| In recent years,with the continuous development of urban underground space,the scale and complexity of the foundation pit project have gradually increased.The deformation generated by the foundation pit excavation has a more significant impact on the safety of the foundation pit itself and the existing structures around it.To correctly grasp the safety status of the foundation pit excavation process and provide a reference for similar projects in the future.This thesis takes the Xi’an station expansion project as the engineering background,the reliability of the pit structure was investigated by using the reliability theory,and the analysis of the deformation of the enclosure structure and the adjacent existing tunnel during the excavation of the pit was carried out by combining numerical model method.The main conclusions of the study are as follows:(1)Based on the stresses inside and outside the pit during the construction of the pit project,the corresponding structural reliability function is established from the three aspects of pit bottom uplift,enclosure structure overturning and pit slip,and the reliability analysis model of the pit system with multiple failure modes is proposed.The single failure mode and multiple failure mode reliability calculation methods based on the one-dimensional reduced dimensional 7-point estimated fourth-order moments are developed.The reliability indexes corresponding to the three failure modes of pit bottom uplift,overturning,and slip are 3.6193,4.2341,and 3.8534,respectively,and the reliability index of the tandem system is 3.5180,all of which are higher than the target reliability index of 2.5,indicating that the reliability meets the requirements.The high-order moment method has the characteristics of high efficiency and accurate calculation results and has good applicability in the foundation pit reliability analysis.(2)A three-dimensional dynamic simulation analysis of the excavation and support construction of a large deep foundation pit adjacent to an existing tunnel is carried out using numerical simulation,and the safety of the pit and the adjacent tunnel is studied from three perspectives: the ground,the tunnel,and the pit support.The deformation of the pit enclosure and the existing tunnel both increase with the increase of the pit excavation depth.The axial force of the anchor cable decreases in the direction of excavation depth and reaches the maximum value in the middle of the ground connection wall in the horizontal direction.With the increase of anchor cable hitting angle,the deformation of the tunnel shows a trend of decreasing and then increasing,but with the decrease of spacing and increase of pre-stress,the deformation of the tunnel decreases obviously,so the deformation of the tunnel can be reduced by decreasing the horizontal spacing of anchor cable hitting and increasing the pre-stress of anchor cable.(3)According to the profile of the foundation pit and the surrounding environment of the first phase,a detailed foundation pit monitoring plan was formulated,and the monitoring frequency and monitoring control standards were set.The horizontal displacement of the pile top,vertical settlement of the pile top,surface settlement,groundwater level,deep horizontal displacement and settlement of the vault of the adjacent underpass tunnel,and horizontal displacement of the side wall was monitored,and the change law of monitoring data was analyzed.The monitoring results show that all monitoring contents during the excavation of the foundation pit meet the control requirements,and the accuracy of numerical simulation and reliability analysis is verified. |
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