Shear Strength Characteristics of Residual Soils in Singapore.” KEYNOTE LECTURE. Proceedings of the International Conference on Prob-lematic Soils, Nottingham, United Kingdom, July 28-30, 2003

1 INTRODUCTION Southeast Asia being in the tropics has a hot and humid climate throughout the year with a high an-nual rainfall. These climatic conditions accelerate the weathering of rocks into residual soils. Residual soils above the ground water table are unsaturated and possess negative pore-water pressures or matric suctions that contribute to the shear strength of soils. Several important unsaturated soil properties of re-sidual soils are soil-water characteristic curve (SWCC), permeability function and unsaturated shear strength parameter φb. As the degree of weath-ering varies with depth, the unsaturated soil proper-ties also vary along the soil profile. Therefore, it is essential to characterize the unsaturated soil proper-ties of residual soils and their relations to the degree of weathering.

Residual soils in Singapore are mainly derived from the weathering of two major geological forma-tions: the Bukit Timah granite and the Jurong sedi-mentary formation (Figure 1). The Bukit Timah granite is acidic igneous rock formed in the Lower to Middle Triassic period (Pitts, 1984). Although

known as granite, it actually varies from granite, adamellite to granodiorite. The Jurong formation is of late Triassic and Lower to Middle Jurassic age. It is sedimentary in origin and the rock types include various types of conglomerate, sandstone and mud-rocks.

The Bukit Timah residual soil varies from silty or

clayey sands to silty or sandy clays but is commonly

Shear Strength Characteristics of Residual Soils in Singapore Rahardjo, H. Leong, E.C. & Rezaur, R. B. School of Civil & Environmental Engineering, Nanyang Technological University, Singapore.

ABSTRACT: Unsaturated soil properties of residual soils from two major geological formations in Singapore were assessed using index property, permeability, multistage consolidated drained triaxial test, consolidated drained direct shear test, constant water content triaxial test and shearing infiltration triaxial tests on speci-mens from the granitic Bukit Timah and sedimentary Jurong formations. Results indicate that index and engi-neering properties of the residual soils at different depths are influenced largely by the pore-size distributions that vary in accordance with the degree of weathering. Degree of weathering of residual soils from the gran-itic Bukit Timah formation appears to be uniform and gradually decreases with depth. Degree of weathering of residual soils from the sedimentary Jurong formation is variable and dependent on different types of parent rock such as mudstone, siltstone and limestone. The angle of internal friction, φ' for residual soils from the granitic Bukit Timah formation was found to be 32o. The highest φb angle for the soil was determined to be 28o at matric suctions lower than 25 kPa. Beyond 25 kPa matric suctions the φb angle decreased to almost zero for matric suctions above 150 kPa. The angle of internal friction, φ' for residual soils from the sedimentary Ju-rong formation was found to be very high (51o). This very high friction angle could be attributed to cementa-tion of soil particles.

Figure 1. Map of Singapore showing geological formations and locations of sampling sites (after Pitts, 1984)

sandy clayey silt (Poh et al., 1985). The residual soils derived from the Jurong sedimentary formation exist as interbedded layers of predominantly clayey silt of medium plasticity, sandy clay and clayey to sandy materials (Chang, 1988).

The objective of this study was to investigate the shear strength characteristics of residual soils from the two major geological formations in Singapore.

2 METHODOLOGY Soil samples for this study were obtained from typi-cal residual soil slopes of two major geological for-mations in Singapore: the granitic Bukit Timah for-mation and the sedimentary Jurong formation. Residual soils of the granitic Bukit Timah formation were taken from a slope at Yishun site while residual soil samples of the sedimentary Jurong formation were taken from NTU-CSE slope located in the Nanyang Technological University campus. The pro-files of the slopes at these two sites are shown in Figure 2. Soil samples were collected from boreholes using a Mazier sampler.

A complete set of samples from a borehole was

taken from each formation. Index property tests were used to identify basic soil properties. Grain size analysis, specific gravity, density and Atterberg limit tests are the main index property tests and the tests were performed in accordance with ASTM (1997) and BS 1377 (1990) procedures.

Permeability of the residual soils was determined for samples from different depths corresponding to different degrees of weathering. Pressure plate tests were conducted to obtain the soil-water characteris-tic curves (SWCC) of the residual soils. Saturated coefficient of permeability was calculated from one-dimensional oedometer test results. Unsaturated co-efficient of permeability function could then be es-tablished using the SWCC and the saturated coeffi-cient of permeability.

Consolidated drained (CD) and constant water content (CW) triaxial tests and CD direct shear tests were performed on undisturbed and compacted soil specimens from the two major geological formations in Singapore.

Triaxial test specimens were prepared from undis-turbed soil samples from Mazier sampler by trim-ming the samples to the required dimensions (diame-ter 50 mm and length 100 mm). Matric suctions were controlled using the axis translation technique (Hilf, 1956) by controlling the pore-air and pore-water pressures of the specimen. To allow the control of pore-water pressure a high air-entry porous ceramic disk of air-entry value of 500 kPa was used.

After saturation, specimens were allowed to con-solidate under an isotropic confining pressure (σ3), a pore-air pressure (ua), and a pore-water pressure (uw). The magnitudes of the selected pressures for consolidation were based on the desired values of net confining stress (σ3−ua) and matric suction (ua−uw). Tests were performed at three different net confining pressures (σ3−ua) of 50, 100, 150 or 200 kPa. A sin-gle specimen was used for each net confining pres-sure. For each specimen a multi-stage test was car-ried out under matric suctions (ua−uw) of 50, 100, 150 and 200 kPa.

The specimens were sheared at a constant strain rate in triaxial compression. A strain rate of 9×10−3 %/min for CW tests and a strain rate of 9×10−4 %/min for CD tests were adopted. The strain rates were found to be sufficient for dissipation of pore-water pressures in CD tests as judged from the uniform water content profiles along the height of the specimens after shearing.

Total and water volume changes were monitored throughout the shearing process. Total volume changes were measured by monitoring the volume of water flowing in or out of the triaxial cell surround-ing the specimen. A correction was applied to allow for the volume change induced by the movement of

Figure 2. Typical weathering profile of residual soil from (a) granitic Bukit Timah formation; (b) sedimentary Jurong formation (from Rahardjo et al. 2000)

the loading ram. Water volume changes were moni-tored by measuring the flow of water from the specimen. Diffused air that accumulated underneath the porous ceramic disk was flushed regularly and measured using a diffused air volume indicator in order to minimize any error in the volume change measurement.

3 TYPICAL SOIL PROFILES & PROPERTIES Typical profiles of residual soils from the granitic Bukit Timah and the sedimentary Jurong formations, based on six-grade weathering classification pro-posed by Little (1969) are shown in Figure 2. The mineralogy of residual soils may vary considerably depending on parent rocks and weathering processes. Index properties depend on the mineralogy of soil and they can reflect the history of weathering (Tuncer and Lohnes, 1977). Typical residuals soil properties of the two major geological formations in Singapore are briefly described in the following sec-tions. 3.1 Specific Gravity The variation of index properties with depth and hence with the degree of weathering of residual soils from the granitic Bukit Timah and the sedimentary Jurong formations are shown in Figures 3 and 4, re-spectively.

The specific gravity of the residual soils from the

granitic Bukit Timah formation range from 2.55 (at greater depths) to 2.78 (at shallower depths).

Specific gravity of residual soils from the sedi-mentary Jurong formation ranges from 2.65 to 2.75 and its value increases slightly with depth (Figure 4). This could be accounted for by the presence of min-erals other than Quartz in deeper layers that have a higher specific gravity than Quartz (Aung, 2001).

3.2 Void Ratio and Total Density Weathering leads to a porous structure due to con-siderable leaching of minerals in the soil. Void ratio appears to decrease with depths (Figures 3 and 4) re-flecting the variation in the degree of weathering. Water and air replace the soluble minerals resulting in a porous structure as rocks are weathered. In the upper layers of residual soils, porosity and void ratio are higher therefore water and air phases occupy more space as compared to the lower layers. As a re-sult, total density is lower near the surface. At deeper depths, porosity decreases resulting in an increase in total density. Therefore, the variation in total density as well as the variation in dry density reflects the variation in degree of weathering. The total density of residual soils from the granitic Bukit Timah for-mation ranges from 1.6 to 2.0 Mg/m3. The total den-sity of residual soils from the sedimentary Jurong formation ranges from 1.9 to 2.5 Mg/m3.

3.3 Atterberg Limits

In general, plasticity index (liquid limit minus plastic limit) of residual soils decreases with depth as the degree of weathering decreases. In addition, natural water content is very close to or less than plastic limit throughout the depth indicating the unsaturated condition of the residual soils. However, the decreas-ing trend of plastic limit and water content with depth is not obvious in the Jurong sedimentary for-mation (Figure 4). This could be due to the less weathering in the soil profiles of the Jurong sedi-mentary formation at this particular site. 3.4 Permeability Typical saturated coefficients of permeability of re-sidual soils from different depths of the granitic Bu-

Figure 3. Index properties of residual soil profile of the granitic Bukit Timah formation (from Aung, 2001)

Figure 4. Index properties of residual soil profile of the sedimentary Jurong formation (from Aung, 2001)

Bukit Timah and the sedimentary Jurong formation are shown in Table 1. The saturated coefficients of permeability of residual soils from the granitic Bukit Timah formation appear to be higher than the satu-rated coefficients of permeability of residual soils from the sedimentary Jurong formation. This could be attributed to the smaller pore sizes of the Jurong residual soils when compared with the pore sizes of the Bukit Timah residual soils.

4 CONSOLIDATED DRAINED TRIAXIAL TESTS

Consolidated drained triaxial tests have commonly been used to obtain shear strength properties of re-sidual soils. Specimens from 3-4m depths of the granitic Bukit Timah formation were used for the CD triaxial tests. The sand and fines contents of the specimens were 53 and 47%, respectively. The liquid and plastic limits were 42 and 28%, respectively and the bulk density was 1.86 Mg/m3. The specimens were classified as silty sand (SM) based on the Uni-fied Soil Classification System (USCS). For the sedimentary Jurong formation, specimens from 4-5m depths were used for the CD triaxial tests. The sand and fines contents of the specimens were 13 and 87%, respectively. The liquid and plastic limits were 21 and 15%, respectively. The specimens were clas-sified as silty clays and clayey silts (CL-ML). The bulk density of the specimens from the Jurong sedi-mentary formation was very high with an average value of 2.24 Mg/m3 indicating a low degree of weathering of the materials. 4.1 Soil-Water Characteristic Curve Soil-water characteristic curve (SWCC) is a measure of the water storage capacity of a soil at various suc-tion values. The ability of a soil to retain water de-creases with increasing soil suction. Therefore, the coefficient of permeability is not a constant in an un-saturated soil but is a function of soil suction be-cause water can only flow through water-filled pores. Suction at which a soil starts to become un-

saturated is called air-entry value. The air-entry value of a soil depends on the soil structure. Its value varies with grain size distribution as well as pore size distribution. The coarser the grain size and the larger the inter-particle and intra-particle pores of a soil, the lower the air-entry value of the soil will be. When a soil becomes more weathered, more clay minerals are formed and the ability of the soil to re-tain water content under high matric suctions in-creases or the air-entry value of the soil increases. Therefore, the SWCC of a soil reflects its pore size and hence the degree of weathering.

The SWCC for the undisturbed residual soil from

the granitic Bukit Timah formation is shown in Fig-ure 5. The SWCCs for the undisturbed residual soils from two different depths of the sedimentary Jurong formation are shown in Figure 6. All the specimens were taken from the same boreholes where the specimens for the CD triaxial tests were retrieved. The SWCC for the granitic Bukit Timah formation does not seem to indicate any clear air-entry value. It can only be deduced that the air-entry value for the residual soil from the granitic Bukit Timah formation

Table 1. Saturated coefficients of permeability of residual soils from different depths

Depth (m) Weathering Soil type ks (m/s)

Granitic Bukit Timah Formation

8.5−9.5 Grade VI Clayey Silt 4.0 × 10−9 16−17 Grade V Silty Sand 5.9 × 10−9

20.5−21.5 Grade V Silty Sand 7.0 × 10−9

Sedimentary Jurong Formation

4.5−5.5 Grade IV Silty Sand 0.5 × 10−9 12−13 Grade IV Silty Sand 1.3 × 10−9 21−22 Grade III Sandy Silt 0.3 × 10−9

Figure 5. Soil-water characteristic curve for undisturbed residual soil from the granitic Bukit Timah formation (from Ong, 1999)

Figure 6. Soil-water characteristic curve for undisturbed residual soil from the sedimentary Jurong formation (from Ong, 1999)

is less 50 kPa since the SWCC (Figure 5) appears to undergo a steep decrease in water content at matric suctions less than 50 kPa. The low air-entry value for the Bukit Timah granitic soil can be attributed to the higher percentage of sand (53%) present in the soil which constitutes larger pore sizes in between the soil particles. As for the residual soil from the sedi-mentary Jurong formation, its air-entry value can be as high as 400 kPa as demonstrated by Lim et al. (1996). The high air-entry value of the Jurong sedi-mentary soil can be attributed to the high fines con-tent of the soil (87%) and the high density of the soil (2.24 Mg/m3) indicating the small pore size distribu-tion of the soil. As a result, the soil remains saturated even at high matric suctions. It could also be re-flected by the fact that the changes in volumetric wa-ter content upon increasing matric suctions are small in the Jurong sedimentary soil (Figure 6) as com-pared with those in the Bukit Timah granitic soil (Figure 5). 4.2 Stress strain characteristics

4.2.1 Granitic Bukit Timah formation Typical results of multistage consolidated drained triaxial tests at a constant net confining stress of 50 kPa and increasing matric suctions on undis-turbed specimens from the granitic Bukit Timah formation are presented in Figure 7. The results are illustrated in the form of graphs of deviator stress, total volume change and water volume change ver-sus axial strain.

Figure 7a shows that the stiffness of the specimen increased and the failure strain decreased at the sub-sequent stages of shearing. The increase of stiffness and the decrease of the failure strain are due to the increase in matric suction at subsequent stages of the multistage test. The specimen also became stiffer af-ter each stage of shearing as only part of the volu-metric strain was recovered when the load was re-leased for the next stage of consolidation.

The specimen also compressed during shearing as noticed in Figure 7b. A comparison of Figures 7b and 7c indicate that the volume of water flowing out of the specimen was greater than the total volume change of the specimen. This implies that the speci-men desaturated as the matric suction was increased at each stage and also during shearing. Therefore, the degree of saturation of the specimen decreased as the test progressed. 4.2.2 Sedimentary Jurong formation Typical results of multistage consolidated drained triaxial tests at a constant net confining stress of 50 kPa and increasing matric suctions on undis-turbed specimens from the sedimentary Jurong for-mation are presented in Figure 8.

The deviator stress versus axial strain curves

(Figure 8a) for soil specimens from the sedimentary Jurong formation show that the specimens exhibited very stiff and brittle behavior. Because of this, multi-stage triaxial tests were very difficult to perform as excessive strains could have caused failure in a brit-tle manner during the early stages of shearing. The strength of the Jurong soil was much higher than the Bukit Timah soil under the same stress conditions. The large difference in soil strength between the two formations can be seen from a comparison between

(a) Deviator stress versus axial strain

(b) Total volume change versus axial strain

(c) Water volume change versus axial strain

Figure 7. Results of multistage consolidated drained triaxial tests on undisturbed residual soil (depth: 3−4 m, UYC50) from the granitic Bukit Timah formation under a net confining stress of 50 kPa and increasing matric suctions (from Ong, 1999)

Figures 7a and 8a. Under a net confining stress of 50 kPa and a matric suction of 100 kPa, the soil from the sedimentary Jurong formation exhibits a much higher peak deviator stress (600 kPa, see Figure 8a) as compared to the soil from the granitic Bukit Ti-mah formation (280 kPa, see Figure 7a). The failure mechanism for soils from the sedimentary Jurong formation was also more like that of rock behavior as a longitudinal crack developed throughout the specimen rather than an inclined failure plane.

The soils from the sedimentary Jurong formation

compressed during shearing (Figure 8b). The magni-tude of compression was small and almost equal to the water volume change during consolidation and shearing as shown in Figures 8b and 8c. This low magnitude of compression and water volume change

was a consequence of the high total density and low initial water content of the soil from the sedimentary Jurong formation. The almost equal magnitudes of the total and water volume changes indicate that the specimens had high degrees of saturation even under high matric suction values. This confirms the finding of Lim et al. (1996) that the air-entry value of the re-sidual soil from the sedimentary Jurong formation can be as high as 400 kPa. 4.3 Mohr Circles and Failure Envelopes

4.3.1 Granitic Bukit Timah formation Figure 9 shows the Mohr circles for three net

normal stresses (50, 100, 150 or 200 kPa) and at each matric suction for soils from the granitic Bukit Timah formation. For residual soils from the granitic Bukit Timah formation, an average angle of internal friction, φ' of 32o is observed in relation to net nor-mal stress for all four matric suctions (Figures 9a to 9d).

From a plot of total cohesion (obtained from the extended Mohr-Columb failure envelope) versus ma-tric suction (Figure 10), the highest φb angle for the undisturbed residual soils from the granitic Bukit Timah formation was determined to be 27.5o at ma-tric suctions lower than 25 kPa. This suggests that the undisturbed residual soils from the Bukit Timah granitic formation desaturates near a matric suction of 25 kPa since the φb angle (27.5o) obtained at a ma-tric suction of 25 kPa is already lower than the φ' an-gle (32o) of the soil. Beyond the matric suction value of 25 kPa the φb angle was found to decrease and the tangent value of φb falls to almost zero for matric suctions above 150 kPa. 4.3.2 Sedimentary Jurong formation

Figure 11 shows the Mohr circles for three net normal stresses (50, 100, 150 or 200 kPa) and at each matric suction for soils from the sedimentary Jurong formations. For residual soils from the sedi-mentary Jurong formation a very high angle of inter-nal friction (φ') of 51o in relation to net normal stress for all four matric suctions is observed (Figures 11a to 11d). Compared to the friction angle of soils from the granitic Bukit Timah formation that contains a higher percentage of sandy materials the friction an-gle observed in the fine-grained residual soils from the Jurong sedimentary formation is exceptionally high. This high internal friction angle can be attrib-uted to the high total density and cementation of specimens for soils from the sedimentary Jurong formation. As noted earlier, the specimens from the sedimentary Jurong formation manifest brittle and rocklike behavior during shearing, indicative of a high degree of cementation. However, a φ' value of 51° seems to be unrealistically high. A possible ex-planation to this exceptionally high value of φ' was

(a) Deviator stress versus axial strain

(b) Total volume change versus axial strain

(c) Water volume change versus axial strain Figure 8. Results of multistage consolidated drained triaxial tests on undisturbed residual soil (depth: 4−5 m, UJC50) from the sedimentary Jurong formation under a net confining stress of 50 kPa and increasing matric suctions (from Ong, 1999)

that although the three specimens tested were from the same sample tube, they had different degrees of cementation leading to different c' values (Toll et al., 2000).

Multi-stage CD triaxial tests conducted by

Hritzuk (1997) on soils from the sedimentary Jurong formation indicated a high effective cohesion but φ' values were typically around 36o. Toll et al., (2000) made an alternative interpretation of the data from the sedimentary Jurong Formation for a matric suc-tion of 50 kPa by assuming that φ' was 36° (typical value from Hritzuk, 1997) as shown in Figures 12 and 13. This resulted in different effective cohesion values for different specimens.

Figure 13 shows the values of total cohesion ob-tained by extending the approach used in Figure 12 to other matric suctions. This alternative approach made by Toll et al. (2000) suggests an almost linear relationship between total cohesion and matric suc-tion over the suction range of 50 to 200 kPa. The φb values are 22o for two specimens and 10o for the third specimen. A backward projection of the linear trends in Figure 13 indicates prediction of the effec-tive cohesion values between 91 and 173 kPa, simi-lar to the results of Hritzuk (1997).

Either of these interpretations could be correct, but clearly each interpretation suggests very different values of φb for residual soils from the sedimentary Jurong formation. These highlight the great difficul-ties in determining soil parameters for residual soils where natural heterogeneity poses uncertainty about similarity of specimens tested (Toll et al., 2000).

5 CONSOLIDATED DRAINED DIRECT SHEAR TEST RESULTS

Consolidated drained direct shear tests require a rela-tively short time of testing due to the thin specimen used in the direct shear box. Consolidated direct shear tests were performed on residual soils from the granitic Bukit Timah formation. The specimens were obtained from 5.5-6.5m depths and the sand and fines contents of the specimens were 7 and 93%, re-spectively. The liquid and plastic limits were 64 and 43%, respectively and the bulk density was 1.8

Figure 9. Extended Mohr-Coulomb failure envelopes for undisturbed residual soils (depth: 3−4 m) from the granitic Bukit Timah formation obtained from multistage consolidated drained triaxial tests under various matric suctions (from Ong, 1999)

Figure 10. Total cohesion versus matric suction for residual soils from the granitic Bukit Timah formation

Mg/m3. The specimens were classified as highly plastic silt (MH).

Figure 14 illustrates the results of multistage con-

solidated drained direct shear tests on undisturbed residual soil specimens from the granitic Bukit Ti-mah formation under a net normal stress of 275 kPa and increasing matric suctions of 25, 50, 100 and 200 kPa

Figure 14a shows the relationship between the shear stress, τ and horizontal displacement during shearing. The shear stress increased with increasing matric suctions. Peak shear stress was mobilized at a smaller horizontal displacement when a higher ma-tric suction was applied to the specimen. This means that the specimen became stiffer with the increase in matric suction.

Figure 14b shows the vertical displacement ver-sus horizontal displacement during shearing of the specimen. The specimen compressed during shearing under matric suctions of 25, 50 and 100 kPa. How-ever, the specimen swelled slightly during shearing under the matric suction of 200 kPa.

Figure 11. Extended Mohr-Coulomb failure envelopes for undisturbed residual soils (depth: 4−5 m) from the sedimentary Jurong formation obtained from multistage consolidated drained triaxial tests under various matric suctions (from Ong, 1999)

Figure 12. Alternative interpretation of data from the sedimentary Jurong formation soil based on an assumed φ' of 36o (from Toll et al., 2000)

Figure 13. Total cohesion versus matric suction for residual soils (depth: 4−5 m) from the sedimentary Jurong formation based on an assumption that φ' is 36o (from Toll et al., 2000)

Figure 14c shows the water volume change be-havior in the soil with respect to horizontal dis-placement during shearing. In general the pore-water drained out from the specimen during the application of matric suction and during the shearing process.

6 CONSTANT WATER CONTENT TRIAXIAL TEST RESULTS

Constant water content triaxial tests were performed on compacted soil specimens from the sedimentary Jurong formation. The constant water content tests can be carried out with a relatively faster strain rate than the rate used in the consolidated drained tests. The specimens used in the tests were taken from 3-4m depths and the sand and fines content of the specimens were 34 and 66%, respectively. The liquid and plastic limits were 36 and 22%, respectively, and the specimens were classified as low plastic silt (ML).

Results of CW triaxial tests carried out under a constant net confining stress of 50 kPa are presented in Figure 15. The specimens were designated accord-ing to their net confining stresses and initial matric suctions. For instance, CW50-400 represents a specimen that was tested under a constant water con-tent conditions at a net confining stress of 50 kPa and an initial matric suction of 400 kPa.

(a) Shear stress versus horizontal displacement

(b) Vertical displacement versus horizontal displacement

(c) Water volume change versus horizontal displacement

Figure 14. Results of multistage consolidated drained direct shear tests on undisturbed residual soil (depth: 5.5−6.5 m, DC275(2)) from the granitic Bukit Timah formation under a net normal stress of 275 kPa and increasing matric suctions (from Wong, 2000)

(a) Deviator stress versus axial strain

(b) Total volume change versus axial strain Figure 15. Results of constant water content triaxial tests on compacted residual soil (depth: 4−5 m) from the sedimentary Jurong formation under net confining stress of 50 kPa and various initial matric suctions (from Ong, 1999)

Figure 15a presents the relationship between the deviator stress and axial strain. Figure 15b illustrates the total volume change of the specimens with re-spect to axial strain. Figure 15a shows that the stiff-ness and brittleness of the residual soils from the sedimentary Jurong formation increased as their ini-tial matric suctions increased. The graph also sug-gests that the shearing behavior of the specimen changed as the initial matric suctions were increased under the same net confining pressure. The speci-mens CW50-170, CW50-200 with low initial matric suctions did not show a distinct peak deviator stress. The higher the initial matric suction of the specimen, the more distinct is the peak deviator stress. Post-peak softening behaviors are observed in specimens with initial matric suctions higher than 200 kPa (Figure 15a).

7 SHEARING INFILTRATION TEST RESULTS

Shearing infiltration tests on residual soils from granitic Bukit Timah formation were performed in a modified triaxial apparatus both under saturated and unsaturated conditions. Shearing infiltration tests were intended to simulate stress paths followed by a soil element in a residual soil slope during rainfall and infiltration. The soil element is essentially under a constant shear stress and only the pore-water pres-sure increases during the infiltration process.

The triaxial specimen used in this study was 50 mm in diameter and 100 mm in height. Miniature pore-water pressure transducers were installed at three different heights of the soil specimen (top, middle and bottom) and at the water compartment at the base for pore-water pressure measurements. Modification and ancillary works on the triaxial ap-paratus needed to position and install the miniature pore-water pressure transducers at different heights of the soil specimen are described in detail by Wong et al. (2001).

Soil specimens for the shearing infiltration tests were taken from the granitic Bukit Timah formation at depths of 1-2 m for the saturated shearing infiltra-tion test and 7-8 m for the unsaturated shearing infil-tration tests. The sand and fines contents of the specimens from 1-2 m depths were 10 and 90%, re-spectively. The specimens had liquid and plastic lim-its of 65 and 43%, respectively and the specimens were classified as highly plastic silt (MH). The specimens from 7−8 m depths had 100% fines con-tent and their liquid and plastic limits were 74 and 48, respectively and the bulk density was 1.8 Mg/m3. The specimens were classified as highly plastic silt (MH).

Shearing infiltration stage consisted of three parts, i.e. shearing, creep monitoring and infiltration. Upon attaining equilibrium, the specimen was sheared to a pre-determined stress level (i.e. 85 to 90 % of the

peak strength) at a constant axial strain rate. The force actuator was programmed to maintain the de-viator stress at the pre-determined stress level for at least ten hours. This was to ensure that any increase in axial strain due to creep was completed before starting the infiltration. Failure was considered to have occurred when there was a sudden increase in axial strain.

Results from the shearing infiltration tests indi-cated that soil specimens failed during infiltration due to an increase in pore-water pressure or a de-crease in matric suction. Shearing infiltration tests results are presented in Figures 16 and 17. Figure 16 shows results of the shearing infiltration under a saturated condition. A cell pressure, σ3 of 300 kPa was used in the test. Figure 16a shows the relation-ship between the deviator stress, (σ1–σ3) and axial strain, εy. Figure 16b illustrates the changes in the axial strain, εy and pore-water pressures with time throughout the shearing-infiltration stage. During shearing, the linear slope of the axial strain in the graph indicates a constant axial shearing rate (0.054 mm/h). During creep monitoring, the specimen com-pressed slightly in the beginning as indicated by the small and slow decrease in the axial strain. However, the axial strain stabilised eventually and this shows that the effect of creep was negligible. The axial strain decreased (specimen compressed) once infil-tration commenced and the rate of decrease in axial strain increased with time. The axial strain decreased drastically when failure took place.

The pore-water pressures from the three transduc-ers (top, middle and bottom) remained constant dur-ing shearing. This proves that the drained shearing rate used was appropriate under the saturated condi-tion and the shearing did not result in any build up of excess pore-water pressure. The pore-water pres-sures also remained constant during the creep moni-toring. Once infiltration started, the pore-water pres-sures increased. However, the base pore-water pressure increased faster than the 'top', 'middle' and 'bottom' pore-water pressures. The 'top', 'middle', 'bottom' and 'base' pore-water pressures increased before failure occurred. Once failure due to infiltra-tion occurred, all the pore-water pressures decreased first and then increased drastically.

Figure 16c shows the water volumetric strain, εw versus elapsed time. Water volumetric strain is de-fined as the percentage of water volume change rela-tive to the initial total volume of the specimen. The decrease in water volumetric strain during shearing indicates that the pore-water flowed out from the specimen. The increase in water volumetric strain during infiltration shows that water was injected into the specimen.

Figure 17 shows the results from a specimen un-der 50 kPa net confining pressure and 50 kPa matric suction during a shearing-infiltration test. The cell

pressure, σ3 used was 400 kPa. The behaviours were similar to the ones observed in the saturated condi-tion. The base and mid-height matric suctions were measured as 13 kPa and 21 kPa, respectively at fail-ure. This suggests that the pore-water pressure at the base was higher than that of the mid-height and there was an upward water flow from the base to the top due to the pressure gradient across the height of the specimen.

(a) Deviator stress, (σ1–σ3) versus axial strain, εy

(b) Axial strain, εy and pore-water pressures versus elapsed time

(c) Water volumetric strain, εw versus elapsed time

Figure 16. Results of shearing-infiltration test on soil specimen (depth: 1.0−2.0 m, TI100−0) from granitic Bukit Timah formation under 100 kPa effective confining pressure and saturated condition during shearing-infiltration stage (Wong et al., 2001)

(a) Deviator stress, (σ1–σ3) versus axial strain

(b) Axial strain, εy and base and mid-height matric suctions versus elapsed time

(c) Water volume strain, εw versus elapsed time

Figure 17. Results of shearing infiltration test on soil specimen (depth: 7.0−8.0 m, TI50−50(2)) from granitic Bukit Timah formation under 50 kPa net confining pressure and 50 kPa matric suction during shearing-infiltration (Wong et al., 2001)

8 CONCLUSIONS Results of index and engineering property tests indi-cate that the variation in the index and engineering properties of the residual soils at different depths are influenced largely by the pore-size distributions that vary in accordance with the degree of weathering.

Consolidated drained (CD) and constant water content (CW) triaxial tests under a constant net con-fining stress and controlled matric suctions, and con-solidated drained direct shear tests were performed to determine saturated and unsaturated shear strength parameters of residual soils from the two major geo-logical formations in Singapore.

The angle of internal friction, φ' for residual soils from the granitic Bukit Timah formation was found to be 32o. The highest φb angle for the soil was de-termined to be 28o at matric suctions lower than 25 kPa. Beyond 25 kPa matric suctions the φb angle decreased and dropped to almost zero for matric suc-tions above 150 kPa.

The angle of internal friction, φ' for residual soils from the sedimentary Jurong formation was found to be very high at 51o. This very high friction angle could be attributed to the high density and cementa-tion of the specimens. An alternative interpretation was also considered based on the results of the satu-rated tests which showed an average φ' of 36°. This interpretation suggests high effective cohesion val-ues between 91 kPa and 173 kPa, consistent with other test results from the same site. The alternative approach indicated φb values of 10−22o for the resid-ual soils from the sedimentary Jurong formation.

ACKNOWLEDGMENTS This work forms a part of a project funded by the National Science and Technology Board of Singa-pore (NSTB 17/6 16: Rainfall-induced slope fail-ures) and by the Ministry of Defence of Singapore (Research Project No: PTRC-CSE/LEO/99.02). The assistance from the Geotechnics Laboratory staff, School of Civil & Environmental Engineering, NTU, in carrying out this research is gratefully acknowl-edged.

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