Pile driving induced liquefaction instability in sandy soils

Abstract

Sandy soils are characterized by negligible cohesion (compared to clay) and limited drainage capacity (compared to gravel), which makes these soils susceptible to excess pore (water) pressure accumulation induced by short term dynamic loading cycles. Pile driving subjects the soil to such type of dynamic loading. Accumulation of excess pore water pressure continuous until contact between the sand particles is lost, a process known as soil liquefaction.
This research investigates two soil configurations which are vulnerable to pore pressure accumulation and subsequent liquefaction induced by pile driving: slopes and confined aquifers. Slopes are vulnerable because of their geometry. Limited accumulation of excess pore pressures might be sufficient to induce sliding failure. Confined aquifers are vulnerable because of the lack of drainage boundaries. This can result in significant excess pore pressure accumulation in the aquifer.
Pile driving induced liquefaction instability is modelled by considering the pile-soil-plug interaction, the emission, propagation and attenuation of waves into the soil domain and the resulting generation of excess pore pressures. Two models are developed for this purpose.
The first is an cylindrically symmetric damped elastic pile-soil-plug model. This model calculates the vertical and radial displacements in the soil domain as a function of space and time. Shear stresses, which are the driving parameter for the second model, are then calculated as a function of the spatial derivatives of the two displacement components.
The second (liquefaction) model is a combination of the governing differential equation for cylindrically symmetric soil consolidation and an empirical model describing generation of excess pore pressures as a function of shear stress amplitudes. The output of this model is the steady state relative overpressure distribution. The relative overpressure distribution can then be used as input into slope stability analysis software in order to investigate whether a slope failure will occur given the reduced soil strength.
The biggest uncertainty in the second model is the coupling with the first model. This coupling is achieved by adjusting the elastic shear stresses (which are unrealistic and can therefore not be directly used as input for the liquefaction model) calculated in the first model. Three variables are defined for this purpose. Analysis of the sensitivity of these variables show that the variable beta, which is the ratio between yield shear stress and initial effective stress of a liquefied interface, is the most sensitive and therefore most uncertain variable in the model. Experimental validation of beta is recommended to the increase accuracy of the liquefaction model.