An evaluation of vacuum consolidation performance using observational techniques
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Soft clays in coastal areas normally have low shear strength and high compressibility. The construction activities for near shore infrastructure on these deposits often pose geotechnical problems due to large time dependent settlement and lateral movement. Ground improvement techniques are used in this terrain to reduce the water content of soft clays by preloading with vertical drains. Depending on the magnitude of the surcharge, substantial and immediate settlement with lateral movement can occur during preloading, which then causes problems of undrained stability in the loaded areas. Vacuum assisted preloading has now become a popular method of ground improvement in Australia where substantial loads must be applied to meet a desired rate of settlement and mitigate undrained failure. To assist vacuum propagation at significant depths, vertical drains are usually used in conjunction. At the Port of Brisbane and the Ballina Bypass, 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. Field monitoring data are presented to demonstrate how the embankment performed during construction. The paper also evaluates the relative performance of the two contrasting preloading systems (i.e. vacuum and non-vacuum system) using a dimensionless analysis. Here the dimensionless parameter can be considered to act as a 'filter' to distinguish the relative performance of the improved foundations for vacuum combined surcharge loading, even if the shape of the time settlement curves and the degree of consolidation are similar.