Ground improvement at the Port of Brisbane, Australia using vertical drains and vacuum assisted preloading

Abstract

Soft clays in coastal areas have low shear strength and high compressibility. Thus construction activities for infrastructure developments in these deposits often pose geotechnical problems due to large time dependent settlements and lateral movements. Ground improvement techniques are adopted to reduce the water content of the soft clays by preloading techniques with vertical drains. Depending on the magnitude of the surcharge used substantial immediate settlement with lateral movements can takes place during preloading. This in turn causes stability problems in the loaded areas. The use of vacuum assisted preloading has now become a popular method in Australia where substantial loads need to be carried out to meet a desired rate of settlement and mitigate undrained failure. To assist the vacuum propagation to significant depths, vertical drains are used in conjunction. At the Port of Brisbane, Australia, vacuum assisted surcharge preloading and conventional surcharge preloading schemes were used to reduce the time required for consolidation and long term settlement in soft Holocene clays. The design of the combined vacuum and surcharge fill system and construction of the embankment are described in this paper. A comparison made on the performance of a combined vacuum surcharge loading system with a standard surcharge fill highlights the clear benefits of vacuum consolidation. Field monitoring data on surface and sub-surface settlements, pore pressures and lateral movements on test embankments performed during construction are presented. An analytical solution for radial consolidation that considers both time-dependent surcharge loading and vacuum pressure to predict the settlement and associated excess pore pressures in soft clay deposits is also proposed.