DEVELOPMENT OF TRIAXIAL SYSTEM FOR SOIL TESTING AT WIDE STRAIN RANGE: PRELIMINARY RESULTS

Transcription

1 International Symposium on Geotechnical Engineering, Ground Improvement and Geosynthetics for Human Security and Environmental Preservation, Bangkok, Thailand DEVELOPMENT OF TRIAXIAL SYSTEM FOR SOIL TESTING AT WIDE STRAIN RANGE: PRELIMINARY RESULTS W. Ratananikom and S. Likitlersuang Department of Civil Engineering, Chulalongkorn University, Thailand S. Yimsiri Department of Civil Engineering, Burapha University, Thailand ABSTRACT This paper presents the development of the conventional triaxial apparatus to be able to perform the soil testing at a wide strain range. Vertical and lateral local strain measurement systems are introduced to the conventional triaxial system. Bender element system is also incorporated to obtain a small-strain (elastic) shear modulus. Isotropically consolidated undrained triaxial compression (CIUC) tests are performed on samples of Bangkok Clay to verify the performance of the developed triaxial system. The preliminary results are presented with an attention to non-linear and anisotropic characteristic. The results are also compared with other published experimental data. INTRODUCTION In the past, geotechnical analysis focused on strength characteristics of soils. However, recent geotechnical design considers soil-structure interaction under working condition. This analysis involves the prediction of deformations of structures and stresses, both in the surrounding soil mass and over areas of contact with the loading boundaries. In doing so, it needs a correct stress-strain relationship of soil under working load condition. Many recent research works have suggested that the stress-strain behavior of soil is highly non-linear with increasing in strain level. Strain levels can be categorized into 3 zones, namely small, intermediate and large (Georgiannou et al., 1991; Lo Presti, 1989). Moreover, there are a number of evidences showing that under working load condition, the strains in ground surrounding foundation and excavations are relatively small (Simpson et al., 1979; Jardine et al., 1986). Burland (1989) summarized various field data showing that large ground mass beneath and around structures experienced direct strains of less than.1%. Therefore, the stress-strain behavior of soils at small and intermediate strain ranges is an essential input to obtain more accurate prediction in geotechnical design. The conventional triaxial test is the most popular technique to study the stress-strain behavior of soil in the laboratory. However, it is often found that the stiffness of soil which is obtained from the conventional triaxial apparatus is far below the stiffness of soil which is derived from back calculation using the measured displacement occurring in the field. There are a lot of factors accounted for this error but the most important factor is an incorrect consideration of the strain level from the test data and an inability of the conventional triaxial system to measure the stress-strain behavior at smaller strain level consistent with what happens in the real structure (Figure 1). The limitation of conventional triaxial apparatus results from bedding errors involves in the measurement. Because of the small-strain range 395

2 occurring beneath and around structures is beyond the capability of the conventional triaxial apparatus. Up to present, a number of researchers have investigated the behavior of many soils at small to intermediate strain ranges. However, the small to intermediate train behavior of Bangkok Clay remains poorly understood. Therefore, this study aims to develop the existing conventional triaxial apparatus to be able to investigate the behavior of Bangkok Clay in a wide strain range. The obtained non-linear behavior of Bangkok Clay is an essential data for improving the advanced constitutive model to better represent the real soil behavior. Fig. 1 Stiffness degradation curve with ranges of strain in structures and tests (Mair, 1993 and Atkinson, 2) Fig. 2 Sources of errors in external axial deformation measurements (Baldi et al., 1988) 396

3 TRIAXIAL SYSTEM The triaxial testing in this study comprises a triaxial cell and a load frame to exert an axial force on the specimen. The triaxial chamber allows a maximum pressure of 17 kpa and can be subjected 45 kn axial load. The cell pressure and pore/back pressure transducers are controlled by two digital pressure controllers (DPCs) (Menzies, 1988). Additional instrumentations are employed in the triaxial testing system which include internal load cell, proving ring, volume change transducer, cell pressure transducer, pore pressure transducer, back pressure transducer, external LVDT. The characteristics of the transducers are summarized in Table 1, some of them are explained in the next section. The ELE data logger box is used to provide the voltage excitation to the transducers and amplify and filter the output signals from the transducers before passing them to the microcomputer for data processing. Table 1 Characteristics of the transducers used Transducers Capacity Calibration factor Resolution Submersible load cell 3 kn kn/v kn External LVDT 5 cm mm/v mm Cell pressure transducer 15 kpa kpa/v.76 kpa Pore pressure transducer 7 kpa kpa/v.581 kpa Back pressure transducer 1 kpa kpa/v.575 kpa Volume change 6 cm cm 3 /V mm 3 transducer Local LVDT 1 1 mm.498 mm/v mm Local LVDT 2 1 mm.497 mm/v mm Proximity transducer 1 5 mm.942 mm/v mm Proximity transducer 2 5 mm.957 mm/v mm Proving ring 2 kn kn/v kn DEVELOPMENT OF TRIAXIAL SYSTEM In order to study the behavior of soil at small to intermediate-strain range, the conventional triaxial apparatus is improved by incorporating the new local strain measuring devices. Moreover, bender element system is incorporated to obtain the small-strain (elastic) stiffness. The triaxial testing system of this study is shown in Figure 3. LOCAL STRAIN MEASUREMENT SYSTEM During the past two decades, a number of the local strain measurement systems have been developed in order to investigate non-linear behavior of soils in a wide strain range. The reviews of the local strain measurement systems available for triaxial apparatus have been presented by Scholey et al. (1995) and Yimsiri and Soga (22). In this study, submersible LVDTs are used for local axial strain measurement, whereas proximity transducers are used for radial strain measurement. The reasons for selecting submersible LVDT are its robustness and not susceptible to specimen rotation and tilting, whereas the reasons for selecting proximity transducer are its non-contacting nature and ease of set - up. 397

4 Cell pressure DPC Internal load cell External LVDT Cell pressure transducer Pore pressure transducer Back pressure transducer Submersible LVDTs Proximity treansducer Proving ring Back/Pore pressure DPC Function Generator Data logger box Oscilloscope Computer Local axial strain measurement system Fig. 3 Triaxial testing system The submersible LVDTs are used for a local axial strain measurement system, its set-up is as suggested by Cuccovillo and Coop (1997). Local axial strain is derived from the average displacement measured by two submersible LVDTs which are mounted diametrically opposite to each other on two lightweight aluminium hinged mounts. In this test, the mounts are attached by gluing with a flexible contact adhesive. The design of the mounts allows the specimen to barrel or develop a shear plane with out causing the armature to jam. Figure 4 shows the set-up of local strain measuring systems (LVDTs and proximity transducers) on the specimen. The calibration curve of submersible LVDT is linear over a range of displacements of about 1 mm. Their resolution is about 1.2 µm, corresponding to strain of.24% for a 5 mm gage length. Local radial strain measurement system The proximity transducers are used for a local radial strain measurement system, its set-up is as suggested by Hird and Yung (199). The proximity transducers are held in place at midheight of the specimen in diametrically opposite direction by the column. The bottom of the column is connected to the bottom base pedestal of the triaxial cell by screws. A 5 5 cm 2 398

5 square aluminium foil is attached on the robber membrane by silicone sealant as a target of the proximity transducer. The movements of the aluminium foil targets in the radial direction are monitoring by the proximity transducer and the change in the specimen diameter can be derived. The set-up of the proximity transducer is shown in Figure 4. Their resolution is about 1.17 µm, corresponding to strain of.23% for a 5 mm gage length. BENDER ELEMENT SYSTEM Fig. 4 Local strain measuring systems The bender element is an electro-mechanical transducer which converts the mechanical energy to electrical energy or vice-versa. The element itself consists of two piezo - ceramic plates rigidly bonded together as sandwich connected in series when its functions as a receiver and in parallel when its functions as a transmitter. The bender element test is a dynamic method of measuring small-strain shear modulus, G max, in the laboratory test. In this test, two bender elements - receiver and transmitter - were embedded in the base pedestal and the top cap of the triaxial cell respectively as shown in Figure 5. The bender elements protruded 4 mm into a soil sample. The shear wave generated by the function generator is transmitted through the soil specimen from the transmitting element at the top cap to the receiving element at the base pedestal. Sine wave with amplitude of 2 V p - p and frequency of 1 khz is used as an input wave. The time taken for the shear wave to travel through the specimen can be derived from the measured arrival waveform and then the shear wave velocity and the small-strain shear modulus can be calculated from Equation (1). 2 2 L Gmax = ρvs = ρ (1) 2 ts where ρ is the total density of the soil and V s is the shear wave velocity, which is determined from the effective length L which the shear wave travels and the travel time t s. The interpretation of the bender element signal represents the main difficulty of this technique. In this study, the arrival time is estimated as the time between the start of an input signal and the first deflection in the output signal, as presented in Figure 6. Moreover, two 399

6 impulses are generated by changing the excited transmitter element in two opposite directions to be able to better identify the arrival point. Fig. 5 Bender elements in the platens of a triaxial cell Input frequency = 1 khz Input signal (V) First arrival Output signal (mv) Travel time Arrival time (ms) Fig. 6 Typical results for shear wave arrivals as detected by the bender element TESTING PROCEDURE Isotropically consolidated undrained compression (CIUC) tests are performed to evaluate the stress-strain behavior of Bangkok Clay at wide strain levels. Undisturbed samples were taken by piston sampler from Ramkhamhaeng area between the depth of m. The index properties are liquid limit = 77. %, plastic limits = 31.2%, plastic index = 45.7 %, 4

7 natural water content = 57.9 % and total unit weight = 17.3 kn/m 3. The specimen size is 5 mm diameter and 1 mm height. Back pressure of 25 kpa was applied to ensure the specimen saturation. After completion of the saturation process, the Skempton B - value (Skempton, 1954) was typically , which corresponds to the degree of saturation of 1% (Black and Lee, 1973). The sample was subjected to isotropic consolidation under its in-situ mean effective stress. The consolidation was performed by constant rate of - stress increment, by which the cell pressure was slowly increased to ensure fully drainage. After end of consolidation, the rest period of 24 hours was applied before undrained shearing. During isotropic consolidation, the shear wave velocity of bender element was also measured to obtain the relationship between the small-stain shear modulus and isotropic confining stress. TESTING RESULTS Isotropic consolidation characteristic The specimens were isotropically consolidated to the in - situ mean effective stress. The measured isotropic consolidation curves of the undisturbed Bangkok Clay sample are shown in Figure 7. It can be seen that the isotropic consolidation curve which is derived from the local strain measurement and the external strain measurement are nearly similar. The results from thisstudy reasonably consistent with the oedometer test results of Bangkok Clay at m by Shibuya and Tamrakar (1999). Due to the constant rate of - stress consolidation, it is possible to calculate the secant bulk modulus and its degradation with volumetric strain as shown in Figure 8. The results are quite scatter with the average bulk modulus at small strain of approximately 2 kpa. Mean effective stress, p' (kpa) Volumetric strain, ε vol (%) Internal External Shibuya and Tamrakar (1999) 35 Fig. 7 Isotropic consolidation curves 41

8 Secent bulk modulus, Ksec (kpa) Internal External Volumetric strain, ε vol (%) Fig. 8 Bulk modulus degradation curves Because of the incorporation of both local axial and radial strain measurements in the triaxial apparatus, it is possible to directly study the anisotropic deformation of the specimen during the isotropic consolidation. The result is shown in Figure 9. It can be seen that the deformation under isotropic consolidation is anisotropic. The ratio of ε a/ ε r is 1.2. The result indicates that the specimen is stiffer in the horizontal direction. Moreover, the results from other tests show that when the specimens were undergone isotropic consolidation over its in - situ mean effective stress, its deformation under isotropic consolidation becomes less anisotropic. 3 Axial strain, ε a (%) ε a = ε r Radial strain, ε r (%) Fig. 9 Anisotropic deformation during isotropic consolidation 42

9 Undrained shearing characteristic The undrained stress path is shown in Figure 1 where p = (σ a +2σ r )/3 and q = σ a -σ r. The stress ration at failure, η f, is 1.2 which indicating a friction angle at failure, φ f, of 3. The φ f found in this study is a little larger than the value of 26 and 28 reported by Balasubramanian and Chaudhry (1978) and Shibuya and Tamrakar (1999), respectively. The different result may be caused by the difference of the depth of specimen. In this study, the specimens were taken at a depth of m, but the lower depth of m and 13.6 m were collected by Balasubramanian and Chaudhry (1978) and Shibuya and Tamrakar (1999), respectively. Deviatoric stress, q (kpa) p'-q Balasubramanian and Chaudhry (1978) Shibuya and Tamrakar (1999) φ = 3 φ = 28 φ = Mean effective stress, p' (kpa) Fig. 1 Undrained stress path From the deviatoric stress-axial strain relationship shown in Figuire 11, undrained secant Young s modulus, E u,sec are derived. The secant Young s modulus degradation curve is displayed in Fig. 12. The secant Young s modulus degradation curve can be derived to as small as.2% axial strain. The derived secant Young s moduli of Bangkok Clay in this study are scattered and less reliable for axial strain less than.1%. Moreover, it can be seen that the degradation curve derived from the local strain measurement are little higher than those from the external strain measurement. The normalized secant Young s modulus degradation curve in this test is lower than those reported by Shibuya and Tamrakar (1999); however, this result is nearly similar with the data from Teachavorasinskun and Amornwithayalax (22). It is noted that the tests by Shibuya and Tamrakar (1999) and Teachavorasinskun and Amornwithayalax (22) were not conducted with the local strain measurement. Small - strain shear modulus from bender element During isotropic consolidation, the shear wave velocity of bender element was also measured to obtain the relationship between the small-stain shear modulus and isotropic 43

10 confining stress. In Figure 13, the relationship between normalized G max /F(e) and p of Bangkok Clay specimen is presented. The empirical equation in the form suggested by Hardin and Black (1968) is used to fit the relationship between G max and p as: 9 8 Deviatoric stress, q (kpa) Internal External Axial strain, ε a (%) Fig. 11 Relationship between deviatoric stress and axial strain 5 Normalised Secent Young's modulus, Eu,sec/p' (kpa) From bender element test Internal External Triaxial compression (Shibuya and Tamrakar, 1999) Triaxial Compression (Teachavorasinskun and Amornwithayalax, 22) Axial strain, ε a (%) Fig. 12 Comparison of normalized secant young modulus degradation curves.6 G max = F( e) p' (2) where G max and p are in kpa and (2.97 e) F( e) = 1+ e 2 (Hardin and Black, 1968) 44

11 The bender element results this study is lower than the empirical equation proposed by Teachavorasinskun and Amornwithayalax (22). The value of G max/ F(e) from SCPT test at the depth of 15 m is also superimposed (Shibuya and Tamrakar, 1999) and it seems to be more consistent with the data of Teachavorasinskun and Amornwithayalax (22). However, it is noted that the stress condition for SCPT is different than isotropic stress condition in the laboratory. When the G max from bender element test is plotted at the axial strain of.1% together with the triaxial test data (Figure 12), they are consistent. 1 Normalised small-strain shear modulus, Gmax/F(e) (kpa) G max /F(e) = p'.7 (Teachavorasinskun and Amornw ithayalax, 22) G max /F(e) = 58.76p' Mean effective stress, p' (kpa) SCPT Test (Shibuya and Tamrakar, 1999) Fig. 13 Relationship between small-strain shear modulus and isotropic confining stress CONCLUSIONS The development of the conventional triaxial apparatus by incorporating the new local strain measuring devices and bender element system is presented in this paper. The preliminary test results conducted on Bangkok Clay from Ramkhamhaeng area by isotropically consolidated undrained compression (CIUC) test are presented. The consolidation results yield a bulk modulus degradation curve and show anisotropic deformation under isotropic stress. The undrained shear results show that the stress-strain relationship is nonlinear with increasing in strain level. Bender element results give relationship between G max -p consistent with proposed empirical equation. ACKNOWLEDGMENT This study is partly supported by the Department of Civil Engineering, Burapha University, through its Research and Development Fund No. 19/255. REFERENCES Balasubramaniam, A. S. and Chaudhry, A. R. (1979), Deformation and strength characteristics of soft Bangkok clay, Journal of the Geotechnical Engineering Division, Vol. 14, No. GT9, pp

12 Baldi, G., Hight, D. W. and Thomas. G. E. (1988), A reevaluation of conventional triaxial test methods, Advanced Triaxial Testing of Soil and Rock, ASTM STP 977, Eds. Donaghe et al., American Society for Testing Materials, pp Black, D. K. and Lee, K. L. (1973). Saturating Laboratory Samples by Back Pressure, Journal of the Soil Mechanics and Foundations Division, Vol. 99, No. 1, pp Burland, J. B. (1989). Ninth Laurits Bjerrum Memorial Lecture: Small is beatiful - the stiffness of soils at small strains, Canadian Geotechnical Journal 26, pp Dyvik, R. and Madshus, C. (1985). Laboratory Measurements of G max Using Bender Elements, Proceedings of the ASCE Annual Convention: Advances in the Art of Testing Soils Under Cyclic Condition, Detroit, ASCE, pp Georgiannou, V. N., Rampello, F. and Silvestri, (1991). Static and dynamic measurement of undrained stiffness on natural OC clay, Proceedings of the European Conference on Soil Mechanics, Florence 1, pp Hardin, B. O. and Black, W. L. (1968). Vibration modulus of normally consolidated clay, Journal of the Soil mechanics and Foundation Division, ASCE, Vol. 9, No. SM2, pp Hird, C. C. and Yung, P. C. Y. (1989). The use of proximity transducers for local strain measurements in triaxial tests, Geotechnical Testing Journal, ASTM, Vol. 12, No. 4, pp Jardine, R. J., Potts, D. M., Fourie, A. B. and Burland, J.B. (1986). Studies of the influence of non-linear stress-strain characteristics in soil-structure interaction, Geotecnique 36, No. 3, pp Lo Presti, D. C. F. (1994). General report: Measurement of shear deformation of geomaterails in the laboratory, Proceedings of the 1 st International Symposium on Pre - failure Deformation Characteristics of Geomaterials, IS - Hokkaido-94, Eds. Shibuya et al., Balkema, Rotterdam, Vol. 2, pp Mair, R. J. (1993). Unwin Memorial Lecture (1992) Developments in geotechnical engineering research: application to tunnels and deep excavations, Civil Engineering, Proceedings of the Institute of Civil Engineers, UK, Vol. 93, pp Menzies B. K. (1988). A computer controlled hydraulic triaxial testing system, Advanced Triaxial Testing of Soil and Rock, ASTM STP 977, pp Scholey, G. K., Frost, J. D., Lo Presti, D. C. F. and Jamiolkowski, M.(1995). A review of instrumentation for measuring small strain during teiaxial testing of soil specimens. Geotechnical Testing Journal, ASTM, Vol. 18, No. 2, pp Shibuya, S. and Tamrakar, S. B. (1999). In-situ and laboratory investigations into engineering properties of Bangkok clay, Characterization of Soft Marine Clays, Tsuchoda and Nakase (eds), pp

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