Recent advances in soft soil consolidation
Many metropolitan cities are situated along coastal belts, which are composed of very soft alluvial and marine clays. Because of low shear strength and high compressibility, the soft soils in these areas are not suitable for construction without appropriate ground improvement. To ensure stability during construction and reduce long-term settlements, it is necessary to implement a preconstruction technique in the soft soil site on which infrastructures are to be built. A common technique is to use prefabricated vertical drains (PVDs), which, combined with surcharge loading and vacuum, occurs before the construction of many essential coastal line infrastructures such as airports, railway tracks, and commercial buildings worldwide. This technique has been proved to be an effective way to expedite soft soil consolidation. Firstly, the radial drainage paths allowed by PVDs accelerate the dissipation of excess pore pressure under the surcharge loading. Secondly, negative pore pressure created by the vacuum accelerates the consolidation process as well as controlling lateral displacement. This chapter introduces the basic principles of PVDs combined with surcharge loading and vacuum, along with the illustration of two types of PVD systems: the membrane system and the membraneless system. The numerical conversion method from 3D to 2D and a constitutive model for soft soils under cyclic loading is presented, along with case histories of the Port of Brisbane, the Pacific Highway in Ballina, the Sandgate Rail Grade Separation Project in Australia, and the Tianjin Port in China. PVDs with vacuum-assisted preloading was utilized in each of these projects and has made significant contributions to preconstruction consolidation. Design charts are introduced for design of PVDs combined with surcharge loading and vacuum in industry.