Current State of the Art in Vacuum Preloading for Stabilising Soft Soil

In this paper the analytical solutions for radial consolidation that include time dependent surcharge loading and vacuum pressure are proposed, whilst also considering the impact of the parabolic variation of permeability in the smear zone. The use of the spectral method for multilayered soil consolidation is introduced and verified. The Elliptical Cavity Expansion Theory is used to predict the extent of soil disturbance (smear zone) caused by the installation of mandrel driven vertical drains. The predicted smear zone is then compared to the data obtained from large-scale radial consolidation tests. Furthermore, the advantages and limitations of applying a vacuum through vertical drains are discussed using the proposed solutions. The vacuum pressure applied generates a negative pore water pressure that increases the effective stress within the soil, which leads to an accelerated consolidation. Vacuum pressure is modelled as a distributed negative pressure (suction) along the length of the drain and across the surface of the soil. Analytical and numerical analyses that incorporate the Authors' equivalent plane strain solution are conducted to predict the excess pore pressures, lateral and vertical displacement. The application of the theoretical models for selected case histories at the site of the 2nd Bangkok International Airport and the Port of Brisbane, are discussed and analysed. The predictions are compared with the available field data and show that the proposed model can be confidently used to predict the performance with acceptable accuracy through rigorous mathematical modelling and numerical analysis. The research findings verify that the role of the smear zone and vacuum distribution can significantly affect the consolidation of soil, but these aspects need to be modelled appropriately to obtain reliable.