Methods of vacuum consolidation and their deformation analyses

Two methods, namely the air-tight sheet method and the vacuum-drain method (sheetless), are generally used to conduct vacuum consolidation in the field. The advantages and disadvantages of both methods and the techniques for preventing vacuum leakage through a middle sand layer in a deposit, as well as the method for maintaining vacuum pressure when large settlement occurs, have been described. Vacuum pressure is an isotropic consolidation pressure applied to a soil deposit, and the deformation induced is different from that induced by a surcharge (e.g. the weight of an embankment). Two typical case histories in Japan are presented and deformation analyses were conducted. In one case history the vacuum-drain method was used with vacuum pressure alone, and in the other a combination of embankment loading and vacuum pressure was applied employing the air-tight sheet method. It is shown that under vacuum pressure loading, the ground deformation (settlement and lateral displacement) can be calculated reliably using a method proposed previously. For the case involving a combination of vacuum pressure and embankment load, the settlements under the embankment centreline can be estimated reasonably assuming one-dimensional deformation conditions.