Research On The Environmental Impact Analysis And Control Measures Of Confined Water Dewatering In Deep Foundation Pit Of Changzhou Metro

Groundwater is an important risk source for rail transit and underground engineering construction.With the rapid development of domestic subway engineering construction,the depth and scale of subway foundation pit are constantly expanding,and the control of groundwater,especially confined water,has become the key in the construction of subway engineering.Most of the strata involved in the construction of the foundation pit in Changzhou metro include a diving aquifer and a number of confined aquifers.The security risk caused by groundwater is high.Based on the typical foundation pit dewatering project of Changzhou metro and the pumping test of typical projects in Changzhou,this paper systematically analyzed and studied the hydrogeology and distribution characteristics of confined water,the stress history,the influence of confined water dewatering on the surrounding environment and control measures in Changzhou area.The main research achievements are as follows.（1）The hydrogeological conditions and geological stratification in Changzhou area are systematically investigated.The average permeability coefficients of the layer（5）and the layer（8）₂ of the target aquifer in the pumping test of Changzhou Metro Station are 2.79E-03cm/s and2.75E-03cm/s respectively,which are all medium permeability.Pumping tests were carried out on two typical foundation pit engineering sites in Changzhou with and without hanging waterproof curtain respectively,and the hydrological parameters of confined aquifer I₁ were calculated.The results show that the waterproof curtain has obvious barrier effect on groundwater.The average water output without the waterproof curtain is 228m³/d,and the influence radius is 156.2m,the maximum surface settlement is 7.2mm.When the water level drop is similar,the average water output and influence radius of the foundation pit with hanging waterproof curtain are 32m³/d and 90.1m,the maximum surface settlement outside the foundation pit is 1.98mm,which is significantly reduced compared with the former.（2）Based on the observation of pumping experiments and numerical simulation,the prediction methods of ground settlement induced by precipitation of foundation pits are comparatively analyzed,including layered summation method,finite element method,and the method considering the effect of stress history to calculate the settlement caused by dewatering of the foundation pit.Overconsolidated soil with OCR between 1-12 is widely distributed within 30m underground in Changzhou area,which has a great impact on the foundation pit project.According to the calculation results,the layered summation method has a large settlement error,exceeding 200%.The error of the settlement calculation method considering the effect of stress history is about 45%,and the calculation accuracy is relatively higher.This method can be used to estimate the surrounding surface settlement caused by deep foundation pit dewatering in Changzhou.The calculation result of the finite element method has a high degree of fitting with the monitoring value,and the error is about 20%.It can be used as a reliable tool and prediction method when the refined calculation and analysis of the surrounding environment impact caused by foundation pit dewatering.（3）Based on the hydrogeological structure,the foundation pits of Changzhou metro stations are divided into five types,and the typical stations of Changzhou metro are selected to analyze the different types of foundation pit.The numerical simulation was used to predict and analyze the environmental impact of foundation pit dewatering under different design conditions.The results show that the type of aquifer with simple structure and small thickness such as the first type of foundation pits,the maximum surface settlement outside the pit caused by dewatering is 3.2mm and the water level to drop by 0.62m.These types of foundation pits have little impact on the environment and is easy to control.The type of aquifer with complex structure,interconnected layers and large layer thickness such as the fifth type of foundation pits,the maximum surface settlement outside the pit caused by dewatering is 16.4mm and the water level to drop by 2.38m.These types of foundation pits have great impact on the environment,so it is necessary to adopt conservative dewatering design and strict control measures.（4）The design of the waterpoof curtains of Changzhou metro stations are counted,and the most typical second type of foundation pit-Boai Road Station is taken as an example to analyze the environmental impact of different foundation pit dewatering design situations.The impact of foundation pit dewatering on the surrounding environment at different insertion depths of the waterpoof curtains and the effect of different layouts of pumping wells on the dewatering effect of the foundation pit were discussed.The results show that more than 70%of the waterproof curtains of Changzhou metro stations completely cut off the I₂ confined aquifer and the overall design is conservative.As the depth of the waterproof curtain inserted into the confined aquifer increases,the impact range of foundation pit dewatering continues to decrease.The maximum surface settlement outside the pit is 50.2mm,36.5mm,17.1mm and 9.2mm when the waterstop curtain is inserted into the I₂ confined aquifer depth to 25%,50%,75%and 100%.Therefore,it is not necessary to completely cut off theⅠ₂ confined aquifer when designing the waterproof curtain.The specific design depth needs to be determined according to the environmental protection requirements around the foundation pit.For the location of the well,the maximum surface settlement is 8.35mm when dewatering in the foundation pit,and the maximum surface settlement is 28.9mm when dewatering outside the foundation pit.Under the same conditions,the water output of dewatering in foundation pit is smaller than that outside the pit,and the environmental impact caused by dewatering is small,which is the preferred scheme for most foundation pit dewatering.（5）The five typical types of foundation pits in Changzhou metro stations were studied on the dewatering optimization design and environmental control measures.The design recommendations of each type of foundation pit waterpoof curtains and the environmental control measures for foundation pit dewatering are put forward.