Discrete Element Modelling of Pile Penetration and Setup in Crushable Sands


Student thesis: Doctoral Thesis

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Awarding Institution
Award date14 Dec 2017


Driven piles are deep foundation elements that are driven into a design depth or resistance, and widely used to support buildings, towers, bridges and tanks due to their easy construction procedure. However, considerable uncertainties exist in current design approaches because of complex driven mechanism. This complex mechanism results from the large deformation together with particle crushing around the pile tip and migration of crushed particles through the matrix of uncrushed soil. Numerical discrete element method (DEM)-based pile penetration tests, which allow full access to the particle-scale information, offer an alternative way for the investigation of pile penetration.

A two-dimensional (2D) DEM model of driven piles in crushable sand was first developed using PFC2D. A new stress normalization method was adopted to synthesize the data at different driven depths in deep penetration tests. Numerical experiments were conducted to investigate the effects of a few parameters. Specifically, the influence of lateral boundary condition on the shaft resistance of a driven pile was investigated by comparing the simulation results from the pile penetration test and the interface shear test under three different types of normal boundary condition, namely, constant normal load (CNL), constant normal stiffness (CNS), and constant volume (CV) boundary conditions. Through the incorporation of the rate process theory (RPT) based creep contact model considering rolling resistance and a probabilistic particle fracture model satisfying mass conservation into a threedimensional (3D) DEM simulation, the coupled effects of the interparticle sliding and delayed particle fracture, and the influences of a few parameters on the creep behavior in one-dimensional (1D) compression were investigated. Adopting the same contact and fracture model, a large-scale 3D model of driven pile was developed to investigate the mechanism of pile setup.

It is found that the normalized vertical and horizontal stress fields surrounding the pile show invariable pattern of stress distribution, suggesting a unique penetration mechanism independent of the penetration depth. The behavior of a particle group has reached the peak state below the pile tip and the critical state after it reaches the pile shaft. The influence of normal boundary condition on the stress ratio at the critical state is not obvious. The creep deformation in 1D compression is mainly caused by stress redistribution at low vertical stress, whereas particle rearrangement and particle breakage becomes more prevailing with the increase of vertical stress. The positive gain in pile shaft capacity with time is caused by the increase contact number due to rearrangement of particles, decrease in contact stress ratio owing to contact creep at the micro-scale, and the associated decrease in gradient of the stress contour at the meso-scale.