Research On The Environment Impact And Control Countermeasures Induced By Dewatering Confined Water Of Suzhou Metro

The construction of the underground rail system is an important means to solve the traffic problems of large and medium-sized cities. With the development of the construction of the subway, the subway pits are becoming deeper and larger, and the effect of confined water has become an important issue for controlling the security of deep excavation of subway. On the basis of the geological database of Suzhou Metro Line 1 and Line 2, selecting some typical deep dewatering projects of Metro Line 4 to systematically study the features of the confined water in Suzhou、the interaction between the subway deep excavation and groundwater、the surrounding environment impact due to dewatering and its control measures caused. This paper made the following work:1、The hydrogeology and engineering geology survey data of the Suzhou Metro Line 1、2 and 4 were analyzed and summarized to identify the characteristics of the groundwater aquifer system of the Quaternary stratigraphy、the distribution and hydraulic characteristics of aquifer system (within 100m depth range diving, confined water layer) which is closely related with the construction of underground rail in Suzhou;According to the spatial relationship of aquifer (or impermeable layer) and the deep excavation, the pattern of hydrogeological structure in Suzhou typical can be divided into eight kinds, and on this basis and the type of the aquifer within the range of excavation depth and whether the confined aquifer in the bottom of the pit threaten the security and stability of excavation, the foundation pit of Suzhou Metro Line 4 stations can be divided into five typical types.2、The waterproof curtain design of some stations threatened by the confined water of Suzhou Metro Line 4 was analyzed and summarized, then taking the pit dewatering project of Hongzhuang station for example and using the finite element method to study the trends of the change of the groundwater level and land subsidence surrounding the pit due to dewatering in the case of different depths of the Diaphragm Wall.The research results show that:① When the inserted length of disphragram wall is less than 30% of the thickness of the confined aquifer, The trends of drawdown and land subsidence are relatively slow with the increasing depth of the disphragram wall, and it is because the depth of the disphragram wall to insert the confined aquifer is relatively smaller, which is failed to form an effective impermeable boundary in the first confined aquifer and the blocking effect of the disphragram wall can be ignored, for example, the values of drawdown and surface subsidence 10m distance away from the pit are 12.5m and 7.5cm;② When the inserted length of disphragram wall reaches 50% to 80% of the thickness of the aquifer, the trends of drawdown and land subsidence are in a larger rate,especially when it exceeds 70% of the thickness of the aquifer, the changes of them are obvious. It indicates that the disphragram wall has formed an effective impermeable boundary in the first confined aquifer to blocking the seepage of the confined water inside and outside of the pit, for example, the values of drawdown and surface subsidence 10m distance away from the pit are 7.0m and 3.9cm;③ When the inserted length of disphragram wall over 80% of the thickness of the aquifer, the corresponding changes of drawdown and land subsidence decrease in a larger、nearly linear rate, and the impact on the surrounding environment due to dewatering further reduced, for example, when the insertion depth of the disphragram wall up to 80%,the values of drawdown and surface subsidence 10m distance away from the pit are 3.0m and 2.0cm;④ When the disphragram wall completely cut off the confined aquifer, there is almost no impact on the surrounding environment, for example, the values of drawdown and surface subsidence 10m distance away from the pit are only 0.7m and 7mm; 3、When the inserted length of disphragram wall is less than 50% of the thickness of the aquifer, The changing trends of the range of the influence surrounding the pit caused by dewatering decrease relatively slowly with the increasing depth of the disphragram wall, and the range of drawdown is more than displacement, for the former is about 330m while the latter is about 300m;When the inserted length of disphragram wall is more than 50% of the thickness of the aquifer, The changing trends of the range of the influence surrounding the pit caused by dewatering decrease in a large rate with the increasing depth of the disphragram wall; when reach 80% of the thickness of the aquifer, the range of drawdown and displacement are Respectively 80m and 60m, and the former is also a little larger than the latter.4, Selecting typical station pit dewatering project of Suzhou Metro Line 4 on the basis of the devided 5 different kinds of pit, and the three-dimensional finite element models of which are established to analysis their environmental impact, then summarying the drawdown and surface subsidence surrounding the pit caused by dewatering of different types of foundation pit.The research results show that:(1) For the first and second types of excavation, due to cut off the hydraulic connection inside and outside, the maximum subsidence caused by dewatering is about 1.66cm, with a range of about 30m;(2) For the remaining three types of excavation, the point and difficulty is dealing with the confined water and its control technologies, and the inserted length of the waterproof curtain in the aquifer play a crucial role in controlling the drawdown and displacement outside the pit:①For the hanging curtain+detwatering inside, its impact on the environment is relatively larger, for example, the western pit of Bus Terminal Station, which belongs to the third type of excavation, the maximum subsidence caused by dewatering is about 6.6cm, with a range of about 200m;② For the cut-off curtain+detwatering inside, its impact on the environment is relatively smaller, for example, the transfer section pit of Zhu Hui Road Station, which belongs to the forth type of excavation, the maximum subsidence caused by dewatering is about 1.47cm, with a range of about 30～50m;5、According to the position and thickness of aquifer、the depth of waterproof curtain、site construction conditions,combined with the precipitation of the influence caused by dewatering with different depths of the disphragram wall, and from three aspects of "waterproof、"dewatering" and " improve the deformation resistance of surrounding ground " to summarize and analysis the countermeasures to control the influence caused by dewatering groundwater (confined water) of the 5 different kinds of the foundation pit in Suzhou.