Prevention partition for land subsidence induced by engineering dewatering in Shanghai

Land subsidence in shanghai has been found for more than 70 years. In the early years, it was mainly caused by groundwater exploitation. In recent years, engineering dewatering in shallow ground (within 90 m) has become a major source for land subsidence in the rapid urbanization course. A management partition of land subsidence induced by foundation pit dewatering for the first confined aquifer was suggested.


Introduction
Shanghai is the economic and financial center of China.In 1939, land subsidence was first observed and its maximum value has reached 2.0 m until now.In the early years, the subsidence was caused by groundwater extraction for water supply.Later, the exploitation was restricted in order to control the land subsidence.However, during the rapid urbanization in recent years, lots of underground engineering were constructed, the engineering dewatering in shallow ground (within 90 m) has become a major source contributing to increasing land subsidence.Xu et al. (2009) analyzed the geohazards which may occur during the construction and maintenance of infrastructures in Shanghai, and the confined water was one of the main causes.Shen and Xu (2011) used numerical simulation to predict the behavior of land subsidence in Shanghai due to pumping of groundwater.Wang et al. (2012Wang et al. ( , 2013)), Zhang et al. (2013), Luo et al. (2008) and Zhou et al. (2010) used numerical simulation to analyze the land subsidence caused by foundation pit dewatering.However, most previous studies focused on a single foundation pit.The land subsidence of engineering dewatering in a city's scale is not concerned too much.The second confined aquifer

Geological partition for strata combination
The Quaternary sediments (Table 1) of Shanghai belong to the deposit of Yangtze River and tidal current of East China Sea.The foundation pit dewatering in Shanghai mainly relates to the first confined aquifer (layer ).The aquifer is formed in the Late Pleistocene and mainly consists of sand.
In normal deposit environments, layer is distributed above layer , but it is replaced by layers 2 and 3−4 in the ancient river erosion area.Layer is distributed between layers and , but in some area, it does not exist where the layer is connected with layer .So the first confined aquifer was divided into 7 geological partitions (Fig. 1 and Table 2).

Prevention partition for land subsidence
According to the urban development planning (SMCPA, 2012), the land subsidence prevention was divided into 4 partitions: emphasized prevention area I, emphasized prevention area II, second emphasized prevention area and general prevention area.

Prevention partition for land subsidence induced by engineering dewatering
In foundation pit dewatering, the curtain (diaphragm wall) is the most effective way to control the drawdown and land subsidence outside the pit (Wang et al., 2009).Considering the curtain's depth of present technology, the first confined aquifer was divided into 3 partitions in vertical direction.The dividing depth is 60 and 70 m separately.A prevention partition for land subsidence induced by engineering dewatering for the first confined aquifer was suggested (Fig. 2).It includes 11 sub-partitions (Table 3).The maximum drawdown for different foundation pits was suggested for each partition (Table 4).

Conclusions
Engineering dewatering has become a major factor of land subsidence in Shanghai.A prevention partition of land subsidence induced by engineering dewatering for the first confined aquifer was developed and suggested.

Figure 1 .
Figure 1.Geological partition for the first aquifer in Shanghai.

Figure 2 .
Figure 2. Prevention partition for land subsidence induced by engineering dewatering.

Table 1 .
Stratigraphic characteristics of central city in Shanghai.

Table 2 .
Geological partition for strata combination.

Table 3 .
Prevention partition for land subsidence induced by engineering dewatering.

Table 4 .
Control indexes of drawdown for partitions for land subsidence induced by engineering dewatering.