CHAPTER 6 SOIL COMPACTION
Omitted Sections 6.6, 6.7, 6.8 SOIL COMPACTION
In Geotechnical engineering practice, the soils at a given site are often less than desirable for the intended purpose. They may be: Weak (strength) Highly compressible Have a high permeability Solution Relocate the project Articulate design for structure members Stabilize or improve the properties of the soil The third alternative may be in most cases the most economical alternative. There are different techniques for improvement of soils (This subject is covered in details in CE 486 “Improvement of Geotechnical Materials”). We will consider in this course only compaction. SOIL COMPACTION
Compaction is also very important when soil is used as an engineering material, that is the structure itself is made of soil.
Ex. .Earth dams .Highways .Airfields .etc.
Definition Compaction is the densification of soils by removal of air through the application of mechanical energy.
The degree of compaction is measured in terms of its dry unit weight. SOIL COMPACTION
Increases unit weight
Increases shear strength
Increases bearing capacity
Increases stability of slopes of embankments
Decreases settlement of structures SOIL COMPACTION
You remember well-graded
reduced Air Air Compaction Water Water
Solid Solid SOIL COMPACTION
General Principle The degree of compaction of soil is measured by its dry unit weight. When water is added during compaction it acts as a softening agent on the soil particles.
gd(max) When the moisture content is Soil Solid gradually increased, the weight of the soil solids in a unit water volume gradually increases.
Soil Solid
Optimum moisture content (OMC) isgthe water content at which the maximum dry unit weight d (max) is attained. SOIL COMPACTION
Soil Solid
water
Soil Solid SOIL COMPACTION
Types of Compaction Methods in the Laboratory •Impact or dynamic (The most common type) •Kneading •Static The laboratory test generally used to obtain the maximum dry unit weight of compaction and the optimum moisture content is called the Proctor compaction test. It is named after R. R. Proctor (1933) (engineer in LA). He established that compaction is a function of: 1.Moisture Content 2.Compactive Effort 3.Soil Type There are two methods or tests: Standard Proctor test (ASTM D-698 & AASHTO T-99) Modified Proctor test (ASTM D-1557 & AASHTO T-180) Standard Proctor Test
• Mold 1/30 ft3 in volume • 3 layers • 25 blows • 5.5 lb hammer • 12 inch drop
Mold Hammer
The procedure for the standard Proctor test is elaborated in ASTM Test Designation D-698 (ASTM, 2007) and AASHTO Test Designation T-99 (AASHTO, 1982). Standard Proctor Test Standard Proctor Test
Process of Compaction Several samples are mixed at different water contents Compact according to the compaction test (standard or modified). W g moist Vmold W = Weight of compacted soil in the mold 3 Vmold = Volume of the mold = (1/30 ft ) For each test find the moisture content of the compacted soil. The dry unit weight is given by g g moist d 1 w
Plot g d vs. w
From the plot, find OMC and g d (max) Standard Proctor Test
In order to avoid a large number of compaction tests, it is desirable to begin the first test at a moisture content that is about 4 to 5% below the approximate optimum moisture content. Standard Proctor Test
REMARKS
1. Each data point on the curve represent a single compaction test.
2. Four or five tests are required
3. The curve is unique for: - A given soil type - Method of compaction - (constant) compactive effort
4. gd(max) is only a maximum for a specific compactive effort and method of compaction. This does not necessarily reflect the maximum dry unit weight that can be obtained in the field. 5. Typical OMC are between 10% and 20%. Outside maximum range 5% to 40%. Standard Proctor Test
6. Increasing the compactive effort tends to increase the maximum dry density, as expected, but also decrease the OMC. (This is why the curve never be to the right of zero curve). 7. In practice less amount of water is used but higher compactive effort or vise versa.
8. For clay soils gd(max) tends to decrease as plasticity increases.
9. The approximation to field is not exact because the lab. test is a dynamic impact type, whereas field compaction is essentially a kneading-type compaction.
10. In the field, compactive effort is the number of passes or “coverage” of the roller of a certain type and weight on a given volume of soil. Standard Proctor Test
Theoretical g d(max)
The maximumg d (max)is obtained when no air in the voids (i.e. s =100%) G g s g d 1 e w
but wGs se for S 100% e wGs G g g s g w z av w 1 1 wGs w Gs
Where gzav = zero air void unit weight.
The relationship between gzav and w can be obtained as shown in the figure across. Compaction curve is always to the left of the zero-air-void curve. No matter how much water is added, the soil never becomes completely saturated by compaction. Standard Proctor Test
To obtain the variation of gzav with moisture content, use the following procedure:
Under no circumstances should any part of the compaction curve lie to the right of the zero-air-void curve. FACTORS AFFECTING COMPACTION
Besides moisture content, other important factors that affect compaction are: 1) Soil type; 2) Compaction effort.
1. Effect of Soil Type
Grain Size Distribution Shape of the soil grains
Gs Amount of clay minerals Type of clay minerals
Fine grain soil needs more water to reach optimum. FACTORS AFFECTING COMPACTION
Effect of Soil type and gradation
Fine grain soil needs more water to reach optimum. FACTORS AFFECTING COMPACTION
Effect of Soil type and gradation Gs g w g g Typical Values z av w 1 1 wGs w 3 Gs g d (max)(kN/m ) OMC (%)
Well graded sand SW 22 7
Sandy clay SC 19 12
Poorly graded sand SP 18 15
Low plasticity clay CL 18 15
Non plastic silt ML 17 17
High plasticity clay CH 15 25
Gs is constant, therefore increasing maximum dry unit weight is associated with decreasing optimum moisture contents.
Do not use typical values for design as soil is highly variable. FACTORS AFFECTING COMPACTION
Compaction Curves Encountered in Soils •The bell-shaped compaction curve is typical for most clayey soils.
Typical
•Some curves have more than one peak others have no peak. FACTORS AFFECTING COMPACTION
2. Effect of Compaction Effort
Standard Proctor
For the standard Proctor test
• The standard Proctor mold and hammer were used to obtain these compaction curves. • For all cases the number of layers was equal to 3. g Compaction effort dmax
wopt. EXAMPLE 6.1
TEXT IN SI UNITS EXAMPLE 6.1
TEXT IN SI UNITS EXAMPLE 6.1
TEXT IN SI UNITS EXAMPLE EXAMPLE Example (2nd Midterm Exam Fall 40-41)
If you are checking the field compaction of a layer of soil and the compaction curve for the soil is shown in Figure 1. From the specifications, the dry density of the compacted soil should be at least 95% of the maximum value and within ± 1% of the optimum water content. When you did the sand cone test, the volume of the soil excavated was 1153 cm3. It weighed 2209 grams wet and 1875 grams dry. a. What is the compacted dry density? b. What is the field water content? c. What is the relative compaction? d. Does the test meet the specifications (explain)? e. If it does not meet, what should be done to improve the compaction so that it will meet the specifications? f. What is the degree of saturation of the field sample? g. If the sample were saturated at constant density, what would be the water content? Modified Proctor Test
Modified Proctor Test (ASTM D-1557, AASHTO T-180)
With the development of heavy rollers (also requirements of heavy aircrafts and trucks) and their use in field compaction, the standard Proctor test was modified for better representation of the field conditions. This is sometimes referred to as modified Proctor test.
•Mold 1/30 ft3 in volume (same as for standard test) •5 layers •25 blows (same as for standard test) •10 lb hammer •18 inch drop
Developed in WWII by U.S. Army Corps of Engineers to better represent the compaction required for airfield to support heavy aircraft. Modified Proctor Test
Modified Proctor Standard Proctor Test Test
Layer 5 Layer 4 Layer 3 Drop = 457.2 mm Layer 2 (18 in) Layer 1
Drop = 304.8 mm (12 in)
hammer hammer = 2.5 kg (5.5 lb) = 4.54 kg (10 lb) Modified Proctor Test
Standard Modified Proctor Proctor Test Test Volume of mold 944 cm3 944 cm3
# of layers 3 5 Mass of hammer 2.5 kg 4.54 kg Drop of hammer 30.5 cm 45.7 cm # of hammer blows 25 25 Compaction Energy for Unit Volume of Soil
Standard Proctor Test
(25)(3)5.5(1) E 12375 ft -lb/ft 3 592.5 kN m/ m3 600 kN m/ m3 (1/ 30)
Modified Proctor Test
(25)(5)10(1.5) E 56250 ft -lb/ft 3 2693.3 kN m/ m3 2700 kN m/ m3 (1/ 30)
. Because it increases compactive effort, the modified Proctor test results in an increase of the maximum dry unit weight of the soil, and this is accompanied by decrease in the optimum moisture content.
. Note: In the field, compactive effort is the number of passes of the roller of a certain type and weight on a given volume of soil. SOIL COMPACTION FIELD COMPACTION FIELD COMPACTION
Most of the compaction in the field is done by means of ROLLERS.
The most common types are:
1. Smooth-wheel rollers (smooth-drum rollers)
2. Pneumatic rubber-tired rollers
3. Sheepsfoot rollers
4. Vibratory rollers FIELD COMPACTION
1. Smooth-wheel rollers (smooth-drum roller) Proof rolling subgrades Finishing operation of fills with sandy &clayey soils Provide 100% coverage Contact pressure 310 – 380 kN/m2 Not suitable for producing high g for thicker layers FIELD COMPACTION
2.Pneumatic rubber-tired rollers
Heavily loaded with several rows of tires
Tires are closely spaced 4 -6 in a row
Provide 70-80% coverage
Contact pressure 600 – 700 kN/m2
Combination of pressure and kneading FIELD COMPACTION
3.Sheepsfoot rollers
Drums with a large number of projections
Area of each projection 25 – 85 cm2
Most effective in compacting clayey soils
Contact pressure 1400 – 7000 kN/m2 FIELD COMPACTION
4.Vibratory rollers Efficient in compacting granular soils Vibrators can be attached to smooth-wheel, pneumatic rubber-tired, or sheepsfoot rollers to provide vibratory effects to the soil.
Figure 6.20 Principles of vibratory rollers FIELD COMPACTION
Handheld vibratory Handheld vibratory plates can be used for effective compaction of granular soils over a limited area. FACTORS AFFECTINGFIELD COMPACTION
There are several factors that must be considered to achieve the desired unit weight of compaction in the field:
Soil type
Moisture content
Thickness of lift
Intensity of pressure
Area over which the pressure is applied
No. of roller passes FIELD COMPACTION
Compaction of Silty Clay FIELD COMPACTION
Vibratory Compaction of Sand FIELD COMPACTION
In most cases, about 10 to 15 roller passes Lack of confining pressure yield the maximum dry unit weight towards the surface economically attainable.
Relationship between dry unit weight Relationship between dry unit weight, and number of passes number of passes, and depth. SPECIFICATIONS FOR FIELD COMPACTION
. Usually it is required for the contractor to achieve a compacted field dry unit weight of say 90 to 95% of the maximum dry unit weight determined in the laboratory by either the standard or modified Proctor test (Recall previous examples).
Relative compaction, R (a)
. For granular soils, specifications can be expressed in terms of relative density. Applicable if the soil (b) contains less than 12% fines (passing No. 200 sieve) From (a) and (b)
where FIELD COMPACTION
Dividing by
R
1 R FIELD COMPACTION
Solve for R
. Approximate formula for granular soils EXAMPLE 6.8 Determination of Field Unit Weight of Compaction
We know that both relative compaction or relative density are both needed for determination of dry density in the field.
Common Methods: 1. Sand cone method
2. Rubber balloon method
3. Nuclear method Sand cone method
1. Sand cone method (ASTM Designation D-1556)
Filling the jar with very uniform dry Ottawa sand
W1 = weight of the jar, the cone, and the sand filling the jar
Excavating a small hole in the area where the soil has been compacted
W2 = weight of the moist soil excavated from the hole.
W3 = the dry weight of the soil = Recall = moisture content = Ww/Ws Sand cone method
The cone with the sand-filled jar attached to it is inverted and placed over the hole.
W4 = combined weight of the jar, the cone, and the remaining sand filling the jar.
W5 = weight of sand to fill both the hole and cone
V = the volume of the excavated hole
Wc= weight of sand to fill the cone only
g d (sand) dry unit weight of Ottawa sand used
The dry unit weight of compaction made in the field is determined as W g 3 d ( field ) V Field Density Test Field Density Test EXAMPLE 6.9
Proctor test
Sand Cone Test EXAMPLE 6.9 EXAMPLE 6.9 EXAMPLE 6.10
Proctor test
Sand Cone Test Proctor test EXAMPLE 6.10
Sand Cone Test RUBBER BALLOON METHOD
2. Rubber Balloon Method (ASTM Designation D-2167) Similar to sand cone method except that the volume of the hole is determined by introducing into it a rubber balloon filled with water from a calibrated vessel.
Determine weight of dry soil Determine volume of the hole (can be read directly) Determine dry unit weight Nuclear Method
3. Nuclear Method (ASTM D6938 - 15 ) . Nuclear density meter (Densometer) o Dense soil absorbs more radiation than loose soil. o Measures the weight of wet soil per unit volume and the weight of water present in a unit volume of soil. o The dry unit weight of compacted soil can be determined by subtracting the weight of water from the moist unit weight of soil.
Operates either in drilled holes or on ground surface Uses radioactive isotope source (Gamma rays) Measure weight of wet soil per unit volume It also measure weight of water per unit volume Determine the dry unit weight of soil Special Compaction Techniques Special Compaction Techniques
Vibroflotation
A technique for in situ densification of thick layers of loose granular soil deposits. Special Compaction Techniques Special Compaction Techniques GSD and compaction by vibrflotation
The most suitable for compaction by The rate of probe Vibroflotation. penetration may be slow and may prove uneconomical in the long run. Difficult to compact
Lower limit of grain-size distribution for which compaction by vibroflotation is effective. Special Compaction Techniques
RATING BACKFILL (Brown , 1977)
SN = Suitability No. for rating backfill where D50, D20, and D10 are the diameters (in mm) through which, respectively, 50, 20, and 10% of the material passes.
The smaller the value of SN, the more desirable the backfill material. Special Compaction Techniques
Typical patterns of Vibroflot probe spacings for a column foundation
Compaction over a large area EXAMPLE 6.11 Special Compaction Techniques
Dynamic Compaction Densification of granular soil deposits Dropping a heavy weight on the ground at regular intervals Weight of hammer 80-360 kN Hammer drop 7.5-30.5 m
Degree of compaction depends on: Weight of hammer Height of hammer drop Spacing of locations at which the hammer is dropped Special Compaction Techniques
Blasting
Compaction (up to a relative density of 80%) up to a depth of about 18 m over a large area can easily be achieved. Usually the explosive charges are placed at a depth of about two-thirds of the thickness of the soil layer desired to be compacted. THE END