Analysis of Consolidation and Deformation of Soft Subsoil under Vacuum

Recently in China the effectiveness and advantage of the vacuum method of subsoil stabilization have been recognized. In this paper the Biot's consolidation theory was used to analyze the coupled consolidation and deformation of clay layers under vacuum in one of improved areas of 50 × 60m at Xingang Harbour. The subsoil in this area consists of three layers of soft clay with overall depth of 15m. Before applying vacuum sand drains with length of 10m were installed and the ground surface was covered with a thin layer of sand. The soils were modelled as an isotropically hardening plastic material. Two models were used, namely the well-known Cambridge model and the double yield surface model （NHRI-model in abbreviation） proposed by the first author of this paper. In addition,a simple elastic model using elastic bulk modulus calculated from the compression index was also used for comparison. The actual three-dimensional problem was simplified as a two-dimensional one by regarding the sand drains as thin sand walls. Computation was carried out in 24 time steps to simulate the vacuum drain process. The surface settlement and the inward lateral displacement of surrounding soils computed by use of simple elastic model and the NHRI-model are comparable to the measured ones, but the Cambridge model gives some lower values.