Coupled Numerical Moelling Of Vacuum Consolidation With Nonuniform Pore Pressure Distribution

Abstract

In this study, Biot’s type hydro-mechanical coupled numerical models are used to examine ground improvement of fine-grained soft soil deposits using prefabricated vertical drains (PVD) and vacuum assisted consolidation methods in combination with embankment preloading. Fully coupled numerical simulations are developed in the context of the traditional unit cell radial consolidation theory commonly applied to PVD and vacuum assisted consolidation. The theoretical justification of nonuniform stress and porewater pressure distribution under an embankment of finite dimension is examined, with reference to field observations from full-scale case studies of PVD/vacuum consolidation in the literature. The impact of nonuniform porewater pressure distribution on the traditional unit cell radial consolidation theory are examined through numerical modelling, and the theoretical compatibility of nonuniform porewater pressure distributions and unit cell radial consolidation theory is discussed. Through numerical modelling, it is observed that the traditional unit cell model of radial consolidation theory, which PVD and vacuum consolidation solutions were developed from, is functionally constrained to the assumption of uniform surcharge in the soil as the initial undrained condition. Deviations from the uniform surcharge assumption, such as nonuniform porewater pressure distribution in the soil that leads to variable porewater pressure gradients with respect to depth below the preloading embankment, or nonuniform applied vacuum pressure with depth, will effectively highlight the theoretical limitation of the traditional unit cell radial consolidation. To adequately address nonuniform stress and porewater pressure distribution in the soil, fundamental revisions to the traditional linear governing equations for PVD and vacuum consolidation are needed considering nonlinearity of the consolidation equation arising from evolving permeability and compressibility of the soil due to change in void ratio during consolidation; non-Darcian flow regime for low permeability soil; and large strain elasto-plastic behavior of the soil. In this study, considering the nonlinear soil stress-strain relationship are approximated using the Modified Cam-Clay model.