Land subsidence due to groundwater drawdown in Shanghai

INTRODUCTION Shanghai is located on the deltaic deposit of the Yangtze River, as illustrated in Fig. 1. Fig. 2 shows the main geological strata. The total thickness of the Quaternary deposit is about 300 m (Shanghai Geology Office, 1976, 1979). Excessive pumping of the groundwater caused compression of the Quaternary deposit, and subsidence in Shanghai. Although several countermeasures, including recharge of the groundwater, have been adopted to mitigate the rate of subsidence, up to the present the recorded cumulative subsidence has been 2–3 m in the central area of Shanghai. This subsidence has caused a lot of social problems. The immediate problem is the increase in the possibility of flooding. From 1981 to 1994, rainfall flooding occurred 22 times, at a rate of almost twice per year (Liu, 2001). The possibility of tidal flooding also increased. From 1956, the height of the dike along the coast line was increased four times, with the crest elevation rising from 5 m to 6.8 m. Other problems caused by the land subsidence are damage to the sewerage system, road, buildings, and subway tunnels etc. In Shanghai the monitoring of land subsidence started in 1921. The history of land subsidence in the urban area of Shanghai can be divided into two periods: namely a rapid subsidence period (1921–1965) and a controlled period (1965–present) (Zhang & Han, 2002). However, there has been a tendency for the subsidence rate to increase after 1990. In this paper, the mechanism of land subsidence due to excessive pumping of groundwater is discussed first. Then the measured field data are presented, and the relationship between groundwater pumping and land subsidence is discussed.