Determining the behaviour of sand under cyclic loading in the direct simple shear apparatus

Abstract

In the past years, significant advances have been made in the analysis of the dynamic behaviour of soils and many analytical models, in-situ and laboratory techniques, and apparatus have been developed. In order to be able to predict the behaviour of a certain soil under cyclic loads, the damping ratio and the shear modulus of that soil have to be determined. The objective of this thesis is to study the factors such as shear strain amplitude, vertical pressure, number of loading cycle and relative density which affect the shear modulus and damping ratio of clean dry sand subjected to cyclic simple shear tests. For this purpose several samples of sand with relative densities of 50% and 65% were subjected to strain-controlled dynamic loading using Direct Simple Shear Apparatus developed by Norwegian Geotechnical Institute. In this study, five different vertical pressures namely 0.5, 1.0 , 2.0 , 4.0 and 8.0 kg/cm2 and the strain levels of about 0.25 % , 0.50 % , and 0.75 % were applied to the specimens. Prescribed deformations were applied to each sample and shear stress-strain hysteresis loops were obtained in order to determine shear modulus and damping ratio at the corresponding strain levels. Shear modulus and damping ratio values obtained from each test were plotted versus shear strain amplitudes under various testing conditions in order to observe the factors affecting shear modulus and damping ratio. The test results have shown that, shear modulus decreases and damping ratio increases with increasing shear strain. Increasing vertical nressure, number of loading cycles, or relative density causes an increase in shear modulus. On the other hand, damping ratio increases with decreasing relative density, vertical pressure or number of loading cycles.