Non-linear analysis of soft ground consolidation at the Ballina by-pass

This study presents a numerical assessment of consolidation under vacuum preloading with a system of vertical drains and membrane, considering the non-linear properties of the soil This membrane system, already widely used in Australia, is where an airtight membrane is placed over the drainage layer to allow a vacuum to be distributed within the sand platform, along the surface of the soil, and down the vertical drains. To date, there have only been a few fundamental investigations and field studies that have tried to assess the way in which the vacuum can propagate in soft clay, both laterally and vertically. In this model, both vertical and horizontal drainage was considered to reflect more realistic in-situ conditions. Moreover, the change of compressibility and permeability during consolidation was taken into consideration, including a possible loss of vacuum along the length of the drain. This model was initially verified for a single drain using large scale laboratory testing, and subsequently applied to a fully instrumented case study, namely the Ballina Bypass (along the Pacific Highway, NSW). The numerical solutions capturing the lateral distribution of the vacuum provided accurate predictions of the pore water pressure and associated settlement.