The city on soft clay
Bangkok rests on a geologically young and unstable foundation. The entire city is built upon the vast, flat delta of the Chao Phraya River. This landscape is the product of thousands of years of river sediment deposition interacting with changing sea levels during the Holocene epoch, which began roughly 11,700 years ago. Around 6,000 years before present, the sea level in the Gulf of Thailand was about 4 meters higher than it is today, turning the area into a wide marine embayment. As the sea slowly receded to its present level, the river deposited enormous quantities of fine-grained sediment, forming the delta.
The most important of these deposits is a layer of soft marine clay known to geologists as "Bangkok Clay". This water-saturated layer blankets the region and has an average thickness of 15 to 20 meters, though it can exceed 30 meters in some areas. It is highly compressible, meaning it compacts easily under weight. Below this soft clay lie at least eight distinct sand and gravel aquifer layers, separated by stiffer clay. The city's very existence relies on this layered sequence of soft clay and water-bearing sand laid down over millennia.
Pumping a city downward
The natural process of sediment compaction is slow. Bangkok's rapid sinking is caused by human activity, primarily the massive extraction of groundwater. Starting in the mid-20th century, rapid urbanization and industrial growth led to an enormous demand for water that public supplies could not meet. As a result, industries and residents began pumping vast quantities of water from the aquifers beneath the city. By the early 1980s, groundwater extraction peaked at over 1.2 million cubic meters per day.
This intense pumping drastically lowered the water pressure within the sand aquifers. The water in the pores of the sand helps support the weight of the overlying clay layers. When the water is removed, that support is lost. The weight of the city above—buildings, roads, and infrastructure—squeezes the water out of the soft Bangkok Clay, causing the clay particles to pack together and the ground surface to sink. During the peak pumping period of the late 1970s and early 1980s, parts of eastern Bangkok were subsiding at a rate of more than 10 centimeters per year. In some areas, the cumulative subsidence since the 1930s has exceeded 1.5 meters.
Government regulations, including the Groundwater Act of 1977 and new pricing policies, have significantly reduced the rate of pumping. This has slowed the subsidence rate to the current 1-2 centimeters per year in most of the city. However, the compression of the clay is a time-delayed process, and the city continues to sink even with reduced pumping. The sinking ground exacerbates flooding, damages building foundations, and makes the city vulnerable to rising sea levels in the Gulf of Thailand.