Suction-Monitored Direct Shear Test on Unsaturated Sand and Sand-Kaolin Clay Mixture

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1 Suction-Monitored Direct Shear Test on Unsaturated Sand and Sand-Kaolin Clay Mixture Purwana, Y.M Sebelas Maret University, Indonesia address Nikraz, H Curtin University, Australia ABSTRACT: One of the common devices for unsaturated soil strength testing is suction-controlled direct shear. In this device, suction is adjusted by controlling water pressure and air pressure during the test. Suction was generated indirectly by adjusting the specimen s water content to the desired value. During the test, suction was monitored using the attached tensiometer. Thus in this method, pore water and pore air pressure control is no longer required. The tests were carried out using sand and sand-kaolin clay mixtures in 95:5 and 90:10 proportions. The results indicates that shear strength with respect to matric suction exhibits a bilinear envelope with an initial value of b is higher than the effective internal friction. Despite the suction capacity of the tensiometer was relatively low, this device was effective for low air entry soils such as sand or sand with relatively small portion of fine grained material. Keywords: Sand-kaolin clay mixtures, suction-monitored direct shear test, unsaturated shear strength ABSTRAK: Salah satu alat yang umum digunakan ntuk pengujian kekuatan tanah tak jenuh adalah alat geser langsung yang dikendalikan dengan suction. Dalam metode ini, suction diberikan dengan mengontrol tekanan air dan tekanan udara selama pengujian. Suction dihasilkan secara tidak langsung dengan menyesuaikan kadar air spesimen kepada nilai yang diinginkan. Selama pengujian, tekanan suction dipantau dengan menggunakan tensiometer yang terpasang pada alat. Jadi dalam metode ini, pengendalian tekanan air pori dan tekanan udara tidak lagi diperlukan. Pengujian dilakukan dengan menggunakan pasir dan campuran pasir kaolin dan tanah liat dengan proporsi 95:5 dan 90:10. Hasil penelitian menunjukkan bahwa hubungan kekuatan geser dengan suction ditunjukkan oleh garis bilinear dengan nilai awal b lebih tinggi dari sudut geser dalam efektif. Meskipun kapasitas suction dari tensiometer relatif rendah, alat ini efektif untuk tanah dengan nilai low entry point yang rendah seperti pasir atau pasir dengan campuran sedikit bahan berbutir halus. Kata kunci: Campuran pasir-kaolin, uji geser langsung yang dikendalikan oleh suction, kuat geser tanah tak jenuh 1. INTRODUCTION In soil laboratory testing, the unconsolidated-undrained, consolidated-undrained and consolidated-drained concept of triaxial testing is very common in practice. Direct shear test in particular uses consolidateddrained condition by controlling shearing rate after consolidation on saturated specimen. Both triaxial and direct shear tests use saturated soil as specimen. However, most of the near-surface soils are in unsaturated condition due to the present of air bubbles in the voids. In such conditions, the effective stress concept is not applicable and the unsaturated soil approach must be used instead. One of the main parameters used in unsaturated soil is suction. The role of this parameter is as very important as pore water pressure measurement is required for the effective stress concept to be used in saturated soil mechanics (Houston et al., 1994). The aim of this study is to review the use of unsaturated direct shear devices and to reintroduce a relatively new method of direct shear for unsaturated soil testing. 20

2 Shear strength (kpa) Purwana & Nikraz / Jurnal Geoteknik HATTI IX(1)(2014) ISSN TESTING METHODS AND SHEAR STRENGTH BEHAVIOUR One of the common devices for unsaturated soil strength testing is suction-controlled direct shear. In this device, suction is generated by controlling water pressure and air pressure during the test. Unsaturated shear strength uses two state stress variables, i.e., ( - u a ) and (u a u w ), and is expressed as: ff = [c + ( -u a ) f tan ] + [(u a -u w ) f tan b ] (1) where c is effective cohesion, ( - u a ) f is net normal stress, is effective angle of internal friction, (u a u w ) f is the matric suction, and b is angle of shearing resistance due to the contribution of matric suction (Fredlund and Rahardjo, 1993). It can be observed from the equation that unsaturated shear strength is a combination of strength in a saturated condition and the additional contribution due to matric suction. The first (saturated strength) can be obtained from the conventional shear strength testing methods such as triaxial or direct shear tests, whereas the second is obtained by unsaturated shear strength testing. The plot of matric suction versus shear strength from the equation forms a linear failure envelope. Further studies on various types of soil indicate that the Eqn. (1) is valid in the very low range of suction between zero and air entry value (AEV). In this range, practically there is no air bubble in the soil. Beyond the AEV, the strength variation with respect to matric suction is nonlinear. The nonlinearity of failure envelope has been studied by Donald (1957) and Escario and Saez (1986) using suction-controlled direct shear. Initially, the angle b was relatively high, and gradually decreased as the matric suction increased. For high range of suction Gan et al., (1998) found that initially the angle b was the same as the effective friction angle. Furthermore, Vanapalli et al. (1996) carried out unsaturated shear test on different type of soil using high matric suction-controlled direct shear, and found that the nonlinearity may be different from one type of soil to another (Figure. 1). Sand exhibits a high decreasing rate of b, while plastic clays may exhibit a low decreasing rate. However in many cases, the initial value of b is higher than (e.g. Likos et al., 2010; Nam et al., 2011). Likos (2010) stated that the higher value of b may be caused by the dilation phenomenon in the development of shear strength. Linear regime Non-linear regime b = b < Plastic clays Silts/clay s Sands Matric suction (kpa) Figure 1 Typical failure envelope of different soils in unsaturated condition (modified after Vanapalli et al., 1996) 3. SUCTION-MONITORED DIRECT SHEAR TEST 3.1 General A relatively new and simpler technique for unsaturated direct shear testing is the suction-monitored direct shear test. The method was firstly developed by Tarantino and Tombolato (2005). They modified the 21

conventional direct shear by attaching high capacity tensiometers on the top cap of the shear box in such a way that matric suction can be monitored during the test.

Suction of the specimen was generated indirectly by adjusting the specimen s water content to the desired value. Jotisankasa et al.

3 conventional direct shear by attaching high capacity tensiometers on the top cap of the shear box in such a way that matric suction can be monitored during the test. This method seems to be simpler due to the fact that controlling of pore water and pore air pressure is no longer required. Suction of the specimen was generated indirectly by adjusting the specimen s water content to the desired value. Jotisankasa et al. (2010) developed a suction-monitored direct shear device by using a low capacity tensiometer (90 kpa). The device has been utilized in Thailand on residual soil and also on sands in Perth, WA for unsaturated soil studies (Jotisankasa and Mairaing, 2010; Purwana et al., 2011). The use of suction-monitored direct shear apparatus in the testing of unsaturated soil is still relatively new, and as literature about this method is still scarce, further study is required. 3.2 Setup and installation Figure 2 shows the cross sectional illustration of suction-monitored direct shear. The tensiometer was made of 16 mm-1 bar air entry ceramic disk, transparent acrylic tube, and pressure sensor. The tensiometer was attached to the shear box through the 18 mm hole made on the top cap, and connected to data logger or readout unit so that suction measurement together with loading and displacement can be recorded continuously during the test. Figure 3 shows the photograph of installation of tensiometer on the top cap of shear box. Figure 2. Cross sectional of suction-monitored direct shear (Purwana et al.,2011) Figure 3. Installation of tensiometer on the top cap of shear box 22

4 4. TESTING PROGRAM 4.1 Specimen preparation Three types of artificial specimens of sand and sand-kaolin clay mixture of 95:5 and 90:10 proportions by weight were prepared. The sieve analysis and Atterberg limits test on sand indicated that the specimen consisted of % fine and medium sand with only 0.26 % of silt and clay. Based on Unified Soil Classification System (USCS) this specimen was classified as poorly-graded sand (SP). The liquid limit and plasticity index of kaolin clay were 58 and 27 respectively. The specific gravity of sand and kaolin clay was 2.58 and 2.63 respectively. The mixture of two or more materials with the different index properties may produce a material with a new property. The bulk specific gravity of a new material of 95:5 and 90:10 mixtures were and respectively. For the simplicity, the specific gravity of 2.63 was applied to all type of specimens. The Atterberg limit tests indicated that the 95:5 mixture was non-plastic material whereas liquid limit and plastic limit of 90:10 mixture were 21.3 and 5.9 respectively. The standard proctor test was conducted using ASTM D 698 to obtain the compaction characteristic of the mixtures. Prior to compaction, all specimens were mixed with distilled water and put in a sealed container for curing for at least 72 hours in order to achieve moisture equilibration. After equilibration, the specimens were poured layer by layer into the shear box, and immediately compacted to achieve the targeted maximum dry density (MDD), optimum moisture content (OMC) and the relevant void ratio, e. These three parameters were used as references when creating the compacted specimens for the direct shear test in both saturated and unsaturated conditions. The results are shown in the Table 1. Table 1. Specimen compaction characteristics Specimen MDD (kn/m 3 ) OMC (%) Void ratio (e) Sand :5 mix :10 mix Kaolin clay Saturation of tensiometer Prior to attachment to the shear box, a tensiometer was saturated by submerging it in the distilled water and vacuumed using desiccators and a vacuum pump. The water reservoir of the tensiometer was made completely free of air bubbles, as the presence of even a small air bubble would have affected the performance of the tensiometer by increasing the response time. Generally, 2-3 hours of vacuuming with 90 kpa is enough for saturation. 4.3 Calibration of tensiometer Once good contact between the specimen and ceramic disk was achieved, interaction between the pore water in the soil and water in the tensiometer reservoir commenced. Tensile stress developed from the pressure difference between the soil in an unsaturated condition and the tensiometer in a saturated condition. Water in the tensiometer was attracted by the soil through the membrane until the pressure in the soil was equal to the pressure of the water in the tensiometer. The tensile stress of water was then read by the assembled sensor transducer, and transferred to the digital readout as voltage. Calibration was required to obtain the relationship between the pressure in the tensiometer and the voltage on the digital readout. The calibration was performed by imposing different values of negative pressure on the tensiometer. The negative pressure was generated by a vacuum pump, while the corresponding voltage was recorded digitally using a multimeter. Figure 4 shows the typical calibration curves of the tensiometer. 23

5 Voltage Purwana & Nikraz / Jurnal Geoteknik HATTI IX(1)(2014) ISSN y = x R² = Suction (kpa) Figure 4. Typical results of calibration curve for tensiometer 4.4 Shearing Test The unsaturated soil strength test consisted of two sets of tests: one set on saturated specimens and one on unsaturated specimens. A series of shear tests on saturated specimen were carried out following the ASTM D 3080 to obtain the effective stress parameters c and. Specimens were compacted directly in the shear box to obtain specified compaction characteristic. Three consecutive normal forces of 4, 14 and 24 kg were applied in this study, giving the initial normal stresses results of 11.2, 39,2 and 67.1 kpa respectively. The displacement rate was determined using a consolidation graph produced from the consolidation process, and a relatively slow horizontal displacement rate of mm/s was sufficient to ensure the tests were in a drained condition. A series of shearing tests on unsaturated specimens was carried out using similar normal forces as per the saturated specimens. However, for the unsaturated test the specimens were not submerged, rather they were remained in unsaturated condition. To obtain different suctions, the specimens were air-dried to achieve different water content. The soil water characteristic curve (SWCC) was a very important tool in predicting specimen water content during the specimen preparation. The shear box was then placed into the direct shear machine, and the tensiometer was attached immediately on the top cap. Petroleum jelly and silicon sealant were smeared on all vulnerable parts of the shear box to avoid any possible of leakage during the test (e.g., between lower and upper half of shear box). Before shearing, the specimens were then left overnight in controlled-temperature room for equilibration. 5. RESULT AND DISCUSSION 5.1 Saturated direct shear test Figure. 5 shows the failure envelopes of the specimens resulted from saturated direct shear test. It can be observed that the presence of kaolin clay contributed to the increase in effective cohesion and to the decrease in effective internal friction. The increase in cohesion came from the contribution of cohesion of kaolin clay, whereas the decrease in internal friction was due to the presence of smaller particles of the clay. 24

6 Figure 5. Failure envelopes of the specimens resulted from saturated direct shear test. 5.2 Hydraulic Conductivity of Residual Soil The shear strength in respect to matric suction was generally presented to depict the strength behavior of unsaturated specimen. The plot was referred to as failure envelope of unsaturated specimen. For this, a lot of unsaturated direct shear tests were required. For suction-monitored direct shear test, the variation of matric suctions was generated naturally by adjusting the specimen water content. For this, air-drying method was employed as previously described. Due to very limited space in this paper, only small part of the result will be presented. Here the discussion will be focused on the 95:5 mixture results as an example. Figure 6 shows the typical result of suction-monitored direct shear test on 95:5 mixture at optimum moisture content. Fig. 6 (a) and (b) show the plot of horizontal displacement versus shear stress and horizontal versus vertical displacements respectively. The plots exhibit the strain hardening until peak stress, followed by strain softening behavior until residual value. The lowest dilation was experienced when the higher net normal stress was applied, whereas the highest one occurred when the lowest normal stress was imposed. During the shearing test, the matric suction was recorded as shown in Fig. 6 (c). It can be observed that during the strain hardening, matric suction was slightly increasing. The author presumed that this phenomenon occurred due to the change in void ratio of the specimen during the hardening process. Matric suction was turning back to initial value when strain softening was occurring. The tests were continued on the same type of specimen for various matric suctions. A series of similar tests were also performed on sand and 90:10 mixture specimens. The results of unsaturated direct shear test with different matric suction combined with the results of saturated test were then plotted on the graphic to produce the failure envelopes of the specimens in unsaturated condition. 5.3 Failure envelopes Figure. 7 (a), (b), and (c) show the unsaturated failure envelopes of sand, 95:5 and 90:10 mixture specimens respectively. It can be observed from those figures that all of the envelopes exhibit the bilinear curve. The curve also explained the strength increase of the specimen due to the increase of matric suction. The angle of the curve is generally denoted as b. 25

7 (a) (b) (c) Figure 6. Result of unsaturated direct shear test on 95:5 mixture at optimum moisture content. (a) Displacement versus shear stress (b) Vertical versus horizontal displacements (c) Displacement versus matric suction 26

8 (a) (b) (c) Figure 7. Unsaturated failure envelopes of (a) Sand (b) 95:5 mixture (c) 90:10 mixture displacements (c) Displacement versus matric suction 27

9 The effect of suction on the shear strength increase was significant. For sand, the increase in suction from zero to 4-5 kpa caused the shear strength increase of about 30%. However, beyond this suction, the increase of suction caused the decrease in shear strength (Figure 7 (a)). For 95:5 mixture, the increase in suction on this particular range caused the increase in shear strength of about 25-50%, and the suction of about 70 kpa caused the increase in shear strength of about % (Figure. 7 (b)). The strength increase of more than 100% also occurred on 90:10 mixture when 70 kpa of suction was taken placed (Figure. 7 (c)). 5.3 Soil-water characteristic curve The behavior of unsaturated soil is strongly affected by matric suction, and matric suction is strongly affected by water content. As a consequence, matric suction and water content are vital parameters, and these are commonly represented by a curve expressing the relationship between matric suction and water content. In geomechanics, the curve is commonly referred to as a soil water characteristic curve (SWCC). Suction is plotted in logarithmic scale and water content is plotted in arithmetic scale. The SWCC quantifies the capacity of the soil to hold water with a particular degree of suction, and the slope of the SWCC describes the rate of water loss corresponding to suction (Agus et al., 2001). One of the important parameter resulted in SWCC is AEV. The AEV is the suction value required for removing water from the largest pores of the soil. Beyond the air entry value, the increase of suction causes a rapid rate of water content loss until the residual water content is reached. Vanapalli et al. (1999) and Siller et al. (2001) elaborated upon the mechanism of the SWCC with regard to a soil sample when it was desaturated from its fully saturated until residual condition. Figure. 8 shows the SWCC of the specimens resulted from the combination of tensiometer and filter paper methods. The result indicated that the AEV of sand, 95:5 and 90:10 mixtures were 1.5, 2.2 and 4 kpa respectively. Figure 2. Soil water characteristic curve of the specimens 6. CONCLUSIONS Based on the result, it is concluded that: 1. Suction-monitored direct shear device has been successfully used for unsaturated shear test. 2. The failure envelopes of unsaturated specimen formed a bi-linear pattern. From zero suction until AEV, the increase of shear strength due to suction (denoted as b ) was higher than the effective friction angle. Beyond the AEV, the slope b was lower than the effective friction angle. 3. Suction was very important parameter in shear strength analysis. The shear strength increase of specimen was significantly contributed by the increase in suction. 28

10 4. In suction-monitored direct shear test, the soil water characteristic curve (SWCC) was very important tool for predicting specimen water content during preparation. 5. Despite the suction capacity of the tensiometer was relatively low, suction-monitored direct shear device was effective for low air entry soils such as sand or sand with relatively small portion of fine grained material. ACKNOWLEDGEMENTS This study was funded by DIKTI scholarship from the Indonesian National Education Department. The authors acknowledge and appreciate this support. The authors also express many thanks to Apiniti Jotisankasa from Kasetsart University for his technical assistance. REFERENCES Agus, S.S., Leong, E.C. and Rahardjo, H. (2001). Soil-water characteristic curves of Singapore residual soils. Geotechnical and Geological Engineering, 19: ASTM-D (2007). Standard test method for laboratory compaction characteristic of soil using standard effort (12,400 ft-lbf/ft3 (600 kn-m/m3)). ASTM International, Pennsylvania. ASTM-D (2004). Standard test method for direct shear test of soils under consolidated drained conditions. ASTM International, Pennsylvania. Donald, I.B. (1957). Effective stresses in unsaturated non cohesive soils with controlled negative pore pressure. M.Eng. Sc. Thesis, University of Melbourne. Melbourne, Australia. Escario, V. & Saez, J. (1986). The shear strength of partly saturated soils. Geotechnique 36(3): Fredlund, D.G. & Rahardjo, H. (1993). Soil Mechanics for Unsaturated Soils. Canada: A Wiley- Interscience Publication. Gan J.K.M., Fredlund D.G., Rahardjo H. (1988). Determination of the shear strength parameters of an unsaturated soil using the direct shear test. Canadian Geotechnical Journal, 25: Houston, S.L., Houston, W.N., Wagner A.M. (1994). Laboratory Filter Paper Measurements. Geotechnical Testing Journal 17(2): Jotisankasa A., Mairaing W. (2010). Suction-monitored direct shear testing of residual soils from landslide-prone areas, Journal of Geotechnical and Geoenvironmental Engineering, 136(3): Jotisankasa, A., Tapparnich, J., Booncharoenpanich, P., Hunsachainan, N. & Soralump, S. (2010). Unsaturated soil testing for slope studies. Proc. International Conference on Slope, Geotechnique and Geosynthetics for Slope, Chiangmai, Thailand: Likos W.J., Wayllace A., Godt J., Lu N. (2010). Modified direct shear apparatus for unsaturated sand at low suction and stress. Geotechnical Testing Journal, 33(4): Nam S, Gutierrez M, Diplas P, Petrie J. (2011). Determination of the shear strength of unsaturated soils using the multistage direct shear test. Engineering Geology 122: Purwana Y.M, Jitsangiam P, Nikraz H, Josinakasa A. (2011). Experimental studies on suction-monitored direct shear apparatus on Perth poorly graded sand. Proc. International Conference on Advances in Geotechnical Engineering. Perth, Western Australia: Sillers, W.S., Fredlund, D.G. and Zakerzaheh, N., (2001). Mathematical attributes of some soil water characteristic curve models. Geotechnical and Geological Engineering, 19(3): Tarantino A., Tombolato S. (2005). Coupling of hydraulic and mechanical behaviour in unsaturated compacted clay. Geotechnique 55 (4): Vanapalli S.K., Fredlund D.G., Pufahl D.E. and Clifton A.W., (1996). Model for the prediction of shear strength with respect to soil suction. Canadian Geotechnical Journal, 33: Vanapalli, S.K., Fredlund, D.G. and Pufahl, D.E., (1999). The influence of soil structure and stress history on the soil-water characteristics of a compacted till. Geotechnique, 49(2):

INTERPRETATION OF UNDRAINED SHEAR STRENGTH OF UNSATURATED SOILS IN TERMS OF STRESS STATE VARIABLES

INTERPRETATION OF UNDRAINED SHEAR STRENGTH OF UNSATURATED SOILS IN TERMS OF STRESS STATE VARIABLES S. K. Vanapalli and D.G. Fredlund Department of Civil Engineering University of Saskatchewan, Saskatoon