Stress‑Strain Behavior and Shear Strength of Unsaturated Residual Soil from Triaxial Tests

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Stress‑strain behavior and shear strength of unsaturated residual soil from triaxial tests

Leong, Eng Choon; Rahardjo, Harianto; Nyunt, T. T.

2012 https://hdl.handle.net/10356/96866

© 2011 Kasetsart University. This paper was published in Asia‑Pacific Conference on Unsaturated Soils: Theory and Practice and is made available as an electronic reprint (preprint) with permission of Kasetsart University. The paper can be found at the following official DOI: [http://www.unsat.eng.ku.ac.th/Proceeding/index.html]. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper is prohibited and is subject to penalties under law.

Downloaded on 06 Oct 2021 13:15:50 SGT Unsaturated Soils: Theory and Practice 2011 Jotisankasa, Sawangsuriya, Soralump and Mairaing (Editors) Kasetsart University, Thailand, ISBN 978-616-7522-77-7 Stress-strain Behavior and Shear Strength of Unsaturated Residual Soil Stress-strainfrom Triaxial behavior Tests and shear strength of unsaturated residual soil from triaxial tests T.T. Nyunt, E.C. Leong & H. Rahardjo School of Civil and Environmental Engineering, Nanyang Technological University, Singapore, [email protected], Nyunt, E.C. Leong [email protected], & H. Rahardjo [email protected] School of Civil and Environmental Engineering, Nanyang Technological University, Singapore, [email protected], [email protected], [email protected]

ABSTRACT: Multi-stage triaxial compression tests were performed on undisturbed samples of residual soils from Bukit Timah Granite formation in Singapore. Tests were carried out under two conditions, namely, consolidated undrained test (CU) (i.e. both pore-air and pore-water pressure were not allowed to drain during shearing) and constant water content test (CW) (i.e. drained pore-air but undrained pore-water condition during shearing). Stress-strain relationships and shear strength of unsaturated soils were studied at different initial matric suctions of 50 kPa, 100 kPa, 200 kPa, and 400 kPa, by performing CU and CW tests. The variations of matric suction during shearing at two different test conditions (CU and CW tests) are also presented. From the experiments, it was observed that the stress-strain relationship from CU test is less stiff than that from CW test. A single relationship of total cohesion intercept with matric suction was obtained from CU and CW tests.

KEYWORDS: stress-strain behavior; shear strength; unsaturated residual soil; matric suction

1 INTRODUCTION Location of sampling site Residual soils cover about the two-thirds of the land area of Singapore. The groundwater tables in these residual soils are well below the ground sur- face and thus the bulk of the residual soils are unsaturated (Leong et al. 2003). The residual soils are derived from the two formations, namely: Bukit Ti- mah Granite residual soils and Jurong Formation se- Legend Old Alluvium Kallang Formation dimentary residual soils. Figure 1 depicts the geo- Jurong Formation residual soils Tekong Formation logical map of Singapore. The Bukit Timah Granite Bukit Timah Granite residual soils Gombak Norite is one of the oldest formations in Singapore and was Sajahat Formation formed approximately 200 to 250 million years ago. The heavily weathered residual soils were formed in Figure 1. Geological map of Singapore with location of sam- the top portions of the Bukit Timah Granite up to pling site. depth of 60 m. Generally, residual soils from Bukit Timah Granite consist of reddish to yellow brown clayey soils. The Jurong formation was formed consolidated drained test (CD),constant water con- about 100 to 200 million years ago. The upper por- tent test (CW), consolidated undrained test with pore tions of the Jurong Formation were heavily wea- pressures measurement (CU) and unconfined com- thered and the residual soils are found approximate- pression test (UC) (Fredlund and Rahardjo 1993). In ly up to depth of 30 m. The residuals soils of Jurong this study, CU (i.e. undrained pore-air and pore- Formation consist mainly of silty clays. In this pa- water pressure condition) and CW (i.e. drained pore- per, the stress-strain relationship and shear strength air pressure and undrained pore-water pressure con- of unsaturated undisturbed residual soils from Bukit dition) triaxial tests were performed to investigate Timah Granite formation are presented. The location the stress-strain relationship and shear strength of of the sampling site is indicated in Figure 1. undisturbed residual soils under different drainage Shear strength parameters for unsaturated soils conditions in terms of pore-air phase. can be obtained by performing triaxial tests, namely:

221 2 SHEAR STRENGTH THEORY FOR Pneumatic Servo UNSATURATED SOILS Actuator

Fredlund et al. (1978) proposed an extended Mohr- Air Pressure Supply Coulomb failure envelope for unsaturated soil as fol- lows: LVDT b Submersible LVDT τ f = c'+ (σ − u a )f tan φ' + (u a − u w )f tan φ (1) Submersible load cell Soil specimen

Local displacement transducer Pore‐water pressure transducer τ ’ φ ’ Proximity transducer where f is shear stress at failure, c and are effec- Control & Data Acquisition tive stress shear strength parameters obtained from Pore‐air pressure transducer System saturated test, (σ-ua)f is net normal stress at failure, b Computer (ua-uw)f is matric suction at failure and φ is angle describing increase in shear strength with matric Amplifier & signal suction. Equation 1 can be written in two- conditioners Figure 2. Schematic drawing of triaxial test set-up (modified dimensional representation as: from Leong et al. 2006).

parameter B obtained was greater than 0.96 for the τ f = c + (σ − u a ) f tan φ' (2) soil specimens during saturation. Near or full saturation was assumed when pore-water pressure parame- b where, c = c'+ (u a − u w ) f tan φ (3) ter B was greater than 0.96 (Head 1980). After saturation, the soil specimens were isotropi- cally consolidated to the in situ effective confining pressure and subsequently at the required net confin- 3 TEST SET-UP ing pressure (σ3-ua) and matric suction (ua-uw) via the axis-translation technique (Hilf 1956). For the A modified triaxial apparatus was used to study the application of matric suction, a constant air pressure stress-strain relationship and shear strength of unsa- of 450 kPa was supplied at the top of the soil speci- turated residual soil. The triaxial apparatus is men through a porous disk and water pressure was equipped with a 3 kN submersible load cell, an ex- controlled via a digital pressure volume controller ternal linear variable differential transformer (DPVC) through a 5-bar high air entry ceramic disk. (LVDT), a submersible LVDT, and a pair of local The amount of water flowing out of the soil speci- displacement transducers (LDT) for axial strain men during matric suction equalization was moni- measurement at different strain levels, a pair of tored via DPVC. The matric suction equalization proximity transducers for radial strain measurement was assumed to have been completed when the and pressure transducers for pore-air and pore-water amount of water flowing out of the soil specimen pressure measurements. A 5-bar high air entry ce- had leveled off. ramic disk was fixed in the bottom pedestal using Multi-stage shearing was adopted to study the epoxy for unsaturated soils testing. Figure 2 shows stress-strain behavior and shear strength of the un- the modified triaxial apparatus used in this study. disturbed residual soil at different initial matric suctions. Shearing was carried out in four stages at initial matric suctions of 50 kPa, 100 kPa, 200 kPa and 4 TEST PROCEDURE 400 kPa under net confining pressure of 50 kPa and constant air pressure of 450 kPa. At the end of each Soil specimens of 50 mm diameter and 100 mm matric suction equalization stage, the specimen was height were trimmed from 70 mm diameter undis- sheared until failure was imminent and unloaded be- turbed residual soil samples. The undisturbed resi- fore imposing the next required matric suction. dual soil samples were obtained using Mazier sam- The soil specimens were tested under two differ- pling. The residual soil specimens used for CU and ent test conditions: consolidated undrained test (CU) CW tests were from depths of 2 to 3 m. Table 1 and constant water content test (CW). A constant summarizes the basic properties of the residual soils. strain rate of 0.5 mm/min was used for both CU and The average initial water content and the average in- CW tests. In CU test, both pore-air and pore-water itial degree of saturation of the undisturbed residual pressure were not allowed to drain during shearing soil sample were 43% and 90%, respectively. and in CW test, air was allowed to flow freely, how- The soil specimens were saturated in the triaxial cell ever, pore-water pressure was undrained during prior to consolidation to ensure identical initial wa- shearing. The variations of pore-air and pore-water ter content in the specimens. The pore-water pressure during shearing were recorded. At the end

222 Table 1. Basic properties of residual soils from Bu- kit Timah Granite Formation 800 Soil properties Value Borehole I-AMA2 600 Depth (m) 2.0-3.0

(kPa) a w i Liquid limit (%) 45 (u ‐u ) = 400 kPa Plastic limit (%) 30 400 stress

Plasticity index (%) 15 (ua‐uw)i = 200 kPa Sand (%) 46 (ua‐uw)i = 100 kPa Silt (%) Deviator 200 45 (ua‐uw)i = 50 kPa Clay (%) 9 Specific gravity 2.74 ML (Low 0 Unified Soil Classification System (USCS) plasticity 0 5 10 15 20 25 silt) Axial strain (%) of the test, water content was measured to obtain the (a) CU test variation of water content with respect to the im- posed matric suction.

800 5 RESULTS AND DISCUSSIONS

Stress strain relationship and shear strength of the 600 (kPa) undisturbed residual soil specimens from CU and (ua‐uw)i = 400 kPa CW tests are presented. The stress-strain relation- stress ship at initial matric suctions, (ua-uw)i, of 50 kPa, 400 100 kPa, 200 kPa and 400 kPa for CU and CW tests (ua‐uw)i = 200 kPa

are shown in Figure 3. Peak deviator stress increases Deviator (ua‐uw)i = 100 kPa with initial matric suction for both CU and CW tests. 200 (ua‐uw)i = 50 kPa The strain at which the soil specimen reaches the peak deviator stress decreases with initial matric suction for both CU and CW tests. 0 The stress-strain relationships obtained for CU 0 5 10 15 20 25 test exhibit less stiff behavior as compared with that for CW test. Moreover, the values of peak deviator Axial strain (%) stress obtained from CU tests are lower than the corresponding values from CW tests. The differences (b) CW test become more significant with higher initial matric Figure 3. Stress-strain relationship at initial matric suctions of suctions. This behavior can be explained by differ- 50 kPa, 100 kPa, 200 kPa and 400 kPa. ences in drainage condition of pore-air phase during shearing. In CU test, excess pore-air pressure was at failure were very close for both CU and CW tests undrained during shearing resulting in higher void with the same initial matric suction. In addition, the ratio at failure under lower (σ3-ua)f compared with excess pore-air pressures at failure in CU tests at dif- the corresponding CW test where pore-air pressure ferent initial matric suction were observed to be sim- was drained and thus, the stress-strain relationship ilar as shown in Table 2. Therefore, matric suction at for CU test was less stiff. In addition, the resulting failure, (ua-uw)f, for CU test was higher than that for higher void ratio at failure due to excess pore-air the corresponding CW tests. Similarly, net confining pressure that developed in the CU test may have in- pressure at failure, (σ3-ua)f, in CU test was lower fluenced the decrease in peak deviator stress com- than that for the corresponding CW test. pared with the corresponding CW test. The effective shear strength parameters (c’and φ ’) The variations of matric suction with axial strain for the saturated residual soils are c’= 22 kPa and φ ’ for CU and CW tests at different initial matric suc- = 33o obtained in another study. The extended tions are plotted in Figure 4. Table 2 summarizes the Mohr-Coulomb failure envelopes for the CU and values of peak deviator stress, pore-air pressure, CW tests were plotted using Equation 2 by drawing pore-water pressure, and matric suction at failure for the tangent line to the Mohr circles of the CU and CW tests. The excess pore-water pressures

223

Table 2. Summary of deviator stress, pore pressures and matric ‐50 suction at failure (ua‐uw)i = 50 kPa (ua-uw)i (kPa) (ua‐uw)i = 100 kPa 50 50 100 200 400 (ua‐uw)i = 200 kPa

(kPa) (σ1-σ3)f (kPa) 156 226 319 469

u (kPa) 472 465 470 468 150 CU test af

suction u (kPa) 467 427 360 92

(ua‐uw)i = 400 kPa (ua-uw)f (kPa) 5 39 110 377 250 matric (σ1-σ3)f (kPa) 159 230 332 518 of

CW uaf (kPa) 450 450 450 450 350 test uwf (kPa) 450 427 362 92

Variation (ua-uw)f (kPa) 0 23 88 358 450 0 5 10 15 20 25 Axial strain (%) 800 (ua‐uw)i c (kPa) (a) CU test (kPa) 50 22 600 100 39 200 68 ‐50 400 108 ‐50 (ua‐uw)i = 50 kPa 400 (kPa) 50 (ua‐uw)i = 100 kPa τ (kPa) 50

(kPa) (ua‐uw)i = 200 kPa

150 suction

200 150 suction

matric 250 (ua‐uw)i = 400 kPa of matric

250 of

0 350 0 200 400 600 800 Variation 350 σ‐ua (kPa) Variation 450 (a) CU test 450 0 5 10 15 20 25 0 5 10 15 20 25 800 Axial strain (%) (ua‐uw)i Axial strain (%) c (kPa) (kPa) (b) CW test 50 14 600 100 30 Figure 4. Matric suction versus axial strain at different initial 200 58 matric suctions of 50 kPa, 100 kPa, 200 kPa and 400 kPa. 400 110

(kPa) 400

unsaturated soil specimens at different initial matric τ suctions with slope of φ ’. Peak deviator stress was used to plot the Mohr circle to obtain the extended failure envelopes for unsaturated soils. Figure 5 200 shows the Mohr circles with total cohesion intercepts for CU and CW tests. The values of total cohesion intercept increase non-linearly with matric suc- 0 tions. Similar non-linear behavior of total cohesion 0 200 400 600 800 intercept with matric suction has been reported by σ‐ua (kPa) Gan et al. (1988) and Thu et al. (2006). b The φ angle for CU and CW tests were obtained (b) CW test by plotting the total cohesion intercept with matric suction at failure as shown in Figure 6. The values Figure 5. Mohr circles at different initial matric suctions of of φ b angle obtained from CU test are close to the 50 kPa, 100 kPa, 200 kPa and 400 kPa.

224 φ b values obtained from CW tests. For non-linear variation of φ b angle with matric suction, Equation 3 1.5 can be written as:

(u a − u w )f = + φ b − (4) c c' ∫ tan d(u a u w )f '

0 φ 1

/ b

or where b o φ /φ' 0.5 w/w w/w ‐CU b o tan φ = f [](u a − u w )f (5) w/wo‐CW

0 Figure 6 shows the calculated and measured total 0 1 100 10,000 cohesion intercept with matric suction for CU and CW tests. Figure 6 also shows that the total cohesion ua‐uw (kPa) intercept with matric suction relationship from CU b and CW tests can be represented by a single curve. Figure 7. Comparison of normalized φ angle and water content with matric suction. The initial φ b angle obtained is 33 same as φ ’ and b approaches 0 at high matric suction. The φ angle was normalized with φ ’ and compared with the 6 CONCLUSION normalized water content, w/w , (where w is the o o water content at low matric suction) obtained from Stress-strain relationship and shear strength of unsa- the experiments as shown in Figure 7. It was ob- b turated undisturbed residual soil from Bukit Timah served that the ratio of φ /φ ’ with matric suction re- Granite formation were studied by performing CU lationship has similar trend as the normalized water and CW tests. From the experimental results, it was content (i.e. w/w ) with matric suction relationship. o observed that the stress-strain relationship from CU

test showed less stiff behavior than the correspond-

ing CW test. A single curve can be used to represent

the relationship of total cohesion intercept with ma- 200 tric suction for CU and CW tests. Non-linear rela- b CU test tionship of φ angle with matric suction was observed and it has similar trend as the water content CW test with matric suction relationship. 150

(kPa) φ ’=33

ACKNOWLEDGEMENTS intercept

100

The work described in this paper is supported under research grant PTRC–CEE/DSTA/2006.01. The first cohesion

50 author gratefully acknowledges the research scholar-

Total ship from Nanyang Technological University, Sin- gapore and financial support from PTRC– CEE/DSTA/2006.01. 0 0 200 400 600

u ‐u (kPa) a w

Figure 6. Total cohesion intercepts versus matric suction for CU and CW tests.

225 REFERENCES

Fredlund, D. G., Morgenstern, N. R., and Widger, R. A. (1978). "The shear strength of unsaturated soils." Canadian Geotechnical Journal, Vol. 15(3), pp.313-321. Fredlund, D. G., and Rahardjo, H. (1993). Soil Mechanics for Unsaturated Soils, Wiley, NewYork. Gan, J., Fredlund, D. G., and Rahardjo, H. (1988). "Determination of the Shear Strength Parameters of an Unsaturated Soil Using the Direct Shear Test." Canadian Geotechnical Journal, Vol. 25(3), pp. 277-283. Head, K. H. (1980). Manual of Soil Laboratory Testing, Effective Stress Tests, Vol. 3, Pentech Press. Hilf, J. W. (1956). "An Investigation of Pore-Water Pressure in Compacted Cohesive Soils," PhD dissertation, Technical Memorandum No. 654, U. S. Department of the Interior, Bureau of Reclamation, Design and Construction Division,Denver. Leong, E. C., Cahyadi, J., and Rahardjo, H. (2006). "Stiffness of a Compacted Residual Soil." Geotechnical Special Pub- lication, No 147, Vol 1, 1169-1180. Leong, E. C., Rahardjo, H., and Tang, S. K. (2003). "Charac- terisation and Engineering Properties of Singapore Residual Soils." Proc., International Workshop on Characterisation and Engineering Properties of Natural Soils, Singapore, Vol 2, 1279-1304. Thu, T. M., Rahardjo, H., and Leong, E. C. (2006). "Shear Strength and Pore-Water Pressure Characteristics during Constant Water Content Triaxial Tests." Journal of Geotechnical and Geoenvironmental Engineering, Vol. 132(3), pp. 411–419.

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