Study On Drainage And Waterproof Of The Foundation Pit At Lize FBD Station In Beijing Subway

With the development of city metro in recent years,more and more deep foundation pits had been built,mostly the pits in metro station constructed by cut and cover method.To these stations,groundwater treatment is always a tough task.Waterproof by grouting and drainage by pumping are often used,each of these two methods has its own advantages and disadvantages.However,in recent literatures,these two methods are not fully compared and analyzed.In this paper,experiment on the effect of grouting in waterproof was performed in the construction process of the transfer passageway at Youanmenwai station.Then comparison and analysis were done on the effect,cost and environmental influence of waterproof and drainage for the foundation pits at Youanmenwai station and Lize Financial Business District(FBD)station in Beijing subway.The findings are hopefully able to provide experience for the foundation pits to be built in similar conditions in the future.Major results are as follow:(1)Test on waterproof by grouting and monitoring on the water leakage were conducted in the process of construction of the foundation pit of the transfer passageway at Youanmenwai station.Waterproof against a water head of 8.3m was achieved by cement-sodium silicate double grout injected with sleeve valve pipe method.Technical principles for the grouting were summarized.(2)Coefficient of permeability of soil layer was back calculated through the measured seepage volume.Calculation results ware verified by numerical simulation.(3)Making use of calculated permeability coefficient,numerical calculation was done to the foundation pit at Lize FBD station,the ground conditions of which are similar to that at Youanmenwai station.The result shows that waterproof by grouting was applicable to the foundation pit at Lize FBD station.After analyzing the quota of the grouting,comprehensive unit price was modified,and the total cost of the grouting was calculated.(4)Numerical analysis was carried out on the pumping effect to the foundation pit at Lize FBD station.Then the cost of water drainage was calculated.Compared with grouting,the cost of pumping will be lower when construction period is short.(5)Technological and economical comparisons were done between the foundation pit at Youanmenwai station and Lize FBD station,which tells as follow:for a large pit,the cost of pumping is lower than grouting in case of short construction period;for a small pit,the cost has just a little difference in two methods as long as the construction period is short.However,since the operating cost of pumping is high,the total cost grows up as time passes by.Therefore,the cost of grouting is relatively low in case construction period is long.