Study On The Theory And Technology Of Recharge And Subsidence Control Of Confined Layer In Excavation Engineering In Tianjin Area

When the diaphragm wall cannot completely cut off the aquifer,dewatering the confined aquifer,different settlement of buildings and underground structures adjacent to the excavation will occur.The artificial recharge can compensate the water head of the confined aquifer outside the excavation to control subsidence.The semi-confined aquifer is mainly consists of silt and silty sand in Tianjin area.The recharge theory of semi-confined aquifer,including characteristic curve of recharge,recharge efficiency,blockage and well loss mechanism under semi-infinite space and boundary truncation condition,lacks intensive study.In this paper,field test,measurement data and numerical simulation are adopted to carry out systematic research on the recharge strategy and theory of semi-confined aquifer in Tianjin area.The semi-confined aquifer in Tianjin is mainly consist of silt and silty sand,its permeability is low and the feasibility of recharging is doubtful.This article first has carried on the certain green field site single well tests,confirmed the feasibility and effectiveness of the semi-confined aquifer recharge in Tianjin area.The single well recharge theory,theory of well loss,pressure recharge test results for different well structures were analyzed.The research shows that although recharge is feasible,it is easy to be blocked,and it needs to be pumped regularly,and the well loss value is significantly higher than pumping condition.The characteristic curves of recharge are close to the drawdown curve of pumping,and there is difference only near the well.Pressure recharge can reduce the number of recharge wells and the cost of recharge.Compared with clay sealing the outer hole of the well wall,grouting sealing can significantly improve the pressure and efficiency of recharge.A series of tests were carried out at a subway station in Tianjin.The results show that the hydrogeological parameters obtained from pumping test can be used to predict the water-level rise of recharge,but the rising cone of recharge is higher than the drawdown curve when it is closer to the central well.Recharge rate and pumping rate at a similar level can effectively control surrounding land surface and building settlement.When recharge stops,the surrounding surface subsidence tends to develop rapidly.Therefore,in actual engineering,the recharge time should be extended appropriately after the pumping stops.The twin-well combined recharge technology proposed in this study can effectively control the decline of water level of aquifer caused by the recharge well pumping.When dewatering in the excavation can lead to the water level drawdown of several aquifers outside the pit.Recharge all layers will lead to a significant increase in cost.Therefore,the strategy of interlayer recharge is proposed.Based on the study of the hydraulic connection between the inner and outer aquifers of the excavation,recharge of semiconfined aquifer Ⅰ outside the pit can effectively raise the head of the upper phreatic layer and the lower II-1 aquifer,and the subsidence caused by the water level drawdown is controlled by the interlayer recharge.For the II-2 aquifer,recharge Ⅰ aquifer also has a certain effect on the water level,but it is still not possible to fully restore the water level.It is recommended to combine backup recharge wells in this aquifer to control the water-level drawdown.In excavation engineering,the diaphragm wall has a great influence on the water level of recharge outside the pit,thus a finite difference model was used to study the line impervious boundary for the recharge blocking effect.Based on the characteristic curve of recharge cone in the case of complete separation of confined aquifer,the calculation expression of the characteristic curve of water level change when uncompletely truncated confined aquifer is proposed.