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In order to investigate the performance behavior of soft ground improvement using different vacuum consolidation methods (VCM) and different PVD thicknesses, four trial sections namely C1, C2, D1, and D2 were constructed. VCM without airtight membrane using cap drains and direct tubing system (VCM-DT) were used for the first two sections with PVD thickness of 3 mm and 7mm for C1 and C2, respectively. VCM with airtight membrane and band drains (VCM-MB) were applied for the last two sections for D1 and D2with PVD thickness of 3 mm and 7 mm, respectively. The soil conditions, construction procedures, instrumentation program, and monitored results of the above trial sections are presented in this paper. The results confirmed that the effective vacuum pressure in PVD mainly depends on vacuum consolidation methods and the assumption of uniform distribution of vacuum pressures along the PVD depth which can be suggested for practical design. For VCM-MB using PVD thickness of 3 mm arranged in triangular pattern of 0.9 m spacing, the degree of consolidation of more than 90% can be achieved in less than 8 months of vacuum pumping. However, for VCM-DT, further investigation is needed for preventing air leakage in vacuum system particularly for the case of thick soft clay deposits with large deformations during the preloading.

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An adequate site investigation of soft clays is important for construction of oil and gas facilities on coastal areas in the Mekong Delta. This paper presents an integrated geotechnical–geophysical study of the soft clay deposit at the Ca Mau site, located on the west coast of southern Vietnam. The geotechnical data were analyzed to provide a detailed characterization of the subsoil profile and soil parameters that can be used in the design works. It is also the first time the electric imaging procedure has been successfully applied in the Mekong Delta to assist the local geotechnical engineers in site investigation of a very soft clay deposit.