MODIFICATION OF TRIAXIAL CELL FOR APPLICATION OF ANISOTROPIC STRESS CONDITION. A. K. Dey 1, M. Paul 2 ABSTRACT

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1 MODIFICATION OF TRIAXIAL CELL FOR APPLICATION OF ANISOTROPIC STRESS CONDITION A. K. Dey 1, M. Paul 2 ABSTRACT Failure of soil layer under tri-axial stress condition occurs in terms of gradual strain accumulation. Strain localization is an important feature of elastoplastic materials undergoing non-homogeneous deformation. Strain accumulation takes place in narrow zones in case of compressive as well as extensive failure. In the context of slope stability, strain softening material behaviour might initiate what is known as a progressive failure. Since soil is considered to be a semi-infinite medium, hence, pressures on a particular soil element beneath the surface are not same in all the three directions i.e. anisotropic stress condition prevails. It is very difficult to simulate the anisotropic stress condition in the triaxial test at the time of application of cell pressure. Thus an isotropic stress condition is applied in the triaxial cell before application of deviator stress. A modification to the existing triaxial cell is therefore required to simulate the exact ground condition. In field condition it is not necessary that every time compressive load will be applied, there may be tensile load or compressive and tensile loads both. One way to apply the anisotropic condition is to use the plunging rod having a diameter same as the sample diameter. In that case the cell pressure will not act from the top and the radial stress and the axial stress may be different. In case cell pressure is more than the axial stress, extension will occur in the sample and reverse stress condition will impose compression to the sample. Extension test, which is rather difficult in normal condition, can be easily carried out with this modification. Different field conditions such as soil layer near a cut slope/excavation, sudden removal of overburden, soil layer just beneath a foundation, etc can be easily modelled. In the present study the plunger diameter is made equal to the sample diameter. As such the triaxial cell is modified. With this modification, cell pressure and the axial stress can be varied independently. Fig 1 shows a comparison of the two types of test apparatus. The soil is of c-φ type and tests are conducted with different water contents. For a comparison purpose, remoulded samples of constant density are tested in both the conventional triaxial cell and modified triaxial cell at varied confining pressures. Test results show that the samples take less axial stress before failure in the modified triaxial stress condition compared to the conventional test condition. Keywords: Anisotropic stress, triaxial test, shear band, critical state line. 1 A. K. Dey, Civil Engineering Department, National Institute of Technology Silchar, India, ashim_kanti@yahoo.co.in 2 M. Paul, Civil Engineering Department, National Institute of Technology Silchar, India, min23paul@gmail.com

2 A. K. Dey & M. Paul Plastic wrinkle pipe (a) Conventional triaxial cell (b) Modified triaxial cell Fig. 1 Comparison between conventional triaxial cell and modified triaxial cell The loading plunger in the modified triaxial cell is encased in a plastic wrinkle pipe having diameter equal to the diameter of the sample.

3 MODIFICATION OF TRIAXIAL CELL FOR APPLICATION OF ANISOTROPIC STRESS CONDITION Ashim Kanti Dey, Professor, National Institute of Technology Silchar, Minakshi Paul, PG student, National Institute of Technology Silchar, ABSTRACT: In the present study the conventional triaxial cell is modified by making the diameter of the loading plunger equal to the diameter of the soil sample. With this modification, axial stress can be applied independent of radial stress. The soil is of c -φ type and tests are conducted at different water contents and different confining pressurews. Tests are conducted in both the normal and modified triaxial test set ups. Test results show that the samples take less axial stress before failure in the modified tri-axial test than that in the conventional test and undergo more strain. Critical state analysis was carried out and a new critical state is found out for the modified triaxial test. INTRODUCTION The failure mechanism of soil is investigated by means of various shear strength test, such as triaxial compression test, direct shear test, vane shear test etc. Amongst all these tests, tri-axial test is widely used to find the shear strength parameters of soil. A number of researchers have considered different aspects like stress-strain inhomo geneities[1], and formation of shear bands in granular material[2], combined axial torsional tests on hollow cylinders[3], strain field determination through photography[4], measurement of two- and three-dimensional surface displacements on plane strain and axisymmetric sand specimens[5], etc. In case of normal tri-axial compression apparatus an all round confining stress is applied to the specimen and thereafter a deviatoric stress is applied to the specimen.this test condition implies that the specimen will be under compressive stress throughout the test. But in practice some situations arise where axial stress is less than the radial stress, such as soil layer adjacent to cut slope/excavation, soil layer immediately below an anchor, soil layer in the passive zone of a retaining wall, etc. To simulate this type of situation in a triaxial test set up, the applied axial stress must be made independent of radial stress. This can be achieved in conventional tri-axial cell if somehow axial stress is made independent of cell pressure. This needs a modification of the conventional triaxial test set up. The simplest way to achieve the goal is to make diameter of the loading plunger equal to the sample diameter. In the present study this condition has been achieved. Once the cell is modified, the critical state of c-φ soil (laterite) is obtained.to investigate the failure mechanism of c-φ soil (laterite) by critical state approach the parameters investigated are σ 1 (major principle stress), σ 3 (minor principle stress) at varied ν (specific volume). Remolded samples having constant moist density with varied water contents such as 12%, 15%, 18% are tested in both the conventional tri-axial cell and also in the modified tri-axial cell at varied confining pressures ranging from 1-3 kg/cm 2. DIFFERENT STRESS STATES Basically three different stress states which are applied for determination of shear strength of soil are shown in Fig.1. In the first case equal confining pressure (i.e. alround pressure) (σ c ) is applied and then deviatoric stress is applied from the top, this is a typical tiaxial test condition. Additional to it another pressure(σ 1 ) also acts axially and this a

4 A. K. Dey & M. Paul typical tri-axial stress state. Second case describes a situation where the radial pressure is independent of the axial stress, which is the main test condition in this study. In the third case pressure acts only from the top and no confining pressure is applied and this is the unconfined compression state SOIL CLASSIFICATION For finding out the grain size distribution curve for the soil both wet sieve analysis and hydrometer analysis were carried out. Fig 3 shows the grain size distribution of the soil used in the present study. Fig.1 Different Stress states MODIFIED TRI-AXIAL STRESS STATE In order to achieve the second stress state as mentioned above, a plastic wrinkle pipe (shown as packing in Fig. 2) is attached to the top of the top platen and the loading plunger is allowed to travel through this packing. Thus when the cell pressure is applied, the axial stress is zero and the sample is subjected to a tension. When the axial stress is equal to the cell pressure, the sample is subjected to an isotropic stress condition. When the axial stress is more the cell pressure, the sample is subjected to a compression. Hence, the same soil sample can be subjected to tensile stress as well as compressive stress Fig.2 Schematic diagram of Modified Tri-axial test set up Fig.3 Grain size distribution curve The grain size distribution curve reveals that it is a well graded soil. According to the sieve analysis and plasticity chart the respective soil falls under the category SC (clayey sand) and CL. Other properties are shown in Table 1. TABLE 1 Properties of soil Specific Gravity 2.6 Liquid limit 28.5 Plastic limit 21 Plasticity index 7.5 Maximum Dry density 1.74 g/cm 3 Minimum Dry density 1.30 g/cm 3 Optimum Moisture 15.8 Content Maximum void ratio 1.00 Minimum void ratio 0.49 Poisson ratio 0.42 Modulus of Elasticity N/mm 2 Unconfined compressive strength NORMAL TRIAXIAL TEST 0.4 N/mm 2 For determining the c-φ value of the soil and investigating the stress-strain relationship, unconsolidated undrained triaxial tests are conducted as per IS 2720 (Part II) Remolded cylindrical specimens having 38 mm diameter and

5 76 mm height are tested in Tri-axial apparatus. The specimen is inserted inside a stretched rubber membrane. Two porous stones are placed at top and bottom of the specimen. The specimen is then placed over the pedestal. The whole tri-axial cell is assembled and fitted properly. Three confining pressures, 1, 2 and 3 kg/sq.cm are applied in the cell by the lateral pressure assembly and foot pump. Now vertical stress is applied by means of upward movement (1.25 mm/min) of the cell base. Displacement transducer, pore pressure sensor and load cell are attached with a digital indicator connected with a laptop to store the load (kg), deformation (mm) and pore pressure (kg/cm 2 ) data. After the completion of the test failed soil sample is observed for evaluating the shearing pattern and shear band inclination. Fig. 4 shows a photograph of the deformation of soil during the test. stress becomes absolute stress along the axial direction and the deviator stress does not come to picture. When the radial stress is more than the axial stress, the sample is subjected to tension and when the axial stress is more than the radial stress the sample is subjected to compression. Fig. 5 shows test set up and deformation of the sample during the modified triaxial test. Normal loading plunger (a) Test set up (b) Deformation of sample Fig. 5. Modified triaxial test set up with loading plunger of diameter equal to the diameter of the sample. COMPARATIVE RESULTS Fig. 4 Sample deformation MODIFIED TRI-AXIAL TEST The modified tri-axial test uses the same sample specification, loading rate, confining pressures as used in the normal tri-axial test. The only difference is that when the cell pressure is applied, no axial load comes to the sample. The sample is subjected to a radial stress with zero axial stress. Subsequently when the axial load is applied, the Stress-strain graph The stress-strain variations of the two types of tests under different water contents and cell pressures are shown in Figs. 6 to 17. The important observations from these tests are summerized below : a) The failure stress is higher in normal triaxial test b) The peak stress is achieved earlier in the normal triaxial test. c) The stress-strain relationship in modified triaxial test is normally flatter.

6 A. K. Dey & M. Paul Fig 6. Stress-strain curve for confining pressure 1kg/sq.cm and moisture content 12% Fig 10. Stress-strain curve for confining pressure 2kg/sq.cm and moisture content 12% Fig 7. Stress-strain curve for confining pressure 1kg/sq.cm and moisture content 15% Fig 11. Stress-strain curve for confining pressure 2kg/sq.cm and moisture content 15% Fig 8. Stress-strain curve for confining pressure 1kg/sq.cm and moisture content 18% Fig 12. Stress-strain curve for confining pressure 2kg/sq.cm and moisture content 18% Fig 9. Stress-strain curve for confining pressure 1kg/sq.cm and moisture content 21% Fig 13. Stress-strain curve for confining pressure 2kg/sq.cm and moisture content 21%

7 Variation of Compressive strength with moisture content Fig 18 shows a comparison of compressive strength of modified test with that of normal test. It is observed that for both normal & modified cases the compressive strength decreases with increase in moisture content. Decrease in compressive strength is of the order of %. Fig 14. Stress-strain curve for confining pressure 3kg/sq.cm and moisture content 12% Fig 15. Stress-strain curve for confining pressure 3kg/sq.cm and moisture content 15% Fig 16. Stress-strain curve for confining pressure 2kg/sq.cm and moisture content 18% 12% 15% 18% 21% Fig 18. Variation of compressive strength for different water contents Comparison of p-q graph The unique line of failure points of both drained and undrained tests is defined as the critical state line which is a state at which large shear distortions occur with no change in stress or in specific volume. The critical state line is important in the sense that failure of isotropically compressed sample will occur once the stress state of the sample reaches the line, irrespective of the test path followed by the sample during axial loading. The p-q curves for the normal triaxial tests and modified triaxial tests are compared through Figs Fig 17. Stress-strain curve for confining pressure 3kg/sq.cm and moisture content 21% Fig 19. Comparison of p-q curves for 12% water content.

8 A. K. Dey & M. Paul Fig 20. Comparison of p-q curves for 15% water content Fig 23. Comparison with unconfined compression test for water content 12%. Fig 21. Comparison of p-q curves for 18% water content Fig 24. Comparison with unconfined compression test for water content 15%. Fig 22. Comparison of p-q curves for 21% water content From the p-q graphs it is observed that for all the moisture contents the p-q line for a modified test lies below than the p-q line for a normal test, i.e. the slope of the p-q line is steeper for normal test than for modified test. Comparison with unconfined compression Unconfined compression tests were conducted for soil samples having water contents 12%, 15%, 18% and 21% and density almost equal to that used in triaxial tests for a comparison of results. Figs show the comparison of results. Fig 25. Comparison with unconfined compression test for water content 18%. Fig 26. Comparison with unconfined compression test for water content 21%.

9 The comparison between the stress-strain graphs of three different stress states reveals the fact that samples under triaxial stress state (normal and modified) can sustain more stress at same strain level compared to the unconfined compression state due to the presence of lateral confinement. Another fact is the shape of the stress-strain curve which is little irregular in case of tri-axial stress state (normal and modified) whereas it is almost smooth at unconfined compression test. Under unconfined condition soils inherent capacity is the only restraining force against the axial deformation for which the stress strain curve is smooth. In case of triaxial stress state (normal and modified) confining pressure acts radially which counteracts the axial deformation along with the soils own shear strength. So in this case axial deformation is the output of force vectors acting on a plane along two different directions. Due to this the shape of the stress-strain curve is irregular. Critical state parameters are considered as constant for a certain soil type for triaxial compression stress state. But in a modified stress state the sample is under tensile stress initially and after that under compressive stress. This is homologous to the situation where at first an elastic cylindrical specimen is elongated by applying a tensile force and subsequently compressed by a compression force. Application of tensile stress at the first case effects the granular orientation of the sample when void spaces between the soil grains increases in the longitudinal direction, subsequent compressive stress reorients the grains reducing the void space to a large extent. As it is a known fact that tensile strength of soil is very less compared to compressive strength; during the first portion of the test, strain propagation takes place very quickly and in later stage this strain gets accumulated in thin zones termed as shear bands. In this situation soil grains can take up same or more compressive stress to a strain level (30%-35%) more than the normal triaxial where soil can take upto 20% strain. In this new condition soil particles reaches critical state earlier than the normal triaxial compression. Soil deformation becomes ductile in nature. Due to this fact a separate critical state is found for the modified condition which is reached earlier than the normal triaxial stress state CONCLUSIONS Following conclusions are drawn from the present study : 1. Failure load reaches earlier in modified triaxial test than in normal test but at a higher strain. 2. With increase in water content the axial stress resisting capacity decreases for both the cases. 3. The Critical state is reached comparatively earlier in case of modified Tri-axial stress state then the normal condition. 4. The shape of the stress-strain curve is flatter in case of modified triaxial test than in normal triaxial test. 5. The shape of the stress-strain curve is smoother in case of unconfined compression whereas it is little irregular for tri-axial stress state (normal and modified) due to the contribution of confining pressure in the later case REFERENCES 1. Sheng D., Westerberg B., Mattsson H. & Axelsson K.(1997), Effects of End Restraint and Strain Rate in Triaxial Tests, Computers and Geotechnics, 21(3), Wolf Henning, KonigDiethard, Triantafyllidis Theodoros (2003), Experimental investigation of shear band patterns in granular material, Journal of Structural Geology, 25, P. Dayakar,Sachan Ajanta, and Prashant Amit (2004), Experimental and Analytical Aspects of Strain Localization for Cohesive Frictional Materials, Jl. of Geotech. And Geoenv. Engg, ASCE, 130 (3), N. Konrad & W. Huang (2004), A study of localized deformation pattern in granular media, Comput. Methods Appl. Mech. Engg., 193, A. L Rechenmacher.(2006) Grain-scale processes governing shear band initiation and evolution in sands Jl of the Mech. and Physics of Solids, 54,