The Bangkok subsoil that forms part of the larger Chao Phraya Plain consists of a broad basin filled with sedimen-tary soil deposits that form alternate layers of sand, gravel and clay. The depth of the bedrock is still undetermined, but its level in the Bangkok area is known to vary between 400 m and 1800 m deep. The aquifer system beneath the city area is very complex, and the deep-well pumping from the aquifers over the last 50 years or so has caused substantial piezometric drawdown in the upper soft and highly compressible clay layer. In this paper, the influence of the piezometric drawdown on the performance of vertical drains with surcharge on the upper clay layer is examined. Instrumented test embankments were built and the performance of large-diameter sand drains, sand wicks and prefabricated vertical drains was examined. A novel interpretation technique was adopted to evaluate the settlements from pore pressure dissipation and to compare with those measured directly from settlement measurements.
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