Modelling of soft ground consolidation via combined surcharge and vacuum preloading

Abstract

A system of vertical drains combined with vacuum and surcharge preloading is an effective approach for promoting radial flow to accelerate soft soil consolidation. Whenever a substantial load is required to meet the desired rate of settlement and if the cost of surcharge application is also significant, application of vacuum pressure with a reduced surcharge fill height can be used instead. The equivalent 2-D plane strain solution for multi-drain analysis is described including the effects of vacuum pressure and smear zone caused by mandrel driven vertical drains. A large-scale radial consolidometer is used to determine the extent of the smear zone, and the laboratory results are explained in detail. The equivalent (transformed) permeability coef- ficients are incorporated in finite element codes, employing the modified Cam-clay theory. The 2D and 3D numerical multi-drain analyses are conducted to predict the excess pore pressures, lateral and vertical displace- ments. The performance of two selected case histories are discussed and analysed, including the sites of Tianjin Port, China and Ska-Edeby embankment, Stockholm, Sweden. The numerical predictions are then compared with the available field data. This research demonstrates that, apart from realistic 3D numerical modelling, the equivalent plane strain solution can now be used as a predictive tool with an acceptable accuracy as a result of the significant process.