PVD IMPROVEMENT OF SOFT BANGKOK CLAY WITH COMBINED VACUUM AND REDUCED SAND EMBANKMENT PRELOADING

Abstract

The proposed site of the Second Bangkok International Airport (SBIA) comprising a total land area of 8.0 km by 4.0 km, is situated on a 16.0 m thick very soft to soft Bangkok clay. At the proposed site, a previous study has been successfully conducted involving the use of prefabricated vertical drain (PVD) with conventional preloading using sand embankment surcharge. Vacuum-assisted consolidation provides an alternative in reducing the length of preloading period. In this method, the soft clay foundation is preloaded by reducing the pore pressures through the application of vacuum pressure in combination with reduced amounts of sand surcharging. Two full scale and fully instrumented test embankments each with base area of 40 m by 40 m were constructed. In Embankment 1, hypernet drainage system combined with 15.0 m PVD length were used. For Embankment 2, perforated and corrugated pipes combined with nonwoven heat-bonded geotextiles were used as drainage system combined with 12.0 m PVD length. Among the foundation instrumentation, vibrating wire piezometers were installed in the foundation subsoil at varying depths to measure both negative and positive pore pressures. The undrained shear strength obtained after improvement was found to be 1.5 to 2.0 times higher than before improvement. Embankment 2 indicated higher drainage efficiency demonstrating 20% to 30% accelerated settlement rate compared to Embankment 1. After 45 days of vacuum pressure application, the test embankments were raised to a maximum height of 2.50 m. The surface settlements in Embankments 1 and 2 were 0.74 m and 0.96 m, respectively, after 140 days. Finite element methods (FEM) was utilized to investigate the influence factors. (A)