Monotonic, cyclic and post-cyclic shear behaviour of granular material under multi-directional simple shear

In most geotechnical applications, soil is subjected to complicated loading conditions. For example, during an earthquake, the soil can be liquefied by randomly oriented ground movements with different loading magnitudes. Currently, this complex in-situ loading conditions can only be duplicated in the multi-directional simple shear devices. With this kind of apparatus, more attentions have been paid on the study of how the multi-directional loading conditions and loading histories can affect the soil behaviour. To undertake a comprehensive investigation on the shear behaviour of granular material subjected to complex loading conditions, in this thesis, several multi-directional simple shear tests were performed by the Variable Direction Dynamic Cyclic Simple Shear (VDDCSS) system. In this testing system, two orthogonal shear actuators can be controlled independently to implement the shear stress in any direction on the horizontal plane. Through this apparatus, the uni- and multi-directional loading paths were conducted on the uniform-sized glass beads in monotonic, cyclic and post-cyclic tests. In the monotonic tests, the effects of relative density and effective vertical stress have been examined in the undrained and drained conditions. More importantly, in another series of tests, the specimens were subjected to two successive shear stresses, between which, the angle varies from 0o to 180o. The results show that the angle between these two shear stresses makes significant impacts on the shear strength, pore pressure buildup, volumetric strain and the level of non-coaxiality. In the cyclic tests, the specimens were cyclically sheared under the uni- and multi-directional loading paths. The results show that the liquefaction resistance is significantly influenced by the relative density, effective vertical stress, shear strain amplitude and loading path. To interpret the difference of liquefaction resistance under different loading paths, an energy dissipation theory is adopted in the discussion, and it is found that the dissipated energy to liquefy a specimen is independent of the loading path. In the post-cyclic tests, two main series of tests were conducted on the glass beads. In the first series of tests, the liquefied specimens were subjected to the undrained monotonic shearing after they were re-consolidated. The effect of the initial liquefaction loading history on the post-liquefaction settlement and shear behaviour have been studied under various relative densities. Conversely, in the second series of tests, the specimens were subjected to the first cyclic shear stress under different liquefaction degrees, loading paths and loading angles (for the uni-directional case). After re-consolidation, the specimens were then subjected to the second cyclic shear stress in the x-direction until liquefaction happened. The results show that the above-mentioned factors in the first cyclic shearing stage can considerably affect the settlement, post-liquefaction shear strength and post-cyclic liquefaction resistance of the tested material