Three-Dimensional Discrete Element Simulation of Cavity Expansion from Zero Initial Radius in Sand

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Rigid inclusions, generally made of structural concrete, are widely employed to reduce the settlement and enhance the bearing capacity of the ground by transferring loads from superstructures through weak soil layers to a firm underlying stratum. However, the installations of rigid inclusions such as driven piles and controlled modulus columns can induce irreversible changes of the soil stress - strain state, and lead to excessive lateral soil movements during the auger penetration or pile driving/hammering process. This thesis proposes a rigorous numerical modelling to investigate the installation effects of rigid inclusions on surrounding ground via cavity expansion simulation adopting discrete element method. The benefits of adopting the discrete element method is attributed to its capability in simulating large displacements and distortions, as well as incorporating the discontinuous nature of granular materials and providing a microscopic insight into the problem. True scale three-dimensional discrete element models simulating the creation of cylindrical cavities from zero initial cavity radius in dry clean and lightly cemented sands are developed. Contact constitutive models mimicking the behaviour of dry clean granular materials and lightly cemented sands are calibrated against existing laboratory experimental results. The numerical models proposed contain up to 500,000 particles with boundary conditions carefully selected to reproduce realistic scenarios. Embedded scripting is adopted to precisely record both the local and global stress and strain variations, as well as the cementation bond breakage during the cavity expansion process. The results confirm that the selection of arbitrary initial cavity radius could significantly influence the soil response at the earlier stages of the cavity expansion. For a given expansion volume, creating a cylindrical cavity from zero initial radius induces larger stresses in the soil compared to expanding existing cavities in the same soil medium from a nonzero given initial radius. This implicates that the estimation of the pile driving force may be largely underestimated adopting the approximation method based on the existing cavity expansion theories, requiring an assumptive initial cavity radius. In addition, the soil lateral displacements, depending on the gradation and the relative density, can reach up to 30 𝑅𝒸 (𝑅𝒸 is the radius of the pile) during the installation, and the loose sand in a plane strain condition can even exhibit dilation during the early stages of the cavity expansion. In the lightly cemented sands, the installation of rigid inclusions or cavity expansion can lead to significant cementation degradations. The influence zone of cementation degradation observed in cemented sand with various cement content can extend to approximately 4𝑅𝒸, in which the shear strength of the soil is significantly reduced due to the cementation bond breakage, which may lead to the reduction in axial capacity, adversely influencing the pile toe stability. Within this influence zone, the displacement induced by the installation is not sensitive to the level of cementations, while soils with higher cement content are expected to experience larger radial displacements beyond this influence zone. Hence, extra care must be taken by practicing engineers when assessing the required pile driving pressure and the installation effects of ground inclusions in the vicinity of existing structures such as pipelines and bridge abutments in both granular materials and lightly cemented sand.
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