Case study and numerical simulation of PVD improved soft Bangkok clay with surcharge and vacuum preloading using a modified air-water separation system

This manuscript presents the case study of soft Bangkok clay improvement and simulations using combined vacuum pressures with pre-existing surcharge embankment preloading, including installations of air-tight membranes and horizontal prefabricated drains connected to the top of the PVDs as well as to the vacuum system with a modified air-water separation system. The 17-m-long PVDs were installed from the top of the embankment in a triangular pattern at a spacing of 0.9 m. Monitoring instruments were installed to measure surface settlements, lateral movements, and pore water pressures in the soft clay layer. The subsequent analyses included settlement calculations, settlement predictions using observational methods, flow parameter backcalculations, soil property comparisons before and after improvement, and FEM simulations. The very soft to soft clay was transformed to medium-stiff clay because its undrained shear strengths and maximum past pressures increased, and its water contents, void ratios, and compression indices decreased. The analyses of the degree of consolidation utilized one-dimensional and observational methods as well as a numerical simulation for the prediction of consolidation settlements and comparison to the field data. The results illustrated the effectiveness of the modified vacuum-PVD system. The findings could be used for guidance of similar soft clay improvement projects.