Application of vacuum preloading with vertical drains in soft soil foundation of rapid-transit tram and numerical simulation

Due to the requirements for the rate of settlement and the increase in the undrained shear strength upon consolidation, the vacuum preloading with vertical drains has been widely used to reinforce soft ground. A field case study improved by the vacuum preloading with vertical drains in Nanjing, China is introduced. A numerical model is established based on the field case. The measurements including vertical settlement at the surface of subsoil, multi-level settlement at the centerline of the subsoil and pore water pressure at different depths are presented and compared with the numerical results. The numerical results overestimate the measurements in the field with the differences in the range of 2.4%~32.5%, which validates the accuracy of the numerical model. By conducting the comparison analysis of parameters including length, permeability coefficient and spacing of prefabricated vertical drains (PVDs), it is found that the length of PVDs is the most influential factor for the maximum surface settlement of soft soils. The degree of consolidation at the final stage of vacuum preloading is 93.4% assessed based on the settlement, and is 88.3% estimated according to the pore water pressure. It is demonstrated that the vacuum preloading with vertical drains is an effective method to reinforce soft soils.