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Shear Strength Parameters of Unsaturated Tropical Residual Soils of Various Weathering Grades
Department of Civil Engineering, University Putra Malaysia, Selangor, Malaysia Department of Civil Engineering, University of Malaya, Kuala Lumpur, Malaysia Ranhill Consulting Sdn. Bhd., Kuala Lumpur, Malaysia |
ABSTRACT
strength envelope for unsaturated soil requires three shear strength parameters that can be defined namely as c', f' and fb. These parameters can be measured in the laboratory. The c' and f' parameters can be measured using standard laboratory equipment on saturated soil specimen [For convenience, these parameters are called "cohesion" and "angle of friction" in the following, when they are referred to individually]. However conventional triaxial and direct shear equipment require modifications prior to their use for testing unsaturated soils, i.e. to measure the fb. Several factors related to the nature of unsaturated soil must be considered in modifying the equipment. The presence of air and water in the pores of the soil causes the testing procedures and techniques to be more complex than those required when testing saturated soils. The modification must accommodate the independent measurement or control of pore air and pore water pressures. In addition, the pore water pressure is usually negative and can result in water cavitation problems in the measurement. This paper describes the modification made to the standard laboratory triaxial test apparatus and outline the test performed for measuring the shear strength parameters of unsaturated residual soil of various weathering grades. The value cohesion, c' is found to increase with increase in matric suction. For a given level of matric suction, the cohesion increases with increase in the soil weathering grade, i.e as the soils/rocks becomes more weathered. The angle of friction, f' however decreases with increase in the soils weathering grade. The angle of friction or change in shear strength with change in suction, fb also increases when the soil/rock becomes more weathered. The values of fb are generally lower than f'.
Keywords: Shear strength, tropical soils, unsaturated residual soils, weathering grades.
INTRODUCTION
The microclimatic conditions in an area are the main factors causing a soil deposit to be unsaturated. Therefore, unsaturated soils or soils with negative pore-water pressures can occur in essentially any geological deposit, such as residual soil, a lacustrine deposit, soils in arid and semi arid areas with deep ground water table, and tropical soils. In Malaysia, residual granite rock soil and sedimentary rock soil occur extensively, i.e. cover more than 80% of the land area. Yet, not much research works have been carried out on these materials. The situation is even worst in the case of unsaturated residual soils.
Tropical residual soils have some unique characteristics related to their composition and the environment under which they develop. Their strength and permeability are likely to be greater than those of temperate zone soils with comparable liquid limits. Most classical concepts related to soil properties and soil behavior have been developed for temperate zone soils, and there has been difficulty in accurately modeling procedures and conditions to which residual soils will be subjected. Engineers appear to be slowly recognizing that residual soils are generally soils with negative in situ pore-water pressures, and that much of the unusual behavior exhibited during laboratory testing is related to a matric suction change in the soil (Fredlund and Rahardjo 1985, 1993). There is the need for reliable engineering design associated with residual soils (Ali and Rahardjo 2004).
When the degree of saturation of a soil is greater than about 85%, saturated soil mechanics principles can be applied. However, when the degree of saturation is less than 85%, it becomes necessary to apply unsaturated soil mechanics principles (Fredlund and Rahardjo 1987). The transfer of theory from saturated soil mechanics to unsaturated soil mechanics and vice versa is possible through the use of stress state variables (Fredlund and Morgenstern 1977).
Shear strength parameters are the key input parameters in any soil stability analysis. For case of unsaturated soils, they are given by the following equations,
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(1) |
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(2) |
where c' and f' are the shear strength parameters [These parameters are simply called "cohesion" and "angle of friction" for convenience, when they are referred to individually], and fb is the shear strength change with change in matric suction. The above equations are the extended form of the Mohr–Coulomb's equation (Fredlund et al., 1978).
The total cohesion intercept c at each matric suction is determined from the point where the failure envelope intersects the shear stress versus matric suction plane. The shear strength of a soil increases as the matric suction increases or degree of saturation decreases. The increase in the shear strength can be considered as an increase in the cohesion intercept because of an increase in matric suction. The increase in the cohesion intercept with respect to matric suction is defined by the intersection between the failure surface and the t (shear stress) versus (ua – uw) plane. This line has a slope of fb that can be measured experimentally. The value of fb is generally equal to or less than f'.
The extended Mohr-Coulomb shear strength envelope for unsaturated soils require that three shear strength parameters be defined namely, c', f' and fb. These parameters can be measured in the laboratory. The c' and f' parameters can be measured using standard laboratory equipment on saturated soil specimen. However conventional triaxial and direct shear equipment require modifications prior to their use for testing unsaturated soils, i.e. to measure the fb. Several factors related to the nature of unsaturated soil must be considered in modifying the equipment. The presence of air and water in the pores of the soil causes the testing procedures and techniques to be more complex than those required when testing saturated soils. The modification must accommodate the independent measurement or control of pore air and pore water pressures. In addition, the pore water pressure is usually negative and can result in water cavitation problems in the measurement.
This paper describes the modification made to the standard laboratory triaxial and outline the test performed for measuring the shear strength parameters of unsaturated residual soil of various weathering grades.
MODIFIED TRIAXIAL TEST APPARATUS
The triaxial test is one of the most common tests performed in the laboratory to measure the shear strength of a soil. This test is basically performed on a cylindrical soil specimen enclosed in a rubber membrane, which is placed in the triaxial cell and subjected to an all-round (confining) pressure. The specimen is then subjected to an axial stress through a loading ram in contact with the top of the specimen.
For saturated soil, the following tests upon the drainage conditions adhered to during the first and the second stages of the test are usually performed. They are the consolidated drained, or CD test, and consolidated undrained or CU test with pore water pressure measurements. For unsaturated soil, the testing procedure is similar to that used in the saturated soil testing except that in this case, in addition to the pore water pressure, the pore air pressure need also to be set and controlled while the sample is loaded to failure, thus enabling the matric suction (ua – uw) to be applied to the soil sample.
To enable test to be carried out in suction induced soil sample, modifications can be made to the conventional Bishop-Wesley triaxial cell. In this study, the top cap of the cell was modified to provide inlet for the air pressure to be applied from the top of the soil sample. Axis translation technique (Hilf 1956) was used in the application of suction to the soil sample. A 5 bar ceramic disc was seated and carefully sealed on a modified base pedestal. This allowed the air and water pressures to be controlled during the tests in order to maintain a constant matric suction, (ua – uw), during the test.
Additional outlets for the air pressure were drilled on the side of the triaxial cell base as shown Figure 1. Other modifications, similar to those of Fredlund and Raharjo (1993), Mabtouk et al. (1995), Wheeler and Sivakumar (1995), and Chiu and Delage (1996), were also done to the base pedestal. Additional ducts were required to facilitate connection in the base pedestal for drainage and flushing of diffused air from below the ceramic disc.
Figure 1. Modified base plate with outlet ports
The existing connections were used for the cell chamber, to sample top cap, and base pedestal for pressure measurement. The base pedestal could be interchanged with adaptors thus enabling the same cell for conducting tests on samples of various diameters. Each outlet terminated at a screwed socket on the edge of the cell base, into which was fitted a valve connecting to the appropriate pressure lines.
The normal base pedestal is not suitable for testing under suction control. A completely new base pedestal with a recess for ceramic disc, including a grooved spiral water-compartment below the disc and inlet and outlet ports for flushing was fabricated. The groove was to improve flushing of diffused air. The specially fabricated base pedestal, which is similar to that of Fredlund and Raharjo (1993), Mabtouk et al. (1995), Wheeler and Sivakumar (1995), and Chiu and Delage (1996), is shown in Figure 2.
Figure 2. Specially fabricated triaxial base pedestal for unsaturated soil testing
The high air entry porous disc needed a good seal. This was done by forming a recess on the pedestal top and placing disc of a smaller diameter and bonded with epoxy resin. It is essential that the whole set up was rendered leak proof; otherwise there would be difficulties in maintaining required suction throughout the tests.
Figure 3 shows a schematic layout of the specially modified triaxial for testing samples the residual soil in an unsaturated state.
Figure 3. Schematic layout of specially modified triaxial testing for unsaturated soil
SOIL SAMPLES
The soil samples were obtained by mean of block sampling of size of 250 x 250 x 250 mm residual soils of two most commonly found rock types found in Malaysia, i.e. igneous (granite) rock at site A, and sedimentary (sandstone) rock at site B.
Site A was a road cut made in a residual soil that had developed over the commonly outcropping Perm-Triassic Mesozonal granite of Peninsular Malaysia (Raj 1984). The road cut was at KM 31 along the Kuala Lumpur-Karak highway. Figure 4 shows location of the sample site. Soil samples were obtained from residual soil of weathering grade VI, according to the commonly used weathering classification for igneous and some sedimentary and metamorphic rocks of Little (1969), as shown in Figure 5. The soil was reddish brown in color and consisted mainly of sandy silty clay.
Figure 4. Location map of the soil samples
Figure 5. Typical classification of weathering profile (after Little 1969)
Site B was a cut slope of approximately 40m high along a link road near the Kuala Lumpur International Airport, Sepang, Malaysia, as shown in Figure 4. The slope basically comprise of residual soils of weathered sandstone, overlying schist and quartzite. The soils are generally yellowish brown in color and consist mainly of fine sand, silt and clay. Samples were obtained for soils of weathering grade V to III.
The block samples were then cut to laboratory sizes (50 mm high by 100 mm diameter) using a specially made split body sampler as shown in Figure 6.
Figure 6. Details of split body sampler
TEST RESULT AND DISCUSSION
Site A
A series of triaxial test using the specially fabricated triaxial cell has been carried out to study the shear strength characteristics of unsaturated granitic residual soil of weathering grade VI obtained from site A. These test include unsaturated consolidated drained (CD) tests with suction, of single stage–single suction, multistage–single suction and multi stage – multi suction. Figure 7 outlines the procedures of the multistage–single suction triaxial tests. While Figure 8 outlines the procedures of the multistage–multi suction triaxial tests. For comparison, the conventional saturated consolidated drained (CD) test is also carried out. Summary of the stresses (s3) and pressure (ua , uw) applied are shown in Table 1.
Table 1. Summary of stresses (s3) and pressure (ua , uw) applied
Figure 7. Flow chart for multistage - single suction triaxial testing
Figure 8. Flow chart for multistage – multi suction triaxial testing
In the unsaturated CD test with single stage-single suction, a suction of 50kPa was applied to the soil samples. While in the unsaturated CD test with multistage-single suction, the tests were carried out in three stages in each suction (ua – uw) plane of 50 kPa, 100 kPa and 200 kPa. The values of shear strength parameter, c' and f' obtained from these tests as well as that of the conventional saturated CD test are tabulated in Table 2.
Table 2. Shear strength parameters of Site A sample
The effective angle of friction, f' obtained from all the tests do not vary much, i.e. ranging from 25o to 27o, with an average of 26o. The value of cohesion (c') however is very dependent on the suction, with c' = 49.9 kPa for conventional saturated CD test, to c' equal to 111.4 kPa at suction of 200 kPa.
Using the unsaturated triaxial CD tests with suction data (i.e. the multistage-single suction test data) to plot shear stress t versus (ua – uw), the angle of friction with respect to suction, fb could be found. As shown in Figure 9, average value of fb obtained is 17o, less than the f'.
Figure 9. Shear stress, t, versus suction, (ua – uw) , (unsaturated CD, multistage-single suction)
A multistage-multisuction CD test was also perfomed on the soil sample, with stresses and pressure applied as shown in Table 1. Figure 10 shows the stress-strain curve. The results obtained is shown in Figure 11. Using f' = 26, the values of cohesion, c' obtained were 38.7 kPa, 61.9 kPa and 85.1 kPa respectively for suction (ua – uw) of 50 kPa, 100 kPa and 200 kPa. The values of c' obtained from this test are apparently slightly lower than those of the multistage-single suction CD test.
Figure 10. Stress – strain curve for multistage – multi suction test
Figure 11. Mohr's plot of multistage-multisuction CD test (using f'= 26o)
Multistage triaxial testing is not recommended for tropical residual soils, especially those with unstable fabric liable to collapse (Geoguide 1996). However this seems not to be the case in this test. The values of f' obtained from both the single stage unsaturated CD and multistage are about similar. Besides, it can be quite difficult to have as set of homogeneous sample, due to the soil variation. The principal advantage of the multistage technique (especially those involving suction) over the conventional one includes the reduction of laborious and often delicate task of sample preparation and test set-up, and time taken for the suction consolidation procedure. The drained triaxial test itself is already generally time consuming due to its slow rate of shearing.
Site B
A series of conventional CIU test with pore water pressure measurements were carried on on samples obtained from site B in order to examine the influence of weathering grades on the shear strenght parameters of residual soils. The results obtained are summarized in Table 3.
Table 3. Shear strength parameters of site B samples
using conventional (saturated) CIU test
The results shows an increase in cohesion but a decrease in angle of friction as the soil/rock becomes more weathered, i.e. as the fine content of the soil increases.
Unsaturated multistage-multi suction CD tests were also carried out on the site B samples. The results obtained are shown in Table 4.
Table 4. Shear strength parameters of Site B samples
(unsaturated multi stage-multi suction CD test)
As expected the value of cohesion, c' increases with increase in the matric suction. For a given level of matric suction, the cohesion increases with increase with the soil weathering grade (i.e. from grade III to V). The angle of friction, f' however decreases as the soil becomes more weathered. The angle of friction or change in shear strength with change in suction, fb also increases when the soil becomes more weathered. The values of fb are generally lower than f'.
CONCLUSIONS
From the results of this study, the followings are observed with regards to the shear strength parameters of unsaturated residual soils.
The value of cohesion, c' increases with increase in the matric suction. For a given level of matric suction, the cohesion increases with increase in the soil weathering grade, i.e as the soils/rocks becomes more weathered. The angle of friction, f' however decreases with increase in the soils weathering grade (i.e. as the fines content of the soil increases). The angle of friction or change in shear strength with change in suction, fb also increases when the soil/rock becomes more weathered. The values of fb are generally lower than f'.
REFERENCES
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