# Evaluation of Land Subsidence during Groundwater Recovery
## 1 Introduction  
The changing groundwater level is an important factor affecting building and infrastructure design. Decreases in groundwater level also affect land subsidence due to pore-water-pressure decreases. Many researchers have summarized the problems of land subsidence in different locations such as Tokyo [1], Osaka [2], Shanghai [3], Taipei [4], Jakarta [5; 6; 7], Hanoi [8], and Bangkok [9; 10]. For example, Jakarta City, Indonesia, is located in an area with a ground surface elevation that is lower than the sea water level. The groundwater level has rapidly decreased due to groundwater pumping, which led to continuous land subsidence. The data collected by the Department of Groundwater Resources (DGR) before 1997 show that the groundwater level in the central part of Thailand has continuously decreased due to groundwater pumping [11]. The growth and expansion of Jakarta have directly increased the demand for groundwater pumping, which is one of the main reasons for land subsidence. To manage this issue, the Department of Groundwater Resources,Thailand, issued a law to control the amount of groundwater pumping by industries. The groundwater level tends to reach the ground surface. The soil surface displacement during the groundwater level recovery has affected civil engineering structures, leading to road and building cracks, especially in old buildings that were designed with a low groundwater level or in dry areas. Differential settlement between a building and the ground surface is also a consequence of land subsidence. [PERSON] [12] reported evidence of damage due to land subsidence, such as ground and footpath separating from the adjoining building, buckling of pavement over footings, and differential settlement between the building and foundation in Bangkok, Thailand. [PERSON] et al. [13] indicated that about 1.60 m of land subsidence occurred from 1933 to 1987, and settlement significantly increased by about 2.05 m in 2003. All settlement occurred during the groundwater pumping period. [PERSON] et al. [14] reported on groundwater pumping in the late 1980s in inner Bangkok. Land subsidence was a result of the time-dependence consolidation behavior of aquitards and the soft clay layer at the surface, induced by the groundwater drawdown in the aquifer layer. Moreover, settlement still occurred slowly as the groundwater level increased, but the rate of settlement was slower than before. After the groundwater level began to recover, [PERSON] [15] analyzed the land subsidence and groundwater pumping data of the whole Bangkok metropolitan area. Between 1994 and 1998, every 1 m\({}^{3}\) of groundwater pumping from underneath the aquifers were related to a ground surface volume loss of about 0.05 to 0.10 m\({}^{3}\). Additionally, the cause of land subsidence was the consolidation of both shallow soft and stiff clay layers (within 50 m of the ground surface) to about 30% to 50% of the total land subsidence. [PERSON] et al. [14] predicted subsurface settlement and compared the results with the measurements after 1995. The calculation results suggested that the compression of the soil layer is up to 25% of the total land subsidence at the surface of the shallow clay layer in the long term (within 50 m of the ground surface). Land subsidence has also occurred in Thailand due to groundwater pumping. The Department of Groundwater Resources collects measurements of the groundwater level, pore-water pressure, and settlement. Many researchers have evaluated and predicted the characteristics of land subsidence in various areas, including Bangkok, Thailand. Finite element analysis is a suitable method of predicting land subsidence. [PERSON] [16] used the finite difference code for one-dimensional coupled flow-consolidation analysis of Bangkok clay from 1960 to 1989, which was the groundwater drawdown period. A decrease in the groundwater level led to a decrease in the permeability of the clay layer with time and an increase in the degree of consolidation.