Numerical modelling of soft soil stabilized by vertical drains, combining surcharge and vacuum preloading for a storage yard

This paper presents a finite element analysis of a case study of a combined vacuum and surcharge load through prefabricated vertical drains (PVD) at a storage yard at Tianjin Port, China. The top 15 m of soil at this site was very soft to soft, and needed to be improved using preloading surcharges of more than 140 kPa. To avoid any stability problems associated with a high surcharge embankment, 80 kPa vacuum pressure combined with fill surcharge was applied (40 and 58 kPa for Sections I and II, respectively). A plane strain analysis was performed using equivalent permeability and transformed unit cell geometry. The converted (equivalent) parameters were incorporated in the finite element code ABAQUS, using the modified Cam-clay theory. The performance of a trial embankment at the site of the storage yard is predicted on the basis of a constant vacuum pressure applied on the soil surface and distributed along the length of the drain. The predictions of settlement, pore water pressure and lateral displacement were compared with the available field data, and an acceptable agreement was found based on this numerical approach. The combination of vacuum and surcharge load can effectively shorten the preloading period, reduce the height of the embankment and counterbalance excessive lateral displacements.