝑚𝑚𝑚𝑚𝑚𝑚

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Where ρ is the bulk density of the soil sample, and ρmax is the maximum bulk density,

while ρmin is the minimum bulk density. The relative density generally accounts for the degree of

particle packing of soil. It therefore greatly influences the shear strength and compressibility of the

soil.

Based on the best estimates of bulk density tabulated above, Mitchell et al. (1974) and

Houston et al. (1974) cited by Carrier et al. (1991) provided estimates of the relative density of

the lunar soil with respect to depth, which is as shown in Table 2-9. These relative densities have

been described in terms of loose to very dense according to Lambe and Whitman (1969). Likewise,

the relative density of the lunar soil tends to increase with increasing depth. Again, it is important

to keep in mind that these corresponding relative densities only represent the lunar soil at the inter- crater areas.

Table 2-8 Relative density of the lunar soil (Carrier et al. 1991)

Relative Density (%) Depth (cm) Soil Description (Lambe and Whitman 1969) 65 ± 3 0 - 15 Medium to Dense 74 ± 3 0 - 30 Dense 92 ± 3 30 - 60 Very Dense 83 ± 3 0 - 60 Dense

Porosity and Void Ratio

Based on the recommended specific gravity value of 3.1 of the lunar soil, in combination

with the best estimates of the bulk density of lunar soil (Table), the in situ porosity and void ratio of soil can be related as below:

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= (1 ) (2-4)

𝑤𝑤 Where ρ is the bulk density of lunar𝜌𝜌 𝐺𝐺soil,𝜌𝜌 G −is 𝑛𝑛3.1, the specific gravity of lunar soil, ρw is the density of water, that is 1 g/cm3, and n is the porosity of the soil, which can be obtained by the expression below:

= 1 (2-5) 𝜌𝜌 𝐺𝐺𝜌𝜌𝑤𝑤 The porosity represents the ratio of𝑛𝑛 the volume− of the voids to the total volume of the soil sample, while the void ratio of a soil represents the ratio of the volume of the voids to the volume of the soil solids within the soil sample. According to the definition, it can be expressed in terms of soil porosity by the following equation:

= (2-6) 𝑛𝑛 The best estimates of the porosity and𝑒𝑒 void1 −ratio𝑛𝑛 of the lunar soil are presented in Table 2-

10. Likewise, the porosity and void ratio of the lunar soil generally tends to decrease with increasing depth.

Table 2-9 porosity and void ratio of the lunar soil (Carrier et al. 1991)

Depth Range (cm) Average Porosity, n Average Void Ratio, e

0 - 15 52 ± 2 1.07 ± 0.07

0 - 30 49 ± 2 0.96 ± 0.07

30 - 60 44 ± 2 0.78 ± 0.07 0 - 60 46 ± 2 0.87 ± 0.07

Similar to relative density, the porosity and the void ratio also refers to the degree of soil packing. Gromov (1998) provides a summary of the bulk density and void ratio of returned lunar soil samples in terms of loose and dense condition, as shown as in Table 2-11.

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Table 2-10 Bulk density and void ratio of returned lunar soils (Gromov 1998)

Lunar Mission Bulk Density (g/cm3) Void Ratio

Soil Condition Loose Dense Loose Dense

Luna 16 1.115 1.793 1.69 0.67

Luna 20 1.040 1.798 1.88 0.67

Apollo 11 1.36 1.8 1.21 0.67

Apollo 12 1.15 1.93 N/A N/A Apollo 14 0.87 – 0.89 1.51 – 1.55 2.37 – 2.26 0.94 – 0.87

Apollo 15 1.1 1.89 1.94 0.71

2.4.3.5 Compressibility

The compressibility of a soil is, by definition, the change of the volume of a soil when a confining pressure is applied. In other words, it is a measure of the change in density of the soil in response to a pressure change. Compressibility plays an important role in the engineering design of structural foundations, especially for settlement analysis. It is therefore a vital parameter in the design of vehicle and excavation tolls in both the lunar and Martian missions.

Compressibility of soil generally occurs in two phases. The first phase takes place for soil under low initial confining pressure or in a state of low density. During this phase, inter-granular slippage occurs and the particles reorient themselves to fill in voids that previously existed in the

“honeycomb ”. The second phase takes place when the confining pressure increased or if the initial confining pressure is fairly high. This phase is characterized by the soil particles being deformed and/ or fractured, in other words, generally breaking near the points of contact.

In general, the compressibility of a soil is described in terms of the compression index, Cc, which by definition can be obtained by the following relationship:

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= (2-7) ∆𝑒𝑒 𝐶𝐶𝑐𝑐 − ∆𝑙𝑙𝑙𝑙𝑙𝑙𝜎𝜎𝑣𝑣 Where e is the void ratio, and σv refers to the applied vertical stress. Typically the compression index is measured by performing an . In an oedometer test, a series of one dimensional loads is applied to a soil sample with a load increment ratio Δp/p = 1.

Concurrently, the deformation in response to the load is measured. When the test is completed, the results are plotted as void ratio versus the logarithm of applied effective vertical stress. Then the compression index Cc can be obtained by the equation above.

Similarly, the swelling index can also be measured through the oedometer test. In this test,

the vertical load on a soil sample is gradually reduced with a constant decrement ratio Δp/p = 1

and the corresponding void ratios are measured. Likewise, based on the measured data from the

unloading line, the swelling index of a soil can also be determined by the relationship as follows:

= (2-8) ∆𝑒𝑒 𝑟𝑟 ∆𝑙𝑙𝑙𝑙𝑙𝑙𝜎𝜎𝑣𝑣 Based on the returned lunar soil sample𝐶𝐶 s −from Apollo Mission 12, Carrier et al. (1972b, and 1973c) studied the compressibility of lunar soil by performing two oedometer and three direct shear tests in vacuum on a 200 g sample. For the first test, the sample was first prepared in a shear box to obtain a medium-dense state. Then the box was later put in the ultra-high vacuum test chamber, being vertically compressed in four increments, and later sheared. After the stage is over, only a small normal load remained on the sample. The sample was then subject to the recompressing load in four increments to achieve a normal stress more than two times that of the previous one, and then sheared. For the second test, the soil samples were re-collected and then replaced in the shear box as loosely as could be obtained within the constraints of the vacuum system. After the sample preparation, the soil sample was also compressed in a series of increments

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and then sheared. It is important to note that the vertical load was applied by adding or removing

weights in the counterweight which was connected to the normal force by a cable-pulley

arrangement. And the normal and shear displacement of the sample was measured by an equipped

Linear Variable Differential Transformer (LVDT). The measured compression index ranged from

0.01 to 0.11, depending on the initial density and the vertical load.

Jaffe (1973) also investigated the compressibility of lunar soil by performing miniature

compression and direct shear tests on a 1.3 g soil sample returned by Apollo 12 astronauts from

the scoop of Surveyor 3. Based on the tests, the compression index was found to vary from 0.04

to 0.21. The details of the test equipment will be discussed in the following section on shear

strength.

Oedometer test was also performed by Leonovich et al. (1975, 1977) on two 10 g soil samples returned from the Luna 16 and 30 missions. The sample was first subject to a series of incremental compression stresses, and then the normal stress was reduced from the sample at each stage of loading. After that, the sample was recompressed with a vertical load multiple times that of the previous one, and then decompressed by reducing the normal load. The compression index was estimated to be from 0.02 to 0.9, with an average value of 0.3.

The compression indexes determined based on the soil samples from the Apollo 12, Luna

16 and 20 missions were summarized by Carrier et al. (1991), as presented in the Table 2-12

Carrier et al. (1991) also provide a summary of compressibility parameters based on the oedometer tests results Carrier et al. (1972) as shown as in Table 2-13 below. He recommended a typical compression index of 0.3 for loose lunar soil and a value of 0.05 for dense lunar soil.

According to Carrier et al. (1991), the compression index of lunar soils generally tends to decrease with increasing initial density.

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Table 2-11 Compressibility parameters of lunar soil (Carrier et al., 1991)

Compressibility Parameters Range Recommended Typical Value

Compression Index, Cc

Loose 0.3

Dense 0.01 – 0.11 0.05

Swelling Index, Cr 0 – 0.013 0.003

Table 2-12 Compressibility index of lunar soils (Carrier et al., 1991)

Density Stress Sample Sample Mission Range Range Cc References Number Weight (g) (g/cm3) (kPa) 0.08 – 0.04 – 1.67 – 1.82 Carrier et al. (1972b) 67.5 0.11 Apollo 0.09 – 0.012 – 12002, 119 200 1.84 – 1.92 Carrier et al. (1972b) 12 31.2 0.062 0.03 – 1.91 – 2.00 1.9 – 69.9 Carrier et al. (1972b) 0.09 0.12 – 1.29 – 1.60 0.21* Jaffe (1973) 28.0 Apollo 0.14 – 12029, 8 1.3 1.4 – 1.64 0.11* Jaffe (1973) 12 28.0 0.14 – 1.58 – 1.68 0.04* Jaffe (1973) 28.0 Leonovich et al. (1975, 0.05 – Luna 16 - ~ 10 1.03 – 1.51 0.3* 1977); Gromov et al. 98.0 (1972) 0.05 – Leonovich et al. (1975, Luna 20 - ~ 10 0.98 – 1.51 0.3* 98.0 1977) * Estimated.

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Table 2-13 Table Measured minimum and maximum densities of lunar soil (Carrier et al., 1991)

Sample Density Specific Void Ratio Sample Mission Weight Gravity, References Number ρmin ρmax emax emin (g) (g/cm3) (g/cm3) G

Apollo Costes et al. 10084 565 1.36* 1.80 3.01 1.21* 0.67 11 (1970a, b)

Cremers et al. 10084, 68 5 1.26 3.01 1.39 (1970) Cremers and Apollo 12001, 19 6 1.30 Birkebak 12 (1971)

12029, 3 6.5 1.15 1.93 Jaffe (1971)

Apollo 14163, 5 1.10 2.9 ± 1 1.64 Cremers (1972) 14 133

14163, 0.89 ± 1.55 ± Carrier et al. 0.97 2.90 ±0.05 2.26 0.87 148 0.03 0.03 (1973a, b)

0.87 ± 1.51 ± 2.90 ± Carrier et al. 14259, 3 1.26 2.37 0.94 0.03 0.03 0.05 (1973a, b)

Apollo Cremers and 15031, 38 5 < 1.30 15 Hsia (1973)

1.10 ± 1.89 ± 3.24 ± Carrier et al. 15601, 82 0.96 1.94 0.71 0.03 0.03 0.05 (1973a, b) Gromov et al. (1972); Luna 16 - ~ 10 1.12 1.79 Lenovich et al. (1974a, 1975) Vinogradov 1.1 – 1.7 – (1972); Luna 20 - ~ 6* 1.2 1.8 Ivanov et la. (1973a, b) Leonovicch et - ~10 1.04 1.80 al. (1974a, 1975) * Estimated.

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2.5 Strength Properties Investigation

2.5.1 Introduction

Cohesion and friction angle combine to compose the ultimate shear strength of a soil, which

directly influences the bearing capacity, slope stability, trafficability. The relationship between

the shear strength, cohesion, and friction angle is expressed in the Mohr- failure criterion

as follows:

= + (2-9)

𝑓𝑓 Where τf refers to the shear strength𝜏𝜏 at 𝑐𝑐failure𝜎𝜎𝜎𝜎𝜎𝜎 on𝜎𝜎𝜎𝜎 the failure plane, σ is the normal stress on the failure plane, c is the cohesion of the soil, and φ is the internal friction angle.

Cohesion

Cohesion refers to the bonding or attraction between particles of certain fine-grained soils

that enhances shear strength and is independent of confining pressure. In other words, cohesion,

as an independent component of the shear strength of a soil, is not influenced by the inter-particle friction. It often refers to the adsorbed film on the surface of the soil in the particulate level.

Haalajian (1963) stated, “Surface tension and viscosity in the adsorbed phase that separates the particles produce cohesive forces between these particles”. Generally speaking, when the adsorbed film on the soil particle is too thick, the particle to particle contact cannot happen. In the meanwhile, when the adsorbed film is decreased to “zero”, the actual particle to particle contact occurs, which is referred as adhesion.

Lunar soil is categorized as granular (cohesionless) soil due to the fact that it does not obtain its cohesion from materials like the traditional cohesive soil which provide inter-particle adhesion after drying (Craig 1992 cited by Sibille et al. 2006). However, as discussed in the

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previous section, lunar soil does contain an apparent cohesion which comes from the angular and

re-entrant particles interlocking with each other during settling as well as from an electrostatic surface charge (Sibille et al. 2006). Vey and Nelson (1963) also commended that the “clean” surface of the lunar soil particle exists due to the low environmental pressure and hard vacuum condition on the moon, which causes the inter-particle forces between the soil particles and therefore increase the apparent cohesion of the soil.

Carrier et al. (1991) stated, that the inter-particle cohesion is “likely to be effective within a wider spectrum of soil grain size,” due to the low gravity environment on the Moon. Vey and

Nelson (1963) also stated that the shear strength of lunar soil is augmented by the cold welding of soil particles, which is believed to be caused by high temperatures and radiation.

Friction

As the other component of the shear strength, friction is of great importance for the research

of lunar and Martian soil, especially for vehicle mobility, since friction contributes greatly to the

gross traction of the vehicle. Friction is, by simple definition, the force resisting movement

between surfaces in contact. Different from the “apparent” cohesion created by the interlocking

mechanism due to the surface roughness as discussed before, friction exists between two “sliding”

surfaces due to the chemical bonding or molecular attraction between the asperities of the surfaces.

Therefore, friction force is proportional to the normal force exerted on the two surfaces to push

them together. The greater the normal forces, the greater deformation of the surficial asperities,

resulting in a larger contact area, which in turn increases the adhesion between the two particles

(Carrier et al., 1991). Lambe and Whitman (1969) summarized a few basic laws of frictional

behavior during the shearing of soils. This includes the concepts that the shear resistance between

two bodies is proportional to the normal force exerting on them and the shear resistance between 74

two bodies is independent of the dimensions of the bodies. During the shearing of soils, the

frictional resistance between particles comes from a number of mechanisms, including the sliding

and rearrangement of particles, resistance to volume change and dilatancy effects, particle

interlocking, particle crushing and strain rate dependent inertial effects. They make the frictional

process in soil mechanics far more complicated and much less understood than that of large

sliding over one another.

As stated before, the lack of adsorbed films on the surface of the lunar soil particle exists

due to the low environmental pressure and hard vacuum condition on the moon, which increases

inter-particle contact. According to Hinners (1964), “No doubt a mineral surface cleaned of

lubricating moisture and gases, will have a higher coefficient of static friction than a contaminated

surface.” An increased mechanical friction occurs on the moon due to the space weathering process

characterized by the absence of atmosphere and water, resulting in the lunar soil particles being

more angular instead of becoming rounded by abrasion as the terrestrial soils. It therefore can be

concluded that the “overall effect of the harsh lunar environment would be one of the factors

increasing the shear strength of the lunar soil and its bearing capacity rather than reducing it” (Vey

and Nelson 1963).

2.5.2 Techniques for Measuring Soil Strength Properties

Both of the future lunar and Martian missions require an in-depth investigation and study of the strength properties of the soil. According to Wills (1966), there are three major categories of tests to measure the shear strength of a soil, including translational, torsional, and triaxial laboratory tests. Other testing techniques are also available to investigate the strength properties of soil, such as standard penetration tests and cone penetration tests.

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2.5.2.1 Direct Shear Test

The direct shear test is known as the oldest and simplest form of shear test. As a

translational test, the direct shear test is basically a linear two dimensional test, which “simulates

the effects of shear loads acting on a predetermined failure surface” (Wills, 1996; Bardet 1997). A direct shear test is generally used to measure the shear strength properties (undrained) of soil, that is, the cohesion c and friction angle φ.

In a typical direct shear test, a soil sample is placed in a confined metal shear box (see

Figure 2-12 and Figure 2-13). The shear box can be either square or circular in plan and is split horizontally into halves. During the test, a normal force is applied on top of the shear box, resulting in a normal stress within the specimen. In the meanwhile, a shear force is also applied by moving the top half of the box relative to the bottom, causing the soil specimen to fail due to shearing.

The direct shear test can be controlled via either stress or strain depending on the testing

apparatus. In stress-controlled tests, a soil sample is subject to a series of shear loads with a constant load increment until the specimen fails. The corresponding specimen deformation can be

obtained by measuring the shear displacement of the top half of the box via a horizontal dial gauge.

In strain-controlled tests, a shear displacement of constant rate is applied to one-half of the box

and the corresponding resisting shearing force of the soil to the shear displacement is also

measured by a horizontal proving ring or load cell. The volume change of the specimen during the

test is also simultaneously measure in a similar way as the stress-controlled tests.

In order to obtain accurate strength parameters, a minimum of three direct shear tests with varying normal loads is required. For each test, the maximum shear stress versus the corresponding normal stress is plotted. By drawing a best fit line to connect these points, the friction angle can

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be obtained as the angle of best fit line from the horizontal axis, while the cohesion is interpreted

as the intercept of the best fit line on the vertical axis.

As the oldest and one of the most widely used testing method, direct shear testing has many

merits, including its simplicity and quick determination compared to other tests. It generally provides fairly reliable strength values (friction angle φ and cohesion c). It is important to note that the direct shear test gives best results on dry sandy materials. The test becomes less reliable with possessing friction angles under 35 degrees. According to Bowles (1992), the difference between the test results from direct shear and triaxial are negligible for sands with friction angles less than 35 degree, while for sands with a friction angle higher than 35 degree, the differences vary from one to four degrees between direct shear and triaxial test results, with the direct shear test overestimating the friction angle. This is due partly to the fact that the soil sample fails along the horizontal plane in the direct shear test instead of the weakest plane. Therefore, the stress state of the soil in a direct shear test cannot be completely defined as in a triaxial test. In addition, the direct shear tests have many other shortcomings. The direct shear test generally utilizes a small amount of soil sample, which can easily be influenced by operator and soil preparation errors. The low deformations at which the soil sample fails are also hard to accurately measured, resulting in a less accurate measurement of the volume change.

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Figure 2-12 Overview of direct shear test (Das 2002)

Figure 2-13 Typical direct shear test setup (Bardet 1997)

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2.5.2.2 Torsional Ring Shear Test

The torsional ring shear test, also known as the ring shear test, is another widely used laboratory testing method to measure the strength properties of soil. In this test a soil specimen is prepared in a thin solid circular or annular circular shear cell with specific dimensions and confined by two rings. A shear cell lid with bent bars is then placed on top of the soil sample in order to prevent slipping of the soil at the lid, and a vertical load is then applied on top of the lid. The soil specimen then is subject to torsion by rotating the lower part of the cell, while the upper part is restrained by calibrated loading rings, which also measure the corresponding restraining torque.

When the stress is reduced to zero, which indicates the steady state shear value has been reached, the normal load will be reduced and the sample will be subject to shearing until the maximum shear value is reached and then dropped.

One point of yield locus can be obtained from each torsional shear test for the specific soil and soil conditions. A curve of the yielding stress (the maximum stress before failure) versus the normal stress can be plotted by combing all the yield point. A minimum of six shear tests is suggested to determine one such yield locus. After the yield locus has been obtained, a best fit straight line is drawn by the method of least squares to connect these yield points. The friction angle can be obtained as the inclination angle of the best fit line from the horizontal axis, while the cohesion is interpreted as the intercept of the best fit line on the vertical axis.

Compared to the direct shear tests, the torsional ring shear tests have several advantages.

First, the failure plane of the soil specimen will remain the same during the test with a constant cross-sectional area and the same rotational direction. Secondly, the displacement of the sample can be better measured. In addition, the torsional ring shear test is generally exempt from the bulldozing effect, a typical issue of the direct shear tests. According to Bekker (1969), in a direct

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shear test, the soils which are pushed in front of the rectangular plat usually result in increasing

shear values, requiring further correction to determine the yield locus. However, the torsional shear

test, compared with the direct shear test, requires more skills and more complex equipment, and is

generally more difficult and time consuming with regards to test preparation.

2.5.2.3 Triaxial Test

The triaxial test, also referred to as the triaxial shear test, is a common laboratory test method used to obtain the stress-strain strength characteristics of soils under controlled drainage conditions. The triaxial shear test is a test that results in stresses along one axis being different from the stresses in perpendicular directions, which simulates the in situ condition of soil in a more

realistic way than the direct and torsional shear tests. According to Wills (1996), the triaxial test

provides, “the most accurate method for the determination of the maximum shearing resistance [of a soil] and the stress-strain relationship up to failure.”

In a typical triaxial test, a soil sample encased within a cylindrical rubber latex membrane is placed in a confined chamber between two parallel flat, circular platens, closing off the top and

bottom ends (refer to Figure 2-14 and Figure 2-15). The chamber is filled with an incompressible

confining medium, typically water, air, or oil, to apply pressure on the specimen along the sides

of the cylinder. The top platen can then be mechanically driven up or down along the axis of the

cylinder to apply normal stress on the specimen in the perpendicular directions. A drainage system

is generally connected to the bottom platen to saturate and/or drain the soil sample depending on

the type of test being conducted. The distance which the upper platen moves is measured with the

corresponding normal load. For a saturated soil sample, the net change in volume of the sample

can also be directly determined by measuring how much water moves in or out of the sample.

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During the test, the soil specimen is subject to a confining pressure provided by surrounding fluid

and a normal stress provided by the platen with a constant strain rate in the axial direction until it

fails. The application of stresses with different directions causes a shear specimen fail. The pore

pressures of fluid within the sample can also be measured by using pore pressure measurement

apparatus.

The conventional triaxial test (CTT) is the most common and simple triaxial test, including

three different variations: the undrained, unconsolidated test (UU), the consolidated, undrained

test (CU), and the consolidated, drained test (CD). The application of different types of triaxial

tests depends on the in situ stress state of the soil and the drainage conditions. Generally speaking, the CD test allows the sample and the pore pressures to fully equilibrate to the surrounding stress

to avoid pore pressures built up by the shearing. Depending on the materials of the sample, the test

may take a long time, especially for low permeability materials. CU testing does not allow the

sample to drain during the second phase of the test when it is subject to normal stress. Therefore,

the shear parameters are measured under undrained conditions and the pore pressures in the sample

can also be measured to obtain the CD strength. The UU test is the quickest and simplest among

the three, allowing no consolidation during the whole test. The unconfined compression test is a

variation of the UU test, typically used for cohesive samples such as concrete and rocks with no

confining pressure. This requires less complicated and less expensive apparatus and sample preparation. It is important to note that for soil samples in a completely dry condition, such as lunar soil and Martian soil, the three testing methods are essentially the same since no fluid exists within the soil.

In the conventional triaxial test, a constant confining pressure, σc is maintained within the

cell during the test. Typically, a minimum of three tests is required with different confining

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pressures σc. And for each test, the stress versus strain curve can be plotted based on the load and

deformation readings, from which the maximum shear stress τmax can be determined. Then the

stress state of the soil sample at failure can be obtained for each test: the confining pressure σc

representing the minor principal stress, σ3 (identical to the intermediate principal stress, σ2 in most cases), and the major principal stress, σ1, defined by summing the confining pressure σc and the

axial load at failure, that is, the maximum shear stress τmax from the stress-strain curve, also known

as the deviator stress or principal stress difference, maximum shear stress Δσ1. Each test can

generate a different Mohr’s stress circle by using the respective principal stresses, σ1 and σ3. After

at least two (but typically three) Mohr’s circles with different confining pressures have been

obtained, a best fit tangent line is drawn. The friction angle φ can be obtained as the inclination

angle of the best fit line from the horizontal axis, while the cohesion is interpreted as the intercept

of the best fit line on the vertical axis.

In addition to Mohr’s circles, Lambe (1964, cited by Bowles, 1992) suggested a new way

to identify the stress state of soil by interpreting the results of triaxial tests in terms of stress paths.

According to Lambe (1964), the stress path plots q versus p, where p and q are the coordinates of the top of Mohr's circle, which by definition can be defined by the major and minor principal stress values, which is expressed as follows:

= (2-10) 𝜎𝜎1+𝜎𝜎3 𝑝𝑝 2 = (2-11) 𝜎𝜎1−𝜎𝜎3 A locus of pointes defined by p and𝑞𝑞 q can 2be plotted based on the triaxial test results. A best fit line is drawn to connect these points, referred to as the Kf line. The cohesion and friction

angle of soil can be obtained based on the intercept and slope of the Kf line.

= (2-12)

𝑠𝑠𝑠𝑠𝑠𝑠∅ 82𝑡𝑡𝑡𝑡𝑡𝑡 𝑡𝑡

= (2-13) 𝑎𝑎 𝑐𝑐 𝑐𝑐𝑐𝑐𝑐𝑐∅ Where α is the angle of the Kf line from the p (horizontal) axis, and a is the intercepts of

the Kf line with the q (vertical) axis. The construction of the stress path is generally much simpler

than that of the conventional Mohr’s circle method, where the circles may overlap or not align

perfectly, and requiring best judgment to minimize error from the discrepancy.

Triaxial tests have many advantages over the direct shear and torsional shear tests,

including the accurate measurement of the volume changes of the specimen and the versatility as

well as adaptability to special testing requirements. Last but not least, in a triaxial test, the complete

state of stress is known for the soil specimen, whereas in the direct shear test and the torsional shear test only the stress at failure can be obtained.

Figure 2-14 Diagram of triaxial test equipment (Das 2002)

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500

400 ) kPa (

300

τ

200 Shear Stress Shear

100

0 0 100 200 300 400 500 600 700 σ Normal Stress 3 (kPa)

Figure 2-15 Mohr stress circles and developed strength envelope.

2.5.3 Strength Properties of Lunar Soil

Due to the importance of the strength properties of lunar soil, intensive studies have been focused on the estimates of the strength parameters of lunar soil, cohesion c and friction angle φ.

A summary of the studies’ results has been presented in table below. Shown in the table are some of the best estimates of the strength properties of lunar soil. Before the Apollo missions, the values tended to be under-conservative with regard to the cohesion c and friction angle φ. For example,

Scott and Roberson estimated the friction angle of lunar soil to be from 35 to 37 degrees and the cohesion to vary from 0.35 to 0.70 kPa, based on the data collected from the Surveyor soil mechanics surface sampler. After the Apollo mission, Mitchell et al. (1972, 1974) performed a thorough study of the geotechnical properties of lunar soil with a focus on estimating the more representative shear strength parameters, based on a variety of data, including the data from the manned Apollo missions. According to Mitchell et al. (1972, 1974), the recommended friction

84 angle ranges from 30 to 50 degrees, and the cohesion varies from 0.1 to 1 kPa. It is important to note that these parameters were determined according to the relative densities, that is, the higher the relative density of the lunar soil corresponding to higher shear strength values. Carrier et al.

(1991) summarized the typical shear strength parameters for the lunar soil of inter-crater areas as shown as in Table 2-15. Likewise, the friction angle and cohesion increase with the increasing density of lunar soil, which is a result of increasing depth.

Table 2-14 typical values of lunar soil cohesion and friction angle for the inter-crater areas (Carrier et al. 1991).

Depth Range Cohesion, c (kPa) Friction Angle, φ (degrees) (cm)

Average Range Average Range 0 – 15 0.52 0.44 – 0.62 42 41 – 43 0 – 30 0.90 0.74 – 1.1 46 44 – 47 30 – 60 3.0 2.4 – 3.8 54 52 – 55 0 – 60 1.6 1.3 – 1.9 49 48 - 51

2.5.3.1 Laboratory Measurements of Shear Strength

Based on the returned lunar soil samples from Apollo and Luna missions, various laboratory testing techniques have been developed and performed to investigate the strength properties of lunar soil based on a limited amount of sample.

The first laboratory measurement of the shear strength of lunar soil was conducted in 1969 at the Lunar Receiving Laboratory at the NASA Manned Spacecraft Center. It is part of the preliminary physical-chemical examination program with a focus on the returned lunar sample from Apollo 11 (Carrier et al., 1991). The measurement was performed by pushing a simple spring-

85 loaded penetrometer into the compact lunar soil sample, whose particle size is mainly finer than 1 mm, while keeping the penetrometer normal to the surface of the soil with either its front or end in touch with the sample. The shear strength parameters, cohesion c and friction angle φ, can then be deduced from the penetrometer readings based on Terzaghi bearing capacity theory, with some adjustments to account for the circular contact areas. Costes et al. (1969, 1970a, b) and Costes and

Mitchell (1970) reported the results from the penetration tests. Further analysis was made on these data for the bearing test conducted on the Apollo 11 soil sample at the maximum density by utilizing the bearing capacity theory (Durgunolu and Mitchell, 1975). The friction angle was found to be from38 to 42 degrees, and the cohesion to vary from 0.25 to 0.85 kPa.

Similar penetrometer tests were performed on a 1.3 g lunar soil sample recovered from the scoop of the Surveyor 3 soil mechanics surface sampler returned in October 1969 by the Apollo

12 astronauts (Jaffe, 1971). In addition, the load and the corresponding deformations were also recorded by the commercial vertical, screw-driven, tension/compression testing machine.

However, no shear strength parameters have been deduced from these penetration test results.

Carrier et al. (1972b, and 1973a) investigated the strength properties of lunar soils by performing three direct shear tests in vacuum on a 200 g returned lunar soil sample from Apollo

Mission 12 (Sample No. 12001, 119). The sample was first prepared in a shear box to obtain a medium-dense state and then transferred to an ultra-high vacuum test chamber, being sealed, vertically compressed in four increments, and later sheared at a pressure less than 5 X 10 ^(-8)

Torr. The measured cohesion of the lunar soil sample ranged from 0.0 to 0.7 kPa and the friction angle ranged from 28 to 35 degrees, depending on the density of the soil sample. Comparison with the shear strength of a basalt simulant with the same void ratio found that the lunar soil sample possessed considerably less strength, being only about 65% of the simulant strength. This is

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believed to be partly due to the weakly cemented particles contained in the lunar soil, such as

microbreccias and agglutinates, which can be easily broken down into smaller particles.

Miniature compression and direct shear tests were also performed by Jaffe (1973) on a 1.3

g soil sample returned by Apollo 12 astronauts from the scoop of the Surveyor 3 mission. Figure

below shows the miniature standard soil mechanics direct shear box utilized in the tests. The

sample was prepared in a steel cup, split horizontally halfway up, with a size of 5 mm in diameter

and 5 mm in length. The sample cup was placed between upper and lower platens which connected

with vertical flexures to support the upper and lower halves of the cups separately to avoid friction.

During the test, the lower half of the sample was driven away by a motor at a constant rate. The displacement of the lower platen was recorded by a dial gauge, while the horizontal shear load response to the motion was transmitted through soil to the upper half of the cup and recorded by a strain gage continuously. Soil samples were prepared in five different densities, ranging from 0.99 to 1.87 g/cm3. Based on the test results, the cohesion was found to be from 0.1 to 3.1 kPa, and the friction angle ranged from 13 to 56 degrees. It was also concluded that the shear strength

parameters increased with increasing initial bulk densities. According to Jaffe (1973), the

miniature direct shear tests were performed at 20ºC and 40 – 50% relative humidity in the

laboratory air. He stated that the shear strength properties of the lunar soil were not appreciably

affected by exposure in air. However, Carrier et al. (1991) pointed out that most of the friction angles measured by Jaffe (1973) were significantly lower than those obtained by other experiments.

Scott (1987) further studied the strength properties of lunar soil by performing miniature triaxial tests on a 1.1 g soil sample returned by Apollo 12 astronauts from the scoop of Surveyor 3 mission in 1972 at Cambridge University Engineering Department. The miniature triaxial apparatus was designed to be able to test a soil sample of less than 1 g (approximately 0.8 g) in a

87 triaxial cylinder of 0.25 in (6.23 mm) in diameter and 0.5 in (12.7 mm) high. During the test, the specimen was subjected to vacuum to obtain the desired confining pressure in order to achieve its natural state with minimal disturbance. Calibration tests were conducted on fine Leighton Buzzard sand in a standard triaxial test of samples with 1 in diameter and 4 in diameter. Part of the testing results suggested a cohesion value of lunar soil ranging from 0 to 1 kPa, and a friction angle between 51 and 59 degrees. Scott (1987) also concluded from the results that the failure behavior of the material was not influenced by a scale effect, which was confirmed by centrifuge tests on model piles. However, it is important to note that measurement of the pressure and displacement from the miniature triaxial tests are not very accurate due to the limitations of measurement sensors at that time.

Direct shear and coulomb device tests were performed by Lenovich et al. (1974, 1975) on the returned lunar samples from Luna 16 and 20 missions to investigate the shear strength properties of lunar soil. The direct shear device Lenovich used had a fixed shear surface and was a type of flat single-shear instrument. It tested soil samples of 25.2 mm in diameter and 12 mm in height. The measured friction angle ranged from 20 to 25 degrees, and the cohesion ranged from

3.9 to 5.9 kPa. The coulomb device had a moving partition to induce the soil deformation and was designed in a way that allowed the observance of the soil strain through the transparent walls of the device in order to investigate the character of soil destruction. The sample for the tests was collected from the highland region of the Moon with an average particle size of 70 to 80 µm. The test results showed a deformation at relative volumetric strain of 0.8 and an increase in bulk density from 1.04 to 1.3 g/cm3. Lines of shear or slide were observed during the tests while the surface remained horizontal and smooth. Another similar test was performed on packed soil sample with an initial bulk density of 1.63 g/cm3, which showed a visible shear plane at failure with an internal

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friction angle of 32 degrees. In order to investigate the shear strength of lunar soil in terms of packing, more tests were performed by a “method of additionally packed soil which made it possible to obtain shear strength graphs with practically constant bulk weight and to the

change in the parameters of soil’s shear as a function of the degree of soil packing.”(Lenovich et

al., 1976). It can be concluded from the test results that an increase in packing pressure will result

in an initial increase in cohesion and internal friction angle which later taper off and eventually

become stable at packing pressures greater than 0.4 to 0.5 kg/cm2. According to Lenovich et al.

(1976), this is due to the fact that the particle to particle contact increase greatly in the beginning

by sticking together and then the increase of particle contact dropped significantly, thus the

increase of shear strength due to contact becomes negligible.

Based on the soil investigation on Luna 16 and Luna 20, Gromov (1998) further studied

the shear strength of lunar soil. Gromov (1998) stated that the geotechnical properties of the lunar

soil are similar in spite of the various sampling locations. He also found that the majority of

compression of lunar soil occurred during the initial phase of loading. In addition, “the main factors

that control the lunar soil packing process are particles sliding and tighter compression of soil

particles and aggregates” along with “distortion of the particles at their points of contact” (Gromov,

1998). For loosely prepared lunar soil, the failure was characterized by local or punching shear

failure, while for densely compacted soil, failure is typically due to general shear failure. He also

noted that the cohesion and internal friction angle generally increased with increasing soil density.

Other researchers also investigated the shear strength of lunar soil based on laboratory

measurements and reported their findings in Russian (Vedenin et al., 1974; Douchowskoy et al.,

1974, 1979). A summary of the shear strength of lunar soil in terms of cohesion and friction angle

is presented as below:

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Table 2-15 Summary of lunar soil cohesion and friction angle (Carrier et al., 1991)

Cohesion, c Friction Angle, φ Sources References (kPa) (degrees) Early inferred by remote sensing 24 – 240 0 Halajian (1965)

> 0.00 > 28 Jaffe (1965a) > 0.00 > 25 Jaffe (1965b)

Inferred from boulder tracks 0.35 33 Nordmeyer (1967)

0.1 10 – 30 Moore (1970) Hovland and Mitchell 0.5~ 21 – 55 (39*) (1971) Apollo 17 - North, East and South 1~ 26 – 50 (37**) Mitchell et al. (1973) Massifs Surveyor : early estimates 1. TV and Landing Data 10 0 Halajian et al. (1967) 0.15 -15 55 Jaffe (1967) Christensen et al. 0.4 – 0.13 30 – 40 (1967) 2. Soil Mechanics Surface Sampler, TV, and Landing Data Scott and Roberson > 35 (1968a) Christensen et al. 0 45 – 60 (1968a) Christensen et al. 10 0 (1968a) Christensen et al. 6. Vernier Engine > 0.07 35 (1968b) Christensen et al. Attitude Jets 0.05 – 1.7 (1968b) 3 and 7. Soil Mechanics Scott and Roberson 0.35 0 0.70 35 – 37 Surface Sampler (1969) Scott and Roberson Surveryor Model: Best Estimate 0.35 35 – 37 (1969) Apollo 11 LM Landing, Boot Prints, Consistent with Surveyor Model Costes et al. (1969) Crater Slope Stability Core Tube, Pole, SWC 0.75 – 2.1 37 – 45 Costes et al. (1971) Shaft Penetration Apollo 12

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Cohesion, c Friction Angle, φ Sources References (kPa) (degrees) LM Landing, Boot Prints, Consistent with Surveyor Model Scott et al. (1970) Crater Slope Stability Core Tube, SWC Shaft 0.56 – 0.75 38 -44 Costes et al. (1971) Penetration LUNOKHOD 1 Lenovich et al. (1971, Vane Shear 3.9 – 4.9 N/A 1972) 0.26 -1.1 50 – 25 Mitchell et al. (1972d) 1.2 – 4.8 50 – 25

0.64 – 2.6 50 - 25 Cone Penetrometer 0.17 – 0.10 45 – 25 Mitchell et al. (1972d) 0.52 – 2.7 45 – 25

0.34 – 1.8 45 – 25 Apollo 14 Soil Mechanics Trench < 0.03 – 0.1 35 – 45 Mitchell et al. (1971) Equal to or greater than Surveyor Apollo Simple Penetrometer Mitchell et al. (1971) Model MET Tracks 37 – 47 Mitchell et al. (1971) Apollo 15 SPR Data and Simulation 47.5 – 51.5 Mitchell et al. (1972a) Studies SPR Data and Soil Mechanics 1.0 50 Mitchell et al. (1972a) Trench Apollo 16 SPR (Station 4, 10 – 20 cm 0.6 46.5~ Mitchell et al. (1972b) depth) SPR (Station 10) 0.37 49.5 Mitchell et al. (1972b)

SPR (Station 10) 0.25 – 0.60 50 -47 Mitchell et al. (1972b) Drill Core Open Hole 1.3 46.5* Mitchell et al. (1972b)

Apollo 17 Drill Core Open Hole (Neutron 1.1 – 1.8 30 -50 Mitchell et al. (1973) Flux Probe) LRV Mitchell et al. (1973) Mitchell et al. (1972d, Apollo Model: Best Estimate 0.1 -1.0 30 -50 1974)

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Cohesion, c Friction Angle, φ Sources References (kPa) (degrees) Lenovich et al. (1974a, LUNOKHOD 1 and 2 (AVE) 0.4*** 40 *** 1975) Returned Lunar Samples Costes et al. (1969, 1970a, b); Apollo 11: Penetrometer 0.25 – 0.85 42 – 38 Costes and Mitchell (1970) Apollo 12: Penetrometer (Surveyor N/A Jaffe (1971) 3) Apollo 12: Vacuum Direct Shear Carrier et al. (1972b, 0 – 0.7 28 -35 1973c) Apollo 12: Direct Shear (Surveyor 0.1 – 3.1 13 -56 Jaffe (1973) 3) Apollo 12: Triaxial Shear 0 – 1 51 – 59 Scott (1987) (Surveyor 3) Lenovich et al. (1974a, Luna 16 and 20: Direct Shear and 3.9 – 5.9 20 – 25 1975): Coulomb Device Gromov et al. (1972) ~ Assumed; * Mean of 69 values; ** Mean of 25 values; *** Estimatted.

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2.6 Review of lunar soil simulants

Various lunar soil simulants were developed in order to support lunar research and related

lunar technology projects, since the scientific value of lunar soil is too great to be consumed in

larger-scale destructive laboratory investigations. Lunar soil simulants are created in a way to

replicate the physical and/or chemical properties of lunar soil based on the current lunar soil studies

and will be applied in future lunar projects for those studies requiring a large amount of soil sample.

Lunar soil simulant is defined as “any material manufactured from natural or synthetic terrestrial

or meteoritic components for the purpose of simulating one or more physical and/or chemical

properties of a lunar rock or soil” (Sibille et al., 2006).

The current lunar soil simulants have two major categories, including root simulants and

derivative simulants. The root simulants are designed to represent an end-member in terms of

physical properties (often with regard to the grain size distribution) and mineralogical properties

(with minimal degree of processing) of the targeted regolith. Root simulants are created to

represent soils from specific regions on the Moon and generally are manufactured from terrestrial

rock, minerals, and/or synthetic sources. A derivative simulant is created from a root simulant by

modification or addition of material to better replicate specific characteristics of lunar regolith to

meet the needs of specific processes to be tested, such as vehicle mobility (Sibille et al., 2006).

Generally speaking, root lunar regolith simulant was applied for predominantly physical activities,

such as drilling and excavation; while derivative simulant typically serves the needs for specific

chemical activities, such as oxygen extraction.

The quality of a simulant is typically defined by simulant fidelity. A higher fidelity simulant corresponds to a more representative material of the actual lunar soil. The following section will present some of the relatively high fidelity simulants and their current status.

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2.6.1 MLS-1

MLS-1, also known as Minnesota Lunar Simulant-1 was a standardized lunar soil simulant, first developed at the Space Science Center at the University of Minnesota in the 1970s. Instead of representing the whole spectrum of lunar soil geotechnical properties, MLS-1 was created mainly to serve the purpose of replicating the bulk of Apollo 11 lunar regolith. MLS-1 consists of a crushed terrestrial basalt material mined from an abandoned quarry in Duluth,

Minnesota, which contains a high composition of titanium, very similar to the lunar regolith from

Apollo 11. Other mineralogy of the quarried basalt includes plagioclase, olive, ilmenite, titanomagnetite, and clinopyroxene as shown as in Figure 2-16. It is important to note that the basalt contains no glass or agglutinates, which is prominent of the Apollo 11 regolith. According to Batiste and Sture (2005), the influence of the contents of glasses is negligible, whereas the agglutinates play a significant role in soil strength in terms of the stress-strain relationship.

According to Sibille et al. (2006), MLS-1 was produced by crushing and milling the basalt rock to obtain a similar particle size distribution and particle size characteristics as the Apollo 11 lunar regolith material. MLS-1 generally represents the coarse particle size of the lunar , whereas it contains a much lower fraction of fine-grain particles than lunar soil. This is due to the milling process in which the rock is crushed into a finer fraction or dust component of the simulant, which often results in the release of relatively large mineral fragments. In contrast, lunar soil consists mainly of lithic or rock fragments as small as the finer dust fraction of the material.

Therefore, the particle size distribution of MLS-1 has been regarded to be more accurately representative of the particle size distribution of Apollo soils and was classified as highly angular, silty sand material as shown as in Figure 2-17. Due to the different mineral fragment composition as well as the absence of a glassy agglutinate fraction, the physical and chemical behaviors of

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MLS-1 vary from that of the lunar regolith, which may be problematic for certain investigation and research project. However, MLS-1 is still regarded as a successful lunar soil simulant, although

it is no longer readily available.

Figure 2-16 Backscattered electron image of lunar simulant MLS-1 (Sibille et al., 2006)

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Figure 2-17 Particle size distributions of MLS-1 and JSC-1 (Perkins and Madson, 1996)

The engineering properties of MLS-1 were determined by geotechnical tests at the

University of Colorado (Batiste and Sture, 2005). According to Perkins et al. (1992), the specific gravity of MLS-1 is determined to be 3.2, while the maximum and minimum bulk densities were

2.20 and 1.50 g/cm3 respectively, which correspond to void ratios of 1.13 and 0.454, respectively.

The lower maximum void ratio of MLS-1 was believed to be due to the lack of glassy agglutinates

in MLS-1. The shear strength of MLS-1 was determined by conventional triaxial compression test,

and the test results are summarized in the Table 2-17. It can be seen from the results that the lower

bound of the internal friction angle of MLS-1 is similar to that of the lunar soil, however the upper

bound of MLS-1 is higher than that of lunar soil. According to Perkins et al. (1992), this is due to

the differences in testing conditions with regard to the different confining pressure. The cohesion

96 value of MLS-1 is found to be much lower compared to the returned lunar soil, which is believed to be due to the lack of agglutinate particles and the relative electroneutrality of Earth’s atmosphere.

Table 2-16 Comparison of friction angle and cohesion of lunar soil and MLS-1 (Perkins 1992, Batiste and Sture 2005)

Density, Confining Stress, Angle of Internal Friction, Cohesion, Material g/cm3 kPa degree kPa 1.89 26.0 48.8 -

1.71 52.6 40.7 - Lunar Regolith 1.50 - 42.0 0..52 1.75 - 54.0 3.0

1.90 13.8 49.8 - 1.90 34.5 48.4 - MLS-1 1.70 34.5 42.9 - 1.70 68.9 41.4 0 – 0.1 2.17 - 66.7 1.5

Willman and Boles (1995) also investigated the strength properties of MLS-1 by performing conventional triaxial compression tests. Results of the tests indicated an internal friction angle of 37 degrees and a cohesion value of 0.9 kPa according to the Mohr-Coulomb failure criterion. The measured friction angle is found to be much lower than that determined by

Perkins et al. (1992) and Batiste and Sture (2005), which is believed to be due to the different testing methods, especially with regard to the confining pressure.

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2.6.2 JSC-1

JSC-1 was first developed and produced at the Johnson Space Center (hence the name JSC-

1) in 1993 to be applied to specific scientific and engineering investigations, specifically investigation, which demands soil sample in large quantity. As a mare basalt simulant, JSC-1 is a silty terrestrial soil, rich in basaltic ash mined from volcanic ash deposits located in a cinder ash quarry on the edge of Merriam Crater in the San Francisco volcano field near Flagstaff, . JSC-1 is designed to emulate the chemical, mineralogical, and textural properties of the lunar mare regolith in the Apollo exploration areas. More specifically, it matches the average mineralogical composition of Apollo 14 mare basalt material, being low in composition of titanium and high in composition of and phosphorous (refer to Figure

2-18). JSC-1 contains mineralogical compositions similar to MLS-1, including plagioclase, , and ilmenite, however it also contains basaltic glass, which is absent in MLS-1.

Compared to MLS-1, JSC-1 is much improved in terms of the texture and mineralogy of the lunar soil. It is important to note that JSC-1 is generally regarded as a root mare simulant, however it can be used in research requiring a simulant representing both lunar mare and highland regolith, due to the fact that it was created as an “average” Apollo 14 soil (Sibille et al., 2006).

JSC-1 was also produced by milling and crushing like MLS-1 to obtain a similar grain size distribution as that of typical lunar soils. The grain size distribution of JSC-1 is shown in comparison to that of MLS-1 (refer to Figure 2-19). According to Mckay et al. (1994), the median particle size for JSC-1 is 98 to 117 µm and the mean particle size is 81 to 105 µm. Approximately

40 percent of particles passed the number 200 sieve (<75 µm). JSC-1 is therefore classified as non- plastic, well graded silty sand (Klosky et al., 2000). The average specific gravity of JSC-1 is found to be 2.9, which corresponds to the low bound of lunar soil according to Carrier et al. (1991). The

98 maximum and minimum bulk densities of JSC-1 were found to be 1.91 and 1.43 g/cm3 (Klosky et al., 1996).

Figure 2-18 Backscattered electron image of lunar simulant JSC-1 (Sibille et al., 2006)

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Figure 2-19 Particle size distribution of lunar regolith and JSC-1 (Mckay et al., 1994)

The strength properties of JSC-1 were determined via conventional triaxial compression tests on samples with various initial bulk densities and under different confining pressures. The reported shear strength parameters are summarized in Table 2-18. Mckay et al. (1994) determined the friction angle of JSC-1 to be approximately 45 degrees and cohesion to be approximately 1.0 kPa according to the Mohr-Coulomb failure criterion with data from soil samples under confining pressures of 34.5, 69, and 103 kPA. Compared to the shear strength of lunar soil, JSC-1 presents a “good mechanical analog to lunar soil” (McKay et al., 1994). Very similar strength parameters, a friction angle of 45 degrees and a cohesion smaller than 1 kPa, were also obtained on samples with bulk density close to or below the critical void ratio by Willman et al. (1995). It is important to note that as the critical state is approached, samples prepared at or above the critical void ratio tend to result in a similar friction angle, in spite of different confinements. Perkins and Madson

(1996) also performed triaxial tests on JSC-1 soil samples with a density of 1.78 g/cm3 and

100 confining pressure ranging from 20 to 700 kPa. The internal angle of friction was determined to be from 48 to 64 degrees while the cohesion was assumed to be zero. Klosky et al. performed triaxial tests on JSC-1 at two different densities of 1.62 and 1.72 g/cm3, corresponding to 40 and

60 percent relative density. The samples were prepared 20 cm in height and 10 cm in diameter.

And the test results indicated that the shear strength parameters strongly related to the initial density of JSC-1. Later Klosky et al. (2000) again measured the shear strength parameter of JSC-

1 with three different initial densities: 1.62, 1.72, and 1.81 g/cm3, which correspond to relative densities of 53%, 75%, and 95%. The internal friction angle was determined to be from 44.4 to

53.6 degrees, and the cohesion was from 3.9 to 14.4 kPa, similar to the previous finding from

Klosky et al. (1996).

Table 2-17 Summary of shear strength parameters of JSC-1.

Relative Angle of Internal Density, Cohesion, Density, Friction, Reference g/cm3 kPa % degree - - 45 1.0 Mckay et al. (1994) 1.50 - 45 <=1.0 1.60 - 45 <=1.0 Willman et al. (1995) 1.65 - 45 <=1.0

1.78 - 48 - 64 0* Perkins and Madson (1996)

1.62 40 44.4 3.9 Klosky et al. (1996) 1.72 60 52.7 13.4

1.62 53 44.4 3.9 1.72 75 49.5 6.2 Klosky et al. (2000)

1.81 95 53.6 14.4 * Assumed

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Approximate 12,000 kg of JSC-1 was produce in 1990s, and it was regarded as one of the

most successful lunar simulants, being widely applied to various research and educational

programs (Sibille et al., 2006). However, the supply of JSC-1 is now unavailable for distribution

to the space exploration community due to poor recordkeeping and unsupervised distribution of

the lunar soil simulant (Schlagheck et al., 2005).

2.6.3 JSC-1A

In order to develop a replacement simulant for JSC-1, JSC-1A was created and produced by Orbital Technologies Corporation of Madison Wisconsin to be used as a lunar soil analog in hardware development and testing. Similar to JSC-1, JSC-1A is also mined from a volcanic ash deposit in a commercial cinder quarry located in the San Francisco volcano field near the Merriam

Crater just outside of Flagstaff, Arizona. As a mare basaltic simulant, it is low in titanium and contains major crystalline silicate phases of plagioclase, pyroxene, and olivine, with minor oxide phases of ilenite and chromite, and traces of clay. It has been recognized and attested that JSC-1A

serves as a standard lunar mare regolith simulant to support NASA’s future exploration and

research of the lunar surface. Three variants of this material is available including JSC-1A, JSC-

1AF, and JSC-1AC, which represent unfractionated JSC-1A, the fine grained fraction of JSC-1A,

and the coarse grained fraction of JSC-1A, respectively. According to the material safety data sheet

(MSDS) provided by Orbital Technologies Corporation reports, the JSC-1A simulant family is a

set of odorless /grain sand-like materials (as shown in Figure below), with a specific gravity

of 2.9, cohesion of 1.0 kPa and internal friction angle of 45 degrees.

In order to characterize the geotechnical properties of JSC-1A simulant for future Earth-

based studies of lunar operations, a group of laboratory tests were conducted at Case Western

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Reserve University and NASA Glenn Research Center to establish the benchmark properties of

JSC-1A (Zeng et al., 2007). The laboratory tests investigated the geotechnical properties of JSC-

1 A, including particle size distribution, specific gravity, bulk density, compaction, shear strength and compressibility of the material. It is important to note that all the testing procedures followed the standard testing methods specified by ASTM (1991), and all tests were repeated multiple times to ensure the repeatability of the data.

The particle size distribution of JSC-1A was determine by sample preparation (ASTM

D421), for coarse particles (ASTM D422), and tests for fine particles

(ASTM D423). The test results are compared with the typical particle size distribution of lunar soil as shown as in Figure 2-20. It can be seen from the Figure 2-20 that the particle size distribution of JSC-1A falls between the +1 standard deviation and -1 standard deviation of lunar regolith, but does not exactly fall into the average range. According to Zeng (2007), JSC-1A has the value of

D10 equal to 0.017 mm, D30 equal to 0.042 mm, and D60 equal to 0.11 mm, which stands for the particle size (in terms of diameter) larger than 10, 30, and 60 percent of particles by weight, respectively. The coefficient of uniformity, Cu is found to be 6.47, and the coefficient of curvature,

Cc is 0.94. It was also found that JSC-1A shows little plasticity and the test could not produce consistent results. Based on the particle size distribution and negligible plasticity of soil, JSC-1A

is therefore classified as a poorly graded silty sand (MS) with 53 percent sand and 47 percent silt

according to the Unified Soil Classification System (USCS).

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Figure 2-20 Particle size distribution of lunar regolith and JSC-1 (Zeng, 2009)

The specific gravity of JSC-1A was determined according to ASTM standard D854, which found the specific gravity to be 2.875 for JSC-1A, which is somewhat lower than the working values of specific gravity for the lunar soil 3.1 recommended by Carrier et al. (1991). Following

ASTM D4253 and ASTM D4254, the maximum and minimum densities of JSC-1A were determined to be 2.03 g/cm3 and 1.57 g/cm3, respectively. And the corresponding maximum and

minimum void ratios for JSC-1A were obtained as 0.826 and 0.410, respectively, based on the

maximum and minimum bulk densities as well as the specific gravity.

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According to ASTM D698, compaction tests were performed on JSC-1A to investigate its

optimum moisture content and the maximum dry density for JSC-1A, in order to determine the best way to attain compaction for the material to simulate extreme conditions possibly encountered on the Moon for engineering related work. It was found that a maximum dry density of 1.75 g/cm3

could be obtained by compaction with an optimum moisture content of 13.5%.

The shear strength of JSC-1A was investigated by standard triaxial tests according to

ASTM D2850. A total of nine triaxial tests were performed on JSC-1A samples of three different

dry densities and each under 100, 150, and 200 kPa confining pressure. According to the Mohr-

Coulomb failure criterion, the friction angle of the soil was determined and the results are

summarized in the Table 2-19. It can be concluded that the friction angle of JSC-1A tends to

increase with the increasing density of the soil. In addition, the friction angle of JSC-1A represents

a good analog with the friction angle reported for the lunar soil (Carrier et al., 1991). It is important

to note that the cohesion measured from the Mohr circle based on the triaxial test results was too

low to make a meaningful conclusion. Later, the cohesion value of JSC-1A was obtained by

performing critical height tests (Li, 2011)

Table 2-18 Results of triaxial tests performed on JSC-1A (Zeng et al., 2009)

Average Bulk Density, Relative Density, % Peak Friction Angle, degree g/cm3

1.66 24.6 41.87

1.79 54.7 46.48

1.94 84.6 56.7

The compressibility of JSC-1A was investigated by performing consolidation tests

following the standard procedures described in ASTM D2435. The compression index of JSC-1A

105 was determined to be 0.068, and the swelling index was found to be 0.001. These relatively low values indicate that JSC-1A presents less compressibility as well as lower swelling properties compared with most of the conventional terrestrial soils. However, these compressibility parameters agree well with the recommended compressibility data of lunar soil provided in the

Lunar Source book (Carrier et al., 1991).

2.6.4 GRC-3

GRC-3 lunar soil simulant was developed to support the equipment analysis for in situ resource utilization (ISRU) operations on the moon, which requires a large quantity of soil sample to test prototypes under simulated conditions. GRC-3 was created by mixing Bonnie silt, a natural mined in Burlington, Colorado, with four different types of sands provided by the Best Sand

Corporation of Chardon, Ohio. Due to all of the raw materials being commercially available at low cost, GRC-3 is available in large quantities for ISRU operation testing at a lower price compared to other lunar soils simulants.

Similar to JSC-1A, the geotechnical properties of GRC-3 were determined at Case Western

Reserve University and NASA Glenn Research Center by performing laboratory tests following the standard testing methods specified by ASTM (1991). The particle size distribution of JSC-1A was shown as in Figure 2-21 as well as the typical particle size distribution of lunar soil. Similar to JSC-1A, the particle size distribution of GRC-3 falls between the bounds of ±1 standard deviation of lunar regolith, but varies from the average range. It was also noted that the medium particle size range slightly exceeds -1 standard deviation of typical lunar regolith, and a small fraction of fine particles (smaller than two µm) of GRC-3 were not reported in the particle size distribution results. From Atterberg limit tests, it was found that GRC-3 shows low plasticity and

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is therefore classified as well-graded silty sand (MS) with 70 percent sand and 30 percent silt

according to the Unified Soil Classification System (USCS).

Figure 2-21 Particle size distribution of lunar regolith and GRC-3 (He, 2011)

The specific gravity of GRC-3 was found to be 2.633 for GRC-3, which is lower than that of the typical lunar soil of 3.1 recommended by Carrier et al. (1991). The maximum and minimum densities of GRC-3 were determined to be 1.94 g/cm3 and 1.52 g/cm3, respectively, and the

corresponding maximum and minimum void ratios for were obtained as 0.732 and 0.358,

respectively. The results from the compaction tests showed an optimum moisture content of 10.11%

at the maximum dry density achieved by compaction, 1.88 g/cm3. The shear strength parameter of

GRC-3 was determined by standard triaxial tests. The obtained friction angles of the soil are

summarized in the Table 2-20. It can be concluded that the friction angle of GRC-3 increases with

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the increasing density of the soil. In addition, the relatively high friction angle of GRC-3 is also

comparable with that of the lunar soil (Carrier et al., 1991). Again, the measured cohesion was too

low to present a meaningful conclusion. By performing consolidation tests, the compression index

of GRC-3 was found to be 0.075, and the swelling index was found to be 0.001, which agrees with the compressibility of typical lunar soil (Carrier et al., 1991).

Table 2-19 Results of triaxial tests performed on GRC-3 (Zeng et al., 2007).

Average Bulk Density, g/cm3 Relative Density, % Peak Friction Angle, degree

1.63 30.4 37.8 1.73 57.2 42.0 1.84 80.3 47.8

2.6.5 Other Lunar Soil Simulants

In addition to the aforementioned lunar soil simulants MLS-1, JSC-1, JSC-1A, and GRC-

3, various other simulants have been developed and produced to support the Earth-based lunar

studies. Jensan Scentific, LLC designed and produced a lunar soil simulant, JS-Lunar Simulant,

specifically as a geotechnical material, in order to approximate the lunar mare basalt material

(Sibille et al., 2006). This simulant contains 10 percent aged brecciated basalt from the Colorado

Rocky Mountain range, 40 percent unweathered vesicular basalt from , 40 percent basalt

from Pullman, , and five percent anorthite from the San Gabriel Mountains in

California.

In 1998, the Japanese Exploration Agency (JAXA) along with the Shimizu

Corporation developed two lunar soil simulant, which are referred as FJS-1 and MKS-1,

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respectively. Both of the simulants were created from a mixture of Mount basaltic lava, olivine,

and ilmenite, and they are designed to replicate the mechanical and chemical properties of the

Apollo lunar sample recovered from the mare region. Both FJS-1 and MKS-1 is comparable to

JSC-1, with a higher SiO2 and lower MgO content.

In order to support China’s Lunar Exploration Program, two lunar soil simulants have

recently been developed in China, referred to as CAS-1 and NAO-1. CAS-1, designed by the

Chinese Academy of Sciences, was created by crushing a low-Ti basaltic scoria, a volcanic ash of alkaline basaltic composition mined from an impact mill at the Jinlongdingzi Volcano in the

Changbai Moutains of Northeast China. CAS-1 is designed to emulate the chemical and mineralogical properties of the lunar regolith from the Apollo 14 exploration areas. Similar to JSC-

1, CAS-1 contains a mineralogical composition of minor plagioclase, olivine, pyroxene and 20%

to 40% modal glass. NAO-1 was developed in National Astronomical Observatories (NAO),

Chinese Academy of Sciences. It was created via gabbro mined from the north bank of the

Yarlungzangbo River in Tibet. NAO-1 is designed to emulate the chemical properties of the lunar

highland soil from the Apollo 16 mission. The major mineralogical compositions of NAO-1 are

pyroxene and anorthite.

Another lunar highland simulant, OB-1, has been developed and produced by the Northern

Center for Advanced Technology Inc. in (Richard et al., 2007). OB-1 was created from a mixture of olivine slag and , which is mined from Avalon Ventures Quarry in Foleyet,

Ontaria. Its design is to replicate the particle size distribution of Apollo 16 highland samples to support the RESOLVE project (Schlagheck et al., 2005).

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3. DEVELOPMENT OF A LIGHT WEIGHT MARTIAN SIMULANT

3.1 Introduction

As has been discussed in the introduction section, the of space exploration is

Mars. Autonomous wheeled robots (rovers) have proven to be effective tools in Martian

exploration (Voilpe, 2003). In a natural terrain, wheel-terrain interaction plays a critical role in vehicle mobility (Bekker, 1956). Prediction of wheel-terrain interaction from terramechanics knowledge is of great importance in the development of planetary rovers (see Figure 3-1) that are able to adapt to challenging conditions while performing complex scientific tasks.

Figure 3-1 Flight Rover “Spirit” in the JPL Spacecraft Assembly Facility (Lindemann et al., 2006)

The interaction between the surface soil and the vehicle wheel can be studied by means of sinkage tests. A typical sinkage test is conducted by applying a constant penetration rate to a plate

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directed into a soil sample by means of a hydraulic ram or other force application device. Pressure-

sinkage curves can be deduced from measured plate displacement and applied force, which are

used to characterize soil behavior for vehicle mobility (Peters, 2002).

The most widely applied pressure-sinkage model was developed by Bekker (1956), which can be expressed as:

= + (3-1) 𝑘𝑘𝑐𝑐 𝑛𝑛 𝐵𝐵 𝛷𝛷 where the pressure and sinkage were𝑝𝑝 linked� by 𝑘𝑘independent� 𝑧𝑧 soil constants , , and plate

𝑐𝑐 𝛷𝛷 dimension . The model𝑝𝑝 takes into𝑧𝑧 consideration the cohesion and internal 𝑛𝑛friction𝑘𝑘 𝑘𝑘 of soil by introducing𝐵𝐵 and , respectively.

𝑘𝑘𝑐𝑐 𝑘𝑘𝛷𝛷 The N2M relationship (Gotteland, 2003) can also be used to model sinkage tests. It

combines elastic and plastic phases of the settlement of soil by linking the linear behavior for small

sinkage to behavior of large sinkage by an exponential function. The N2M is presented with four

parameters , , and .

𝑚𝑚 𝐶𝐶𝑚𝑚 𝑠𝑠𝑚𝑚 𝑠𝑠0 = + (1 exp ( )) (3-2) 𝐶𝐶𝑚𝑚 𝑠𝑠𝑚𝑚 𝑠𝑠0 𝑧𝑧 1−𝑚𝑚 1−𝑚𝑚 𝑝𝑝 � 𝐵𝐵 𝐵𝐵 𝑧𝑧� − − 𝐶𝐶𝑚𝑚 𝐵𝐵 The exponent m is valued between 0 and 1, corresponding to a soil with a cohesive behavior and

a soil with a frictional behavior, respectively. The parameter characterizes the elastic phase by

0 an analogy to the elastic modulus E. The parameters and 𝑠𝑠 characterize the plastic phase by

𝑚𝑚 𝑚𝑚 an analogy to the bearing capacity and hence, the cohesion𝐶𝐶 and𝑠𝑠 internal friction of soil.

Due to the differences between planetary rovers and terrestrial vehicles with respect to

physical dimensions, payload, and terrain, the conventional terramechanics theory must be

111 improved to apply to planetary rovers. Therefore testbeds and platforms were developed according to the special conditions of planetary rovers to conduct wheel-terrain interaction experiments.

A simple and effective approach to create such a light weight simulant is to mix GRC-3 lunar simulant with Styrofoam spheres. As has been discussed in the literature review, GRC-3 is one of the widely used lunar simulants created by mixing bonnie silt and four different sands from

Best Sand Corporation: BS110, BS530, BS565 and BS1635. The geotechnical properties of GRC-

3 are determined by laboratory tests with estimation of the bulk density ranging from 1.52 g/cm3 to 1.94 g/cm3 and the friction angle ranging from 37.8 degree to 47.8 degree (He et al., 2011).

Other mechanical properties of GRC-3 are also provided in He et al. (2011), including grain size distribution, specific gravity, and compressibility. Since both GRC-3 lunar simulant and

Styrofoam spheres are safe, durable, and commercially available in large quantities, the mixture can therefore satisfy the first three requirements. This study is directed towards the development of Martian soil simulant to better replicate Martian soil, based on known mechanical properties, with an emphasis on the unit weight range and compressibility.

This chapter will describe both the development and geotechnical characterization of the new light-weight Martian soil simulant CWRU-1. It will begin with the introduction of existing knowledge with regard to the mechanical properties of Martian soils from past Viking, Pathfinder,

Mars Exploration Rover Opportunity, Spirit, and Phoenix missions, recognizing that limited relevant knowledge has been gained so far. The geotechnical properties of past Martian soil simulants including JSC Mars-1 and the Mars Simulant (MMS) are also considered. Later, this chapter will present the laboratory experimental procedures and testing results. Finally, comparison will be made between the measured parameters and mechanical properties of Martian soils as well as Martian soil simulants.

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3.2 Mechanical Properties of Martian Soil

The principal source of data about the physical properties of Martian soils has been derived

from orbital/remote observations and experiments that were performed by the Viking Landers.

According to Moore et al. (1977, 1978, 1979, 1982, 1987), Martian surface materials can be

categorized into five distinct types: drift material, clods and rusts, blocky material, rocks, and

features thought to be outcrops of bedrock. Since no Martian lander instruments were designed to

measure the mechanical properties of the Martian surface materials, the mechanical properties can

only be crudely estimated.

3.2.1 Particle Size Distribution

The particle size distribution is one of the most important geotechnical properties for a granular material such as Martian soil, affecting the strength and compressibility of the material, as well as its optical, thermal, and seismic properties. Based on the Viking, Patherfinder and MER

data, Martian surface material mostly consists of particles less than 50 µm in size. Observations

made during past missions have suggested that ubiquitous dust is suspended in the Martian

atmosphere. Atmospheric observations of the dust indicated particle size on the order of 1-2 µm

( et al., 1979; and Lemmon, 1999; Clancy et al., 2003; Lemmon et al., 2004).

Inferred from the results of gas exchange experiments from Viking landing missions, the physical

grain size of individual Martian soils including the drift and clods and crusts range from 0.1 to 10

µm in diameter, while blocky material ranges up to 1.5 mm in grain size (Ballou et al., 1978;

Christensen and Moore, 1992). Images from MER landing sites were used to determine that the

particle size of fine grained dust ranges up to 45 µm. Less-fine sand deposits were found by

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Opportunity, with particle size approximately 130-160 µm. Similar sand bedforms with particle

size approximately 60-160 µm was also observed by Spirit.

3.2.2 Bulk Density

The bulk density, defined as the mass of materials contained within a given volume, is a

fundamental property in predicting vehicle mobility and analyzing road stability since it influences

the soil’s strength, compressibility, and permeability. Based on results from Viking Landing and

Pathfinder Rover, the best estimates of the Martian soil bulk density were provided regarding

different materials, as shown in Table 3-1 (Arvidson et al., 1989; Moore et al., 1999).

According to the data achieved from Viking Landers, the bulk density of the drift material

is determined to be 1150 ± 150 kg/m3 by X-ray fluorescence spectrometer (XRFS) analysis ( et al., 1977) as a best estimate. The average angle of internal friction of crusty to cloddy material is consistent with a lunar regolith simulant that has a bulk density near 1400 ± 200 kg/m3, which

is thus taken as the best estimate for crusty to cloddy material. A large bulk density, 1600 ± 400

kg/m3, has been taken as a best estimate for blocky material by analogy with a lunar simulant

which shows consistent analyses of footpad 3 penetration and with a bulk density of 2300 kg/m3.

Little information about the rocks has been determined so far because rocks were too strong to be

sampled. A bulk density of 2600 kg/m3 has been assigned to the rocks by analogy with common

dense terrestrial rocks (Moore et al., 1987; Arvidson et al., 1989).

Pathfinder observations also provide similar results, with a rough estimate of the bulk

density ranging from 1285 to 1518 kg/m3 for drift material and 1422 to 1636kg/m3 for crusty

material (Moore et al., 1999).

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3.2.3 Shear Strength

Shear strength parameters, including cohesion and friction angle, are crucial soil

mechanical parameters that exert significant influence on the issues of vehicle mobility and

excavation. The principal way to characterize the stress-strain characteristics of a soil is the Mohr-

Coulomb failure criterion, which can be expressed as:

= c + tan (3-3)

f Two Viking Landings provide valuesτ of cohesionσ c andφ internal friction angle φ of Martian

soil through analysis of trench walls slopes of artificial piles of material, and footpad imprints

(Moore et al., 1977, 1982, 1987; Arvidson et al., 1989). Because the Pathfinder lander did not

have a surface sampler arm nor foot pads, φ and c values were derived from rover wheel tracks

and wheel-trenching sites (Moore et al., 1999). Dump pile slopes were used to estimate the internal

friction angle by Phoenix Lander and soil cohesions were calculated based on an excavation model

in the Phoenix landing site (Shaw et al., 2009). The Mars Exploration Rovers Spirit and

Opportunity investigated the physical properties of Martian soil by means of rover wheel trenches

and wheel scuffs (Sullivan et al., 2011). The shear strength parameters of Martian surface materials

are summarized in Table 3-1.

3.3 Mechanical Properties of Martian Soil Simulant

Mechanical Martian soil simulants have been primarily sourced from two locations, the

Hawaiian volcanic regions and the Mojave Desert. JSC Mars-1 was developed from weathered ash

deposits of Hawaiian volcanoes and manufactured in large quantity throughout the 1990s. The

common application of JSC Mars-1 is due to its spectral similarity to bright regions on Mars and

composition of volcanic ashes (Allen et al., 1998). However, in more recent years, increasing data

115 on a broad range of from wide-spread Marian surface location have been collected by

Martian missions (Viking, Pathfinder, MER and Phoenix missions) (Peters et al., 2008), requiring development of a subsequent simulant that can better mimic the physical properties of Martian regolith. The Mojave Mars Simulant (MMS) was developed in response by mechanically crushing basaltic boulders sourced from the Mojave Desert. MMS more closely resembles soil shaped by the weathering process on Mars and presents mechanical parameters closer to the measurements from more recent Martian missions.

3.3.1 Particle Size Distribution

The grain size distribution for MMS sand and JSC Mars-1 as determined through standard test methods for particle size analysis (ASTM D422) is shown in Figure 3-2 (Peters et al., 2008).

Compared with the sand and JSC Mars-1, the MMS sand has a larger fraction of smaller particles.

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Figure 3-2 Particle size distribution curve of light weight simulant, GRC-3 lunar simulant, MMS sand, and JSC Mars-1 (Peters et al. 2008)

3.3.2 Bulk Density

The values of dry density of MMS simulants and JSC Mars-1 are summarized in Table 3-

1. The density of both the MMS sand and dust are closer to that of Martian soil than JSC Mars-1

is (Peters et al. 2008).

3.3.3 Shear Strength

The estimated cohesion c and internal friction angle φ for MMS simulants are presented in

Table 3-1. The parameters of shear strength were determined through conventional direct shear

testing (ASTM D3080). It can be seen from Table 3-1 that the friction angle of both the MMS sand

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(38°) and dust (31°) are comparable to the values from Viking Landers and Pathfinder (Peters et

al. 2008).

Table 3-1 Physical properties of Martian surface material found at different landing sites, the MMS simulants, and JSC Mars-1 simulants (Arvidson et al.1989; Moore et al.1999; Shaw et al.2009; Sullivan et al.2011; Peters et al.2008)

Grain Sizes Bulk Density Angle of friction Cohesion µm kg/m3 Degrees kPa Viking Lander drift material 0.1-10 1150 ± 150 18.0 ± 2.4 1.6 ± 1.2 crusts and clods 0.1-10 1400 ± 200 34.5 ± 4.7 1.1 ± 0.8 blocky material 0.1-1500 1600 ± 400 30.8 ± 2.4 5.1 ± 2.7 rocks 35 × 103 2600 40 to 60 — Pathfinder drift material — 1258 to 1518 15.1 to 27.9 0.18 to 0.53 crusty material — 1422 to 1636 37.0 ± 2.6 0.17 ± 0.18 Phoenix crusts and clods — — 38 ± 5 0.2 ± 0.4 to 1.2 ± 1.8 Opportunity and Spirit (Trenching) — — 30 37 0 2 (Scuffing) — — — 0 11 − − MMS sand — 1384 38 0.81 − MMS dust — 1078 31 0.38 JSC Mars-1 — 835 47 1.91

3.4 Method for Creating the CWRU-1 Simulant

The CWRU-1 light-weight simulant is made by mixing GRC-3 soil particles with 2 mm to

4 mm Styrofoam spheres (available at http://www.stevespanglerscience.com/). The Styrofoam balls were first coated with glue on the surface and then GRC-3 simulants were introduced. The glue applied in the tests was 's Craft Multi-Purpose Spray Adhesive. After that, the two components were mixed by hands until all the Styrofoam balls were fully covered with GRC-

3 particles. After 30 minutes, the GRC-3 particles were attached to the Styrofoam sphere uniformly as shown in Figure 3-3.

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Figure 3-3 Image of light weight simulant

It is well known that the gravity magnitude on Mars accounts for 38% that on the earth. As

a consequence, the mass density of light weight simulant needs to be the same fraction of typical

Martian soil to satisfy the unit weight requirement. Since the bulk density of Martian soil can vary

depend on different types of surface materials and different measurement methods (Table 3-1), a

mass density range of 0.6 to 0.7 g/cm3 is assigned as the aimed densities of the mixed soil sample to simulate the blocky material on Mars. Several mixing ratios of the GRC-3 simulant versus

Styrofoam spheres have been tested to develop the new mixture. A ratio of 5:6 of the GRC-3

simulant to Styrofoam sphere by volume was found to be the most suitable one, leading to an

average density of 0.675 g/cm3. A total of 4000 g of the light weight simulant is made of 2 mm to

4 mm Styrofoam spheres mixed with GRC-3 sand at a ratio of 6:5 by volume, bounded by 1850 g of spray adhesive glue.

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3.5 Geotechnical Properties of CWRU-1

Several geotechnical laboratory tests were conducted to measure the geotechnical properties of the newly developed Martian soil simulant. The test procedures generally followed the commonly applied standards in the field specified by ASTM (1991), so as to ensure the comparability of the results. Repeatability of the data was checked through repeating tests multiple times. It is important to note that air-dried soil samples instead of oven- dried ones were utilized in the test because the Styrofoam ball will shrink under high temperature.

The water content for the cohesion-less soil is small enough for the purpose of the tests.

3.5.1 Specific Gravity

The specific gravity of a soil is defined as the ratio of the mass density of solid particles to the mass density of pure water at 4ºC. It is an important mechanical parameter, base on which other geotechnical properties such as porosity and void ratio can be estimated.

In this study, a total of the three samples of the light weight simulant were tested generally following ASTM standard D854. The results are summarized in Table 3-2, showing very little difference. The average specific gravity was found to be 1.51 from the test. It is worth noting that the simulants were not subjected to vacuum for the sake of Styrofoam balls which would blow up if subjected to vacuum. The specific gravity determined by the tests is therefore smaller than the actual value.

Table 3-2 Test Results of Specific Gravity of CWRU-1 Light weight simulant Specific Gravity

Test 1 1.518 Test 2 1.529 Test 3 1.498 Average 1.515

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3.5.2 Particle Size Distribution

Replication of the grain size distribution plays a significant role in the development of soil

simulant. According to the past testing results from Martian missions, the Martian soil is classified

as a fine-graded cohesion-less to rocky soil (Banin et al.,1992).

The particle size distribution of the light weight simulant is determined by sieve analysis

(ASTM D422). Since it contains little fine particles, hydrometer tests (ASTM D423) are not

applied and reported here. A group of sieving tests were also conducted on the GRC-3 soil, which

was utilized as sourcing material for the light weight simulant.

The results of the measurements are shown in Figure 3-2, together with that of GRC-3, MMS sand and JSC Mars-1. As shown in the figure 3-2, the results from the two samples are almost identical, suggesting a good uniformity of the newly developed soil sample. Compared with the grain size distribution of MMS simulants and JSC Mars-1, the particle size of the new simulant is larger due to the combination of the Styrofoam balls and the GRC-3 simulant particles. The main characteristics of the soil are:

a. It has D10= 0.19 mm, D30 = 0.503 mm, and D60 = 1.22 mm.

2 b. The coefficient of uniformity Cu = D60/D10 = 6.42, and the coefficient of curvature Cc = D30 /

(D60×D10) = 1.09.

Therefore, the soil can be classified as poorly-graded coarse sand.

It is also useful to know whether the soil can be repeatedly used. After a soil sample went

through a compaction test (using a standard Procter device), its particle size distribution was studied again to investigate the effect of mechanical compaction on particle size distribution.

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Moreover, the particle size distribution was also measured again after a soil sample went through

a shear test up to failure. The results of particle size analysis after soil samples were subjected to

mechanical compaction and shear are shown in Figure 3-2. Mechanical compaction and shearing

after a single repetition produced no significant change of the simulant particle size. We conclude

that a single compaction or shear test repetition had little impact on the particle size distribution.

Thus the simulant can be used more than once in most soil tests, as far as size distribution is

concerned.

3.5.3 Maximum and Minimum Density

One of the most important properties in the development of a light weight simulant is the

density of soil that controls the ratio of styrofoam spheres to the GRC-3 simulant particles. As stated before, the unit weight of the light weight simulant must be one third that of typical soil, and the density of the mixed soil sample should therefore be in the range of 0.6 to 0.7 g/cm3.

In order to define the state of density of a cohesion-less soil like the light weight simulant, it

is important to determine the maximum and minimum densities. From the measured in situ density

of the soil, it is possible to calculate its relative density as

= × × 100% (3-4) 𝜌𝜌𝑚𝑚𝑎𝑎𝑎𝑎 𝜌𝜌−𝜌𝜌𝑚𝑚𝑚𝑚𝑚𝑚 𝑅𝑅𝐷𝐷 𝜌𝜌 𝜌𝜌𝑚𝑚𝑚𝑚𝑚𝑚−𝜌𝜌𝑚𝑚𝑚𝑚𝑚𝑚 where, is the maximum bulk density, is the minimum bulk density, and is the bulk

𝑚𝑚𝑚𝑚𝑚𝑚 𝑚𝑚𝑚𝑚𝑚𝑚 density 𝜌𝜌of a soil sample. The relative density,𝜌𝜌 generally referred to as the degree ofρ soil particle

packing, has a significant impact on the strength and stiffness of a soil. It is a crucial parameter in

predicting rover mobility as it influences the soil’s bearing capacity and slope stability.

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A total of six tests were performed in the laboratory at CWRU in order to determine the

maximum and minimum bulk density of the light weight simulant. The tests followed ASTM

D4253 and ASTM D4254 for minimum index densities.

The results are summarized in Table 3-3. The average maximum and minimum dry densities

of the light weight simulant are 0.916 g/cm3 and 0.621 g/cm3, respectively. Considering that the

mass density of Martian soil ranges from 1000 kg/m3 to 2000 kg/m3 (Table 3-1), this density is

roughly one third of a typical regolith, a good simulation for blocky material.

Table 3-3 Test Results of Maximum and Minimum Densities of CWRU-1

Light weight simulant Minimum density (g/cm3) Maximum density (g/cm3)

Test 1 0.607 0.917 Test 2 0.627 0.912 Test 3 0.631 0.919 Average 0.621 0.916

3.5.4 Compressibility

Both the design of a vehicle and excavation tools require the knowledge of a soil’s load- settlement relationship. The soil parameters that are used to describe this relationship can be determined in a consolidation test conducted following the standard procedures described in

ASTM D2435. In a consolidation test, the corresponding deformation is measured when a soil

sample is subject to a series of one-dimensional loads with a load increment ratio = 1. From

the recorded data, a diagram of void ratio versus log () can be made∆𝑝𝑝.⁄ According𝑝𝑝 to

the straight-line part of the relationship, the compression index, Cc, can be determined as

Cc = (e1 – e2) / log (p1 / p2) (3-5)

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where e1 and e2 are the void ratio of the soil corresponding to effective vertical stress of p1 and p2,

respectively. Then, the vertical loading is gradually reduced in steps and the resulting void ratio is

measured. From the recorded data on the unloading part, the recompression index, Cs, is

determined as

Cs = (e1 – e2) / log (p1 / p2) (3-6)

It should be noted that there is no need to test the consolidation index for a particular light-weight

simulant.

A total of two oedomter tests were conducted on the light-weight simulant at two initial densities: one loose and one dense. The testing results are shown in Figure 3-4. From the data it is found that for the loose sample with initial void ratio of 1.04, the compression index Cc = 0.345,

and the recompression index Cs = 0.012. For the dense sample with initial void ratio of 0.88, the

compression index Cc = 0.137, and the recompression index Cs = 0.008. Both values are quite high, indicating the soil is more compressible than most conventional soils.

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Figure 3-4 Results of 1D compression test for light weight simulant

3.5.5 Shear Strength

Shear strength parameters, cohesion c and internal friction angle of the newly-developed simulant were determined through conventional triaxial test, the most φcommon way to describe the strength of a soil.

A total of nine triaxial tests were conducted on light weight Martian soil simulants of three different dry densities: 0.64 g/cm3, 0.87 g/cm3 and 0.89 g/cm3, each at 20, 40 and 80 kPa confining pressure following the standards described in ASTM D2850. In addition, the shear strength was measured once more after a sample was tested in the triaxial tests up to failure, with soil sample density of 0.65 g/cm3.

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The principal stresses at failure of three samples at different confining pressures for each

density are used to generate Mohr’s circles at failure, from which the failure envelopes can be

drawn. Then the cohesion and friction angle of the soil can be determined. A typical result of

triaxial tests is presented in Figure 3-5. Based on the strength recorded, deformation is somewhat

greater than 15 percent strain or load holds constant, the Mohr’s circles at failure can be drawn

and the friction angle and cohesion value determined as shown in Figure 3-6.

The results of triaxial tests are summarized in Table 3-4. According to the results, the friction

angle is found to increase with increasing relative density. The range of friction angle is similar to

that reported for Martian surface material (crusty material and blocky material) (Table 3-1). In

comparison with the typical Martian soil simulant and Martian soil, the measured cohesion is a

little higher, which is mainly due to the glue applied on the simulant particles and difference in the

particle size distribution. As shown in Figure 3-2, the JSC Mars-1, MMS sand, and new light weight simulant are in different particle size categories. The light weight simulant dominates the range from 2.7 to 0.17 mm, while the JSC Mars-1 covers the range from 1.2 to 0.1 mm and MMS sand covers the range from 0.6 to <0.075 mm. Due to the wide range in particle size of the light weight simulant, fine particles can fill in the voids when the soil goes under compaction which leads to a higher cohesion value. The results of Mohr’s circle analysis after soil samples were subject to shear to failure are shown in Table 3-4 marked as Sample 3. By comparing Sample 3 and Sample 4, it can also be concluded that the shearing process after a single repetition had little impact on the shear strength of the light weight simulant.

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Table 3-4 Triaxial test results of CWRU-1 Average Bulk Sample No. Friction Angle Cohesion Density g / cm3 degree kPa 1 0.89 39.9 5

2 0.87 38.4 11

3 0.65 31.6 10

4 0.64 30.6 10

Figure 3-5 Deviator stress versus axial strain recorded in traxial tests on light weight simulant (density = 0.64 g/cm3)

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Figure 3-6 Mohr stress circles for light weight simulant (density = 0.64 g/cm3)

3.6 Conclusion

A new light weight simulant, CWRU-1, is developed by mixing GRC-3 lunar simulant

with 2 to 4 mm Styrofoam Spheres at a ratio of 5:6 to achieve typical Martian regolith mass density

range. The geotechnical properties of a new light-weight Martian soil simulant were determined in the laboratory following methods used widely in geotechnical engineering. The results are compared with Martian regolith and Martian soil simulant, which forms the basis to judge the effectiveness of the simulant in simulating the mechanical behavior of Martian soils. Based on the results of this study, the following conclusions can be drawn:

a. The average specific gravity of the simulant was found to be 1.515.

b. The particle size of the simulant is larger than that of GRC-3, as well as the average

of typical Martian soil simulants; single compaction and shear test had little effect on the

particle size distribution.

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c. The maximum density of the simulant is 0.916 g/cm3 and the minimum density of

the simulant is 0.621 g/cm3, which are about 38% that of typical soil regolith on earth,

similar to 38% that of Martian soils, especially Martian blocky material.

d. The compression and recompression indices of the light weight simulant are lower

than typical soils.

e. The friction angle of the light weight simulant increases with density, which is also

similar to that of Martian soil, particularly to Martian crusty material and blocky material.

The measured cohesion is higher than the typical Martian soil simulant and Martian soil

due to the glue applied on the surface of the particle and the wide range in particle size.

Single shear had little effect on the shear strength of the light weight simulant.

In summary, the geotechnical properties of CWRU-1 generally match the geotechnical requirements. CWRU-1 presents similar mechanical properties to Martian soil and can serve as a good analog for portions of the Martian crusty and blocky material. The light weight simulant will be translated into useable soil bins that are validated for the study of high shrinkage mobility tests.

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4. DEVELOPMENT OF A MINIATURE TRIAXIAL TESTING APPARATUS

4.1 Introduction

As discussed in Chapter 2, the triaxial test is one of the most widely used methods in

Geotechnics to investigate the physical and mechanical properties of soils and rocks under

different confining stresses. The triaxial test is of vital importance for soil characterization, vehicle

mobility, and excavation. Triaxial test results also play an important role for numerical modeling

studies and the development of suitable constitutive models by providing input parameters.

Scott (1987) was one of the pioneers to study the strength properties of lunar soil via triaxial

tests. Based on a 1.1 g soil sample returned by Apollo 12 astronauts from the scoop of Surveyor 3

mission in 1972, Scott (1987) conducted the first miniature triaxial tests at Cambridge University

Engineering Department (see Figure 4-1). The miniature triaxial apparatus was designed for a soil sample of less than 1 g in a triaxial cylinder of 0.25 in (6.23 mm) in diameter and 0.5 in (12.7 mm) high. According to Scott (1987), the testing results suggested the failure behavior of the material was not influenced by a scale effect, which was confirmed by centrifuge tests based on model piles.

However, Scott (1987) also admitted that the measurement of the pressure and displacement from the miniature triaxial tests were not very accurate due to limitations of the measurement sensors at that time. Part of the testing results (Scott, 1987) suggested a cohesion value of the lunar soil ranged from 0 to 1 kPa, and the friction angle from 51 to 59 degrees, but the detailed results have never been published.

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Figure 4-1 Small and large samples after failure (Scott, 1987)

With current advances in sensor technology, a much more accurate measurement of the

axial load and sample displacement can be obtained in triaxial testing. A new miniature triaxial

system has therefore been developed and housed at the Department of Civil Engineering

Laboratories, Case Western Reserve University. This miniature triaxial apparatus was designed to

investigate the mechanical properties of lunar soil simulant and lunar soil. Due to the fact that only

a limited quantity of lunar soil sample can be used for the investigation, this miniature device was designed for performing triaxial tests on a soil sample of less than 10 g in a triaxial cylinder of 10 mm in diameter and about 20 mm high. The design caters for undrained, unconsolidated (UU)

triaxial studies of soil strength properties.

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4.2 Apparatus Description

The miniature triaxial testing system is as shown in the Figure 4-2. The apparatus allows continuous monitoring of axial stress and displacement. It consists of four parts in the set up:

(1) The pressure cell

(2) The loading unit

(3) The vacuum and confining system

(4) The measurement system

These primary components of the setup are each introduced below.

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Figure 4-2 Miniature triaxial testing system

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4.2.1 Pressure Cell

There are two main parts of the cell; the base and cover. The cell base consists of an approximately 12 mm thick, 195 mm diameter circular plate and a 10 mm high and 10 mm diameter cylinder sample platen which is mounted in the center of the plate (Figure 4-3). The cell base has a total of four ports through which lines are used for air extraction, drainage, confinement oil and pore pressure measurement. Since only dry soil sample was utilized for lunar soil and lunar soil simulant strength tests, and the confining pressure is provided by compressed air in the tests, only one port is put into use and the other three were capped for our purposes. A transparent perspex cylinder is used as cell cover to sit on an ‘O-ring’ within a seating cut into the cell base to provide a seal for the pressure cell system. The cell cover has an oil filler with dowty bonded seal and one port for connection with the compressed air to provide confining pressure (Figure 4-6). It is worth noting that typically liquid is used as the confining medium, however, compressed air was used here to better simulate the outer space environment; free from any free water source. Another advantage of the compressed air lies in that it is not as sensitive to membrane leaks. An 11 mm diameter, solid cylindrical loading shaft runs through the cell cover and applies the axial load onto the soil sample. The cell base and cover are connected via three rods with thread fittings at bottom to properly fix them to the cell base to ensure a tight seal.

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Figure 4-3 Cell base for the miniature triaxial testing system

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Figure 4-4 Cross section of cell base for the miniature triaxial testing system

Figure 4-5 Plane view of cell base for the miniature triaxial testing system

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Figure 4-6 Cell cover for the miniature triaxial testing system

Figure 4-7 shows the setup for a soil sample inside the cell, prior to testing. The sample is placed between 10 mm diameter stainless steel bottom platens and sample top cap. The top cap has a 12 mm internal diameter plug with a 19 mm diameter flange at its top where the loading shaft can be seated to ensure perfect coupling between the loading shaft and the top cap. The air vacuum is connected to the bottom plate and air pressure injection lines to the cell cover. Two 2 mm thick porous disks are placed between the top and bottom platens to ensure uniform distribution of the injection air pressure into the sample. The soil sample is enclosed in a 0.025 mm latex membrane.

High pressure vacuum grease is pasted over the ‘O-ring’ joints between the cell base and cell cover to complete the seal between the sample and the cell.

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Figure 4-7 Setup of a soil sample

4.2.2 Loading Unit

The loading unit consists of a loading frame and load cell. The loading frame has been designed to withstand up to 2.5 kN of axial load which acts, through the loading ram and onto the sample on a circular bottom plate. The cell base rests on the circular bottom plate and load is applied by movement of the circular plate upward. The loading rate and direction can be controlled using the control box, which has been mounted to the loading frame. A load cell is placed between the loading shaft and the top of the loading frame using thread fittings. This load cell is rated to

2.5 kN of axial force. Load readings are continuously reported to the from the load cell.

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4.2.3 Vacuum and Confining System

4.2.3.1 Vacuum Unit

Prior the test, the soil specimen was subjected to vacuum to obtain the desired confining

pressure in order to achieve its natural state with minimal disturbance. The vacuum environment

for the soil sample preparation is created using a DuoSeal 1400 model vacuum pump. This vacuum

unit has high contamination tolerance, and provides ultimate vacuum of less than 0.0001 Torr

(0.00013 mbar).

4.2.3.2 Confining Unit

The confining unit is used to provide the required air pressure to the testing soil in the

confinement cell. The pressure of compressed air is controlled by a pressure gauge, which has a

measurement range of up to 60 psi (about 413 kPa) and an accuracy of measurement of 0.2 psi

(about 1.4 kPa).

Figure 4-8 Vacuum unit for the miniature triaxial testing system

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4.2.4 Measurement System

4.2.4.1 Displacement Transducer and Load Cell

The measurement system consists of a displacement transducer and a load cell. For the

setup, the triaxial apparatus is fitted with a linear potentiometric transducer to measure the axial

displacements. Given the fact that during the tests, the whole pressure cell will move vertically

upward with deformation of the sample, the C is therefore mounted on the loading ram (above the

pressure cell) to measure axial displacement. The displacement transducer is Model 82-P0322,

with a range of ±25 mm, and is capable of carrying an accuracy of measurement to 0.002 mm.

The axial load for the miniature devices is designed to be applied by a combination of load cell and an 11 mm diameter stainless steel loading ram. One end of the loading ram is connected to the bottom plate of the load cell. The other end of the loading ram is connected to the top of the sample. The top part of the load cell is connected to the loading frame using a 40 mm height cylindrical steel rod. The load cell utilized here is Model 82-P0370, can measure up to 2.5 kN axial loads with a measuring accuracy of ±0.03% of full range.

Figure 4-9 Displacement transducer and load cell for the miniature triaxial testing system

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4.2.4.2 Data Acquisition Instrument

The analogical data from the load cell, and displacement transducers are collected using a

76-Q0802/C-CZ model, high-, multifunction data acquisition instrument, the

Digimax Plus. The data acquisition instrument is connected to a computer through a RS232 serial port by using a pin to pin type cable and has the ability to transfer data at rates of 38,400 bit per second, 8 data bit, no parity, 1 stop bit, no flow control. The data acquisition instrument has another serial output channel for connection of a serial printer. There are two differential analogical input channels to collect data. One channel is for connection to the load cell with resolution of 130,000 points. Another one is for connection to the linear displacement transducer with resolution of

130,000 points. The function of the acquisition instrument is managed by its installed software, organized in various menus. The user interface consists of a graphical display with 240X128 pixel resolution and a ten push-button key board, of which four are interactive with the icons which appear on each display. A simple and convenient version of the “Hyper Terminal” program for the

Windows operating system is used to receive the analogical data from the load cell and displacement transducers, transferring it to the computer and plotting it during acquisition. The software has an efficient digital interface that allows the operator to monitor the real-time data change during the tests. The data acquisition instrument also has a graphical user interface to present the load versus the displacement graph updated in real time. The two axes are adjusted by the software function with steps of 5%.

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Figure 4-10 Displacement transducer for the miniature triaxial testing system

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4.2.5 Others

Additional tools which were utilized in the miniature triaxial testing are listed below:

Figure 4-11 Squeeze bottle for sample preparation

Figure 4-12 Sample mold for sample preparation

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Figure 4-13 Sample top cap

4.3 Types of Testing for the Miniature Triaxial Apparatus

Since the outer space environment is free from any free water source, the water component will not be considered in the tests so as to simulate the lunar and Martian environments. Only dry soil sample will be considered in the tests. Therefore, the new miniature triaxial apparatus was primarily designed to conduct unconfined, unconsolidated (UU) tests of dry soil sample.

4.4 Testing Procedures

The procedures outlined as below were generally followed with standards described in

ASTM D2850 to determine the shear strength of soil samples (dry sand) via the triaxial tests under

UU conditions:

1. In the squeeze bottle weigh a representative amount of dry soil.

2. Put a rubber sleeve membrane over the base platen and sealed it with rubber bands.

3. Place a specimen mold upon the latex membrane and tighten it with a metal latch. Fold

back the membrane over the top rim of the mold. Place a porous stone and filter paper into

the membrane and make sure they enclose the top of the base platen.

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4. Connect vacuum pump to the specimen mold to evacuate the air between the latex

membrane and the membrane stretcher. The latex membrane should be attached to the

inside wall of the specimen mold tightly due to the negative pressure.

5. In order to obtain different desired density ranges, three different sample preparation

methods were utilized as below:

(A) Carefully pour the soil from the squeeze bottle onto the rim of the specimen mold,

subsequently adding a little more soil gently.

(B) Place soil from the squeeze bottle into the specimen mold and tap lightly.

(C) Place soil from the squeeze bottle into the specimen mold in increments. Tap hard after

each increment.

6. When the soil reached to the desired height, carefully place another filter paper and porous

stone above the soil specimen and add the top cap on top of the porous stone. Next, roll the

membrane off the specimen mold over the top cap and seal the membrane to the top cap

with rubber bands.

7. Weigh remaining soil in the squeeze bottle to obtain the sample weight.

8. Remove the vacuum from the outside of the specimen mold and quickly connect the

vacuum to the inside of the soil specimen via the connection of the cell base.

9. Carefully unlock the latch and remove the specimen mold. The sample then should be free

standing as shown in Figure 4-7.

10. Obtain three height measurements approximately 120 degrees apart, and use the average

value for the initial specimen height. Using a pair of calipers to take three diameter readings

120 degree apart each at the top, at mid height, and at the base, compute the average

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diameter of the specimen at each height and then compute a final average specimen

diameter as

= ( + 2 + )/4 (4-1)

𝑎𝑎𝑎𝑎 𝑡𝑡 𝑚𝑚𝑚𝑚𝑚𝑚 𝑏𝑏 Where is the measured𝑑𝑑 top diameter,𝑑𝑑 𝑑𝑑 is 𝑑𝑑the measured bottom diameter,

𝑡𝑡 𝑏𝑏 is𝑑𝑑 the measured diameter at mid𝑑𝑑 high and is the average specimen

𝑚𝑚𝑚𝑚𝑚𝑚 𝑎𝑎𝑎𝑎 diameter.𝑑𝑑 Calculate the corresponding value of initial sample𝑑𝑑 volume using the average

height and the average diameter just obtained. Determine the soil bulk density by dividing

the mass of used soil by the initial sample volume.

11. Put on a clean ‘O-ring’ within the base seating, make sure that the base is free of soil grains

and then add some airtight lubricant in the seating. Assemble the triaxial cell to obtain an

airtight seal.

12. Place the assembled cell in position in the loading device. Attach the pressure-maintaining

system and motor to the triaxial system. Raise the cell until the load shaft sits slightly below

the load cell. Adjust the displacement transducer to the machine. Set the loading device to

the desired strain rate of 0.8 mm/min.

13. Simultaneously connect the compressed air to the cell to provide confining pressure to the

sample while removing the vacuum from inside of the sample. After the chamber pressure

is applied, wait approximately 10 minutes to allow the specimen to stabilize.

14. Record and plot the simultaneous axial stress and sample displacement via the data

acquisition device.

15. Start the loading device with the loading shaft slightly above the specimen cap. Readings

may be taken until the sample fails as follows:

a) The sample bulges outward at the center.

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b) The sample deformation is greater than 20 percent axial strain.

c) The load peaks and falls off.

d) The load holds constant.

16. After the sample has failed, release the chamber pressure, and remove the sample load.

17. Prepare a new specimen with similar density and repeat steps 1 through 16 for a total of

three different confining pressures. We used confining pressures of 100, 150, and 200 kPa.

4.5 Testing Results

To verify the performances of the apparatus, a series of case test programs were carried out. Two

kinds of soils were taken as study objects: standard sand and JSC-1A lunar soil simulant. The geotechnical properties of JSC-1A was summarized in Chapter 2

4.5.1 Triaxial Strength Tests for JSC-1A

Strength testing was carried out for JSC-1A lunar soil simulant sample under 100, 150, and

200 kPa confining pressure. When the confining pressures stabilized, axial stress was applied using a strain rate of 0.08 mm per min to sample failure, as indicated by an ultimate strength or peak strength in axial stress in the axial stress versus axial strain plot. Failure took approximately 50 min. The axial strain and axial stress were continuously recorded during the test. Figure 4-14 shows soil sample before and after failure.

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Figure 4-14 JSC-1A soil sample before and after failure

A typical result of triaxial testing is shown in Figure 4-15. As shown in the figure, JSC-1A

shows both peak and ultimate strength. It is also noticeable that the shear stress versus displacement curves were somewhat noisy. The irregularities are probably partly due to friction between the loading ram and pressure cell. A motor system has been utilized to reduce the effect of the piston friction, however, the small size of the soil sample made the piston friction comparable to the obtained axial stress. The irregularities are also partly due to the statistical

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fluctuations in grain structure at the shear plane and in interactions with the membrane interior

side.

Based on the peak or ultimate strength recorded, the maximum deviator stresses can be

calculated and the major principal stress for each test can therefore be obtained as:

= +

𝜎𝜎1 𝜎𝜎3 ∆𝜎𝜎1 Where is the major principal stress, is the corresponding cell confining pressure, and

1 3 is the maximum𝜎𝜎 deviator stress. 𝜎𝜎

∆𝜎𝜎1 Based on the principal stress obtained at failure of three different confining pressures from

three different samples with one density, the corresponding Mohr’s circles at failure can be drawn.

Therefore, the shear strength parameters, cohesion and friction angle of the soil samples can be

obtained. The Mohr stress circles for JSC-1A is as shown as in Figure 4-16. It is also noted that similar to the testing results from the conventional triaxial tests, the cohesion measured from the

miniature triaxial tests were also too low to be determined. The cohesion from the testing is

therefore considered to be zero.

.

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1200

1000

800

600 Deviator Stress (kPa)

400

200

0 0% 5% 10% 15% 20% 25%

Axial Strain (%)

100 kPa 150 kPa 200 kPa

Figure 4-15 Deviator stress versus axial strain recorded in triaxial tests on JSC-1A

(Density = 1791 kg/m3)

150

600

500

400

300

200 Shear kPa Stress,

100

0 0 200 400 600 800 1000 1200

Normal Stress, kPa

Figure 4-16 Mohr stress circles for JSC-1A (Density = 1791 kg/m3)

The results of triaxial tests are summarized in Table 4-1. It can be concluded from the testing data that the friction angle increases with increasing density of the soil. The range of friction angles is similar to that reported through the conventional tests.

Table 4-1 Results of miniature triaxial tests performed on JSC-1A

Average Bulk Density, Peak Friction Angle, Relative Density, % g/cm3 Degree 1691 32.4 39.5 1791 55.1 43.8

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Table 4-2 Results of triaxial tests performed on JSC-1A (Zeng et al., 2007)

Average Bulk Density, Peak Friction Angle, Relative Density, % g/cm3 Degree 1659 24.6 41.9

1789 54.7 46.5

1.94 84.6 56.7

4.5.2 Triaxial Strength Tests for Standard Sand

Strength testing was also carried out for standard sand under 100, 150, and 200 kPa confining pressure. In this test, the standard sand with a dry density of 1771 kg/cm3 was used.

Figure 4-17 shows the stress–strain of the standard soil under different confining pressure.

It can be seen that similar noisy results were obtained due to the same reason. Based on the principal stress obtained at failure, the corresponding Mohr’s circles at failure can be drawn as

Figure 4-18. A friction angle of 37.2 degree can be obtained from the tests.

152

700

600

500

400

300 Deviator Stress (kPa) 200

100

0 0% 5% 10% 15% 20% 25% 30% Axial Strain (%)

100 kPa 150 kPa 200 kPa

Figure 4-17 Deviator stress versus axial strain recorded in triaxial tests on standard sand (Density = 1771 kg/m3)

153

400

350

300

250

200

150

100 Shear kPa Stress,

50

0 0 100 200 300 400 500 600 700 800 900

Normal Stress, kPa

Figure 4-18 Mohr stress circles for standard sand (Density = 1771 kg/m3)

4.6 Calibration Tests

Calibration tests were attempted to get rid of the friction noise by performing the miniature triaxial tests on cotton. Due to the shear strength of the cotton being very low, the shear strength measured from the tests can be regarded as noise created by the friction between the loading ram and pressure cell. However, due to the high vacuum capacity, all the cotton samples were largely deformed before the tests as shown in Figure 4-19, and the tests results show high irregularity, which makes no meaningful conclusion and is therefore not reported here.

154

Figure 4-19 Cotton sample for calibration tests before loading

4.7 Conclusions

A description has been given of the construction of a new miniature triaxial test which has been set up with four major parts: (1) the pressure cell; (2) the loading unit; (3) the vacuum and confining system, and (4) the measurement system. The miniature triaxial apparatus is a simplified apparatus when compared to modern triaxial cells and it was designed to perform UU triaxial tests on dry sandy soil.

The features of the apparatus have been displayed: (1) It was able to perform triaxial tests on a soil sample of less than 10 g in a triaxial cylinder of 10 mm in diameter and about 20 to 30 mm high. (2) It is equipped with a measurement system, by which the shear stress versus displacement curves can be simultaneously recorded during the loading. (3) The time to prepare 155

and run a triaxial test on a cohesion-less sand sample with this miniature apparatus is about 1.5

hours.

To verify performances of the apparatus, a series of case testing programs were performed.

These experiments show the behavior of the JSC-1A in the miniature triaxial apparatus conforms

to that obtained from the conventional tests. It is therefore believed that the present equipment

measures the true stress and strain behavior of the soil being tested.

Nevertheless, it is also shown from the test results that this apparatus had some drawbacks.

First of all, the measurement of the axial stress is not very precise due to the piston friction which results in noisy shear stress vs displacement curves. Secondly, no radial pressure or radial strain has been measured during the tests, which could lead to a less precise measurement of confining pressure and interpretations of axial strain. Thirdly, the triaxial device can only be utilized for dry soil sample. Future work is therefore suggested to improve the accuracy of the testing device.

Based on the obtained testing results, the validity of the used apparatus was confirmed. The apparatus described was more convenient, more compact, as well as cheaper compared with contemporary triaxial apparatus. The reported apparatus allowed us to perform UU triaxial tests on lunar soil, lunar soil simulant, Martian soil simulant and any dry cohesion-less soil with limited quantity. The setup of this apparatus can provide the shear strength parameters needed for engineering design applied to outer space exploration.

156

5. CONCLUSIONS AND RECOMMENDATIONS

5.1 Introduction

With the development of new technologies, space exploration, returned lunar exploration

and Mars exploration has become the frontier of the new century. Two essential components of

the program “Moon, Mars and Beyond”, in situ resource utilization (ISRU) and surface mobility,

require a thorough understanding of the mechanical and engineering properties of lunar and

Martian soils. In keeping with these goals, the first objects of this research was to develop a light- weight Martian soil simulant that can be produced in large quantities at a reasonable price and to characterize the geotechnical properties of this simulant for the study of high shrinkage mobility tests. The second objective of this research is to construct a new miniature triaxial testing device

to perform triaxial tests on dry, cohesion-less soil of less than 10 g to investigate the shear strength

of lunar regolith and lunar simulants.

In this dissertation, a comprehensive investigation was performed on the geotechnical

properties of the lunar regolith, lunar soil simulants, Martian soil and Martian soil simulants. A

light weight Martian soil simulant was developed and a series of laboratory tests were used to

measure the geotechnical properties of the simulants including particle size distribution, specific

gravity and density ranges. The test procedures generally followed the standards specified by 1991

ASTM. The standards are commonly used in the geotechnical engineering community, and the comparability of the testing results can therefore be verified. A miniature triaxial testing device

was constructed and a series of case testing programs were performed on standard sand and JSC-

1A. In addition, the testing results were compared to the shear strength obtained through the

conventional triaxial tests to confirm the validity of the new device. Conclusions based on the

testing results and the recommendations for future work are presented in the following sections. 157

5.2 Summary of Conclusions

A comprehensive study was carried out to develop a light weight Martian soil CWRU-1 as well as a miniature triaxial testing apparatus. Several conclusions can be drawn regarding the obtained results of laboratory tests and analysis.

5.2.1 Geotechnical Properties of CWRU-1

Based on the testing results and the analytical comparison of CWRU-1 with Martian soil and Martian soil simulant, it can be concluded that CWRU-1 is an effective simulant with regards to the mechanical behavior of Martian soils. CWRU-1 is developed by mixing GRC-3 lunar simulant with 2 to 4 mm Styrofoam Spheres at a ratio of 5:6 to achieve typical Martian regolith mass density range. It can be easily reproduced in large quantities at a relatively low cost, which satisfied the requirement of the study for high shrinkage mobility tests. According to the results of this investigation, the following additional conclusions can be drawn:

1) The average specific gravity of the stimulant was found to be 1.515.

2) The particle size of the simulant is larger than that of GRC-3, as well as the average of

typical Martian soil simulants; single compaction and shear test had little effect on the

particle size distribution.

3) The maximum density of the simulant is 0.916 g/cm3 and the minimum density of the

simulant is 0.621 g/cm3, which are about 38% that of typical soil regolith on earth, similar

to 38% that of Martian soils, especially Martian blocky material.

4) The compression and recompression indices of the light-weight simulant are lower than

typical terrestrial soils.

158

5) The friction angle of the light weight simulant increases with density, which is also similar

to that of Martian soil, particularly close to Martian crusty material and blocky material.

The measured cohesion is higher than the typical Martian soil simulant and Martian soil

due to the glue applied on the surface of the particle and the wide range in particle size.

Single shear had little effect on the shear strength of the light weight simulant.

6) In summary, the geotechnical properties of CWRU-1 Martian soil simulant were

comparable with that of Martian regolith and Martian soil simulant, and can serve as a good

analog for portions of the Martian crusty and blocky material for high shrinkage mobility

tests.

5.2.2 Miniature Triaxial Testing Apparatus

A new miniature triaxial device has been developed to perform triaxial tests on dry, cohesion-less soil of less than 10 g. The features of the apparatus can be concluded as below:

(1) It was able to perform triaxial tests on a soil sample of less than 10 g in a triaxial cylinder of

10 mm in diameter and about 20 to 30 mm high.

(2) It is equipped with a measurement system, by which the shear stress versus displacement curves can be simultaneously recorded during loading.

(3) The time to prepare and run a triaxial test on a cohesion-less sand sample with this miniature

apparatus is about 1.5 hours.

A series of case testing programs were performed to confirm the validity of the new

apparatus. Based on the experimental results of the analysis, the shear strength parameters of JSC-

1A lunar soil simulant obtained through the miniature triaxial apparatus is comparable to that

159 obtained from conventional device tests. It is therefore believed that the new triaxial apparatus is able to conduct effective measurements of stress and strain behavior of the soil being tested.

In summary, the aforementioned apparatus presents more advantages compared to the conventional triaxial device, including being more convenient, more compact, as well as cheaper.

Most importantly, it allowed us to perform UU triaxial tests on any dry cohesion-less soil with limited quantity. The setup of this apparatus can provide the shear strength parameters needed for engineering design of lunar soil, lunar soil simulant and Martian soil simulant. We anticipate the principles advanced in our study to benefit further outer space exploration.

5.3 Recommendations for Future Study

Recommendations for future work in this research shall include but are not limited to:

1) In this research, all the laboratory tests and physical model tests were conducted on

CWRU-1 created within two months. Additional testing programs can be developed to test

CWRU-1 within a longer time frame to study its physical and mechanical behavior over

time to establish a more complete database for further studies and research.

2) A reliable calibration testing programs needs to be developed to reduce the noise from

obtained axial stress from the miniature triaxial tests. New materials need to be utilized to

obtain a reliable testing result.

3) Only two ranges of bulk densities have been obtained in the miniature traixial test. A more

comprehensive soil preparation program is suggested to obtain a larger range of bulk

densities.

4) Consider using an internal vacuum as the confining pressure on the lunar simulant or

Martian soil simulant for the miniature triaxial tests to better simulate the vacuum space

160

environment. The shear strength parameters obtained from the low confining pressures via

the vacuum system may be a better simulation of the conditions on the moon and Mars.

5) Volume change characteristics plays an important role in the measurement of shear

strength of soil sample. It is suggested to equip the miniature triaxial device with a radial

strain measurement device to record the radial deformation. The volumetric strain can

therefore be measured more accurately for further analytical study of the lunar soil and

others.

6) In order to obtain a more precise confining pressure at failure for the miniature triaxial tests,

additional transducers are suggested to equip the miniature triaxial device to record the

radial pressure in the triaxial cell.

161

REFERENCES

Allen, C., Jager, K., Morris, R., Lindstrom, D., Lindstrom, M., & Lockwood, J. (1998). JSC Mars- 1: A Martian Soil Simulant. Lunar and Conference, (p. 1690).

Anonymous. (2015a). Stonehenge. Retrieved from Wikipedia: https://en.wikipedia.org/wiki/Stonehenge

Anonymous. (2015b). History of the Telescope - Binocular. Retrieved from About.com: http://inventors.about.com/library/inventors/bltelescope.htm

Anonymous. (2015c). Space Exploration. Retrieved from Wikipedia: https://en.wikipedia.org/wiki/Space_exploration

Anonymous. (2015d). Sputnik 1. Retrieved from Wikipedia: https://en.wikipedia.org/wiki/Sputnik_1

Anonymous. (2015e). Timeline of Space Exploration. Retrieved from Wikipedia: https://en.wikipedia.org/wiki/Timeline_of_space_exploration

Anonymous. (2015f). . Retrieved from Wikipedia: https://en.wikipedia.org/wiki/Exploration_of_the_Moon#cite_note-9

Anonymous. (2015g). . Retrieved from Wikipedia: https://en.wikipedia.org/wiki/Exploration_of_Mars

Anonymous. (2015h). Lunar Exploration Timeline. Retrieved from Universities Association: https://en.wikipedia.org/wiki/Exploration_of_Mars

Anonymous. (2015j). Mars Exploration. Retrieved from NASA: http://mars.nasa.gov/programmissions/missions/log/

Anonymous. (2016a). . Retrieved from Wikipedia: https://en.wikipedia.org/wiki/Climate_of_Mars

Anonymous. (2016b). Wikipedia: Mare Ibrium. Retrieved from https://en.wikipedia.org/wiki/Mare_Imbrium

Arvidson, R., E, G., M, D.-B., J, A., M, S., P, C., & R, S. (1989). Nature and distribution of surficial deposits in and vicinity. Journal of Geophysical Research, 94(B2), 1573-1587.

Ballou, E., Wood, P., Wydeven, T., Lehwalt, M., & Mack, R. (1978). Chemical interpretation of Viking Lander 1 life detection experiment. Nature, 271, 644-645.

162

Banin, A., Clark, B., & Wanke, H. (1992). Surface chemistry and mineralogy. In Mars 1 (pp. 594- 625).

Bardet, J. (1997). Experimental Soil Mechanics. New Jersey: Prentice Hall.

Barsukov, V. (1977). Preliminary data for the regolith core brought to earth by the automatic Luna 24. Proc. Lunar Sci. Conf. 8th, (pp. 3303-3318).

Basu, A., Wentworth, S., & McKay, D. (2001). Submillimeter grain-size distribution of Apollo 11 soil 10084. Meteorit. Planet. Sci., 177-181.

Bates, R., & Jackson, J. (1980). The glossary of geology, 2nd edition. Washington: American Geological Institute.

Batiste, S., & Sture, S. (2005). Lunar regolith simulant MLS-1: production and engineering properties. Lunar regolith simulant materials workshop: book of abstracts., (p. 42717).

Bekker, M. (1956). Theory of Land Locomotion. . Mieczyslaw .

Bekker, M. (1969). Introduction to terrain-vehicle systems. Mieczyslaw Gregory.

Bowles, J. (1992). Engineering properties of soils and their measurement. . Boston: McGraw-Hill, Inc.

Cadenhead, D., & , B. (1972). The adsorption of atomic hydrogen on 15101,168. The Apollo 15 lunar samples, (pp. 272-274).

Cadenhead, D., & Stetter, J. (1975). Specific gravities of lunar materials using helium pycnometry. Proceedings of the 6th Lunar Science Conference, (pp. 3199-3206).

Cadenhead, D., Stetter, J., & Buergel, W. (1974). Pore structure in lunar samples. Journal of Colloid and Interface Science, 322-336.

Cadenhead, D., Wagner, N., Jones, B., & Stetter, J. (1972). Some surface characteristics and gas interactions of apollo 14 fines and fragments. Proceedings of the 3rd Lunar Science Conference, (pp. 2243-2257).

Carrier, W. (1970). Lunar soil mechanics on the Apollo missions. Texas Civil Engineering, 7.

Carrier, W. (1973). Lunar soil grain size distribution. The Moon, 250-263.

Carrier, W. (1974). Apollo drill core relationships. The Moon, 183-194.

Carrier, W. (2003). Particle size distribution of lunar soil. Journal of Geotechnical and Geoenvironmental Engineering , 956-959.

163

Carrier, W. (2005). The four things you need to know about the geotechnical properties of lunar soil. Lunar Geotechnical Institute, pp. 2-9.

Carrier, W., Bromwell, L., & Martin, R. (1972b). Strength and compressibility of returned lunar soil. Proceeding of Second Lunar Science Conference, (pp. 3223-3234).

Carrier, W., Bromwell, L., & Martin, R. (1973c). Behavior of returned lunar soil in vacuum. Journal of Soil Mechanics & Foundations, 979-996.

Carrier, W., Johnson, S., Carrasco, L., & , R. (1972a). depth relationships: Apollo 14 and 15. Proceeding of Second Lunar Science Conference, (pp. 3213-3221).

Carrier, W., Johnson, S., Werner, R., & Schmidit, R. (1971). Disturbance in samples recovered with apollo core tubes. Proceeding of Second Lunar Science Conference, (pp. 1959-1972).

Carrier, W., Mitchell, J., & Mahwood, A. (1973a). The nature of lunar soil. NASA STI/Recon Technical Report A 75 .

Carrier, W., Mitchell, J., & Mahwood, A. (1973b). The relative density of lunar soil. Proceedings of the 4th Lunar Science Conference, (pp. 2403-2411).

Carrier, W., Olhoeft, G., & Mendell, W. (1991). Physical Properties of the Lunar Surface - Lunar sourcebook . New York: Cambridge University Press.

Carter, J., McKay, D., Taylor, L., & David, C. (2004). Lunar simulant JSC-1 is Gone: the need for new standardized root simulants. Proceedings of Space Resources Roundtable VI. (p. 211). Johnson Space Center.

Carusi, A., Cavarretta, G., Cinotti, F., Civitelli, G., Coradini, A., Funiciello, R., . . . Taddeucci, A. (1972). Lunar glasses as an index of the impacted sites lithology: The source area of Apollo 15 " glasses". Geol. Romana, 137-151.

Cherkasov, I., Vakhnin, V., Kemurjian, A., Mikhailov, L., Mikheyev, V., Misatov, A., . . . Shvarev, V. (1968). Determination of the physical and mechanical properties of the lunar surface by means of Luna 13 automatic station. Moon and Planets II, (pp. 70-76).

Christensen, E., Batterson, S., Benson, H., , C., Jones, R., Scott, R., . . . Sutton, G. (1967). Lunar surface mechanical properties—Surveyor 1. Journal of Geophysical Research, 801-813.

Christensen, E., Batterson, S., Benson, H., Choate, R., , R., & Jaffe, L. (1968). Lunar Surface Mechanical Properties. Surveyor VI Mission Report, Part II: Scientific Results. 32-166: Jet Propulsion Lab. Tech. Rept.

Christensen, P., & Moore, H. (1992). The Martian surface layer. University of Arizona Press, Space Science Series. 164

Clanton, U., McKay, D., Taylor, R., & Heiken, G. (1972). Relationship of exposure age to size distribution and particle types in the Apollo 15 drill core. In The Apollo 15 lunar samples. Vol. 1. (pp. 54-56).

Clark, B., Baird, A., Rose, H., Toulmin, P., Christian, R., Kelliher, W., . . . Huss, G. (1977). The Viking X-ray fluorescence experiment: Analytical methods and early results. Journal of Geophysical Research, 4577-4594.

Costes, N. (1972). Mobility performance of the lunar roving vehicle terrestrial studies Apollo 15 results. Washington DC: NASA.

Costes, N., & Mitchell, J. (1970). Apollo 11 soil mechanics investigation. Geochimica et Cosmochimica Acta, 2025-2044.

Costes, N., Carrier, W., Mitchell, J., & Scott, R. (1969). Apollo 11 soil mechanics investigation. 85-122.

Costes, N., Carrier, W., Mitchell, J., & Scott, R. (1970a). Apollo 11 soil mechanics investigation. Journal of Soil Mechanics and Foundations Division, 2045-2080.

Costes, N., Carrier, W., Mitchell, J., & Scott, R. (1970b). Apollo 11 soil mechanics investigation. Science, 739-741.

Costes, N., Cohron, G., & Moss, D. (1971). Cone penetration resistance test— An approach to evaluating the in-place. Proc. Lunar Sci. Conf. 2nd, (pp. 1973-1987).

Diego, D. (n.d.). Stonehenge. Condado de Wiltshire, Inglaterra. 2014.

Duke, M., Woo, C., Bird, M., Sellers, G., & Finkleman, R. (1970a). Lunar soil: size distribution and mineralogical constituents. Science, 648-650.

Duke, M., Woo, C., Sellers, G., Bird, M., & Finkelman, R. (1970b). of lunar soil at Tranquillity Base. Geochimica et Cosmochimica Acta, 347.

Fletcher, J. (1973). Self-recording portable soil penetrometer. 1971-06: United States Patent Application 161 .

Florensky, C., Basilevsky, A., Ivanov, A., Pronin, A., & Rode, O. (1977). Luna 24: Geologic setting of landing site and characteristics of sample core. Lunar and Planetary Science Conference Proceedings, (pp. 3257-3279).

Frondel, C., Klein, C., & Ito, J. (1971). Mineralogical and chemical data on Apollo 12 lunar fines. Lunar and Planetary Science Conference Proceedings, (pp. 719-726).

165

Frondel, C., Klein, C., Ito, J., & Drake, J. (1970). Mineralogical and chemical studies of Apollo 11 lunar fines and selected rocks. Geochimica et Cosmochimica Acta, 445-474.

Gotteland, P., & Benoit, O. (2006). Sinkage tests for mobility study, modeling and experimental validation. Journal of Terramechanics, 451-467.

Graf, J. (1993). Lunar soils grain size catalog. In NASA Reference Publication No. 1265 (p. 216).

Gromov, V. (1998). Physical and mechanical properties of lunar and planetary soils. Earth, Moon, and Planets, 51-72.

Gromov, V., Leonovich, A., Lozhkin, V., Rybakov, A., Pavlov, P., Dmitryev, A., & Shvarev, V. (1972). Results of investigations of the physical and mechanical properties of the lunar sample from Luna 16. Space Research Conference, (pp. 43-52).

Halajian, J. (1965). The case for a cohesive lunar surface model. Annals of the New York Academy of Sciences, 671-710.

Halajian, J. (1967). Mechanical, optical, thermal, and electrical properties of the surveyor I landing site. Journal of the Astronautical Sciences, 270.

Halajian, J. (2007). Moon Stories. Oklaboma: State Publishing & Enterprises.

Hapke, B., Cohen, A., Cassidy, W., & , E. (1970). Solar radiation effects on the optical properties of Apollo 11 samples. Geochimica et Cosmochimica Acta, 2199.

Haskin, L., & Warren, P. (1991). Lunar chemistry - The Lunar Sourcebook. New York: Cambridge University Press.

He, C., Zeng, X., & Wilkinson, A. (2011). Geotechnical Properties of GRC-3 Lunar Simulant. Journal of , 528-534.

Heywood, H. (1971). Particle size and shape distribution for lunar fines sample. Lunar and Planetary Science Conference Proceedings, (pp. 1989-2001).

Hinners, N. (1964). Lunar soil mechanics. NASA-CR-116588.

Horai, K., & Winkler, J. (1976). Thermal diffusivity of four Apollo 11 samples. Lunar and Planetary Science Conference Proceedings, (pp. 3183-3204).

Horai, K., & Winkler, J. (1980). Thermal diffusivity of Apollo 11 samples. Lunar and Planetary Science Conference Proceedings, (pp. 1777-1788).

Hörz, F., & al., e. (1991). Lunar surface processes. In Lunar sourcebook: A user’s guide to the Moon (pp. 61-120).

166

Houston, W., & Mitchell, J. (1971). Lunar core tube sampling. Lunar and Planetary Science Conference Proceedings, (pp. 1953-1958).

Houston, W., Mitchell, J., & Carrier, W. (1974). Lunar soil density and porosity. Lunar and Planetary Science Conference Proceedings, (pp. 2361-2364).

Hovland, H., & Mitchell, J. (1971). Mechanics of rolling sphere-soil slope interaction. Berkeley, CA: University of , Berkeley.

Jaffe, L. (1965a). Depth of the lunar dust. Journal of Geophysical Research, 6129-6138.

Jaffe, L. (1965b). Strength of the lunar dust. Journal of Geophysical Research, 6139-6146.

Jaffe, L. (1967). Surface structure and mechanical properties of the lunar maria. Journal of Geophysical Research, 1727-1731.

Jaffe, L. (1969). Lunar surface material: Spacecraft measurements of density and strength. Science, 1514-1516.

Jaffe, L. (1971). Bearing strength of lunar soil. The Moon, 337-345.

Jaffe, L. (1973). Shear strength of lunar soil from . The Moon, 58-72.

Johnson, S., & Carrier, W. (1971). Soil mechanics results of Luna 16 and Lunokhod 1: A preliminary report.

Johnson, S., Koon, M., & Carrier, W. (1995). Lunar soil mechanics. Journal of British Interplanetary Society, 43-48.

King, E., Butler, J., & Carman, M. (1972). Chondrules in Apollo 14 samples and size analyses of Apollo 14 and 15 fines. Lunar and Planetary Science Conference Proceedings, (pp. 673-686).

Klosky, J., & al, e. (2000). Geotechnical behavior of JSC-1 lunar soil simulant. Journal of Aerospace Engineering, 133-138.

Klosky, J., & al., e. (1996). Mechanical properties of JSC-1 lunar regolith simulant. Engineering, Construction, and Operations in Space (pp. 680-688). ASCE.

Lambe, T., & Whitman, R. (1969). Soil mechanics, series in soil engineering. New York: John Wiley & Sons.

Langseth, M., Keiham, S., & Chute, J. (1973). Heat-flow experiment. In Apollo 17 Preliminary Science Report (pp. 91-94).

167

Leonovich, A., Gromov, V., Dmitriyev, A., Penetrigov, V., Semenov, P., & Shvarev, V. (1977). The main peculiarities of the processes of the deformation and destruction of lunar soil. In NASA Special Publication (pp. 735-743).

Leonovich, A., Gromov, V., Rybakov, A., Petrov, V., Pavlov, P., Cherkasov, I., & Shvarev, V. (1971). Studies of lunar ground mechanical properties with the self-propelled Lunokhod-1. In Peredvizhnaya Laboratoriya na Luna-Lunokhod-1 (pp. 120-135).

Leonovich, A., Gromov, V., Rybakov, A., Petrov, V., Pavlov, P., Cherkasov, I., & Shvarev, V. (1972). Investigations of the mechanical properties of the lunar soil along the path of Lunokhod I. Space Research Conference, (pp. 53-64).

Leonovich, A., Gromov, V., Rybakov, A., Petrov, V., Pavlov, P., Cherkasov, I., & Shvarev, V. (1976). Investigations of the physical and mechanical properties of the lunar soil brought by Luna- 20 and along the route of motion of Lunokhod 2. In Space activity impact on science and technology (pp. 321-332).

Leonovich, A., Gromov, V., Semyonov, P., Penetrigov, V., & Shvartov, V. (1975). Luna 16 and 20 investigations of the physical and mechanical properties of lunar soil. Space Research XV, pp. 607-616.

Lindsay, J. (1971). Sedimentology of Apollo 11 and 12 lunar soils. Journal of Sedimentary Research, 780-797.

Matsushima, T., Katagiri, J., Uesugi, K., & al, e. (2009). 3D shape characterization and image- based DEM simulation of the lunar soil simulant FJS-1. Journal of Aerospace Engineering, 22(1), 15-23.

McKay, D., Carter, J., Boles, W., Allen, C., & Allton, J. (1994). SC-1: A new lunar soil simulant. Engineering, construction, and operations in space IV, 857-866.

McKay, D., Heiken, G., Basu, A., Planford, G., Simon, S., Reedy, R., . . . Papike, J. (1991). The Lunar Regolith. In Lunar sourcebook: A user’s guide to the Moon (pp. 285-356).

McKay, D., Heiken, G., Taylor, R., Clanton, U., Morrison, D., & Ladle, G. (1972). Apollo 14 soils: Size distribution and particle types. Lunar and Planetary Science Conference Proceedings, (pp. 983-994).

Meyerhof, G. (1951). The ultimate bearing capacity of foudations. Geotechnique2.4, 301-332.

Mitchell, J., Bromwell, L., Carrier, W., Costes, N., & Scott, R. (1971). Soil mechanics experiment. In Apollo 14 Preliminary Science Report (pp. 87-108). NASA SP-272.

Mitchell, J., Bromwell, L., Carrier, W., Costes, N., Houston, W., & Scott, R. (1972a). Soil mechanics experiment. In In Apollo 15 Preliminary Science Report (pp. 7-28). NASA SP-289. 168

Mitchell, J., Carrier, W., Costes, N., Houston, W., Scott, R., & Hovland, H. (1973). Soil mechanics experiment. In Apollo 17 Preliminary Science Report (pp. 8-22). NASA SP-330.

Mitchell, J., Carrier, W., Houston, W., Scott, R., Bromwell, L., Durgunoglu, H., . . . Costes, N. (1972b). Soil mechanics experiment. In In Apollo 16 Preliminary Science Report (pp. 8-29). NASA SP-315.

Mitchell, J., Houston, W., Carrier, W., & Costes, N. (1974). Apollo soil mechanics experiment S- 200. In Final report, NASA Contract NAS 9–11266 (p. 135).

Mitchell, J., Houston, W., Scott, R., Costes, N., Carrier, W., & Bromwell, L. (1972c). Mechanical properties of lunar soil: Density, porosity, cohesion, and angle of friction. Lunar and Planetary Science Conference Proceedings, (pp. 3235-3253).

Moore, H. (1970). Estimates of the mechanical properties of lunar surface using tracks and secondary craters produced by blocks and boulders. US Dept. of the Interior, Geological Survey.

Moore, H., & Jakosky, B. (1989). Viking landing sites, remote-sensing observations, and physical- properties of Martian surface materials. 81 (1), 164-184.

Moore, H., Bickler, D., Crisp, J., Eisen, H., Gensler, J., Haldemann, A., . . . Pavlics, F. (1999). Soil-like deposits observed by , the Pathfinder Rover. Journal of Geophysical Research: Planets 104.E4 , 8729-8746.

Moore, H., Clow, G., & Hutton, R. (1982). A summary of Viking sample trench analyses for angles of internal friction and cohesions. Journal of Geophysical Research 87, 10043-10050.

Moore, H., Hutton, R., Clow, G., & Spitzer, C. (1987). Physical properties of the surface materials at the Viking landing sites on Mars. U.S. Geol. Surv. Prof.

Moore, H., Hutton, R., Scott, R., Spitzer, C., & Shorthill, R. (1977). Surface materials of the Viking landing sites. Journal of Geophysical Research 82.28, 4497-4523.

Moore, H., Liebes, S., Crouch, D., & Clark, L. (1978). Rock pushing and sampling under rocks on Mars. U.S. Geol. Surv. Prof.

Moore, H., Spitzer, C., Bradford, K., Cates, P., Hutton, R., & Shorthill, R. (1979). Sample fields of the Viking landers, physical properties, and . Journal of Geophysical Research 84, 8365-8377.

Nordmeyer, E. (1967). Lunar surface mechanical properties derived from track left by nine-meter boulder. MSC Internal Note No. 67–TH-1.

Oravec, H. (2009). Understanding mechanical behavior of lunar soils for the study of vehical mobility. Case Western Reserve University. 169

Papike, J., Taylor, L., & Simon, S. (n.d.). Lunar minerals. In The Lunar Sourcebook.

Perkins, S., & Madson, C. (1996). Mechanical and load-settlement characteristics of two lunar soil simulants. Journal of Aerospace Engineering, 9.1.

Perkins, S., Sture, S., & Ko, H. (1992). Experimental, physical and numerical modeling of lunar regolith and lunar regolith structures. Proceedings of the 3rd international conference on engineering, construction, and operations in space. Denver, Colorado.

Peters, G., Abbey, W., Bearmana, G., Mungasa, G., Smith, J., , R., . . . Beegle, L. (2008). Mojave Mars simulant—Characterization of a new geologic Mars analog. Icarus 197, 470-479.

Peters, J. (2002). The fundamentals gap in vehicle–terrain interaction: the between inapplicable field tests and irrelevant theory. Proceedings of the 14th International Conference of the ISTVS, . Vicksburg, MS.

Quaide, W., Oberbeck, V., Bunch, T., & Polkowski, G. (1971). Investigations of the natural history of the regolith at the Apollo 12 site. Lunar and Planetary Science Conference Proceedings, (pp. 701-718).

Richard, J., Sigurdsonand, L., & Battler, M. (2007). OB-1 lunar highlands physical simulant evolution and production. Lunar and Dust Regolith Simulant Workshop.

Schalgheck, R., Sibille, L., & , P. (2005). The status of lunar simulants. Workshop on Lunar Regolith Simulant Materials.

Schrader, C., Rickman, D., Mclemore, C., & al, e. (2008). Extant and extinct lunar regolith simulants: modal analyses of NU-LHT-1M and-2m, OB-1, JSC-1, JSC-1A and-1AF, FJS-1, and MLS-1.

Scott, R. (1968). The density of lunar surface soil. Journal of Geophysical Research, 73, 5469- 5471.

Scott, R. (1969). Soil Mechanics Experiment: Final Report.

Scott, R. (1987). Failure. Geotechnique, 37, 423-466.

Scott, R., & Roberson, F. (1967). Soil mechanics surface sampler: Lunar tests, results, and analyses. JPL Tech. Rpt.

Scott, R., & Roberson, F. (1968a). Surveyor III—Soil mechanics surface sampler: Lunar surface tests, results and analysis. 4045-4080: J. Geophys. Res., 73.

Scott, R., & Roberson, F. (1968b). Soil mechanics surface sampler. JPL Tech. Rpt.

170

Scott, R., & Roberson, F. (1969). Soil mechanics surface sampler. Results, NASA SP-184.

Scott, R., Carrier, W., Costes, N., & Mitchell, J. (1970). Mechanical properties of the lunar regolith, Part C of Preliminary geological investigation of the Apollo 12 landing site. Apollo 12 Preliminary Science Report, NASA SP-235.

Scott, R., Carrier, W., Costes, N., & Mitchell, J. (1971). Apollo 12 soil mechanics investigation. Geotechnique, 21, 1-14.

Sellers, G., Woo, C., Bird, M., & Duke, M. (1971). Composition and grainsize characteristics of fines from the Apollo 12 double-core tube. Lunar and Planetary Science Conference Proceedings, (pp. 665-678).

Shaw, A., Arvidson, R., Bonitz, R., Carsten, J., Keller, H., Lemmon, M., . . . Trebi-Ollennu, A. (2009). Phoenix soil physical properties investigation. Journal of Geophysical Research114 (E1).

Sibille, e. a. (2006). Lunar regolith simulant materials: recommendations for standardization, production and usage. NASA.

Stenger, R. (2001). Man on the moon: Kennedy speech ignited the dream. Retrieved from CNN.

Vaniman, e. a. (1991a). Exploration, samples, and recent concepts of the Moon. In Lunar Sourcebook (pp. 5-26).

Vaniman, e. a. (1991b). The Lunar Environment. In Lunar Sourcebook (pp. 27-60).

Vedenin, A., & Markachev, V. (1974). Results of investigations of the mechanical properties of the lunar ground with the TOR-1 apparatus. In Metrologiya i metody optiko-fiz. izmerenij (pp. 28- 29).

Vey, E., & Nelson, J. (1965). Studies of lunar soil mechanics. In IITRI Project (p. 272).

Vinogradov, A. (1971). Preliminary data on lunar ground brought to Earth by automatic probe “Luna-16.”. Lunar and Planetary Science Conference Proceedings, (pp. 1-16).

Vinogradov, A. (1972). Preliminary data on lunar regolith returned by automatic probe “Luna- 20.”. Geokhimiya, 7, 763-774.

Willman, B., Boles, W., McKay, D., & Allen, C. (1995). Properties of lunar soil simulant JSC-1. Aerosp. Engrg., ASCE, 8(2), 77-87.

Wills, B. (1966). Horizontal shear in soil vehicle mechanics. TjEC-jt-CAL REP0P-No'9560 TECHNICAL.

Wilson, J. (2007). Exploration: NASA’s plans to explore the Moon, Mars and beyond. 171

Zeng, X., He, C., Oravec, H., & al, e. (2009). Geotechnical properties of JSC-1A lunar soil simulant. Journal of Aerospace Engineering, 23(2), 111-116.

Zheng, Y., Wang, S., Ouyang, Z., Zou, Y., Liu, J., Li, C., . . . Feng, J. (2009). CAS-1 lunar soil simulant. Advances in Space Research 43, 448-454.

172