The Effects of Different Earth Pressure Coefficient at Rest in Triaxial Shear Tests on Clay

University essay from KTH/Jord- och bergmekanik

Abstract: Triaxial shear test is the most accurate test for deciding the undrained shear strength of clay. In every test the ratio between the horizontal and vertical stresses, the coefficient of earth pressure at rest (K0′), has to be decided. It’s widely believed that the choice of this parameter will influence the results, but it’s not known to what extent. In this thesis 20 consolidated undrained active triaxial shear tests on clay has been con- ducted with a K0′ at 0.6 and 0.8. The clay was collected with a 54 mm piston sampler at the Norwegian Geo-Test Site in Trondheim, Norway, on depth of 3.0 to 7.8 meters. Besides the triaxial testing, index tests and oedometer tests was conducted on every cylinder to do a gen- eral classification of the clay. The clay has a sensitivity of 9-20, a water content of 35-51 %, a plasticity index of 27-65 % and an over consolidation ratio of 2.6-6.8. The consolidation phase of the triaxial test was conducted in five loading steps with a rest time in-between to monitor the amount of pore water expelled at each stress level. The loading steps was 50 %, 75 % and 100 % of maximum cell pressure and thereafter at 50 % and 100 % of the maximum deviator stress. The shear phase was done at a speed of 1.5 % per hour to a total of 10 % axial strain. It is not possible to reach an unambiguous conclusion from the results, but the maximum shear strength of tests with a K0′ at 0.8 is 17 % higher, while the total amount of pore water extortion is equal between the two values. The amount of creep in the latest steps is on the other hand smaller for a K0′ at 0.8. This indicates that the soil is handling the stress level better than with a K0′ at 0.6.

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