Abstract

Bangkok, the capital city of Thailand, has experienced rapid growth over the last two decades and is relatively prosperous by Asian standards. The city is situated in the lower half of the flat deltaic plain of the River Chao Phraya and lies at ground elevations of between 0.5 and 1.5m above mean sea level, although some areas are even lower and lie below mean sea level. Drainage of the area is poor and takes place by way a network of sluggishly flowing waterways known as ‘klongs’.

A recent report by the Asian Institute of Technology (A. S. Balasubramaniam et al. 1992)indicated that subsidence phenomena were beginning to be experienced in the city in the early 1960s when rapid population growth resulted in excessive deep well pumping to satisfy the increased demand for water. At present there are approximately 15000 tube wells drilled through the upper 200 m of superficial deposits, with approximately 600 new abstraction licences granted in 1993.

The area in and around the city is underlain by recent marine clay (known as Bangkok Clay) which overlies older deposits of Pleistocene age. These older deposits, which comprise an upper stratum of stiff clay overlying alternating clays, sands and beds of gravel, are of significant thickness. The total depth of superficial deposits overlying the basement rocks in the Bangkok area varies between 500 and 2000m (Anon 1981).

The upper sequence of very soft to soft Bangkok Clay and the underlying stiff clay stratum is consistent over much of Bangkok and

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