The influence of Piezometric Draw Down on Settlements in Test Embankments with Vertical Drains and Surcharge in the Bangkok Sub-soils

Abstract

The Bangkok sub-soils which is a part of the larger Chao Praya Plain consists of a broad basin filled with sedimentary soil deposits which form alternate layers of sand, gravel and clay. The profile of the surface of the bedrock is still undetermined, but its level in the Bangkok area is known to be between 550 m to 2000m. The aquifer system beneath the city area is undoubtedly very complex and the deep well pumping from the aquifer over the last fifty years or so have caused substantial piezometric draw down in the upper clay layer, which is soft and highly compressible. In this paper the influence of the piezometric drawdown on the performance of vertical drains with surcharge in ground improvement works is examined in detail on two sites for construction of a Naval Dockyard and a new international airport. Also , the earlier work of Eide (1977)on a test section at a third site in the Bangkok-Siracha Highway is briefly included to complete the full presentation of the performance of test embankments with vertical drains in the Bangkok sub-soils. Test embankments with sophisticated instrumentation were built and the performance of sandwicks, large diameter sand drains and prefabricated vertical drains (PVD) were examined. A novel interpretation technique was adopted in evaluating the settlements from pore pressure dissipation and to compare with those measured directly. Also, the long term settlements and the rate of decay in settlements and lateral movements were also determined to illustrate the progress of primary consolidation and the onset of secondary consolidation. This is to illustrate that the primary cause for the settlement on a long term basis is due to secondary consolidation, rather than due to hydraulic connections or undrained creep.