An Investigation of the Origin of Rock City and Cause of Piping Problems at Mountain Lake, Giles County, Virginia

AN INVESTIGATION OF THE ORIGIN OF ROCK CITY AND CAUSE OF PIPING PROBLEMS AT MOUNTAIN LAKE, GILES COUNTY, VIRGINIA

A thesis submitted

To Kent State University in partial

fulfillment of the requirements for the

degree of Master of Science

Nidal Walid Atallah

December, 2013

Thesis written by

Nidal Walid Atallah

B.S., Earlham College, 2009

M.S., Kent State University, 2013

Approved by

______Abdul Shakoor, Ph.D., Advisor

______Daniel Holm, Ph.D., Chair, Department of Psychology

______Janis Crowther, Associate Dean, College of Arts and Sciences

TABLE OF CONTENTS

Page

ACKNOWLEDGMENT ...... vii ABSTRACT ...... 1 CHAPTER 1: INTRODUCTION ...... 3 1.1 Overview ...... 3 1.2 Historical and Cultural Significance of Mountain Lake ...... 4 1.3 Hydrologic and Geomorphic Settings of the Study Area ...... 8 1.4 Geologic Setting of the Study Area ...... 9 1.5 Review of Previous Hypotheses on the Origin of Mountain Lake and Rock City ...... 14 1.6 Water-Level Fluctuations at Mountain Lake ...... 20 1.6.1 Past Episodes of Water-level Drops as Documented by Previous Studies ...... 20 1.6.2 Hypotheses on Mountain Lake’s Water-level Fluctuations ...... 25 CHAPTER 2: METHODOLOGY ...... 31 2.1 Investigations of Rock City ...... 31 2.1.1 Analysis of the Outcrop Scarp Complex ...... 32 2.1.2 Mapping of Rock City ...... 32 2.1.2 Discontinuity Measurements and Stereonet Analysis ...... 33 2.2 Investigations of the Piping Potential of Lake-bottom Sediment ...... 36 2.2.1 Sample Collection ...... 36 2.2.2 Laboratory Investigations ...... 39 CHAPTER 3: ROCK CITY: MAPPING, ORIGIN, AND MODE OF DISPLACEMENT ...... 48 3.1 Introduction ...... 48 3.2 Results for the Outcrop Scarp Complex ...... 48 3.3 Analysis of Discontinuity Orientation Data ...... 53 3.3.1 Description of the Overall Debris Field ...... 53 3.3.2 Bedding-Plane Orientations of Rock Blocks in Rock City ...... 59 3.3.3 Joint Orientations within Rock Blocks in Rock City ...... 63 3.4 Discussion and Interpretations ...... 66

3.4.1 Interpretation of the Outcrop Scarp Complex ...... 66 3.4.2 Comparison of Discontinuity Orientations between the Rock Blocks and Scarp Outcrops ...... 68 3.4.3 The Origins of Rock City and Mountain Lake ...... 71 CHAPTER 4: EVALUATION OF PIPING SUSCEPTIBILITY OF LAKE-BOTTOM SEDIMENT ...... 76 4.1 Data Presentation ...... 76 4.1.1 Grain Size Distribution and Atterberg Limits ...... 76 4.1.2 Density and Compaction-mold Permeameter Test Data ...... 83 4.2 Discussion and Interpretations ...... 91 4.2.1 Discussion of Limitations ...... 91 4.2.2 The Role of Piping in Lake-level Fluctuations ...... 92 CHAPTER 5: CONCLUSIONS AND RECOMMENDATIONS ...... 96 5.1 Conclusions ...... 96 5.2 Recommendations ...... 97 REFERENCES ...... 99 APPENDIX A ...... 104 APPENDIX B ...... 107 APPENDIX C ...... 116 APPENDIX D ...... 125

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LIST OF FIGURES

Figure 1.1: Location of the study area ...... 5 Figure 1.2: Geologic map of the study area ...... 5 Figure 1.3: Mountain Lake Lodge with main sandstone structure in the middle ...... 7 Figure 1.4: Mountain Lake when full in 2005 ...... 7 Figure 1.5: Location of Mountain Lake within USGS hydrologic units ...... 10 Figure 1.6: Topographic Map showing stream flow directions around Mountain Lake ...... 10 Figure 1.7: Block Diagram of Mountain Lake and its vicinity ...... 11 Figure 1.8: SE-NW regional lineation ...... 19 Figure 1.9: Seismic refraction line and profile at the northern end of lake ...... 21 Figure 1.10: Aerial images of Mountain Lake at different water levels ...... 22 Figure 1.11: Drastic water level drop in December, 2012 ...... 22 Figure 1.12 The locations of lake-bed depressions ...... 23 Figure 1.13: Examples of piping holes within the depressions ...... 24 Figure 1.14: Lake-bed bathymetric map ...... 28 Figure 1.15: Footage of lake-bottom piping holes during scuba diving missions in June, 2012 ... 28 . Figure 2.1: An example of the two principal joint sets along a rock block in Rock City representing the orientations of its sides...... 34 Figure 2.2: Soil sample locations...... 37 Figure 2.3: Hydrometer test apparatus...... 43 Figure 2.4: Casagrande’s plasticity chart...... 43 Figure 2.5: Shelby tube sampling...... 45 Figure 2.6: Apparatus set-up for the compaction-mold permeameter test...... 45 Figure 3.1: Rock City Map ...... 49 Figure 3.2: An oblique 3D view and east-facing photo of Mountain Lake, Pond Drain, and Rock City...... 50 Figure 3.3: Examples of scarp outcrops ...... 51 Figure 3.4: Locations of GPS readings along scarp outcrops ...... 52 Figure 3.5: Bedding plane attitudes for all scarp outcrops ...... 54 Figure 3.6: Density concentrations of poles for joint set orientations in the scarp outcrops ...... 54 Figure 3.7: Examples of rock block sizes ...... 56 Figure 3.8: Rock City streets and alleys of varying widths ...... 57 Figure 3.9: Examples of boulder fields and the processes supplying them with material ...... 58 Figure 3.10: Comparison between poles of bedding plane attitudes for the scarp outcrops and the rock blocks mapped in Rock City...... 60 Figure 3.11: Examples of rock blocks dipping in directions other than to the west ...... 62 Figure 3.12: Comparisons between the principle joint sets within the rock blocks in Main Street and the principal joint sets in the headscarp ...... 64

Figure 3.13: Comparison between the principal joint sets of selected rock blocks from all portions of Rock City and the principal joint sets in the headscarp ...... 65 . Figure 4.1: Grain size distribution curves of all samples ...... 77 Figure 4.2: A plot of Atterberg limits of all samples on the Casagrande Plasticity Chart...... 81 Figure 4.3: Permeability values at different hydraulic gradients ...... 86 Figure 4.4: Discharge values at different hydraulic gradients ...... 86 Figure 4.5: Amount of sedimentation per 100 ml output water collected ...... 89 Figure 4.6: Amount of sedimentation per 60 second period ...... 89

LIST OF TABLES

Table 1.1: Simplified stratigraphic column in the study area ...... 11 Table 1.2: Approximate percent of full Mountain Lake from 1751 to 2003 ...... 24 . Table 2.1: List of soil samples gathered………………………………………………...... 37 . Table 3.1: Bedding plane orientations of rock blocks in Rock City……………………………... 61 Table 3.2: Direction and angle of lateral rotation for rock blocks compared to the headscarp….. 70 . Table 4.1: Grain size distribution results as percent passing...... 77 Table 4.2: Grain size percentages and other grain size distribution properties...... 79 Table 4.3: Plasticity characteristics of fine materials in all the samples...... 79 Table 4.4: USCS Classification of soil samples...... 82 Table 4.5: Theoretical and actual critical hydraulic gradients for tested samples...... 90

ACKNOWLEDGMENT

I would like to thank my advisor, Dr. Abdul Shakoor, for his guidance, devotion and moral support throughout the period of my study at Kent State University. I am also grateful for all the time he has dedicated to the successful completion of this research project, including joining me on several long trips to the research site. I am honored to have been his student and fortunate to have had the opportunity to learn from his wealth of knowledge and extensive experience in the different fields and applications of engineering geology. I would also like to thank my committee members Dr. Daniel Holm and Dr. Neil Wells for taking interest in my research topic, their support, enthusiasm, and help at different points of the research project.

I am especially grateful to Dr. Chester Watts “Skip”, Radford University, for his extensive contributions to my research project, from providing information on the study site and the equipment used in the field, to his unrelenting help and on all aspects of the project. It has certainly been a privilege to work with him and it goes without mention how much I have learned from the many discussions and “brain storming sessions” we have had in the past two years. I am especially indebted to him and his wife Libby for their kindness, hospitality, and generosity for hosting me in their home on numerous visits to the research site. I am also very thankful to all of those who helped me from

Radford University: George Stephenson (Paki) for his help with lake-bottom sediment sampling and the operation of different equipment, Ken Dunker for volunteering for a scuba diving mission to obtain lake-bottom samples, Dr. Elizabeth McCellan for her

vii

contribution to reconnaissance and the development of field methods, James Freeman for his help in field mapping and operation of the GPS unit, and Chris Bolgiano for his help with GPS data processing. It has been a pleasure meeting all of them and sharing some great experiences at Mountain Lake. Their contributions to this research project have made it a special one and I am humbled by their kindness to help.

I would also like to thank all my peers at Kent State University who took interest in my project and volunteered to help with different aspects. In this regard, I would like to thank Krysia Kornecki, Chelsea Lyle, and Emine Onur for accompanying me to the field and helping with data collection. Additionally, as project time was winding down yet more soil samples were coming in, I was fortunate to have received help from

Chelsea Windus, Yuchen Shen, Evan Green, and Roger Sicker with laboratory testing.

The quantity and quality of laboratory results could not have been possible without their relentless efforts. I would like to thank them all for being the supportive friends that they are and hope that I can repay their kindness.

I am grateful to the Fulbright Program for sponsoring my study at Kent State

University and all the workers at AMIDEAST for their follow up and support throughout the duration of my study. I would also like to thank AEG Foundation, Sigma Gamma

Epsilon (SGE), and the Department of Geology at Kent State University for providing funding that made field work possible.

Finally, I would like to dedicate this research project to my wonderful parents and family members who have supported me unconditionally in pursuit of a higher degree in geology and have stood by me throughout all challenges.

viii

ABSTRACT

Mountain Lake is one of only two natural lakes in the state of Virginia. The lake’s

origin has been attributed to either a natural solution-collapse basin, or to a landslide

damming the valley of northwesterly flowing Pond Drain, or to a NW-SE trending fracture lineation. The lake is located within the breached northwest limb of a gently plunging anticline, a part of the larger Valley and Ridge physiographic province. In recent years, the lake drained almost completely, exposing the lake bottom and revealing the presence of four sinkhole-like depressions, containing piping holes at their sides and bottoms, at the northeastern and northwestern margins of the lake. This study focuses on the most likely origin of large sandstone blocks present at the northern end of the lake in an area locally referred to as “Rock City”, including mapping of the block locations and analyzing the mode and extent of displacement that they have undergone. An additional objective is to investigate the piping potential of the lake-bottom sediment and its role in seepage out of the lake basin causing lake-level fluctuations.

Mapping of Rock City was conducted by taking GPS readings at the corners of the rock blocks and using ArcMap Software. Investigations of the displacement mode of the rock blocks was done by comparing the measured orientations of principal discontinuity sets, forming the rock-block boundaries, with discontinuity orientations of undisturbed outcrops within the headscarp, using stereonet analysis. Grain size analysis,

Atterberg limits, and a compaction-mold permeameter test were used to evaluate lake sediment’s susceptibility to piping.

Field observations and discontinuity data analysis indicate that Rock City is a

landslide that dammed the valley of Pond Drain, consequently forming the lake. The

primary mode of slope movement involves lateral spreading that is associated with

extension occurring along discontinuities. The Tuscarora Sandstone rock blocks

comprising Rock City were detached from the scarp face along a northwest-southeast

trending joint set and were displaced laterally towards the west. A seismic event appears

to be the most likely triggering mechanism for slope movement.

Laboratory testing reveals that lake-bottom sediment is susceptible to piping, which is the primary mechanism responsible for the formation of the lake-bed depressions and lake-levels fluctuations. Grain size analysis reveals that lake-bottom sediment consists predominantly of fine sand and silt, both of which are highly susceptible to piping. Results of the compaction-mold permeameter test show that the hydraulic gradient at which lake-bottom sediment starts to pipe, the critical hydraulic gradient, ranges between 1 and 10, depending on the density, grain size distribution and cohesive properties of the sediment.

CHAPTER 1

INTRODUCTION

1.1 Overview

Mountain Lake, located in Giles County, Southwestern Virginia (Figure 1.1), is

one of only two natural lakes in the state of Virginia, and even more unusually it exhibits

rare self-draining (self-dumping) behavior, with recent episodes in 2008, 2011, and 2012

that have left the lake almost completely empty. Because lakes are so rare in the non-

glaciated portion of the Appalachian Mountains, the origin of Mountain Lake is of

interest, but has remained enigmatic: suggestions include subjacent karst collapse and

damming of the valley by a landslide, in either case possibly aided by a northwest-

southeast trending fracture, the existence of which is controversial. The northeastern

(deeper) end of the lake abuts an area of heterogeneous colluvial debris (the presumed

landslide), including substantial rectangular blocks of hard Tuscarora (Clinch)

sandstones, known as “Rock City” because of the way that the gaps between the blocks resemble streets and alleys in a city (footprints of some of the larger blocks exceed 140 m2). The purpose of this study was to investigate the orientations of the large rock blocks

and use discontinuity measurements to understand their mode and extent of displacement.

A secondary objective was to investigate the role of piping through the lake-bottom

sediment in seepage out of the lake basin as a cause of lake-level fluctuations.

In greater detail, the lake is located at an approximate distance of 27 km (17 mi) from the township of Blacksburg, Virginia. The lake sits at an elevation of 1180 m (3875

ft) near the summit of Salt Pond Mountain within the northwestern limb of a gently plunging, breached, anticline with two homoclinal ridges on both sides. The anticline is associated with the Narrows thrust fault and the larger Valley and Ridge physiographic province. The lake is underlain by three major geologic units: the Silurian Tuscarora

Sandstone (Stu) at the northern end, the Ordovician Juniata Sandstone (Oj) in the middle portion, and the Ordovician Reedsville-Trenton Formation (Ort), alternatively referred to as the Martinsburg shale, at the southern end, in a top to bottom sequence (Figure 1.2).

1.2 Historical and Cultural Significance of Mountain Lake

Mountain Lake holds a rich heritage of cultural and historical significance that has attracted guests and tourists alike for the better part of the past two centuries. The lake is famous for the shooting of the box office hit movie Dirty Dancing in 1986. Upon arrival to the Mountain Lake Lodge today, guests are welcomed with a brochure giving a self- guided tour to the movie locations in the hotel and its grounds. The Mountain Lake area is also a focal point of environmental protection and scientific research as it encompasses the Mountain Lake Conservancy (MLC) and the Mountain Lake Biological Station

(MLBS). Private property covers 2,600 acres adjacent to 15,096 acres of the federally- designated Mountain Lake Wilderness. This wilderness is managed by the U.S. Forest

Service and is part of the Eastern Ranger District of the George Washington and

Jefferson National Forests.

According to the Mountain Lake Lodge website, the British surveyor Christopher

Gist, of the Ohio Land Surveying Company, first described the lake in 1751. Henley

Chapman, an early commonwealth attorney who helped frame Virginia’s constitution in

Figure 1.1: Location of the study area.

Bartholomew, M. J., Schultz, A. P., Lewis, S. E., McDowell, R. C., and Henika, W. S. (2000), A digital geologic map of the Radford 30 by 60 minute quadrangle, Virginia and West Virginia, 1:100,000-scale Digital Geologic Map, edited, Virginia Department of Mines, Minerals and Energy, Virginia Division of Mineral Resources, unpublished draft.

Srh

Figure 1.2: Geologic map of the study area [Bartholomew et al., 2000b].

1829, was the first known owner of the lake. Chapman would allow settlers to “salt” their cattle in the lake, which fittingly was referred to as “Salt Pond”. The name was soon changed to Mountain Lake by its new owner General Herman Haupt of Pennsylvania, but

Salt Pond Mountain retains its name to the present day. In 1851, Haupt erected the first lodge at Mountain Lake, which became a pleasure resort by 1857. Often visited by stagecoach travelers, the lake attracted many more visitors with the construction of the

Virginia and Tennessee Railroad in the 1850’s.

In 1930, the Mountain Lake property was purchased by William Lewis Moody from the state of Texas, then a frequent lodge guest. In 1936, Moody constructed the main hotel sandstone structure (Figure 1.3) that stands today in place of the previous wooden one. Following Mr. Moody’s death, the property was passed on to his daughter

Mary Moody Northen, who admired Mountain Lake as she was growing up. The Mary

Moody Endowment was established after she passed away in 1986 and operates the lodge to the present day. Ms. Northen’s final wishes were to “maintain the lodge and surrounding land as a place where people could connect with nature, as she had as a young woman” (Mountain Lake Lodge website, 2013). In 1989, the Mountain Lake

Conservancy (MLC) was inaugurated as a fulfillment to Ms. Northen’s wishes. The

Endowment continues to fund environmental research programs and other non-profit organizations. In 2012, the Endowment renovated the main lodge and expanded its outdoor recreational program. The Mountain Lake Biological Station, founded by the

University of Virginia at the top of Salt Pond Mountain in 1930, remains a location for scientific research, training and education. Another historically significant event occurred

Main Sandstone Structure

Figure 1.3: Mountain Lake Lodge with main sandstone structure in the middle (2008) [Virginia Places, 2013].

Figure 1.4: Mountain Lake when full in 2005 [Google Earth, 2013].

in 1857 when Union General George Crook and his troops crossed Salt Pond Mountain

following the battle of Dublin.

1.3 Hydrologic and Geomorphic Settings of the Study Area

Positioned near the summit of Salt Pond Mountain, Mountain Lake sits at an

elevation of about 1180 m AMSL (3870 ft) and approximately 665 meters (2182 feet)

above the New River. This makes it the highest-altitude lake east of the Mississippi and south of New England (GNIS, 2010; Roningen, 2011). Mountain Lake is an elongated, club-shaped, body of water, oriented from south to northwest (Figure 1.4), where it meets

a breach in the northwestern limb of an anticline, an outlet through a ridge of Tuscarora

Sandstone into Pond Drain. When full, the lake covered a total surface area of about 1.9x

105 m2, an approximate total volume of 1.87x 106 m3, and a maximum water depth of 33

m (108 ft) at the northern end, which shallows towards the south (Cawley et al., 2001b;

Roningen, 2011).

Mountain Lake occupies a small local watershed extending about 1.3 km2.

It is replenished by (1) direct precipitation, (2) surface runoff from five streams

(one of which is perennial), and (3) a line of lake-bottom springs found at the

contact between the Reedsville-Trenton and Juniata Formations (Cawley et al.,

2001b). The area of this watershed is only 7.4 times the surface area of Mountain

Lake when full (Jansons et al., 2004). Aside from loss through evapotranspiration and drainage through Pond Drain at the northwestern end (only when lake is full), it has been noted that water escapes through subterranean pathways (see Section 1.6).

Mountain Lake is located within the Ohio [River] Hydrologic Unit, but it is

only 1.2 km away from the Eastern Continental Divide with the Mid-Atlantic

Hydrologic Unit (Roningen, 2011), as shown in Figure 1.5. This divide separates the Roanoke River and its tributaries from the New River and its tributaries. The headwaters of three major streams lie close to Mountain Lake: namely, Sartain

Branch, Pond Drain and Doe Creek (Figure 1.6). To the east of the lake, Sartain

Branch drains into the Mid-Atlantic Hydrologic Unit (Seaber, 1987; Roningen,

2011). To the southwest of Mountain Lake, Doe Creek flows all the way to the

New River. To the northwest, Mountain Lake meets Pond Drain, but it is separated by landslide debris material from the Tuscarora Sandstone that has been proposed to have dammed an ancient valley associated with the stream, consequently forming the lake. In the past when Mountain Lake reached high water levels, water flowed over the dam material into Pond Drain. The latter flows northwestward for approximately 4 km before pouring into Little Stony Creek, which delivers its waters to the New River while giving the beautiful scenery of the Cascade Falls along the way.

1.4 Geologic Setting of the Study Area

As mentioned earlier, Mountain Lake is located within the crest and breached northwest limb of a gently plunging anticline, a part of the Narrows thrust fault and the larger Valley and Ridge physiographic province. A breached resistant arch of

Tuscarora Sandstone divides Salt Pond Mountain into Doe Mountain and Bald

Knob, which are homoclinal ridges whose steep scarp slopes oppose each other

(Sharp, 1933). Figure 1.7 shows a block diagram of Mountain Lake and its vicinity.

Figure 1.5: Location of Mountain Lake within USGS hydrologic units [National Atlas of the United State, 2010 (from Joyce, 2012)].

Hunters Branch Sartain Branch

Pond Drain

Doe Creek

Figure 1.6: Topographic Map showing stream flow directions around Mountain Lake [US Geological Survey (USGS), the National Map Viewer, 2013].

Figure 1.7: Block Diagram of Mountain Lake and its vicinity [Sharp, 1933].

Table 1.1: Simplified stratigraphic column in the study area.

Thickness Formation Age Rock Type (Mills, 1990)

Rose Hill (Srh) Silurian Sandstone 45-60 m

Sandstone/ Tuscarora (Stu) Silurian 15-45 m Orthoquartzite

Juniata (Oj) Ordovician Sandstone 60-110 m

Reedsville- Ordovician Shale (mostly) 425-490 m Trenton (Ort)

The axial plane of the anticline trends about N60°E (Mills, 1988), plunges

northeast at about 7.5° (Parker et al., 1975), and is also described as a doubly-

plunging Bane anticline (Bartholomew et al., 2000a; Joyce, 2012). The lake is

underlain by three major geologic units: the Silurian Tuscarora Sandstone (Stu) at

the northern end, the Ordovician Juniata Sandstone (Oj) in the middle portion, and

the Ordovician Reedsville-Trenton Shale (Ort) at the southern end, in a top to

bottom sequence (Figure 1.2). Table 1.1 provides a simplified stratigraphic column

of the study area.

Overlying the Tuscarora Formation is the Rose Hill Formation (45-60 m thick),

which is a fine-grained, reddish-to-brown, hematite-cemented sandstone with irregular

layers of shale (Mills, 1990). At Mountain Lake, the Rose Hill occurs in the form of

colluvial deposits (float) as it has mostly been eroded away leaving the Tuscarora as the

main cap rock on surrounding ridges. However, some Rose Hill outcrops were identified

at the top of Salt Pond Mountain, especially on ridge tops northeast of the lake.

The Tuscarora Formation (15-45 m thick), alternatively referred to as the Clinch

Formation, is a white-to-light grey, fine-to-coarse, thickly-bedded, locally conglomeratic, upward-graded, and silica-cemented orthoquartzite (Mills, 1990; Eckroade, 1962; Folk,

1960; Cawley, 1999). It is a highly resistant cliff-former that often contains cross-bedding and trace fossils such as skolithos. At the northeastern end of the lake, the Tuscarora

Sandstone occurs in the form of a belt of complex scarps or outcrops as well as colluvial deposits of large rock blocks and boulders, all clustered in an area known as “Rock City”.

The rock blocks are hypothesized to constitute the primary material that dammed a valley

at the northern end of the lake, an outlet into Pond Drain, consequently forming the lake

(see section 1.5). Features within Rock City were the primary focus of this study, in order

to evaluate the possible origins of Rock City and Mountain Lake as well as the mode and extent of displacement involved in the damming process.

Underlying this cap rock is the Juniata Formation (60-110 meters thick) which is

a less resistant formation composed mainly of sandstone, siltstone, and shale (Mills,

1990). It is mainly described as a reddish-brown-maroon, fine-grained, iron-rich quartz

sandstone cemented by both silica and iron (Lesure et al., 1982; Cawley, 1999). The

Juniata Sandstone grades upward into medium-grained sandstone and siltstone (Joyce,

2012; Bartholomew et al., 2000a) and contains cross-bedding and trace fossils.

Below the Juniata Sandstone is the Reedsville-Trenton Formation, also referred to as the Martinsburg Formation, (290-500 m thick or more), which is divided into two

major sub-units; the Reedsville Shale at the top and the Trenton Limestone at the bottom.

It is worth mentioning that while the naming of this formation is probably based on the

type locality where it is described (C.F. Watts, oral commun., 2013), there is still a

degree of ambiguity in previous studies regarding whether both sub-units were included

and differentiated in the naming. Mills (1981) describes the upper part of the formation as

consisting of thin-bedded sandstone and siltstone, the middle part predominantly fissile

shale, and the lower part [possibly the Trenton sub-unit] consisting of light-gray, thin to- thick bedded limestone. Whether the Reedsville-Trenton Formation contains calcareous portions, especially its upper part where it is exposed at Mountain Lake, is at the heart of the debate over Mountain Lake’s origin and specifically regarding the viability of a

solution-collapse basin origin (see section 1.5). While some geologists such as Eckroade

(1962) and Parker et al. (1975) suggested that such calcareous content lies at a considerable depth, others such as Holden (1938), Butts (1940), Marland (1967), and

Roningen (2011) suggest its presence within the uppermost portion of the formation.

Roningen (2011) goes on to confirm “significant carbonate content” in the uppermost section of the formation in Narrows, Virginia. This observation is interesting considering that the hypothesis of a natural-collapse basin origin has long been ruled out by studies suggesting an origin associated with a natural landslide dam and/or a linear fracture feature identified by Cawley (1999) (see section 1.5).

1.5 Review of Previous Hypotheses on the Origin of Mountain Lake and Rock City

Mountain Lake’s unusual formation in the non-glaciated portion of the

Appalachians has prompted geologists to attribute its origin to either a natural solution

collapse-basin, or a landslide damming the valley of a northwesterly flowing Pond Drain, and/or due to a NW-SE trending fracture lineation. Others went as far as attributing a

glacial cirque or volcanic caldera origin to Mountain Lake, but such hypotheses were

quickly abandoned.

Holden (1938) hypothesized that Mountain Lake formed as a result of the

dissolution of a calcareous layer in the upper Reedsville-Trenton Formation. Ferguson et

al. (1939) supported this hypothesis, referring to the origin as a “natural solution collapse

basin”. As mentioned earlier, the presence of calcareous content in the uppermost portion

of the Reedsville-Trenton Formation is central to the viability of this hypothesis, which

has not been confirmed. Based on lake water alkalinity measurements, Roningen (2011)

called for lake origin hypotheses associated with karst dissolution to not be ruled out.

Sharp (1933) considered the possibility of a solution basin for Mountain Lake’s origin, but ultimately rejected it. He argued that considering the soluble limestone existed at a depth of about 1,000 feet, it could not have been possible for dissolution to

reach the surface. He also provided evidence that Mountain Lake was different from all

other limestone sinks in the area. Cooper (1964) and Fielder (1967) argued against the

possibility of collapse in the Reedsville-Trenton Formation on the basis on its strength as no such weaknesses were encountered during the Walker Mountain tunnel project that cut through a complete section of the formation. They also asserted that no other sinkholes existed in the Reedsville-Trenton Formation, including within the lower carbonate-rich portions. Furthermore, Parker et al. (1975) rejected the hypothesis of a collapse basin based on calculations of formational thicknesses underlying Mountain Lake. They argued that “a true sinkhole in the upper Martinsburg [Reedsville-Trenton] would have to extend upward through at least 50 ft (15 m) of Juniata to reach the bottom of Mountain Lake, then further dissolve through 1500 ft (457 m) of Martinsburg to make a sinkhole.”

However, Williams (2003) mentions that stoping can cause subjacent karst collapse sinkholes (caprock sinkholes) from caverns deeper than 1000 m in Canada and Russia.

Williams also notes that although subjacent karst collapse sinkholes are typically narrow and steep walled, they can be up to 700 m long and 400 m deep. The deepest collapse sinkhole containing a lake, 528 m deep, is the Creveno Jezero (Red Lake) in Criatia.

Nonetheless, the hypothesis of a landslide or rockslide damming the stream valley

of Pond Drain, consequently causing the formation of the lake, has been the more popular

explanation. The major works promoting this hypothesis were Rogers (1884), Hutchinson

and Pickford (1932), Sharp (1933), Eckroade (1962), Marland (1967), and Parker et al.

(1975). Most of these studies suggested that the headwaters of the northwesterly running

Pond Drain cut through the resistant sandstone ridges, hence breaching the northwestern end of the anticline, carving out a narrow valley that was later dammed by colluvial deposits, namely blocks from the Tuscarora Sandstone. It should be noted that this

hypothesis is directly related to the origin of Rock City, which is a debris field at the northeastern end of the lake that consists of the same Tuscarora Sandstone blocks

involved in the hypothesized damming process. From this perspective, Rock City can be

perceived as the natural landslide dam that impounds Mountain Lake till the present day.

The mode of displacement by which the damming occurred has been attributed to

different types of mass movement and is the primary focus of this study. Hutchinson and

Pickford (1932) quoted a Mr. G.A. Stose as suggesting that the damming occurred as a

result of the “caving in of overhanging ledges of hard rock that were undermined by the

stream”, a form of vertical displacement. Sharp (1933) suggests that the Tuscarora blocks

crept downward from the ridge, probably in the form of talus, but mentioned the

possibility of a rockslide as well. Eckroade (1962) agreed with Sharp’s explanation but

added that frost heaving generated more Tuscarora blocks that were displaced, probably

by solifluction, further damming the stream valley. Solifluction in this case corresponds

to the definition provided by Easterbrook (1999) as the “downslope movement induced

by alternate freezing and thawing of debris of slopes” or “gelifluction”. Marland (1967)

used 14C-dating of core bottoms to confirm that the colluvial material was produced by

solifluction during frost climate about 9,180±330 YBP. He also added that the leaky

nature of the damming material prevented the formation of a “permanent lake” until

about 2,000 YBP (see section 1.6). Parker et al. (1975) incorporated a number of modes

of displacement suggested by earlier works in explaining the damming process and

consequent lake formation. They agreed that the damming occurred via talus or slide rock

(Sharp, 1933; Eckroade, 1962) through mass movement by solifluction (Eckroade, 1962;

Marland, 1967) and the vertical collapse of ledges due to undercutting (Rogers, 1884;

Hutchinson and Pickford, 1932). However, they suggested that vertical collapse was the

primary mode of displacement, as they point out that the lake origin corresponds to Type

20a of Hutchinson’s (1957) classification whereby a large landslide blocked a small

stream, in this case by the undercutting of resistant Tuscarora and Rose Hill Sandstone beds through removal of the less resistant Juniata Sandstone. Mills’ investigations of colluvial deposits in the Mountain Lake area, not far from the study site, led him to suggest their lateral transport and the retreat of Tuscarora escarpments through the process of “topographic inversion of hollows and noses” (Mills, 1981; 1988; 1989;

1990). This process takes place when the “hollows” or “dells” become “armored” with colluvial deposits such as the Tuscarora rock blocks that are collectively more resistant than surrounding ridges or “noses”. Water runoff drains around the resistant fill and cuts into the flanks of softer noses or ridges, which causes the lateral migration of hollow walls and the destruction of noses making the hollows the new noses.

Cawley (1999) conducted fracture trace analysis and revealed the presence of a

lineation feature trending from SE to NW (Figure 1.8), which he interpreted as a “fracture feature and probable fault associated with the regional Appalachian fold and thrust tectonics.” Cawley (1999) confirmed this feature using direct current resistivity techniques and concluded that pronounced resistivity lows along the feature were indicative of a “water-filled fracture zone”. As far as lake origin goes, Cawley proposed that the regional fracture feature had a significant role in the carving out of the valley of

Pond Drain at the northwestern end of the lake as well as its damming and consequent lake formation. In a later publication (Cawley et al., 2001b), the damming process was described to have occurred through the “incremental settling and breakup of an overlying resistant rib of Clinch [Tuscarora Sandstone] bedrock in physical contact with the fault lineation”. Additionally, Cawley (1999) and Cawley et al. (2001b) suggested that the basin of Mountain Lake was formed as fine sediments eroded away by water seeping through the fracture feature, which they also incorporated in explaining periodic lake water-level drops (see section 1.6). The enigma of Mountain Lake’s origin appeared to have been resolved, however, in a study by Roningen (2011) the fracture feature identified by Cawley (1999) could not be confirmed using electrical resistivity tomography (ERT), joint sampling, and lineament analysis.

Finally, while the role of karst dissolution and/or the northwest trending fault, identified by Cawley (1999), in the formation of Mountain Lake cannot be ruled out, there is little doubt that damming by colluvial deposits took place at the northern end of the lake.

Most recently, seismic refraction studies performed by Watts and others at Radford

University confirm the presence of these deposits, providing support for the landslide-

Mountain Lake

Figure 1.8: SE-NW regional lineation [Cawley et al., 2001b].

dammed valley hypothesis (C.F. Watts, oral commun., 2013). As shown in Figure 1.9, a seismic refraction line at the northwestern end of the lake reveals a colluvium filled valley at the western side of the profile indicated by deeper bedrock and substantially lower p-wave velocities (Freeman et al., 2012). The result of this damming has been the accumulation of heterogeneous colluvial material in Rock City at the northern end of the lake, the circumstances of which are the primary focus of this study.

1.6 Water-Level Fluctuations at Mountain Lake

In recent years, Mountain Lake has experienced significant water-level fluctuations causing a decline in tourism and a substantial financial loss to the Mountain Lake Lodge.

Figure 1.10 shows areal images of the lake at different water levels. Notably sharp drops in water levels occurred in 2008, 2011 and 2012 which have left the lake almost completely drained, exposing the lake bottom and revealing the presence of four sinkhole-like depressions, containing piping holes at their sides and bottoms, near the northeastern and northwestern margins of the lake (Figures 1.11-1.13). The geological and hydrological controls responsible for this phenomenon have been debated among geologists, often as part of the controversy over the lake’s origin discussed earlier. This section provides a review of previous episodes of water-level drops, hypotheses put forth to explain the phenomenon, and the objectives of this study in this regard.

1.6.1 Past Episodes of Water-level Drops as Documented by Previous Studies

Water-level fluctuations are not unusual for Mountain Lake, as previous studies have documented episodes of lower lake levels during the past several decades. Such

Seismic Line

(a)

(b)

Figure 1.9: (a) Seismic refraction line running east-west over the dam material at the northern end of lake and (b) the seismic refraction profile [Freeman et al., 2012].

(a) (b) (c) Figure 1.10: Aerial images of Mountain Lake at different water levels during (a) September, 2005, (b) October, 2008, and (c) October, 2011 [Google Earth, 2013].

Depressions

Figure 1.11: Drastic water level drop in December, 2012 (right) compared to levels in April, 2012 (left).

(a)

Depression #3 Depression #2 Saddle

Depression #1

(b) (c) Figure 1.12 (a) Aerial image of a dry lake bed showing the locations of the four depressions near northeastern and northwestern margins [Google Earth, 2013], (b) depressions 1 and 2, and (c) depression 3 known as the “Cat’s-paw”.

Figure 1.13: Examples of piping holes within the depressions.

Table 1.2: Approximate percent of full Mountain Lake from 1751 to 2003 based on 24 historical accounts [Parker, 2003].

Year(s) Percent of Full Lake (%) 1751 80 1768-1804 20 1794 50 1820 50 1835 100 1855 100 1861 100 1864 100 1865-1869 20 1871 100 1879 100 1885-1888 95 1898-1904 60, 100 1904-1905 100 1913 95 1930 95 1935 85 1952-1953 85 1959 spring 60 1959 summer 100 1969-1997 100 1997-2000 95 2001-2002 75 2003 summer 100

accounts can be traced all the way back to when it was first described by Christopher Gist in 1751 as “a lake or pond about ¾ of a Mile Long NE & SD, & ¼ of a Mile wide” (Gist,

1751). Other accounts of a smaller lake are found during 1789, the 1800s, mid-late 1950s, seasonal drops during the drought conditions of 1997-2002, and the most recent drastic drops in 2008, 2011 and 2012. Parker (2003) provided a list of approximate percentage of a full Mountain Lake from 1751 to 2003 based on 24 historical accounts (Table 1.2).

Additional pronounced historical lows have been identified through sediment core analysis. Marland (1967) conducted 14C dating and microfossil analysis on sediment cores, and based on the concentrated compositions of littoral cladocerans, identified at least three prolonged periods of low water levels at Mountain Lake. The last of these periods was estimated to have occurred around 1786, an observation supported by a 1794 survey filed in the Montgomery County Court House that suggested a smaller Mountain

Lake (Parker et al., 1975). Similarly, Parker et al. (1975) used14C dating for a yellow pine trunk collected at a 10 meter depth in the then full lake and identified a two decade long episode of low water level at around 1655± 80 YBP. Most recently, Cawley et al. (2001a) conducted detailed analysis on radiocarbon dated sediment cores representing the past

6100 years. Through examinations of diatom and pollen content, sedimentary erosion features, and the presence of wood fragments, plant fibers, and Sphagnum spores, Cawley et al. (2001a) identified at least six extended periods of low water levels or dry conditions at Mountain Lake. These periods were around 100, 400, 900, 1200, 1800, and 4100 YBP.

1.6.2 Hypotheses on Mountain Lake’s Water-level Fluctuations

Aside from water loss through evapotranspiration and surface runoff through

Pond Drain at the northwestern end of the lake (Figure 1.6) (only when lake is full), many

studies have suggested that water escapes through subterranean pathways (Marland,

1967; Parker et al., 1975; Cawley, 1999; Cawley et al., 2001b; Jansons et al., 2004;

Roningen, 2011; Joyce, 2012). Corresponding to previous hypotheses regarding the

origin of Mountain Lake, different studies have suggested that the subterranean outflow

takes place either through the damming material, karst features, and/or through a NW-SE

trending fault or fracture.

A number of the previous studies attributed water-level fluxes and losses to

subterranean pathways through the Tuscarora blocks damming the northern end of the

lake. Sharp (1933) explains a then recent increase in water depth as possibly the result of

“a more thorough sealing of the interstices of the block dam”. Marland (1967) attributed

the loss to seepage through the Tuscarora Sandstone blocks comprising the “slide-rock dam”, which he also refers to as a “leaky landslide dam”. Similarly, Parker et al. (1975) explained the presence of submerged tree trunks and water-level fluctuations as the result of “incomplete damming of the original stream valley” of Pond Drain as well as change in dam permeability. They also suggest that water loss can be accounted for through ongoing “underground seepage” as less than half the water was calculated to leave through Pond Drain runoff and evapotranspiration. Additionally, it has been suggested by some that seepage may also partially be controlled by seismic activity (Parker et al.,

1975; Cawley, 1999; Cawley et al., 2001b). Cawley et al. (2001b) point out previous accounts of a 1951 earthquake readjusting the damming material, allowing silts to seal the crevice and consequently filling the lake up once again. Considering that the Giles

County Seismic Zone has been responsible for some of the largest known earthquakes in southeastern United States (Bollinger and Wheeler, 1988), such controls on the origin and behavior of Mountain Lake should not be disregarded.

Cawely (1999) agreed with previous studies that water loss occurs through Tuscarora

colluvium, but added that seepage and erosion of fine sediment also occurs along a SE-

NW trending regional lineation or linear fracture feature, the same process that he

suggested to have formed the lake. At a depth of 33 m (108 ft), Cawley (1999) identified

a steep walled “crevice-like portion of the lake” at the northeastern end of the lake, in

reference to lake-bottom depressions 1 and 2, which he described as aligned parallel to

and superimposed upon the regional lineation (Figure 1.8). The crevice feature was first

identified by Parker et al. (1975) and is perceived to channel the seeping water and

sediment beyond the lake. This was confirmed by scuba divers, sonar bathymetry, and

would for the first time become exposed in 2008 as the lake drained nearly completely.

Lake-bed bathymetric maps by Joyce (2012) and Watts and others (C.F. Watts, oral

commun., 2013) confirmed the presence of this feature (Figure 1.14), which was dubbed

sinkhole depressions 1 and 2 as a saddle of sediment separates the feature into two

depressions (C.F. Watts, oral commun., 2013) (Figure 1.12b). Two other smaller

depressions, 3 and 4, were also identified at the northwestern end of the lake that had not

been incorporated in Cawley’s hypothesis. Figure 1.12a shows the location of the four

lake-bottom depressions.

Joyce (2012) conducted a dye tracer study in an attempt to delineate the pathways

of subterranean water exiting the lake. The dye injected into the depressions yielded

Figure 1.14: Lake-bed bathymetric map [C. F. Watts, Radford University, 2012].

Figure 1.15: Footage of lake-bottom piping holes during scuba diving missions in June, 2012 [C. F. Watts, Radford University, 2012].

limited detection at monitoring sites placed in the drainage systems surrounding the lake.

However, one noteworthy detection site was identified in Hunters Branch about 3.5 km north of the lake (Figure 1.6), which Joyce interpreted as suggesting “complex flow dynamics”. Jansons et al. (2004) utilized a hydrologic model simulating Mountain Lake’s water budget to confirm that subterranean pathways do in fact account for most of the lake’s water loss and that it takes place during both periods of water level drop and rise.

Roningen (2011) used a different formulation and modified evapotranspiration calculations to reevaluate the water balance at Mountain Lake. The resulting lake hydrograph showed a baseflow that is “constant on a daily time step” over the span of at least five months with a value of 300±0.23mm/day over the watershed. She also calculated that 93-97% of this daily baseflow, equivalent to 44 L/s, was attributed to the net groundwater outflow. This was in agreement with calculations made by Jansons et al.

(2004). Subtracting the 3-4 L/s estimated minimum groundwater inflow, the minimum outflow from the lake corresponds to 47-48 L/s. However, Ronengen (2011) recognized that the annual model used in the study does not explain past lake levels when it was full or nearly so, suggesting that it may be attributed to changing inflow and outflow conditions in the lake. She explains that the change in outflow is possibly the result of

“blockage and/or changes in sedimentation”. Similarly, Cawley (1999) concludes that

“rate of discharge through time is controlled in part by tectonic events and by a balance of hydrologic conditions and sedimentation factors”. He also documents that in 1998, when scuba divers disturbed the lake bottom, silt sediment became suspended and flowed

with water through the holes in the depressions. Recent scuba diving missions by Radford

University captured this phenomenon through pictures and videos (Figure 1.15).

In this regard, the objective of this study is to identify the role of piping in subterranean water loss and the consequent fluctuations in lake levels. Specifically, the engineering properties of lake-bottom sediment as well as its susceptibility to piping will be evaluated. Piping is defined as the internal erosion process by which fine sediment is washed out under the influence of a high hydraulic gradient. This results in the development of channels or “piping holes” in the soil mass that act as open conduits for water and sediment to flow through. Piping is regarded as one of most serious and frequently occurring problems in dams and other impoundment structures.

CHAPTER 2

METHODOLOGY

2.1 Investigations of Rock City

“Rock City” is the area around Mountain Lake’s northeastern corner that encompasses numerous large rectangular rock blocks, boulder fields and isolated boulders, and other colluvial deposits, almost exclusively derived from the Tuscarora

Sandstone. Boulder fields are areas where a substantial number of small to medium colluvial boulders have been deposited. As stated in the previous chapter, many geologists hypothesized that these blocks, alongside colluvial sediment and alluvium filling the spaces between them, constitute the materials forming a natural landslide dam at the northwestern end of the lake, blocking the surface flow down the valley of Pond

Drain. This study included an investigation of a complex of scarps formed by cliffy but irregular and discontinuous outcrops of Tuscarora Sandstone just upslope of Rock City.

Investigations of Rock City included mapping the large rock blocks, the “alleys” and

“streets” separating the blocks along major discontinuities, and the other aforementioned features within its boundaries. Additionally, analysis of discontinuity data, including both bedding planes and joint sets, in the rock blocks and outcrops upslope were conducted in order to evaluate the mode and extent of displacement exhibited by the rock blocks. The results of these studies (presented in Chapter 3) were also applied to an evaluation of the origins of both Rock City and Mountain Lake.

2.1.1 Analysis of the Outcrop Scarp Complex

The orientations of bedding planes and joints were measured along the line of outcrops exposed above Rock City to assess whether these outcrops represent in situ bedrock and to establish a basis for comparison to orientations of discontinuities within the detached and transported block in Rock City, in order to analyze the mode of displacement.

2.1.2 Mapping of Rock City

A map of Rock City was created using GIS and shows the positions and sizes of rock blocks, the widths of alleys separating the rock blocks, and the locations of boulder fields and upslope outcrops present at the northern boundary of Rock City. The main components of these field investigations included taking GPS coordinate readings at the corners of blocks in Rock City and measuring their dimensions using a tape measure. The large rock blocks are roughly rectangular in shape and were categorized into the following size-based groups: very large (with footprints >50 m2), large (>20 to 50 m2), and medium (2.5 to 20 m2). Additionally, GPS readings were taken at the outcrops upslope and at the boundaries of boulder fields.

The GPS unit used was comprised of a Trimble Pro XRT backpack and a

NOMAD data logger with Terrasync V6.x. field software. The points collected were differentially corrected using GPS Pathfinder® Office software and entered into ESRI

ArcMap 10.1 Office software in order to generate maps of Rock City. In ArcMap, a polygon was drawn for each of the rock blocks by connecting the corresponding coordinate points read in the field. Similarly, a generalized line representing the outcrop

belt was drawn by connecting the GPS points taken at individual outcrops. The boundaries of the boulder fields were also drawn to highlight their locations and dimensions.

Furthermore, to represent the dimensions of rock blocks more accurately, their shapes were idealized into rectangles according to the orientations of the two principal and nearly vertical joint sets representing the sides of each block, as shown in Figure 2.1.

Manual measurements of the lengths of the sides of blocks also proved beneficial in improving the accuracy of rock block dimensions in ArcMap. This additionally facilitated analysis of rock block displacement mode and direction. The need for idealizing the shapes of rock blocks using discontinuity data arose from the fact that the final product of connecting GPS points taken at the corners of rock blocks did not accurately represent the shapes and orientations of the rock blocks, most of which are notably rectangular. This may be attributed to inaccurate GPS readings due to overlaying vegetation, clouds, rock overhangs, or other factors causing a limited number of satellite signals. In some cases, the GPS points were accurate yet the shapes of rock blocks were irregular and hence the orientations of the two principal joint sets were not well represented. This was due to either intense weathering and/or disintegration of the original joint surfaces. Hence, the rock block shapes needed to be corrected according to manual measurements and joint face reconstructions prior to displacement analysis.

2.1.2 Discontinuity Measurements and Stereonet Analysis

The orientations of discontinuities forming the rock-block boundaries as well as within the outcrops upslope were measured. The Window Mapping Method

Mountain Lake Joint Set 2 Joint Set 1

Figure 2.1: An example of the two principal joint sets along a rock block in Rock City representing the orientations of its sides.

(Wyllie and Mah, 2004) was used for discontinuity orientation data collection along the entire length of the outcrop belt, whereby all joints encountered within the selected area were measured. Similarly, the orientations of all joints within rock blocks were measured, especially those clearly making up the sides of rock blocks (Figure 2.1). These measurements were made using both a Brunton compass and an Ipad (application GeoID

1.61).

These data were then entered into the stereonet analysis program Dips 6.0 to produce stereographic projections of discontinuity orientations for both rock blocks and scarp outcrops. All stereographic projections were made in the pole vector plot mode and the lower hemisphere equal angle projection. Pole density concentrations were used to identify the orientations of bedding planes and principal joint sets for both the outcrops and individual rock blocks. The discontinuity data analysis is summarized as follows:

1) The orientations of bedding planes, using dip and dip direction, of individual rock

blocks were compared to the averaged bedding plane orientation of the scarp

outcrops. The purpose of this comparison was a) to confirm that the rock blocks have

displaced from their original positions, as indicated by differences in the bedding

plane dip angles between the rock blocks and scarp outcrops, and b) to identify the

directions of tilt exhibited by the rock blocks, as indicated by the dip directions of

bedding planes.

2) The strike direction of joint orientations, using dip and dip direction, within

individual rock blocks were compared to the principle joint sets found within the

scarp outcrops, specifically using an orthogonal joint system. The purpose of this

comparison was to determine the direction, clockwise and counter-clockwise, and

angle of lateral rotation experienced by rock blocks during displacement away from

the scarp outcrops.

3) The mode of displacement and type of mass movement that took place within Rock

City was assessed, and the origins of both Rock City and Mountain Lake were

evaluated based on the results discontinuity data analysis.

4) The possible origin of principal joint sets defined within the outcrop belt and how

they relate to the formation of Mountain Lake and the drastic lake-level fluctuations

were evaluated.

2.2 Investigations of the Piping Potential of Lake-bottom Sediment

2.2.1 Sample Collection

A total of sixteen soil samples were collected from the lake bottom in areas adjacent to the four sinkhole-like depressions at the northeastern and northwestern ends of the lake. Sampling methods included grab sampling, manually and by scuba diving, as well as Shelby tube sampling. Figure 2.2 and Table 2.1 show and list the different sampling locations and methods of sample collection.

The sampling method during the study period was, to a large extent, governed by

Mountain Lake’s water levels. Following is a sequence of the sampling periods and circumstances: In the summer of 2012, scuba diving was necessary for sample collection due to elevated lake levels. Six samples were brought up to the surface in 2000 cm3 (122 in3) plastic containers. In December of 2012, Mountain Lake nearly emptied, with water

Figure 2.2: Soil sample locations.

Table 2.1: List of soil samples gathered.

Sample Collection Type Location Date Collected No. Method Lake bottom #1 Grab sample Scuba Diving June, 2012 (Depression 1) Lake bottom #2 Grab sample Scuba Diving June, 2012 (Depression 1) Lake bottom #3 Grab sample Scuba Diving June, 2012 (Depression 2) Lake bottom #4 Grab sample Scuba Diving June, 2012 (Depression 3) Lake bottom #5 Grab sample Scuba Diving June, 2012 (Depression 3) Lake bottom #6 Grab sample Scuba Diving June, 2012 (Depression 4)

#7 Grab sample Lake-bottom Manual December, 2012

Lake-bottom #8 Grab sample Manual December, 2012 (near Depression 1) Lake-bottom #9 Grab sample Manual December, 2012 (Depression 1) Lake-bottom #10 Grab sample Manual December, 2012 (Depression 3) Lake-bottom #11 Grab sample Manual December, 2012 (Depression 4) Shelby tube and #12 Settlement Trough Manual March, 2013 grab samples Shelby tube and #13 Settlement Trough Manual March, 2013 grab samples Shelby tube and #14 Settlement Trough Manual March, 2013 grab samples Shelby tube and #15 Lake bottom Manual March, 2013 grab samples Shelby tube and #16 Lake bottom Manual March, 2013 grab samples

water occupying just portions of the two deepest depressions present in the northeast

(depressions 1 and 2), so four more samples were collected manually from the exposed lake floor in locations close to the lake-bottom depressions. In March of 2013, five

Shelby tube samples were obtained in order to determine the field bulk and dry density values of lake-bottom sediment. Care was taken to minimize disturbance to sediments.

Two of these samples were collected from exposed sections of the lake floor, while the three other samples were gathered from a “settlement trough” north of the lake where pump outlets had dumped large amounts of fine-grained soil particles during works at the lake in February, 2013. The reasoning behind this was that the trough had seemingly accumulated soil material of fine sand and silt content; the same material that is believed to pipe through the depressions at the northern end of the lake. Additionally, the settlement trough was observed to exhibit similar depositional patterns to the lake bottom, which was assumed to result in similar density values. For each location where a

Shelby tube sample was gathered, a corresponding grab sample was collected for additional laboratory testing.

2.2.2 Laboratory Investigations

A series of laboratory tests were performed on the sixteen soil samples in accordance with the American Society for Testing and Materials (ASTM) standardized testing procedures. This included grain size analysis, Atterberg limits tests, and a compaction-mold permeameter test to evaluate soil particle movement under varying hydraulic heads. The purpose of these tests was to classify soils according to the Unified

Soil Classification System (USCS) and evaluate their susceptibility to piping. The results

of these soil analyses are presented in Chapter 4. As mentioned earlier, piping is the phenomenon where fine cohesionless soil particles, especially silts, are eroded or washed out to an exit point by seepage forces generated by a high hydraulic gradient. The result is the development of open channels or pipes that facilitate seepage.

Sieve Analysis

Grain size distribution of the soil samples was determined by sieve analysis according to ASTM designation D422. The results of the grain size distribution were used to classify the soils according to the Unified Soil Classification System (USCS).

Considering that fine sand and silt are known for their high susceptibility to piping (Holtz et al., 2011), the grain size distributions were also used to evaluate the piping potential of lake-bottom sediment. Piping is a possible mechanism by which the lake-bed depressions were formed and through which water exits the lake.

The samples were first oven dried for a period of 24 hours, weighed and pulverized by hand in preparation for sieve analysis. The weights were checked to ensure that they exceeded 1000 grams or more, depending on the nominal diameter of the largest particle as per ASTM D422. The sieves selected for the purpose of this study were numbers 4, 10, 40, 100, and 200. The sediment retained on each sieve was then weighed, percents retained and passing were calculated, and grain size distribution curves were generated.

Hydrometer Test

The hydrometer test was performed on the fraction of soil that passed the number

200 sieve (75 micrometer), i.e. the silt and clay fraction, in accordance with ASTM designation D422. The test results were used to determine the proportions of silt and clay for each of the soil samples. This information was used to accurately classify the soils according to USCS, and to evaluate their piping potential.

The hydrometers used in this experiment were of the 152H type. The dispersion agent sodium hexametaphosphate (NaPO3) was prepared in lab by dilution with deionized water at a concentration of 40 grams/liter. First, 50 grams of the sediment was placed in a 250 ml beaker containing 125 mm of the dispersion agent and was stirred until thoroughly wetted. It was allowed to soak for a period of 16 hours, after which the slurry was dispersed for about 15 minutes and immediately transferred into a 1000 ml glass sedimentation cylinder (Figure 2.3). A rubber stopper was placed at the open end of the cylinder and was turned upside down and back sixty times for about one minute.

Vigorous shaking was applied in the inverted position if any soil remained at the bottom.

Sufficient deionized water was added to the slurry to reach a total volume of 1000 ml in the cylinder. Hydrometer readings were made at 2, 5, 15, 30, 60, 250 and 1440 minutes.

For each of the readings, the hydrometer was carefully inserted 20-25 seconds before recording the value at the top of the meniscus. After each reading, the hydrometer was rinsed and placed in a cylinder containing deionized water only and a thermometer, in order to make sure temperature remained constant at 20°C (68°F). If temperatures varied

from this standard, hydrometer values were corrected using a table from ASTM specifications was used. Proper calculations were then made and reported.

Atterberg Limits

Atterberg limits of fine-grained soils passing sieve No.200 were determined using the liquid and plastic limit tests according to ASTM designation D 4318. Atterberg limits are the water contents at which marked changes in the engineering behavior of fine- grained soils occur. The liquid limit (LL) is the minimum water content at which the soil behaves as a viscous liquid. The plastic limit (PL) is the minimum water content at which the soil behaves as a plastic material. The plasticity index (PI) is the range of water contents where soil behaves as a plastic material and is calculated by the difference between the liquid limit and plastic limit (PI= LL- PL). The plasticity index is a very important property for fine-grain soils: soils with lower plasticity generally have less cohesion and resistance to piping, erosion, and development of settlement cracks than soils with higher plasticity (Holtz et al., 2011).

Atterberg limits are necessary to accurately classify the fine-grained soils according to the Unified Soil Classification System (USCS). The Atterberg limits were plotted on Casagrande’s plasticity chart (Casagrande, 1948), with the values of the plasticity index on the y-axis and the liquid limit on the x-axis (Figure 2.4). On this chart, silts and clays are separated by the A-line and a vertical line at the 50% liquid limit value separates low plasticity from high plasticity soils, both clays and silts.

Figure 2.3: Hydrometer test apparatus.

50 CH or OH 40

Plasticity Index (PI) Index Plasticity 30

20 CL or OL MH or OH 10 CL-ML ML or OL 0 ML 0 10 20 30 40 50 60 70 80 Liquid Limit (LL) Figure 2.4: Casagrande’s plasticity chart.

Density Determination

The field density of lake-bottom sediment was determined using Shelby tube samples. Shelby tubes are typically used to obtain undisturbed samples. The samples were taken to the lab to determine their bulk density, natural water content, and dry density. The main objective of determining the dry density of soils was to simulate field conditions for the compaction-mold permeameter test, which was conducted to evaluate the lake-bottom sediment’s susceptibility to piping (see next section). The sampling procedure consisted of manually inserting the Shelby tube vertically into the ground until it was filled with soil (Figure 2.5). Then a small trench was dug around the tube in order to reduce the confining pressure and to facilitate pulling out the tube from the ground with minimal disturbance. The tube’s open ends were covered and tapped.

In the lab, the volume of each tube and the total mass of soil contained in it were used to calculate the bulk density (Ϫm). The natural water content (wn) and dry density

(Ϫd) were determined by measuring the mass of soil after it was oven-dried for a period of 24 hours. The equations for these calculations are as follows:

( ) = ( ) 푇표푡푎푙 푀푎푠푠 푀푡 Ϫ푚 푇표푡푎푙 푉표푙푢푚푒 푉푡 ( ) wn = (100) ( ) 푇표푡푎푙 푀푎푠푠 표푓 푤푎푡푒푟 푀푤 ∗ 푇표푡푎푙 푀푎푠푠 표푓 푆표푙푖푑푠 푀푠 = 1 + Ϫ푚 Ϫ푑 푤

Figure 2.5: Shelby tube sampling.

Figure 2.6: Apparatus set-up for the compaction-mold permeameter test.

Compaction-mold Permeameter Test

This test was performed to simulate field conditions of lake-bottom sediment in

order to determine the values of discharge (Q), permeability (k), and susceptibility to

piping at different hydraulic gradients. The experiment was conducted at the hydraulic

heads corresponding to hydraulic gradients of 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, and 7. A

constant head set-up was used for the experiment (Figure 2.6). A filter and porous stone

were placed at the input end (bottom) of the permeameter to facilitate the free flow of

water and prevent sediment from clogging the porous stone. However, the exit end (top)

was provided neither a filter nor a porous stone in order to allow the free flow of water

and fine sediment. Most samples were tested in the loose to semi-loose density state with

minimal compaction applied, as indicated by density measurements in the field. These

density values were achieved by gently tapping the compaction-mold; however a drop

hammer was occasionally used when denser samples were desirable.

The hydraulic gradient at which piping starts, also referred to as the critical

hydraulic gradient, was identified for six samples. The amount of water collected at the

exit end was used to calculate the permeability (k) and discharge (Q) at different

hydraulic gradients. This was done according to the following equations: Q= V/t and k=

Q/Ai, where Q is the flow rate or discharge (cm3/second), V is the volume of water (cm3),

t is the time (seconds), k is the permeability (cm/second), A is the cross-sectional area

(cm3), i is the hydraulic gradient [∆h/L] (dimensionless), ∆h is the difference in hydraulic

head (h1-h2), and L is the length of soil column (cm). Similarly, the mass of fine

sediment collected at the exit end was used to calculate the amount of sedimentation due

to piping at different hydraulic gradients. The amount of piped sediment was determined by evaporating the water collected at the exit end and measuring the weight of dry sediment left out. The results were used to generate graphs showing the weight of sediment collected per 100 ml discharge water as well as per 60 second period.

CHAPTER 3

ROCK CITY: MAPPING, ORIGIN, AND MODE OF DISPLACEMENT

3.1 Introduction

As illustrated in Figures 3.1 and 3.2a,b, Rock City is the area northeast of

Mountain Lake that consists of the characteristic large rectangular rock blocks as well as

significant amounts of additional coarse colluvial deposits, all consisting almost

exclusively of Tuscarora Sandstone, which caps the ridges of Salt Pond Mountain. Rock

City (sensu lato) is divided into two main components for the purposes of this study: the

overall debris field (which is the hypothesized landslide dam material mentioned earlier),

and a discontinuous and complex belt of outcrops presenting multiple and variable scarps

of Tuscarora Sandstone upslope (herein referred to as the Outcrop Scarp Complex,

presumably the source of the debris downslope) (Figure 3.1).

3.2 Results for the Outcrop Scarp Complex

The overall Rock City debris field is bounded to the north by a partly buried and

partly overgrown set of complex cliffy outcrops that are irregular but overall trend at

N40-70°W. This outcrop complex consists of a set of Tuscarora Sandstone scarps that vary in height, width, degree of planarity, elevation, and aspect. They range from small separate outcrops to sizable cliff-like walls (Figure 3.3). They crop out at different elevations with variable set-backs along any particular stretch, in places giving the appearance of steps or benches. To help visualize the varying levels of these outcrops,

Figure 3.4 shows the GPS points taken at outcrops during two separate mapping trips.

Figure 3.1: Rock City Map.

Pond Drain

Rock City

(a)

Mountain Lake

Rock City Colluvial Deposits

(b) Figure 3.2: (a) An oblique 3D view of Mountain Lake, Pond Drain, and Rock City [C.F. Watts, Radford University, 2013], and (b) East-facing photo of the lake and the southeastern edge of Rock City.

(a) (b)

(e) (f) Figure 3.3: Examples of scarp outcrops: (a and b) small separate scarp outcrops, and (c-f) continuous cliff-like walls of scarp of varying heights.

Figure 3.4: Locations of GPS readings along scarp outcrops where discontinuity measurements were taken.

Bedding attitudes in the scarp outcrops are nearly horizontal, as shown in a stereonet plot of the poles of bedding planes (Figure 3.5), which includes color-coded contours based on pole concentrations. Bedding dips vary from 0 to 24º with an average dip of 7º and a standard deviation of 4.5º. This dip value is slightly lower than 10º dip stated for the limbs of the Mountain Lake anticline (Parker et al., 1975; Mills, 1990). Dip directions in the outcrop belt average to 331º but vary somewhat (circular standard deviation =72º), although this reflects only small deviations from horizontal. Dip directions show a slight majority to the NE, and to a lesser extent to the NW and SW

(both equal).

Figure 3.6 presents the attitudes of poles to joints measured in the scarp complex outcrops. At first glance, joints in the scarp area appear to be non-systematic and randomly oriented as indicated by the significant amount of scatter on the stereographic pole plot (Figure 3.6). However, statistical analysis shows the presence of at least four principal joint sets (Figure 3.6). The strike directions of these nearly vertical joint sets are

1) N35ºW, 2) N15ºE, 3) N55ºE, and 4) N83ºE. Of these, sets 1 and 3 are distinctively orthogonal.

3.3 Analysis of Discontinuity Orientation Data

3.3.1 Description of the Overall Debris Field

The field of debris extends at least about 170 m (558 ft) E-W along the lake shore and about 100 m (328 ft) N-S from the lake shore to the outcrop belt directly and upslope

(Figure 3.1). The debris consists of all sizes ranging from fine debris up to huge rock

Figure 3.5: Bedding plane attitudes for all scarp outcrops.

Figure 3.6: Density concentrations of poles for joint set orientations in the scarp outcrops with the corresponding great circles of the principal joint sets labeled in blue.

blocks, the largest of which has a footprint area of 142 m2. Although all the coarse debris

technically qualifies as “large boulders”, everything ≥ 2.5 m2 was designated herein as

“rock blocks” in order to distinguish the blocks that were large enough to be measured in

detail as part of this study. These coarse rock blocks were arbitrarily split into three size groups: medium (2.5 to 20 m2), large (>20 to 50 m2), and very large (>50 m2). Examples

are illustrated in Figure 3.7. Mapping of Rock City revealed the presence of at least 57

rock blocks, of which 10 are very large, 21 are large, and 26 are medium-sized (Appendix

A, Table A-1). Most are markedly rectangular.

The rock blocks are separated by “streets” and “alleys”, which are passageways a few to many meters in width between the blocks (Figure 3.8), depending on the amount of separation between the blocks during transport. The alleys and streets vary from quite rectilinear (thus suggesting city blocks, and thereby prompting the name “Rock City”), to nearly random.

Rocks smaller than 2.5 m2, which were designated as small boulders, were

scattered over the entire area of Rock City, but were only mapped where they were

concentrated into sizable boulder fields (Figure 3.9a and b). Rocks in boulder fields range

from some of the medium-sized rock blocks down to cobbles and smaller debris. These

deposits are irregularly distributed, so boulder fields were also referred to as “jumbles”

during this study. At least six major boulder fields were identified in the vicinity of Rock

City, plus several smaller ones, as labeled on Figure 3.1. Boulder fields appear to form

not only from masses of rock detached from the scarp outcrops, but also from breakage

off large rock blocks. Figure 3.9c-f shows examples of the rock collapse processes

1.7 meters 4.6 meters

6.4 meters 16 meters

Figure 3.7: Examples of rock-block sizes: rock block #16 (top) is of medium size and rock block #28 (bottom) is of very large-size.

Rock Block #26

3 meters Rock Block #41

Rock Block #28 Rock Block #27

14 meters

Figure 3.8: Rock City streets and alleys of varying widths: (top) a small alley (3meters wide) and (bottom) a large alley “Main Street” (14 meters wide).

(a) (b)

Collapsing Roof Collapsing Roof

Collapsed rocks

Overhangs Scarp

Collapsed rocks Sliding rocks

(e) (f) Figure 3.9: Examples of boulder fields (a-b) and the processes supplying material for boulder fields such as collapsing roofs between rock blocks (c-d), collapsing overhangs (e), and sliding rock blocks from the scarp (f).

(overhangs, collapsing roofs, and sliding rocks), all supplying material for the formation of boulder fields. All but one lie directly downslope from the scarp complex, but the additional boulder field is located farther to the north, in the gentle nose of the plunging anticline. This boulder field may be associated with an unnamed thrust fault on Salt Pond

Mountain that was identified by McDowell and Schultz (1990), although it has not been confirmed in the field and does not appear on recent geologic maps.

3.3.2 Bedding-Plane Orientations of Rock Blocks in Rock City

Table 3.1 shows the average dip and dip direction values for the bedding planes of all rock blocks, and Figure 3.10 compares them to bedding orientations in the scarp complex outcrops. Although the bedding planes dip in all directions, Table 3.1 shows that the dip directions for bedding planes in most rock blocks exceed 180º, with the majority of the blocks showing bedding dip directions to the NW, W and SW, but with a slight predominance to the northwest. Exceptions to overall westward tilting include the cluster of rock blocks present in the southeastern edge of Rock City, in which the bedding planes dip mostly south (Figure 3.11a), and a small number of blocks near the scarp complex that dip north (Figure 3.11b).

The dip angles of rock block bedding planes generally increase away from the scarp face, indicating a higher degree of tilt. For example, rock blocks 24A through 29B within “Main Street”, a major alley adjacent to the scarp complex, have gentle dip magnitudes of 7 to 11º (Table 3.1), while rock blocks beyond Main Street have steeper dips. Two major rock-block clusters are noteworthy: the cluster to the southeast of Main

Street, containing rock blocks 14 through 20, has bedding dipping at 11 to 63° and the

Figure 3.10: Comparison between poles of bedding plane attitudes for the scarp outcrops (red) and the rock blocks mapped in Rock City (black).

Table 3.1: Bedding plane orientations of rock blocks in Rock City.

Rock Dip Dip Direction Rock Dip Dip Direction Bock No. (Degrees) (Degrees) Bock No. (Degrees) (Degrees) RB#1 13 213 RB#21A 6 158 RB#2A 21 299 RB#21B 50 84 RB#2B 13 332 RB#22 71 318 RB#3 17 339 RB#23 32 358 RB#4A 11 221 RB#24A 11 315 RB#4B 21 259 RB#24B 10 13 RB#5A 13 300 RB#25 8 318 RB#5B 10 6 RB#26 8 96 RB#6 06 314 RB#27 14 325 RB#7 06 267 RB#28 7 305 RB#8 29 354 RB#29A 9 317 RB#9 28 276 RB#29B 9 25 RB#10 09 344 RB#30A 12 305 RB#11 11 310 RB#30B 20 311 RB#12A 82 201 RB#31 5 293 RB#12B 23 21 RB#32 5 167 RB#12C 30 196 RB#33 24 324 RB#12D 16 255 RB#34A 25 318 RB#12E 36 314 RB#34B 27 315 RB#12F 75 197 RB#35A 13 281 RB#12G 50 200 RB#35B 12 277 RB#13 15 280 RB#35C 12 279 RB#14 32 192 RB#36 19 68 RB#15 18 232 RB#37 07 342 RB#16 16 260 RB#38 09 359 RB#17 63 238 RB#39 18 26 RB#18 26 203 RB#40 20 12 RB#19 20 169 RB#41 33 310 RB#20 11 150

(a)

Scarp outcrop

(b) Figure 3.11: Examples of rock blocks dipping in directions other than to the west: (a) southeastern cluster of blocks dipping towards the south, and (b) northward back-tilting of rock blocks adjacent to the scarp complex.

cluster to the west, containing rock blocks 30 through 34B, has dip magnitudes ranging

from 5 to 27°.

Although bedding dips depend on their locations within Rock City, block size is

also a factor. The smaller blocks, especially those within boulder fields, appear to have

higher, but random, dip angles, which is reasonable considering their smaller masses and

somewhat more irregular shapes. In other words, smaller blocks tilt and rotate much more easily and hence were excluded from the aforementioned analysis of the relationship between the bedding plane dip angle and rock-block location.

3.3.3 Joint Orientations within Rock Blocks in Rock City

Joint orientations were also measured within the rock blocks in Rock City, for

comparison with orientations in the scarp complex outcrops. The two principal joint sets

found in the scarp complex outcrops (the orthogonal joints, sets 1 and 3) were also

identified in the rock blocks as making the sides of most blocks, thereby accounting for

the blocks’ distinctive rectangular shapes. The orientations of those surfaces (plus any

other joints) were measured for each of the rock blocks, and the results are presented in

Appendix A, Table A-1. Measurements for the blocks of particular interest are presented

in black in Figures 3.12 and 3.13. Analyses of the results, with justifications for the

comparison approach, are covered in the Discussion and Interpretations section below.

(a) (b) (c)

(d) (e) (f)

(g) (h)

Figure 3.12: Comparisons between the principle joint sets within the rock blocks in Main Street (black) and the principal joint sets in the headscarp (red). (a) rock block #24A (b) rock block #24B (c) rock block #25 (d) rock block #26 (e) rock blocks #27 (f) rock block #28 (g) rock block #29A (h) rock block #29B.

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

Figure 3.13: Comparison between the principal joint sets of selected rock blocks from all portions of Rock City (black) and the principal joint sets in the headscarp: (a) rock block #7 (b) rock block#10 (c) rock block #32 (d) rock block #33 (e) rock blocks #34A (f) rock block #34B (g) rock block #14 (h) rock block #19 (i) rock block #20.

3.4 Discussion and Interpretations

3.4.1 Interpretation of the Outcrop Scarp Complex

Irregularities in the outcrop scarp complex: Some of the variations in the bedding planes and joint orientations may be original or due to minor variations in folding along the outcrop, but some irregularities may occur because parts of outcrop belt are not precisely in situ. In other words, the scarp complex may include some blocks that have started to detach from in situ strata inside Salt Pond Mountain, but have only tilted or rotated a little, and have not completely detached from the hillside and travelled downhill. The irregular shape of the scarp line implies that blocks detached in an irregular fashion along pre-existing fractures and joints, rather than along a large, smoothly concave failure surface, so they are perhaps better thought of as a scarp complex rather than a simple scarp. Notwithstanding their irregularity and discontinuities in outcrop, they nonetheless constitute an overall continuous line of outcrops that fairly exactly spans the extent of the large rock blocks and boulder fields comprising Rock

City, so they appear to constitute a headscarp for the entire field of debris. Also, notwithstanding some irregularities, the standard deviation of 4.5° for bedding dip magnitudes in the scarp outcrops means that the average dip magnitude of 7° can be used for comparison with bedding-plane orientations within the detached blocks in Rock City

(see below). Although the geologic maps of the area (Bartholomew et al., 2000b) shows local dip to the NE on the northeastern corner of Mountain Lake, this study found dip directions primarily to the NE but also to the NW and SW. The NW directions conform to expectations for the NW limb of the anticline. The SW dips are contrary to the NE

plunge of the anticline, so they possibly indicate cambering out of the local hillside, away from the scarp. The NE dips do not seem to be the result of starting to cross the nose of the anticline as not enough dips were observed towards the NNW. Instead, it may the result of some back-rotation from the start of slumping, or more likely, the original bedding surfaces used for measuring bedding attitudes were not perfectly horizontal when deposited. Bedding planes in the Tuscarora Sandstone likely represent the tops, sides, fronts, and backs of broad sand bars, which could easily have had depositional slopes of a few degrees in any or all directions.

Interpretation of the joints within the scarp complex: The origin of the joints observed in the outcrops may include tectonic stresses, valley stress relief, periglacial frost action, and seismic activity. Of the four principal joint sets identified in the headscarp (Figure 3.6), joint sets 1 and 3 intersect at right angles, forming an orthogonal joint system, with strike directions of N35°W and N55°E. The N35°W joint set is most likely associated with stress relief as the headwaters of Pond Drain carved a sandstone- walled valley prior to its collapse in the Rock City area. Furthermore, this joint set orientation is close to the direction of the fracture lineation identified by Cawley in 1999, which he interpreted as being a water-filled fracture or fault trending at N40°W. Cawley

(1999) also suggested that this feature was oriented parallel to the primary compressive stresses of the Alleghanian Orogeny and perpendicular to the Saltville and Narrows thrust faults (Cawley, 1999). If the fracture lineation described by Cawley (1999) is accepted, it would have facilitated carving out a valley through headward erosion by Pond Drain, and it could also contribute to subterranean drainage of lake waters.

3.4.2 Comparison of Discontinuity Orientations between the Rock Blocks and Scarp Outcrops

Comparison of bedding-plane attitudes of detached blocks vs. scarp outcrops:

Figure 3.10 shows that the bedding planes within the rock blocks in Rock City are much

more scattered than those in the scarp outcrops, so block orientations tend to become

randomized during transport. The rock-block bedding planes also exhibit steeper dip

magnitudes, ranging from 5 to 82°. Compared to the scarp area, the predominantly

westward orientations of rock-block bedding planes in Figure 3.10 indicate primarily

outward tilting, away from the headscarp, with an additional component of tilt toward the

west (Table 3.1). Tilt appears to increase with distance from the scarp, and to be more

randomized with smaller blocks. However, the cluster of blocks at the southeastern edge

of Rock City experienced greater tilting (towards the SE and SW) that is probably caused

by steeper slopes into Mountain Lake in that area (Figure 3.11a). The small number of

northward-dipping blocks very close to the scarp face suggest that initial detachment, or at least the minor detachments of the last blocks to separate from the scarp, involved a greater degree of vertical drop and (more or less) back-rotation than the larger-scale movements farther away from the scarp, which mostly involved lateral spreading and outward tilting (figure 3.11b).

Comparison of joint orientations within rock blocks vs scarps outcrops: Being well developed and distinctively orthogonal, and being primarily responsible for the markedly rectangular shapes of the rock blocks, the orthogonal joints (sets 1 and 3) were considered to be the most suitable for use as references when comparing joint orientations in rock blocks to those in the outcrops. Comparison allows for an evaluation

of the degree of lateral rotation experienced by the blocks during displacement away from the scarp. Table 3.2 shows the direction and degree of rotation for all the rock blocks, the joint orientations of which were compared to orientations of the reference

joints sets in the scarp.

However, this approach becomes problematic as the blocks become more rotated: as lateral rotation exceeds 35 to 40°, distinguishing between the joint faces (and thus

between clockwise and counterclockwise rotation) becomes impossible. For example

rock block #15 shows that it may have experienced either a clockwise rotation of 43° or a

counter-clockwise rotation of 47°. For this reason, this study focused in part on major

rock blocks relatively close to the headscarp, namely those in Main Street (Figure 3.1)

because of their limited rotation (to be shown below) and likewise their low degree of

tilting. Figure 3.12 compares the orientations of joints in the rock blocks clustered

around Main Street to the reference joint sets in the scarp. The findings did not reveal any

particular trend, but rather that these blocks have rotated from 10 to 30° in both clockwise

and counter-clockwise directions (other blocks have rotated more than that).

Additionally, the rock blocks that are adjacent to one another seem to exhibit similar

rotational movement. For example, rock blocks 24A, 24B, and 25 (see locations on

Figure 3.1) show a counter-clockwise rotation of about 15-20°, while rock blocks 26 and

27 show a clockwise rotation of about 15-20°, rock blocks 28 and 41 near the center of

Main Street show a counter-clockwise rotation of 10-15°, and rock blocks 29A and 29B

at the western end of Main Street show a clockwise rotation of 30 and 22°, respectively.

Joint set orientations for a number of selected rock blocks from other locations in

Table 3.2: Direction and angle of lateral rotation for rock blocks compared to the headscarp.

Rock Block ID No. Direction of Rotation Angle of Rotation RB#11 Clockwise 3° RB#32 Clockwise 5° RB#33 Clockwise 5° RB#28 Clockwise 10° RB#26 Clockwise 15° RB#41 Clockwise 15° RB#19 Clockwise 18° RB#27 Clockwise 20° RB#10 Clockwise 21° RB#29B Clockwise 22° RB#29A Clockwise 30° RB#15 Clockwise 43° Average Rotation Angle (Clockwise) 17° RB#21A Counter-clockwise 1° RB#5A Counter-clockwise 7° RB#34B Counter-clockwise 7° RB#22 Counter-clockwise 9° RB#6 Counter-clockwise 10° RB#34A Counter-clockwise 13° RB#2A Counter-clockwise 15° RB#14 Counter-clockwise 15° RB#24A Counter-clockwise 15° RB#24B Counter-clockwise 15° RB#25 Counter-clockwise 20° RB#20 Counter-clockwise 25° RB#7 Counter-clockwise 26° RB#9 Counter-clockwise 32° Average Rotation Angle (Counter-clockwise) 15°

Rock City were also compared to the joint orientations in the scarp (Figure 3.13). The direction and degree of rotation for some of these rock blocks can be described as follows: rock blocks that exhibit a counter clockwise rotation are #7 (26°), #9 (32°), #14

(15°), #20 (25°), #21A (1°), #22 (9°), #34A (13°), and #34B (7°); whereas those that exhibit a clockwise rotation are #10 (21°), #32 (5°), #33 (5°), and #19 (18°). The comparison does not suggest any overall relationship between the locations of the rock blocks and their degrees of rotation, although rock blocks immediately adjacent to Main

Street blocks, both to the east and west, exhibit minimal rotation, of just 1 to 13°, while the rock blocks in the cluster at the southeastern edge of Rock City had high rotation angles ranging from 8 to 43°. However, as mentioned earlier, this approach to comparing the scarp to rock blocks is limited in terms of identifying high rotational angles and cannot be applied to rock blocks whose sides are unrelated to the orthogonal joint sets.

3.4.3 The Origins of Rock City and Mountain Lake

Mapping of Rock City and examinations of discontinuity orientation data from both the headscarp and rock blocks lead to the conclusion that the most parsimonious interpretation for Rock City is that it is a landslide that dammed the ancient valley of

Pond Drain where it leaves the present-day Mountain Lake basin, consequently forming the lake. This agrees with several previous studies (Rogers, 1884; Hutchinson and

Pickford, 1932; Sharp, 1933; Eckroade, 1962; Marland, 1967; Parker et al., 1975) that supported the hypothesis of a natural landslide dam origin for Mountain Lake. This is supported by the predominance of blocks tilted outward from the scarp and down the hillside, somewhat in line with the presumed pre-existing topography, with principal

lateral rotation around an axis perpendicular to slope, in both clockwise and counter- clockwise directions. This is contrary to the original sub-horizontal dip of bedding slightly into the hill found within scarp outcrops. The bedding in the failed area could

have been tilted a little by undercutting and/or cambering prior to failure.

The presence of a scarp feature at the northern edge of Rock City suggests that

displacement of the Tuscarora rock blocks is associated with slope movement rather than

a karst subsidence feature. Although we cannot rule out the possibility that a subjacent

karst collapse through the Reedsville-Trenton Formation played a role in the

development of the valley of Pond Drain (at least without further subsurface

investigations), the absence of a scarp feature along the entire perimeter of Mountain

Lake suggests that it is unlikely for such a collapse to have occurred beneath the entire

area of the lake basin. Thus, a slope failure appears to be the most likely mechanism by

which the rock blocks of Rock City have been displaced. This study has not found

definitive evidence as to whether the blocks failed all at once or over multiple events. On

the one hand, if the lake was caused by damming by mass movement, then a single

calamitous failure would be more likely to create a dam, because fine components in

multiple failures would be more likely to be washed away between events. On the other

hand, the rock blocks in Rock City seem unlikely to have constituted an original belt of

rock wider than 30 to 40 m prior to spreading apart downslope and appear to have slid

fairly easily over finer debris, which might well have been an earlier phase of the

landslide or even earlier slides. In that case, the damming of Mountain Lake could have

occurred over an extended period of time before the lake was able to reach full levels, so

it is noteworthy that core analysis conducted by Marland (1967) suggested that even

though the colluvial material was produced nearly 10,000 YBP, a permanent lake was not

established until 2,000 YBP.

The mass of Tuscarora Sandstone, the blocks of which make up Rock City,

appears to have broken loose from the scarp face generally along a northwest-southeast

trending joint set, one of the pair of orthogonal joint sets, with lesser contributions from

the many other fractures, and perhaps with small initial back-rotational components.

After the initial detachment, the rock mass broke into markedly rectangular blocks, along the joint faces belonging to the orthogonal joint system. While many Tuscarora boulders and blocks appear to have detached from the scarp as rock falls, in some cases as collapsing overhangs, this does not represent the primary slope movement responsible for the formation and physical characteristics of Rock City. The way that adjacent blocks can generally be fitted back together across the “alleys” (at least for bigger blocks near the scarp complex) suggests that the rock mass moved to the west in the form of either a translational slide or a lateral spread. However, considering that the bedding in the

Tuscarora Sandstone, along which sliding could occur, is relatively gentle (7o) and dips towards the hillside and that the Tuscarora Sandstone is underlain by another relatively resistant sandstone formation, the Juniata, a simple rock block sliding model seems unlikely. It therefore appears that lateral spreading (Cruden and Varnes, 1996) was the primary mode of movement. Lateral spreads are common on gentle slopes (USGS, 2013), and typically involve extensional movement of the sort manifested by the “streets” and

“alleys” in Rock City (Figure 3.1). The variations in the alleys and streets from rectilinear

to nearly random additionally imply variations in block movement from translation without rotation relative to each other, to considerable independent rotation. Additionally, the fact that Rock City takes the form of a broken landslide, by virtue of separation between the blocks, as opposed to a single rock mass moving downward provides evidence that slope movement is not associated with a translational landslide. The same conclusion can be reached considering that that the rock blocks are present in all segments of Rock City, including adjacent to the scarp face, as opposed to being concentrated in the toe area as would be expected for translational movement.

It is not clear what triggered the movement that resulted in the formation of Rock

City. Remnants of hurricanes and other conditions leading to oversaturated slopes have certainly been responsible for landslides, debris flows, and the like in the Appalachians, and it is also certainly possible to collapse a mountainside by undercutting it via stream erosion. However, considering the fact that the weaker formations such as the Juniata

Sandstone or Reedsville-Trenton Shale [Martinsburg] are at a significant depth and that the bedding along which a translational movement could occur is very gentle, it may well be that a seismic event was needed to trigger the landslide. Lateral spreads are likely to require considerable strong ground shaking (Keeper, 1984). In this regard, Giles County hosts a highly active seismic zone that is responsible for the massive earthquake of 1897, the second largest recorded earthquake in southeastern United States (Bollinger and

Wheeler, 1989), so a large local earthquake is well within the realm of possibility. In the case of Mountain Lake, the geomorphic setting that would allow for lateral movement to take place under a seismic event appears to be the sandstone-walled valley resulting from

headward erosion by Pond Drain. The possibility of earthquakes having a role in

Mountain Lake’s formation has been suggested by previous studies, such as Parker et al.

(1975), Cawley (1999), and Cawley et al. (2001b). However, this was mainly in the

context of lake-level fluctuations as earthquakes were suggested to adjust boulders within

the landslide dam causing changes in the amount of seepage leaving the lake.

Previous studies have suggested other mechanisms inducing rock block

displacement such as gravitational forces, freeze-thaw cycles (Eckroade, 1962; Marland,

1967; Parker et al., 1975), and runoff from storm events (Mills, 1981), all of which seem

valid but only as secondary mechanisms to the lateral spreading caused by seismic

activity. Some of the suggested types of mass movement associated with these

mechanisms are creep (Sharp, 1933), solifluction (Eckroade, 1962; Marland, 1967;

Parker et al., 1975), and vertical collapse due to undercutting (Rogers, 1884; Hutchinson

et al., 1932; Parker et al., 1975). Mills (1981, 1988, 1989, and 1990) was the only geologist to clearly suggest the lateral displacement of colluvial deposits in the Mountain

Lake area. However, he did not put lateral displacement in the context of a lateral spread, which is a well-defined and specific form of slope movement.

CHAPTER 4

EVALUATION OF PIPING SUSCEPTIBILITY OF LAKE-BOTTOM SEDIMENT

This chapter includes data presentation, analysis, and discussion of laboratory tests conducted on lake-bottom sediment. The primary objective was to evaluate the sediment’s susceptibility to piping, which is the most likely cause of subterranean seepage and water-level fluctuations at Mountain Lake. This was accomplished through analysis of grain size distribution (sieve analysis and hydrometer analysis), determination of Atterberg limits (liquid limit and plastic limit tests), and permeability testing.

4.1 Data Presentation

4.1.1 Grain Size Distribution and Atterberg Limits

Grain size analysis and Atterberg limits tests were performed for lake-bottom soil samples in order to classify the soils according to the Unified Soil Classification System

(USCS) and to determine the percentages of silt and fine sand, which are well known for their high piping potential. Similarly, the plasticity of fine-grained soils may be linked to piping as soils with lower plasticity generally have less cohesion and less resistance to piping.

The grain size distributions of tested samples were determined using sieve analysis, for grains coarser than 0.075 mm and the hydrometer test for sediment finer than 0.075 mm (passing sieve #200). The results of grain size analysis are presented in

Table 4.1 as percent passing for each grain size. The grain size distribution curves for all the samples are shown in Figure 4.1 and separately for each sample in Appendix B.

Table 4.1: Grain size distribution results as percent passing.

% Passing Sieve No. No.4 No.10 No.40 No.100 No.200 < No.200 (Hydrometer) Grain Size Diam. 4.75 2 0.425 0.15 0.075 0.074 0.05 0.02 0.005 0.002 0.001 (mm) #1 100.0 99.8 99.3 94.1 68.3 67.5 48.6 28.4 12.6 8.8 5.8 #2 99.9 99.8 99.6 96.7 74.2 73.3 52.2 26.2 11.1 7.4 4.7 #3 100.0 100.0 99.9 99.9 57.5 56.9 43.2 24.5 10.3 6.4 4.1 #4 99.5 99.2 96.8 74.2 32.8 32.5 25.5 15.8 9.8 6.1 3.8 #5 100.0 100.0 98.8 73.9 40.2 39.9 32.8 25.5 12.4 7.2 4.3 #6 82.1 68.3 42.3 16.8 4.1 4.1 3.3 2.1 1.0 0.6 0.4 #7 99.9 99.5 91.2 55.5 38.2 37.6 23.2 10.3 5.1 3.1 1.9 Sample #8 99.8 98.6 79.4 8.6 1.7 1.5 1.2 0.9 0.6 0.3 0.0 No. #9 100.0 100.0 99.9 67.4 25.9 25.4 14.2 4.7 2.1 1.4 0.9 #10 100.0 100.0 98.8 70.0 48.4 47.7 31.7 15.4 7.7 4.4 2.4 #11 100.0 100.0 98.3 73.9 52.8 52.0 32.4 14.9 6.6 3.6 2.2 #12 99.1 97.3 84.7 51.8 16.0 15.7 9.8 3.9 1.4 1.0 0.6 #13 99.8 99.7 98.6 85.5 35.9 35.6 28.8 11.2 4.4 3.2 2.1 #14 98.0 93.3 72.2 21.0 6.7 6.5 3.4 1.2 0.6 0.4 0.3 #15 100.0 99.5 95.4 78.4 40.2 39.5 23.1 7.4 2.3 1.5 0.9

100 #1 90 #2 #3 80 #4

70 #5 #6 60 #7 50 #8 #9 40 #10 30 #11 Cumulative Passing % 20 #12 #13 10 #14 0 #15 Gravel Coarse Sand Medium Fine Sand Silt and Clay 10 1 Sand 0.1 0.01 0.001 Grain Size (mm)

Figure 4.1: Grain size distribution curves of all samples.

Table 4.2 summarizes the percentages by mass found for each grain size (gravel, sand,

silt, and clay) as well as other grain size distribution parameters (GSDP).

Test results for grain size distribution indicate that the lake sediment consists of

predominantly silty sand and sandy silt, which are well known for their piping potential.

According to the USCS, eight samples classify as silty sand (SM), four as sandy silt (ML

or MH), two as poorly graded sand (SP), and one as poorly graded sand with silt (SP-SM)

(Table 4.4). Fine sand particles are the dominant grain size in most samples, averaging

54% of the total mass and ranging from 25.4-77.6%. Silts are the second most abundant grain size, averaging 32.4% of the total mass and ranging from 3.2-63.1%. It is worth mentioning that in some samples, the soil grains were found to cluster in clumps after being oven-dried during sample preparation. This made it difficult to pulverize or breakup the soil samples into individual grains prior to conducting sieve analysis. While pulverization by hand was applied as best as possible, small fractions of such samples may actually belong to smaller diameters than represented in the grain size distribution curves. Samples #6 and 16 were exceptionally hard to pulverize, so sample #6 may, in fact not be a poorly graded sand with gravel. As for sample #16, it was decided not to run it through sieve analysis at all.

The Atterberg limits, which include the liquid limit (LL), plastic limit (PL), and plasticity index (PI), were used to corroborate the hydrometer analysis results by identifying whether the fines (fraction smaller than 0.075 mm) were made up of silt or clay-size particles. This was necessary to provide a complete USCS classification of the soil samples, which are presented in Table 4.4. The plasticity parameters of all samples

Table 4.2: Grain size percentages and other grain size distribution properties.

Grain Size Percentages Grain Size Distribution Properties Sample Sand No. Gravel Silt Clay D10 D30 D60 Cc Cu Coarse Medium Fine #1 0.0% 0% 1% 31% 55.7% 12.6% 0.01 0.03 0.07 1.50 6.00 #2 0.1% 0.0% 0.3% 25.4% 63.1% 11.1% 0.01 0.03 0.06 1.50 6.00 #3 0.0% 0.0% 0.1% 42.4% 47.2% 10.3% 0.01 0.04 0.08 1.48 6.09 #4 0.5% 0.3% 2.4% 63.9% 23.1% 9.8% 0.02 0.07 0.12 1.68 5.44 #5 0.0% 0.0% 1.2% 58.6% 27.8% 12.4% 0.02 0.06 0.12 1.41 6.39 #6 17.9% 13.7% 26.0% 38.2% 3.2% 1.0% 0.11 0.29 1.50 0.52 13.63 #7 0.1% 0.4% 8.4% 53.0% 33.0% 5.1% 0.02 0.06 0.18 0.96 9.41 #8 0.2% 1.2% 19.3% 77.6% 1.7% 0.16 0.23 0.35 1.00 2.25 #9 0.0% 0.0% 0.1% 74.1% 23.8% 2.1% 0.03 0.08 0.14 1.72 4.71 #10 0.0% 0.0% 1.2% 50.4% 40.6% 7.7% 0.02 0.05 0.12 1.21 7.43 #11 0.0% 0.0% 1.7% 45.5% 46.2% 6.6% 0.01 0.04 0.10 1.27 7.08 #12 0.9% 1.8% 12.6% 68.7% 14.5% 1.4% 0.05 0.10 0.22 1.06 4.65 #13 0.2% 0.2% 1.1% 62.7% 31.5% 4.4% 0.02 0.06 0.11 1.69 5.33 #14 2.0% 4.7% 21.1% 65.5% 6.1% 0.6% 0.09 0.20 0.36 1.18 3.89 #15 0.0% 0.5% 4.1% 55.2% 37.9% 2.3% 0.02 0.06 0.11 1.48 6.10

Table 4.3: Plasticity characteristics of fine materials in all the samples.

Sample No. LL PL PI USCS Symbol (fines) #1 40.7 33.5 7.2 ML #2 46.9 35.4 11.5 ML #3 44.5 34.0 10.5 ML #4 53.0 35.6 17.4 MH #5 67.3 42.9 24.4 MH #6 60.7 41.5 19.2 MH #7 55.0 43.6 11.4 MH #8 N/A #9 36.0 29.7 6.3 ML #10 57.3 37.7 19.6 MH #11 67.2 43.6 23.6 MH #12 27.8 21.8 6.0 CL-ML #13 34.5 28.8 5.7 ML #14 26.8 25.7 1.1 ML #15 33.7 27.4 6.3 ML #16 66.7 59.5 7.2 MH

are presented in Table 4.3. The liquid limit values range between 26.8 and 67.3 and the

plasticity index (LL-PL) range between 1.1 and 24.4, indicating the presence of both low

plasticity and high plasticity silts. The liquid limit flow curves for all the samples are

presented in Appendix C.

The results of the Atterberg limits, when plotted on Casagrande’s plasticity chart

(Figure 4.2), indicate that the fine-grained portions of the soils tested from the lake-

bottom mainly range in composition from low plasticity silts (ML) to high plasticity silts

(MH). Seven samples classify as low plasticity silts, seven as high plasticity silts, and one

as silt of low plasticity to clay of low plasticity (CL-ML), i.e. as a silty clay. While low

plasticity silts are better known for their susceptibility to piping, silts are generally less

resistant to erosion and piping. The reason for this is that clean silts have no cohesion

(Holtz et al., 2011). Sample #8 was predominantly composed of coarse grained sediment

with merely 1.7% fine material, so the Atterberg Limits tests were not possible for this

sample. Results of the Atterberg limit tests for all samples are presented in Appendix C.

Interpreting the aforementioned results in accordance with sample locations, a number of

observations can be made. First, samples gathered from within the vicinity of

depressions 1 and 2 in the northeastern portion of the lake-bottom (Figure 2.2), which are

the larger of the four depressions, classify as low plasticity silts (ML). These include

samples #1, #2, #3 and #9. Sample #9 classifies as a silty sand with approximately 26%

of the material classifying as a low plasticity silt (ML). This suggests that lake-bottom sediment in the vicinity of depressions 1 and 2 is most susceptible to piping.

Second, while samples #12, 13, and 14 classify as silty sand and poorly graded

100 CL: Low plasticity clay 90 CH: High plasticity clay ML: Low plasticity silt 80 MH: High plasticity silt CL-ML: Low plasticity clay to low 70 plasticity silt OL: Organic soil of low plasticity 60 OH: Organic soil of high plasticity : Samples from Depressions 1 and 2 50 : Samples from Depressions 3 and 4 : Samples from Settlement Trough : Samples from other locations CH or OH 40

30 Plasticity Index (PI) Index Plasticity #5 #11 #10 #6 20 #4 CL or OL #3 #7 MH or OH 10 #12 #9 #2 CL-ML #15 #1 #16 ML #14 #13 ML or OL 0 0 10 20 30 40 50 60 70 80 Liquid Limit (LL)

Figure 4.2: A plot of Atterberg limits of all samples on the Casagrande Plasticity Chart.

Table 4.4: USCS Classification of soil samples.

Grain Size Percentages Sample USCS Symbol USCS Classification No. of Fine Material Gravel Sand Silt Clay

#1 0.0% 32% 55.7% 12.6% ML ML, Low Plasticity Silt (Sandy Silt)

#2 0.1% 26% 63.1% 11.1% ML ML, Low Plasticity Silt Silt with Sand)

#3 0.0% 43% 47.2% 10.3% ML ML, Low Plasticity Silt (Sandy Silt)

#4 0.5% 67% 23.1% 9.8% MH SM, Silty Sand

#5 0.0% 60% 27.8% 12.4% MH SM, Silty Sand

#6 17.9% 78% 3.2% 1.0% MH SP, Poorly Graded Sand with Gravel SM, Silty Sand (SC-SM, Silty, Clayey #7 0.1% 62% 33.0% 5.1% MH Sand) #8 0.2% 98% 1.7% - SP, Poorly Graded Sand

#9 0.0% 74% 23.8% 2.1% ML SM, Silty Sand

#10 0.0% 52% 40.6% 7.7% MH SM, Silty Sand MH, High Plasticity Silt (Sandy Elastic #11 0.0% 47% 46.2% 6.6% MH Silt) #12 0.9% 83% 14.5% 1.4% CL-ML SM, Silty Sand

#13 0.2% 64% 31.5% 4.4% ML SM, Silty Sand

#14 2.0% 91% 6.1% 0.6% ML SP-SM, Poorly graded sand with silt

#15 0.0% 60% 37.9% 2.3% ML SM, Silty Sand

#16 - - - - MH -

sand with silt, the fine material plots as low plasticity silt (ML). This indicates high

susceptibility to piping. Interestingly, these samples were gathered from a “settlement

trough” north of the lake, which is a trench-like feature that had received pumped water,

and sediment in suspension, from the vicinity of depressions 1 and 2 during works at the

lake in February, 2013. The mitigation works included filling the lake-bottom depressions with local material mixed with bentonite in hopes that lake levels would rise again. The lake water had to be emptied prior to the works, which was accomplished by pumping water from depressions 1 and 2, the deepest portions of the lake, into the settlement trough. Sampling from this trough was conducted based on the assumption

that the soil material deposited in it was similar to the material that is believed to pipe

through the depressions at the northern end of the lake.

Third, the fine material for all the samples gathered from depressions 2 and 3, the

smaller depressions at the northwestern margin of Mountain Lake (Figure 2.2), classify as

high plasticity silts (MH). These include samples #4, #5, #6, #10, and #11. While these

samples are regarded as susceptible to piping, considering high content of fine sand and

silt, they may be less pipable than sediment gathered from depressions 1 and 2.

Distinctions in the nature of fine materials from all samples are presented in figure 4.2

and color coded according to sample location. Sample locations are also shown in Figure

2.2.

4.1.2 Density and Compaction-mold Permeameter Test Data

The compaction-mold permeameter test was conducted on six samples to evaluate the

piping potential of lake-bottom sediment at Mountain Lake by simulating field

conditions. Measurements of the discharge (Q), permeability (k), and amount of sediment discharge due to piping were made at different hydraulic gradients. The ultimate purpose was to determine the hydraulic gradient at which lake sediment would start to pipe, also referred to as the critical hydraulic gradient (ic), when applying different hydraulic heads

in the experiment. Results of the compaction-mold permeameter test are shown in Table

D-3 of Appendix D.

The number of tested samples was limited to six due to time constraints and

difficulties in collecting sufficient amounts of lake-bottom sediment, especially during periods of high lake level. Shelby tube samples used for density determination, were not collected until the later stages of the study when lake levels had started rising. Although some field densities were obtained and used in the experiment, none were from the vicinity of the depressions at the lake bottom. In this regard, the soil samples that did not have in situ densities available were tested in a loose to semi-loose density state. Table D-

2 of Appendix D shows the dry density values at which the samples were tested as compared to those measured in the field.

The dry densities measured in the field ranged from 1.37 to 1.6 g/cm3 (85.8-100.1

pcf), which are typically considered low to medium density values for silts, silty sands,

and fine sands (Sulewska, 2010; Lambe et al., 1969). However, one sample fell below

this range with a dry density of 0.62 g/cm3 (38.8 pcf), which is considered an extremely

low density value. While deeper soils are expected to be denser by virtue of the

overburden pressure, the limited depth of the soil column above bedrock, about 4.5 to 6

m (15-20 ft), at the lake bottom does not result in significant compaction. Furthermore,

the recently deposited material has not been fully consolidated. Thus, conducting the

compaction-mold permeameter test for soils in the loose to semi-loose states is justified.

The permeability values of the tested samples, at different hydraulic gradients, ranged from 10-6 to 10-3 cm/s (Figure 4.3), which suggests good drainage (Terzaghi et al., 1996).

These results support the occurrence of seepage at the lake-bottom. Additionally, the

permeability values correspond well to those for silts, sandy silts, silty sands, and fine

sands, as documented by Fetter (2001). In most samples the permeability values did not

show much variation at different hydraulic gradients, but piping and clogging affected the

consistency of permeability values in a few of cases (Figure 4.3). For example, much

higher permeability values were exhibited by sample #10 at hydraulic gradients ≥6,

probably due to pronounced piping. On the other hand, sample #1 (loose state)

experienced a steep decrease in permeability at gradients ≥7 due to clogging (Figure 4.3).

The relatively higher permeability value for sample #9 can be attributed to both a higher

percentage of fine sand (74%) and a low density state. Although permeability is typically

categorized in terms of grain size content, a correlation with density was also observed as

samples tested at lower densities had higher permeability values. The discharge for all

samples increased with higher hydraulic gradients (Figure 4.4). This is reasonable

considering that the seepage forces acting on the sample increase with a higher gradient.

Additionally, increased piping appears to have contributed to elevated discharge. Once

again, sample #1 (loose) experienced exceptional conditions of decreasing discharge at

gradients ≥6 due to clogging.

The compaction-mold permeameter test clearly reveals the occurrence of piping

#1 (Loose) #1 (compacted) #9 #10 #13 #15 1.0E-01

1.0E-02

1.0E-03

1.0E-04

1.0E-05 Permeability (cm/sec.) [log [log scale] (cm/sec.) Permeability

1.0E-06 0 1 2 3 4 5 6 7 8 9 10 Hydraulic Gradient

Figure 4.3: Permeability values at different hydraulic gradients.

#1 (Loose) #1 (compacted) #9 #10 #13 #15 10

0.1

0.01 Discharge (ml/sec.) [log [log scale] (ml/sec.) Discharge

0.001 0 1 2 3 4 5 6 7 8 9 10 Hydraulic Gradient

Figure 4.4: Discharge values at different hydraulic gradients.

in all samples. Figure 4.5 shows a quantification of the piping potential of lake-bottom sediment by plotting the mass of sediment collected per 100 ml discharge water at different hydraulic gradients. The curves indicate that the trends are not uniform and show irregularity. These irregularities are most likely associated with sample adjustments during the flow of water as piping channels open and clog. Although fluctuations are frequent, the results indicate that the amount of sedimentation (sediment output) generally increases with a higher hydraulic gradient in most of the samples. The gradient at which piping starts to occur varies from 1 to 10 for the tested samples. This variation can be attributed to differences in the grain size distribution, plasticity characteristics, and density values between the samples. It is worth mentioning that high sedimentation values at gradients 1 through 2.5 represent some form of chemical precipitate rather than sediment from piping, with the exception of sample #1 (loose) which is a very pipable silty soil that started piping at a gradient of 1.

The sedimentation rate for all samples ranged from 0.05 to 17.6 grams/100 ml.

The samples tested at lower densities show a higher susceptibility to piping. This is best exemplified by sample #1, which was tested in both the loose and relatively compacted states corresponding to density values of 1.05 and 1.19 g/cm3, respectively. In the loose state sample #1 started piping much earlier than the compacted sample and had substantially higher sedimentation rates at all gradients (Figure 4.5). Contrary to permeability values, the piping potential of silt-dominated samples was observed to be higher than those predominantly composed of fine sand. For instance, samples #1 (loose) and #10 were tested at approximately similar densities, but sample #1 (loose), composed

of a higher percentage of silt, yielded higher sedimentation rates. Figure 4.6 shows a

quantification of sedimentation due to piping per unit time of 60 seconds.

Finally, it was noticed that the compacted samples, i.e. samples #1 (compacted),

13, and 15, experienced piping at higher hydraulic gradients than samples tested in the

loose state. The reason this was obscured in the previous figures is because the amount of

sedimentation documented for these samples included the chemical precipitate mentioned

earlier. Samples #15 and #1 (compacted) were not observed to experience significant

piping until reaching hydraulic gradients of 7 and 8, respectively. Furthermore, sample

#11 was exceptionally resistant to piping and only a small amount of sediment was

observed to pipe at a gradient of 10. These observations clearly support a direct

relationship between soil density, cohesion properties, and the rate of piping.

The critical hydraulic gradient (ic), described in the compaction-mold

permeameter test as the gradient at which piping starts, is based on Karl Terzaghi’s

(1922, 1943) concept of “heave” which occurs when the uplift pressure increases until it

reduces the effective stress to zero. The critical hydraulic gradient typically ranges from

0.84-1.12 (Holtz et al., 2011), but it is generally assumed to equal 1 for different calculations. When the critical gradient equals one, it is presumed that heave or quick conditions have been reached. However, other authors have suggested that the critical hydraulic gradient necessary to initiate backward erosion actually occurs at lower gradients ranging from 0.048 to 0.34 (Jantzer and Knutsson, 2010). This was not the case in the compaction-mold permeameter test, whereby the critical hydraulic gradient for most tested samples was found to be much higher than 1. The primary reason for this

#1 (Loose) #1 (compacted) #9 #10 #13 #15 100.00

10.00

1.00

0.10

Sedimentation (grams/ 100ml) [log scale] [log 100ml) (grams/ Sedimentation 0.01 0 1 2 3 4 5 6 7 8 9 10 Hydraulic Gradient

Figure 4.5: Amount of sedimentation per 100 ml output water collected.

#1 (Loose) #1 (compacted) #9 #10 #13 #15 1.00E+01

1.00E+00

1.00E-01

1.00E-02

1.00E-03 Sedimentation (g/60 sec.) [log scale]

1.00E-04 0 1 2 3 4 5 6 7 8 9 10 Hydraulic Gradient

Figure 4.6: Amount of sedimentation per 60 second period.

disparity is attributed to the plasticity characteristics of the lake-bottom sediment. The theoretical values of critical hydraulic gradient calculated for the samples ranged between

0.61 and 0.88, which were much lower than what was observed in the experiment (Table

. 4.5). These calculations were based on the following equation = = , where 휌푠푎푡 −휌푤 ∆ℎ 푖푐 휌푤 퐿 ic is the critical hydraulic gradient, ρw is the density of water, ρsat. is the saturated density

of soil, ∆h is the difference in hydraulic head, and L is the length of the soil column. It

should be pointed out that the compaction-mold set-up simulates pressure conditions at

the exit point of the dam where water drains out of the lake. Because the exit point has

not been yet identified in the field, it is not possible to calculate the hydraulic heads (lake

levels) at which the lake-bottom sediment would be susceptible to piping.

Table 4.5: Theoretical and actual critical hydraulic gradients for tested samples. Saturated Density Theoretical Actual Critical Sample No. (g/cm3) Critical Gradient Gradient #1 (loose) 1.66 0.66 1 #1 (compacted) 1.74 0.74 8 #9 1.84 0.84 3.5 #10 1.61 0.61 4.5 #13 1.87 0.87 10 #15 1.88 0.88 7

For three of the tested samples, the compaction-mold permeameter test was

conducted with a deceasing hydraulic gradient, after piping had occurred. In two of these

samples, the permeability and discharge values resembled those recorded during

increasing gradients. However, with regards to piping, sedimentation continued to occur

even at very low gradients, but no clear trends were found once compared to increasing

gradient values. A different behavior was observed for the third sample (#10), whereby

the values of permeability and discharge were found to be significantly higher, yet

steady, compared to the values during increasing gradients. This observation appears to

correspond to previous hydrological studies at Mountain Lake (Jansons et al., 2004;

Roningen, 2011) that suggested a steady seepage rate of lake waters.

4.2 Discussion and Interpretations

4.2.1 Discussion of Limitations

Although the piping susceptibility of lake-bottom sediment has been demonstrated above, several limitations of soil sampling and analysis need further discussion. This is done as follows:

1) During sample preparation for sieve analysis, it was difficult to pulverize grains that

formed clumps during oven-drying. These samples were thoroughly pulverized by

hand as best as possible, but in some cases it was not possible to break-up all the

clumps into individual grains. This was especially the case for samples #6 and sample

#16, which had gravel sized clumps that were hard to pulverize. The clumping did not

allow sieve analysis to be performed on sample #16.

2) Because the compaction-mold permeameter test was adopted in the advanced stages

of this study, many of the samples collected in the early stages were found to be

insufficient in quantity. Time constraints prevented me from developing a set-up

using a smaller compaction-mold that would allow for testing more samples.

3) Another limitation of soil analysis was the difficulty in obtaining field density values

of lake-bottom sediment at Mountain Lake. These density values were needed to best

simulate field conditions in the compaction-mold permeameter test in order to

evaluate the piping potential of lake-bottom sediment. Unfortunately, Shelby tube

sampling was not possible either during lake-level drop in December of 2012 or when

water was pumped out of the lake during works carried out in February of 2013. It

was not until March, 2013 that Shelby tube samples were collected. However,

because lake levels had already started rising at that point, it was only possible to

obtain samples from locations further away from the vicinity of the depressions,

including some from a settlement trough north of the lake. Although samples in both

the loose and relatively compacted states were tested, this limitation might have

reduced the degree of certainty regarding the compaction-mold permeameter test

results. Finally, time constraints prevented additional testing of the samples at varying

densities to establish the relationship between density and piping potential.

4.2.2 The Role of Piping in Lake-level Fluctuations

laboratory investigations of lake-bottom sediment from the vicinity of the sinkhole-like depressions suggest that soil piping is a major mechanism by which water seeps out of the lake basin, consequently causing lake-level drops. It is also hypothesized that the piping process is responsible for the formation of the four depressions in the northeastern and northwestern portions of the lake, which differs from the assertion by

Parker et al. (1975) that the depressions are locations that were “not entirely filled by the damming process”. Evidence of piping was confirmed by grain size distribution analysis

showing that the lake-bottom sediment is predominantly composed of fine sand and silt, both highly susceptible to piping. Additionally, piping was documented to occur at different hydraulic gradients in the compaction-mold permeameter test performed to simulate lake conditions. Furthermore, field observations suggest that evidence of piping is abundant, as indicated by the depressions and piping holes observed by scuba divers.

These features became exposed at the surface during December of 2012 when lake levels dropped dramatically resulting in a nearly empty Mountain Lake.

While it cannot be confirmed whether a karst feature and/or a northwest trending fracture feature, as identified by Cawley (1999), provide subterranean pathways for water exiting the lake, seepage through such pathways could not be possible without piping through the lake-bottom sediment. Recent seismic refraction studies by Watts and others at Radford University (C.F. Watts, oral commun., 2013) indicate that the most likely pathway for water seepage is conduits through the dam material at the northern end of the lake. This appears to be valid considering that the dam consists of Tuscarora blocks filled in with colluvial and alluvial sediment, which would be prone to experiencing piping erosion under high hydraulic heads, provided there is an exit point. Seepage through the dam material has been suggested to occur by some previous studies such as Sharp (1933),

Marland (1967) and Parker et al. (1975). Although Cawley (1999) and Cawley et al.

(2001b) explained that the water loss is associated with the northwest trending fracture lineation, they also stated the possibility of seepage through the dam material. These two studies as well as Parker et al. (1975) suggested that earthquakes may affect water levels by adjusting the Tuscarora blocks making up the landslide dam. In light of lab

experimentation, this explanation seems reasonable since earthquake vibrations would be expected to help clog existing piping channels and possibly allow for the development of new ones.

As far as the hydraulic gradient at which lake-bottom sediment is susceptible to piping, the compaction-mold permeameter test indicates that piping could occur at a hydraulic gradients ranging from 1 to 10, depending on the degree of compaction and amount of cohesion. Since lake-bottom sediment is expected to be in loose and semi- loose density conditions, the risk of piping holes developing starts at a hydraulic gradient as low as 1. This means that if the lake level is equal to the length of the seepage path, piping is expected to occur. Unfortunately, the length of the seepage path is currently unknown since the exit point for seeping water has not been found. Joyce (2012) conducted a dye tracer study for the purpose of determining the exit points, but was only able to detect a small fraction in Hunters Branch. However, the fact that Hunters Branch is about 3.5 km away from the northern end of the lake suggests either “complex flow dynamics” (Joyce, 2012) whereby the drainage path may follow conduits through loose material and/or fractures and faults, or that other closer exit locations may exist. Joyce

(2012) recommended that further studies be conducted using different types and concentrations of fluorescent dye.

As stated in chapter 3, four principle joint set were found in the headscarp.

Interestingly, one of these joint sets, striking at about N35°W, appears to be parallel to the fracture lineation identified by Cawley (1999), which may indicate possible subterranean pathways for water exiting Mountain Lake. However, the fracture feature

identified by Cawley (1999) cannot be confirmed without the use of proper geophysical techniques. Such an undertaking would be necessary especially considering the observation by Roningen (2011) that the fracture lineation feature could not be confirmed using electrical resistivity tomography (ERT), joint sampling, and lineament analysis.

CHAPTER 5

CONCLUSIONS AND RECOMMENDATIONS

5.1 Conclusions

The conclusions of this study can be summarized as follows:

1) Rock City is a landslide. The large rectangular blocks of the Tuscarora Sandstone and

other colluvial deposits comprising Rock City dammed the valley of Pond Drain that

breached the northwestern limb of the Mountain Lake anticline, consequently

forming the lake.

2) The primary mode of slope movement in Rock City involves lateral extension

associated with lateral spreading. The Tuscarora rock mass was detached from the

scarp face in an irregular fashion along a northwest-southeast trending joint set as

well as other pre-existing fractures and joints, with the individual rock blocks

displacing laterally towards the west. During the process of detachment the rock

blocks separated from each other by a few to many meters, forming streets and alleys,

and rotating laterally in both clockwise and counter-clockwise directions.

3) A seismic event appears to be the most likely triggering mechanism for slope

movement resulting in Rock City formation. The geomorphic settings resulting from

Pond Drain carving a valley through the Tuscarora Sandstone ridges appears to have

provided a gap for collapsing rock to relocate to.

4) Water level drops at Mountain Lake are associated with seepage at the northern end

of the lake caused by soil piping. Laboratory analyses of lake-bottom sediment

indicate that it consists predominantly of fine sand and silt, both of which are highly

susceptible to piping. The four sinkhole-like depressions at the northeastern and

northwestern margins of the lake were formed via piping action. It is not clear where

the exit points for the piped material are. Bedrock joints, conduits thorough the dam

material, and karst features, if any present at the site, may facilitate the piping

process.

5) The hydraulic gradient at which lake-bottom sediment is susceptible to piping, also

referred to as the critical hydraulic gradient, ranges from 1 to 10 depending on the

density, grain size distribution, and cohesive properties of the sediment. Considering

that lake-bottom sediment is predominantly composed of fine sand and silt in a loose

to semi-loose density state, piping becomes a risk at a hydraulic gradient as low as 1.

5.2 Recommendations

1) Subsurface investigations (drilling, geophysical surveys) should be conducted to

identify any surfaces of weakness along which a translational movement may have

occurred or any large size solution cavities. Subsurface investigations will help in

accurately defining the type of slope movement and mode of displacement,

identifying any subterranean water pathways, and confirming the origins of Rock City

and Mountain Lake.

2) LIDAR imagery should be used to generate a more accurate map of Rock City

showing the positions of large rock blocks and colluvial deposits.

3) Detailed stratigraphic correlations between the rock blocks and the scarp face should

be established to assess and quantify the amount of vertical displacement.

4) The lake bed should be monitored in order to mitigate the development of new piping

holes. Although the existing depressions were filled in February of 2013, the soil

piping continues to be a risk. This risk is expected to increase in the future as lake

levels are projected to rise.

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APPENDIX A

DISCONTINUITY DATA FOR ROCK BLOCKS IN ROCK CITY

104

105

Table A-1: Rock block sizes and discontinuity orientations.

Block Discontinuity Orientations Rock Size Size Bedding Joint Sets Bock No. 2 Category (m ) Plane JS#1 JS#2 JS#3 JS#4 RB#1 21.2 Large 13/213 N70E N20W N15E N60W RB#2A 35.0 Large 21/299 N40E N50W N4W - North- RB#2B 28.1 Large 13/322 N40E N53W N34E South RB#3 9.8 Medium 17/339 N70E N13W N85E - RB#4A 34 Large 11/221 N45E N52W N70E N6W RB#4B 9.3 Medium 21/259 N60E N20W N5E - RB#5A 18.0 Medium 13/300 N48E N42W - - RB#5B 9.0 Medium 10/006 N35E N55W N70W RB#6 19.0 Medium 06/314 N45E N45W N12W - RB#7 26.2 Large 06/267 N60W N30E - - RB#8 10.5 Medium 29/354 N85E N5E - - RB#9 14.2 Medium 28/276 N23E N67W - - RB#10 31.8 Large 9/344 N77E N14W - - RB#11 31.9 Large 11/310 N55E N35W - - RB#12A 14.4 Medium 82/201 N25E N65W - - RB#12B 9.7 Medium 23/21 N25E N65W - - RB#12C 6.3 Medium 30/196 N5E N80W N42E - RB#12D 13.6 Medium 16/255 N85E N5W - - RB#12E 8.8 Medium 36/314 N18E N72W - - RB#12F 5.5 Medium 75/197 N12E N85E N15W - RB#12G 10.6 Medium 50/200 N76E N10W - - RB#13 12.6 Medium 15/280 N35E N65W - - RB#14 48.1 Large 32/192 N40E N50W N75E - RB#15 19.3 Medium 18/232 N10E N80W - - RB#16 7.8 Medium 16/260 N84E N6W - - RB#17 25.3 Large 63/238 N35E N80W - - RB#18 38.5 Large 26/203 N70E N3E N35W N40E RB#19 36.6 Large 20/169 N73E N17W N82W RB#20 34.6 Large 11/150 N60W N30E Very RB#21A 52.4 6/158 N54E N36W N15E Large RB#21B 2.7 Medium 50/084 N5E N85E N25W N20E

106

Very RB#22 54.6 71/318 N46E N44W - - Large RB#23 9.6 Medium 32/358 N35E N40W - - Very RB#24A 66 11/315 N40E N50W N63W - Large RB#24B 47.3 Large 10/013 N40E N50W N70W - Very RB#25 93.2 08/318 N35E N55W N34W N66W Large RB#26 42.9 Large 8/096 N70E N20W - - RB#27 31.9 Large 14/325 N70E N15W - - Very RB#28 102.4 07/305 N65E N25W - - Large Very RB#29A 64.9 09/317 N86E N5W N30E - Large Very RB#29B 57.4 09/025 N78E N15W N8W - Large Very RB#30A 104.0 12/305 N85W N20W - - Large RB#30B 19.3 Medium 20/311 N5E N70W - - RB#31 11.2 Medium 05/293 N82W N8W - - Very RB#32 142.5 05/167 N60E N38W - - Large Very RB#33 57.1 24/324 N60E N30W - - Large RB#34A 35.1 Large 25/318 N42E N48W - - RB#34B 45.3 Large 27/315 N48E N42W - - RB#35A 6.6 Medium 13/281 N45E N42W N7W - RB#35B 4.3 Medium 12/277 N70E N20W - - RB#35C 6.5 Medium 12/285 N65E N25W - - RB#36 18.5 Medium 19/068 N70E N15W - - RB#37 18.6 Medium 07/ 342 N80E N30W N55E - RB#38 33.1 Large 09/359 N70E N25W N55W - RB#39 26.7 Large 18/026 N50E N30W N10W - RB#40 41.3 Large 20/012 N20E N70E - - RB#41 26.5 Large 33/310 N45E N35W - -

APPENDIX B

RESULTS OF THE GRAIN SIZE DISTRIBUTION ANALYSIS

107

108

100 Cc=1.50 90 Cu= 6.00 80 USCS Classification= ML 70

60 50 40 30

Cumulative % Passing Cumulative 20 10 0 10 1 0.1 0.01 0.001 Grain Size (mm)

Figure B-1: Grain size distribution curve for sample #1.

100 Cc=1.50 90 Cu= 6.00 80 USCS Classification= ML 70 60 50 40 30 20 Cumulative % Passing Cumulative 10 0 10 1 0.1 0.01 0.001 Grain Size (mm)

Figure B-2: Grain size distribution curve for sample #2.

109

100 Cc=1.48 90 Cu= 6.09 80 USCS Classification= ML 70 60 50 40 30 20 Cumulative % Passing Cumulative 10 0 10 1 0.1 0.01 0.001 Grain Size (mm)

Figure B-3: Grain size distribution curve for sample #3.

100 Cc=1.68 90 Cu= 5.44 80 USCS Classification= SM 70 60 50 40 30 20 Cumulative % Passing Cumulative 10 0 10 1 0.1 0.01 0.001 Grain Size (mm)

Figure B-4: Grain size distribution curve for sample #4.

110

100 Cc=1.41 90 Cu= 6.39 80 USCS Classification= SM 70 60 50 40 30 20 Cumulative % Passing Cumulative 10 0 10 1 0.1 0.01 0.001 Grain Size (mm)

Figure B-5: Grain size distribution curve for sample #5.

100 Cc=0.52 90 Cu= 13.63 80 USCS Classification= SP 70 60 50 40 30 20 Cumulative % Passing Cumulative 10 0 10 1 0.1 0.01 0.001 Grain Size (mm)

Figure B-6: Grain size distribution curve for sample #6.

111

100 Cc=0.96 90 Cu= 9.41 80 USCS Classification= SM 70 60 50 40 30 20 Cumulative % Passing Cumulative 10 0 10 1 0.1 0.01 0.001 Grain Size (mm)

Figure B-7: Grain size distribution curve for sample #7.

100 Cc=1.00 90 Cu= 2.25 80 USCS Classification= SP 70 60 50 40 30 20 Cumulative % Passing Cumulative 10 0 10 1 0.1 0.01 0.001 Grain Size (mm)

Figure B-8: Grain size distribution curve for sample #8.

112

100 Cc=1.72 90 Cu= 4.71 80 USCS Classification= SM 70 60 50 40 30 20 Cumulative % Passing Cumulative 10 0 10 1 0.1 0.01 0.001 Grain Size (mm)

Figure B-9: Grain size distribution curve for sample #9.

100 Cc=1.21 90 Cu= 7.43 80 USCS Classification= SM 70 60 50 40 30 20 Cumulative % Passing Cumulative 10 0 10 1 0.1 0.01 0.001 Grain Size (mm)

Figure B-10: Grain size distribution curve for sample #10.

113

100 Cc=1.27 90 Cu= 7.08 80 USCS Classification= MH 70 60 50 40 30 20 Cumulative % Passing Cumulative 10 0 10 1 0.1 0.01 0.001 Grain Size (mm)

Figure B-11: Grain size distribution curve for sample #11.

100 Cc=1.06 90 Cu= 4.65 80 USCS Classification= SM 70 60 50 40 30 20 Cumulative % Passing Cumulative 10 0 10 1 0.1 0.01 0.001 Grain Size (mm)

Figure B-12: Grain size distribution curve for sample #12.

114

100 Cc=1.69 90 Cu= 5.33 80 USCS Classification= SM 70 60 50 40 30 20 Cumulative % Passing Cumulative 10 0 10 1 0.1 0.01 0.001 Grain Size (mm)

Figure B-13: Grain size distribution curve for sample #13.

100 Cc=1.18 90 Cu= 3.89 80 USCS Classification= SP-SM 70 60 50 40 30 20 Cumulative % Passing Cumulative 10 0 10 1 0.1 0.01 0.001 Grain Size (mm)

Figure B-14: Grain size distribution curve for sample #14.

115

100 Cc=1.48 90 Cu= 6.10 80 USCS Classification= SM 70 60 50 40 30 20 Cumulative % Passing Cumulative 10 0 10 1 0.1 0.01 0.001 Grain Size (mm)

Figure B-15: Grain size distribution curve for sample #15.

APPENDIX C

RESULTS OF THE ATTERBERG LIMIT TESTS

116

117

20 LL=37.5

Water Contet (%) Contet Water PL= 33.5 10 PI= 4 USCS Classification= ML 0 25 blows 10 100 Number of Blows "N" Figure C-16: Liquid limit flow curve for sample #1.

20 LL=44.5

Water Contet (%) Contet Water PL= 35.4 10 PI= 9.1 USCS Classification= ML 0 25 blows 5 50 Number of Blows "N" Figure C-17: Liquid limit flow curve for sample #2.

118

20 LL= 44

Water Contet (%) Contet Water PL= 34 10 PI= 10 USCS Classification= ML 0 25 blows 10 100 Number of Blows "N" Figure C-3: Liquid limit flow curve for sample #3.

20 LL= 52

Water Contet (%) Contet Water PL= 35.6 10 PI= 16.4 USCS Classification= MH 0 25 blows 10 100 Number of Blows "N" Figure C-4: Liquid limit flow curve for sample #4.

119

30 LL= 66.5

Water Contet (%) Contet Water 20 PL= 42.9 PI= 23.6 10 USCS Classification= MH 0 25 blows 10 Number of Blows "N" 100

Figure C-5: Liquid limit flow curve for sample #5.

30 LL= 60

Water Contet (%) Contet Water 20 PL= 41.5 PI= 18.5 10 USCS Classification= MH 0 25 blows 10 100 Number of Blows "N" Figure C-6: Liquid limit flow curve for sample #6.

120

20 LL= 54

Water Contet (%) Contet Water PL= 43.6 10 PI= 10.4 USCS Classification= MH 0 25 blows 10 100 Number of Blows "N" Figure C-7: Liquid limit flow curve for sample #7.

20 LL= 35

Water Contet (%) Contet Water PL= 29.7 10 PI= 5.3 USCS Classification= ML 0 25 blows 10 100 Number of Blows "N" Figure C-8: Liquid limit flow curve for sample #9.

121

30 LL= 56

Water Contet (%) Contet Water 20 PL= 37.7 PI= 18.3 10 USCS Classification= MH

0 25 blows 10 100 Number of Blows "N" Figure C-9: Liquid limit flow curve for sample #10.

30 LL= 65

Water Contet (%) Contet Water 20 PL= 43.6 PI= 21.4 10 USCS Classification= MH 0 25 blows 10 100 Number of Blows "N" Figure C-10: Liquid limit flow curve for sample #11.

122

LL= 27.8

Water Contet (%) Contet Water 10 PL= 21.8 PI= 6 USCS Classification= CL-ML

0 25 blows 5 Number of Blows "N" 50

Figure C-11: Liquid limit flow curve for sample #12.

20 LL= 33

Water Contet (%) Contet Water PL= 28.8 10 PI= 4.2 USCS Classification= ML 0 25 blows 5 Number of Blows "N" 50

Figure C-12: Liquid limit flow curve for sample #13.

123

LL= 26.8

Water Contet (%) Contet Water 10 PL= 25.7 PI= 1.1 USCS Classification= ML 0 25 blows 10 100 Number of Blows "N" Figure C-13: Liquid limit flow curve for sample #14.

15 LL= 31.9

Water Contet (%) Contet Water 10 PL= 27.4 PI= 4.5 5 USCS Classification= ML 0 25 blows 5 50 Number of Blows "N" Figure C-14: Liquid limit flow curve for sample #15.

124

30 LL= 64

Water Contet (%) Contet Water 20 PL= 59.5 PI= 4.5 10 USCS Classification= MH 0 5 50 Number of Blows "N" Figure C-15: Liquid limit flow curve for sample #16.

APPENDIX D

RESULTS OF THE COMPACTION-MOLD PERMEAMETER TEST AND THE SOIL DENSITY VALUES USED

125

126

Table D-1: Field densities measured.

Field Bulk Field Dry Density Sample No. Sampling Location Density (g/cm3) (g/cm3) #12 1.87 (116.7 pcf) 1.42 (88.5 pcf) #13 Settlement Trough 1.87 (116.7 pcf) 1.37 (85.8 pcf) #14 1.99 (124.2 pcf) 1.6 (100.1 pcf) #15 Lake Bed 1.85 (115.4 pcf) 1.39 (86.8 pcf) #16 Lake bed 1.26 (78.6 pcf) 0.62 (38.8 pcf)

Table D-2: Densities of samples tested in the compaction-mold permeameter test as compared to field densities.

Level of Compaction Sample Description Field Lab Dry Dry Relative Sampling Densit Densit Compaction USCS Sample No. (%) Classific Location y y Description Notes (g/cm (g/cm [R.C.= ρ ation 3) 3) (field) / Symbol ρ (lab) * 100] Slight tamping This sample was a of mold was combination of Samples D1-1 1.048 applied, but & D1-2, both gathered from #1 (Loose) - (65.4 N/A sample was the vicinity of depression #1. pcf) Depression mainly prepared They range between low ML #1 in the loose state plasticity sandy silt and silt Compacted by with sand. The average #1 1.18 tamping and percentage of sand was (compacted - (73.9 N/A compaction 28.2% and silt 59.4%. No ) pcf) hammer field densities were available. This sample is predominantly composed of fine sand Slight tamping (74.1%). It was obtained near Depression of mold was 1.34 a large piping hole in #1 (near applied, but #9 - (83.9 N/A SM depression #1 when the lake piping sample was pcf) was almost completely empty hole) mainly prepared and lake-bottom was exposed. in the loose state No field densities were available. Slight tamping of mold was Composed of 50.4% fine sand Depression 0.98 applied, but and 40.6% silt. Sampled #10 #3 ("Cat - (61.1 N/A SM sample was when lake was almost Paw") pcf) mainly prepared completely empty. in the loose state Compacted to Composed of 62.7% fine sand 1.37 1.38 field density Settlement and 31.5% silt. Sampled from #13 (85.8 (85.9 using 99.9 SM Trough settlement trough north of the pcf) pcf) compaction lake. hammer Compacted to Composed of 55.2% fine sand 1.39 1.41 field density Settlement and 37.9% silt. Sampled from #15 (86.8 (87.8 using 98.9 SM Trough settlement trough north of the pcf) pcf) compaction lake. hammer

127

Table D-3: Results of the compaction-mold permeameter test.

Dry Hydraulic Gradient (i) Sampl Dens ity Param e No. (g/c 1 1.5 2 2.5 3 3.5 4 4.5 5 6 7 8 9 10 m3) eters

K 9E- 8.3E- 8.32E 7.38E 7.35E 7.71E 7.2E- 1.01E 8.1E- 7.8E- 2.83E 1.18 2E- - (cm/sec 06 06 -06 -06 -06 -06 06 -05 06 06 -06 E-06 06 .) Q 8.3 1.04 1.15E 1.54E 1.71E 2.04E 2.50E 2.67E 4.21E 3.75E 4.33E 1.83E 8.75 1.67 #1 (ml/sec. 3E- - 8 -02 -02 -02 -02 -02 -02 -02 -02 -02 -02 E-03 E-02 (Loos (54.4 ) 03 e) pcf) 5.0 g/60 5.00E 5.00E 1.00E 5.00E 7.50E 1.00E 8.00E 1.75E 4.00E 8.00E 9.25 1.20 0E- - sec. -03 -03 -02 -03 -03 -02 -02 -02 -02 -02 E-02 E-01 03 g/100 1.0 12.0 0.72 0.54 0.98 0.41 0.50 0.63 3.17 0.78 1.54 7.27 17.62 - ml 0 0 K 2.5 2.34E 2.27E 2.28E 2.19E 2.25E 2.20E 2.20E 2.25E 2.44E 2.54 2.80 (cm/sec - - 2E- -06 -06 -06 -06 -06 -06 -06 -06 -06 E-06 E-06 .) 06

Q 2.3 4.33E 5.25E 6.33E 7.08E 8.33E 9.17E 1.02E 1.25E 1.58E 1.88 2.33 #1 1.18 (ml/sec. - - 3E- -03 -03 -03 -03 -03 -03 -02 -02 -02 E-02 E-02 (comp (73.9 ) 02 pcf) acted) 1.5 g/60 2.50E 2.50E 5.00E 1.00 5.50 - - 0 0 0 0 0 0 0E- sec. -03 -03 -03 E-02 E-02 02 g/100 1.0 - - 0 0 0 0.59 0 0.45 0 0 0.53 0.88 3.93 ml 7 K 4.61E 4.80E 4.78E 4.53E 4.53E 4.48E 4.46E 4.69E 4.62E 4.76E (cm/sec - - - - -04 -04 -04 -04 -04 -04 -04 -04 -04 -04 .)

Q 1.34 0.00E 0.00E 0.00E 0.00E 0.00E 0.00E 0.00E 0.00E 0.00E 0.00E (ml/sec. - - - - #9 (83.9 +00 +00 +00 +00 +00 +00 +00 +00 +00 +00 pcf) ) g/60 1.63E 6.59E 5.71E 5.67E 5.82E 6.76E 1.43E 3.16E 6.24E 8.41E - - - - sec. -02 -03 -03 -03 -03 -03 -02 -02 -02 -02 g/100 - 0.46 0.13 0.09 0.08 0.07 0.08 0.14 0.26 0.45 0.49 - - - ml K 4.39E 4.59E 4.23E 4.48E 5.18E 1.95E 1.50E 1.00E 7.09E (cm/sec ------05 -05 -05 -05 -05 -04 -04 -02 -03 .)

Q 0.98 0.00E 0.00E 0.00E 0.00E 0.00E 0.00E 0.00E 0.00E 0.00E (ml/sec. - - - - - #10 (61.1 +00 +00 +00 +00 +00 +00 +00 +00 +00 pcf) ) g/60 1.44E 2.29E 1.07E 1.15E 1.33E 1.50E 4.23E 1.88E 5.40E - - - - - sec. -03 -03 -03 -03 -03 -02 -02 +00 +00 g/100 - - 0.33 0.41 0.17 0.17 0.17 0.35 1.15 0.64 2.22 - - - ml K 7.7 7.2E- 7.2E- 7.2E- 7.15E 7.24E 7.2E- 7.47E 7.5E- 7.43E 7.51 7.1E (cm/sec - - 4E- 06 06 06 -06 -06 06 -06 06 -06 E-06 -06 .) 06

Q 1.38 0.05 0.0 (ml/sec. - - 0.013 0.017 0.020 0.023 0.027 0.030 0.035 0.042 0.048 0.056 #13 (85.9 9 72 pcf) ) g/60 0.00 0.0 - - 0.002 0.005 0 0.002 0.005 0.007 0.005 0.005 0.005 0.030 sec. 8 08 g/100 0.1 - - 0.31 0.50 0 0.18 0.31 0.42 0.24 0.20 0.17 0.90 0.21 ml 7 K 1.5E- 1.38E 1.39E 1.4E- 1.47E 1.46E 1.43E 1.41E 1.23E 1.24 1.25 (cm/sec - - - 05 -05 -05 05 -05 -05 -05 -05 -05 E-05 E-05 .)

Q 1.41 2.78E 3.21E 3.86E 4.52E 5.43E 6.10E 6.62E 7.81E 8.00E 9.17 1.04 (ml/sec. - - - #15 (87.8 -02 -02 -02 -02 -02 -02 -02 -02 -02 E-02 E-01 pcf) ) g/60 5.00E 1.92E 0.00E 2.71E 6.52E 1.83E 3.97E 4.69E 4.80E 8.26 6.25 - - - sec. -03 -03 +00 -03 -03 -03 -03 -03 -03 E-03 E-03 g/100 - - 0.3 0.1 0 0.1 0.2 0.05 0.1 0.1 0.1 0.15 0.1 - ml