Study On Stability Of Pile Anchor Support Structure In Deep Foundation Pit Considering Coupling Effect

Foundation pit engineering is a traditional and complex geotechnical engineering.It not only involves the strength and stability of foundation pit,but also strictly controls the deformation of foundation pit.With the development of economy,various kinds of super large foundation pit rise in the cities.As the excavation depth of foundation pit increases,the groundwater problem gradually emerges.Therefore,it is very important to study the variation of soil and water pressure,the deformation of retaining structure and the ground subsidence in the excavation process of foundation pit,which caused by dewatering.In this paper,the deep foundation pit project of a rail transit tunnel at Beijing New Airport is taken as the research object.The methods of theoretical analysis,monitoring data analysis,numerical simulation results and actual measurement are adopted in this paper.The deformation law of foundation pit during excavation and the influence of groundwater on the stability of foundation pit are systematically studied.The results can provide valuable experience for the design and excavation of deep foundation pit with the same pile-anchor support structure in this area in the future.The main research results of this paper are as follows:(1)On the basis of comprehensive analysis of engineering support methods,the monitoring contents and monitoring methods of this foundation pit project are formulated.The horizontal displacement of the top of the supporting pile,the deep deformation of the pile and the axial force of the bolt are mainly monitored.Then the monitoring data are analyzed and the deformation law of foundation pit excavation is obtained.(2)In this paper,two numerical models of foundation pit excavation with considering dewatering and without considering dewatering and are established.By comparing the difference of horizontal displacement of supporting pile,axial force of anchor rod and ground subsidence outside foundation pit in excavation,it is concluded that groundwater is an important factor affecting the stability of retaining structure of deep foundation pit.(3)With the increase of the depth of the excavation,horizontal displacement of pile body increases gradually and it shows a concave on both sides,middle convex shape.The maximum horizontal displacement of pile is nearly at 1/2 depth of foundation pit excavation.The maximum horizontal displacement of the pile considering fluid solid coupling is 2~3 times of that without considering fluid solid coupling;The anchor force generally has three stages;The settlement of the soil outside the pit increases gradually,and the maximum settlement is about 10 m from the edge of the foundation pit.(4)It is found that the simulation results with fluid-solid coupling are closer to the actual monitoring results.The design considering the dewatering will make the foundation pit design more safe.The pile anchor support system can effectively control the foundation pit deformation and pit surface settlement.