Seepage Stability And Monitoring Analysis Under Dewatering Conditions For A Deep Foundation Pit Of A Metro Station In Lanzhou

Since reform and opening up, China’s economy rapid development, construction of urban rail transit becomes increasingly prosperity and foundation pit engineering has become an important research subject. With the deepening of the excavation depth of the foundation pit, the problem of groundwater is gradually increasing, and the deformation of the foundation pit dewatering seepage can not be ignored. According to statistics, some of our foundation pit dewatering depth has reached 40 meters, in the engineering construction, a series of engineering accidents are caused by the defects in the way of thinking and method of groundwater management. In foundation pit engineering, ground water control has become another major technical difficulty and hot spot in geotechnical engineering.Based on the actual engineering of deep foundation pit of a subway station in Lanzhou, this paper summarized the real-time monitoring of supporting system, supporting pile and its surrounding environment in the process of excavation. According to the monitoring results of the horizontal displacement of supporting structure, supporting forces and the ground settlement, the groundwater level varied with time and the changing rule of the depth in excavation could be obtained. Meanwhile, the coupling analysis of seepage field and stress field in the excavation process of the foundation pit excavation was carried out, the deformation law of the supporting form( "bored piles within supporting system ") in the collapsible loess area of subway deep foundation pit in construction was obtained. Compared with the deformation of maintenance structure and the measured value. Main study findings of the paper are shown in the following:(1)The interaction of support axial force during erection and dismantling is analyzed. The supporting removal process has a great influence on the axial force of the adjacent support, and has less influence on the other supporting axial force.(2)The deformation law of supporting piles in the course of dewatering and excavation are analyzed. The deformation of the pile is large in the middle and small at both ends, and the maximum displacement point is near the excavation face. With the deepening of the excavation, the active earth pressure of pile is increased, the passive earth pressure decreases. And the depth of foundation pit excavation, the larger the horizontal displacement of the pile, the maximum horizontal displacement usually occurs at the depth of excavation 1/2 to 2/3.(3)The calculation results of displacement under dewatering conditions is greater than the calculated value which has ignored the influence of groundwater.And the former is more close to measured data. Effect of groundwater on horizontal displacement of pile is greater than the impact on surface subsidence and uplift of the basement.(4)Numerical simulation results are close to field monitoring results, the form of bored piles within supporting system could effectively control the deformation, and have some applicability in the supporting system of deep foundation pit of the subway station in Lanzhou. The research results of this paper can provide guidance for the design and construction of deep foundation pit of Metro.