Land Subsidence due to Pumping of Ground Water

Abstract

Recently the subsidence has been found extraordinarily rapid in large cities such as Tokyo and Osaka. It amounted to as large as 22 cm in a year in Osaka. In these cities, the lowered coastal areas have suffered frequently from invasion of the sea water raised by typhoons.For the purpose of observation of subsidence, precise leveling has been performed almost yearly in these cities. Besides it, many iron pipes were installed penetrating clay layers and having foundation on an aquifer. The pipes have been uplifting from the surface of the ground and their amounts were recorded automatically. The upheaval corresponds to the compaction of clay layers penetrated by the pipes.These data reveal that, in Osaka, the half of the amount of the compaction occurs in the upper 30 m layer, while the whole compaction is confined to the layer above 200 m depth.In the eastern part of Tokyo, the depth of alluvial formation reaches 50 m, which in Marunouchi, the central part of Tokyo, it is about 20 m. These districts have suffered from subsidence for about 20 years.Some of Japanese physicists and engineers have noticed the effect of water table depression on subsidence. Dr. K. Wadati, based on his observation at Osaka, found that the subsidence velocity is approximately proportional to the water table depression, and the rapid subsidence is a result of overpumping of ground water. Further, his theory suggested that, when the water table is raised sufficiently by any means, subsidence is to be ceased, though the ground surface can not be uplifted. This was accidentally verified by the severe American bombing in these two cities near the end of the World War II, ruining factories and stopping the ground water use. Immediately after that the water table recovered and subsidence was nearly stopped. But as soon as the ground water use was again reopened, the subsidence began again.Land subsidence is also caused by pumping of irrigation water, though it is not so serious as in the case of industrial water. In the reclaimed land along the lower course of the River Kiso, there are several number of wells for irrigation, and subsidence provoked of serious saline disaster by sea water invasion.The mechanism of clay layer compaction is outlined according to Drs. Wadati and Hirono's theory.