Influence Of Foundation Pit Excavation On Adjacent Building Foundation In Soft Soil Area

At present,the excavation of foundation pit is inevitable in the process of development and construction in various cities.Especially in soft soil area,foundation pit excavation will lead to soil unloading.If there is a building pile foundation around the foundation,the soil around the pile will change,causing additional deflection and bending moment of the pile foundation.As a result,the upper structure will be damaged due to insufficient bearing capacity,and even endanger people’s life and property safety.By using the methods of literature research,theoretical analysis,field measurement and numerical simulation,this paper systematically explores the influence of foundation pit excavation unloading on adjacent building foundation in soft soil area.The main results are as follows:(1)The influence of foundation pit excavation unloading on adjacent building pile foundation is a typical passive pile problem.The failure mode of pile foundation under horizontal load is different according to the pile material,geometric size,constraint conditions and soil properties around the pile.The advantages and disadvantages of theoretical analysis method and numerical analysis method for foundation pit excavation are systematically summarized.Among the theoretical analysis methods,the composite foundation reaction method(p-y curve method)is the most effective horizontal nonlinear deformation analysis method of pile foundation.ABAQUS and PLAXIS software based on finite element principle in numerical analysis method have strong applicability for foundation pit excavation engineering.(2)Based on ABAQUS software,the influence of foundation pit excavation under different working conditions on adjacent building foundation is studied.The research results show that with the pile foundation becoming more and more far away from the edge of foundation pit,the horizontal displacement and bending moment of pile body are also becoming smaller and smaller.L / H = 2 can be taken as the influence limit of foundation pit excavation on pile foundation.With the increase of excavation depth of foundation pit,the horizontal displacement and bending moment of pile body gradually increase.The maximum value of horizontal displacement of adjacent pile foundation is about 0.1% ～ 0.5% of excavation depth;the change of pile foundation stiffness has great influence on pile bending moment,but has little influence on horizontal displacement.When the length of the pile is less than the excavation depth of the foundation pit,the maximum horizontal displacement occurs at the bottom of the pile.When the pile length increases to the same depth as the foundation pit excavation,the horizontal displacement at the pile head increases,and the horizontal displacement at the pile bottom decreases,and the maximum displacement appears at the pile head;the internal support can reduce the impact of foundation pit excavation on adjacent pile foundation;when the stiffness of the supporting structure increases,it can effectively control the soil deformation outside the pit,and can play a good protection role for the adjacent pile foundation.(3)Combined with the foundation pit engineering example of a real estate development project in Cixi City,the corresponding foundation pit support scheme is proposed,and its stability is checked by using Lizheng software based on M method.The real-time monitoring of surrounding buildings,structures and pile foundations caused by the foundation pit is carried out.The monitoring results show that the horizontal displacement and vertical displacement of the slope top of the foundation pit change with the excavation time in a ladder shape,and the development law and the location of the inflection point correspond to the excavation progress of the foundation pit.The changes of road vertical displacement,building corner point and groundwater level caused by foundation pit excavation are small,which do not exceed the alarm value.The results of field measurement and numerical simulation of the horizontal displacement of adjacent pile foundation are compared.Due to the embedded effect of upper structure,the maximum value of horizontal displacement calculated by numerical simulation is located at the top of pile,while the maximum value of horizontal displacement measured on site is about 2m below the ground.The influence of the disturbance of soil between piles on the pile foundation is ignored in the numerical analysis,resulting in the simulation value being slightly lower than the measured value.(4)Based on the control standard of foundation pit deformation,the deformation control of foundation pit is divided into active control and passive control.Active control includes optimization of foundation pit support scheme and foundation reinforcement of passive area in pit;passive control mainly refers to reinforcement of soil outside the pit or setting of isolation pile wall outside the pit.The protection suggestions of foundation pit excavation unloading to adjacent building foundation are given.Before the excavation of the foundation pit,the adjacent protected objects can be actively reinforced to enhance the stiffness and anti deformation ability of the object structure,and the information tracking monitoring can be carried out in combination with the excavation of the foundation pit,and the supporting and construction measures can be adjusted in real time.