Evaluation Of The Behaviour Groundwater Seepage And Land Subsience Via Considering Infrastructures Penetrated Into Aquifers

This study was undertaken to evaluate the impact behaviour on groundwater seepage and land subsidence when infrastructures such as deep buiding foundations, underground structures, subway tunnels, underground path structures, were constructed in the multi-aquifer-aquitard system (MAAS) such as in Shanghai. In order to investigate the mechanism of these impacts, laboratory element tests and numerical simulations based on groundwater seepage model were conducted to analysis cutoff behavior on groundwater seepage of infrastructures. Laboratory element tests were also conducted to determine the parameters in numerical simulation. The obtained mechanism based on the laboratory tests and numerical simulation was employed to investigate the patterns of land subsidence in Shanghai via considering the impact of infrastructure in MAAS of Shanghai. Based on the research results, countermeasures of future construction plan of infrastructures in MAAS were proposed. The gist and primary new findings of this dissertation includes:1) Land subsidence in Shanghai since 1990 was still related to drawdown of groundwater level. A serious of regression analyses was carried out to investigate the relationship among subsidence, groundwater level of each aquifer, and volume of groundwater withdrawal. The results show that subsidence since 1990 is not correlated with the volume of groundwater withdrawal; however, it is still correlated with the drawdown of groundwater level.2) Land subsidence in Shanghai is also correlated with activities of urban construction. Factors resulting in land subsidence during activities of urban construction include additional load during and after structural construction and drawdown of groundwater level, e.g. groundwater pumping during excavation, leaking of underground sturctures. The existence of infrastructure in aquifers in the urban region is a significant reason, which results in the drawdown of groundwater level.3) A series of laboratory investigation on the cutoff behavior of an impervious wall in a phreatic aquifer was conducted. Test results show that drawdown of groundwater level at the lower side of the wall increases with the increase of the inserted depth of the wall in soil and increases with the increase of the width the wall. The optimal insertion depth ratio of the wall (ratio of the depth over the thickness of soil layer) is about 70%.4) A series of numerical analyses was conducted to investigate the obstruction behaviour of underground structures in MAAS. The analytical results show that the existence of underground structures changes the groundwater level, groundwater flow direction, and the rate of subsidence and causes leakage of groundwater among aquifers through aquitards. Influential factors include depth of infrastucture penetrated into the aquifers, the width of underground structures, and the relative position between infrastructure and groundwater discharge or recharge well. The optimal ratio of penetration depth of infrastructure in aquifer II is about 56%.5) A series of numerical analyses was conducted to analyze behavior of seepage and land subsidence via considering the impact of understructures in MAAS of Shanghai. Since it is difficult to make sure the distribution of understructures and it is also difficult to allocate each understructure in numerical model, following two extreme situations are assumed in this study.i) Distributed underground structure: In this situation, the whole aquifer with underground structures is assumed as a uniform material with so-called effective hydraulic conductivity and effective compressibility. This method changes the aquifer with underground structures into another material with lower coefficient of hydraulic conductivity and compressibility. The results revealed that the rate of subsidence in the urban area increased via considering existence of underground structures. When the volume ratio of underground structures in aquifer II increases 10%, subsidence increases about 32%. However, if the volume ratio of underground structures in aquifer I and low-pressure aquifer increases 10%, subsidence increases only about 3%.ii) The second assumed situation is so-called concentrated allocation of underground structure in aquifer, which embowels the heavily urbanized area. In this situation, part of soil material in aquifer is directly replaced by structure material to investigate the impact of concentrated infrastructures on the emboweled urban area. Results showed that additional subsidence within the emboweled urban area increases with the increase of volume ratio of underground structure (VROUS). However, when VROUS is controlled within a critical value, additional subsidence in the emboweled urban area can be controlled within the allowable value. The critical value of VROUS for low-pressure aquifer is 65%, for aquifer I it is abot 62%, and for aquifer II it is about 58%.