Vacuum consolidation and deformation analyses - case studies in Japan
Two methods, namely the air-tight sheet method and the vacuum-drain method, are generally used to conduct vacuum consolidation in the field. The air-tight sheet method involves the placement of an air-tight sheet on the ground surface and embedding its periphery in the ground. A vacuum is then applied to the air or water in the soil below the sheet to induce soil consolidation. In the vacuum-drain method a surface or subsurface clayey soil layer is used as a sealing layer without the need for an air-tight sheet. These two methods have their various advantages and disadvantages. For example, for the case where an underwater clay deposit is consolidated by application of a vacuum pressure, the vacuum-drain method has an obvious advantage; while for the case requiring consolidation of a soil layer that extends from the ground surface, the air-tight sheet method should be used. Two typical case histories in Japan are presented: one in which the vacuumdrain method was adopted, and the other which involved pre loading. as part of a highway construction, using a combination of embankment loading and vacuum pressure employing the air-tight sheet method. It is shown that under vacuum pressure loading, the ground deformation can be calculated reliably using the method proposed by Chai et al. in 2005. For the case involving a combination of vacuum pressure and embankment load, the settlements under the embankment centreline can be reasonably calculated assuming one-dimensional (1 D) deformation conditions. However, it is also noted that there is no well-established method for calculating lateral displacements of the ground associated with these ground improvement methods and further research in this area is needed.