Micro-mechanical study of sand fabric during earthquake

The seismically induced interface friction capacity of piled foundations is a function of the soil-pile interface friction, the relative stiffness of the pile and the soil and the volume changes in the soil adjacent to the pile which affect the strength and stiffness of the soil. In order to fully understand this interface it is necessary to study the behaviour at the micro level which is possible with Discrete Element Modelling. A 2D DEM model was developed that simulated a section of an inflexible pile in a coarse grained soil that was subject to cyclic shear load. The model had to be representative of typical soil behaviour so it was necessary to carry out a sensitivity analysis to investigate the effect the micro behaviour had upon the macro properties, properties that are typically interpreted from laboratory tests such as triaxial tests. It was necessary to develop appropriate boundary conditions that allowed shear to be applied and dynamic deformable boundary particles to absorb some of the energy. In order to appreciate the stability of the particles, a new fabric quantity called the “symmetric geometric deviation index” was developed to show the deviation of the contact points from a symmetric, stable distribution.
The results of the sensitivity analysis showed that the macro stress strain response and dilation behaviour with deformable boundary particles is more representative of actual behaviour than that with rigid boundaries. Further, the symmetric geometric deviation index was constant post peak for deformable boundaries, suggesting critical state conditions whereas it continued to change with rigid boundaries.
A study of the impact of size of the element and the boundary conditions led to the development of DEM model which could be used to simulate the effect of a horizontal cyclic shear load applied to the base of the element. A comparison between a soil element with deformable boundaries and a soil element adjacent to a pile showed that the pile increased the shear stress in the soil which would lead to greater deformation consistent with observations in practice.