| With the implementation of the strategy of "the Belt and Road",the national economy has entered a new era of rapid development.The rapid development of the economy has promoted the construction and development of large infrastructure projects such as high-rise buildings,highways,and subways.Modernized urban construction often adopts the method of "inserting needles into seams".Especially for the demolition and construction areas in old urban areas,the buildings to be built may be located in the built buildings around them.This requires the buildings to be built to minimize the disturbance to the surrounding buildings in the construction process and ensure the normal life of the surrounding residents.Deep foundation pit engineering involves excavation and unloading of soil mass,which is easy to cause disturbance to the surrounding environment.If improper construction,it is easy to cause engineering accidents.In this paper,the elastic fulcrum method of planar strut structure is used to design the foundation pit based on the soil-rock composite foundation pit of a nearby building.Firstly,the parameters of the foundation pit support scheme are explained,and the selection principle of the support design is clarified.Based on the calculation and analysis of the selected scheme of micro steel pipe pile,pre-stressed anchor bolt and soil nail,a supporting design method suitable for the soil-rock composite foundation pit of adjacent buildings is formed.PLAXIS finite element software is used for modeling and analysis.The silty clay layer is simulated by Mohr-Coulomb model(MC),modified Cambridge model(MCC),hardened soil model(HS),small strain hardened soil model(HSS),and the adjacent buildings are simulated by frame structure.The grid deformation under different constitutive models is analyzed.The vertical displacement and horizontal displacement caused by excavation of foundation pit.Finally,HSS model is used to study the influence of anchor prestressing coefficient and bending stiffness of steel pipe pile on the horizontal displacement of pile body.The main contents include:(1)The selected design unit is adjacent to underground pipeline,which requires high deformation control.Before excavation,micro-pile steel pipe pile is used to advance support.During excavation,prestressing anchor bolts and soil nails are laid at intervals of layers.The circular slip surface method is used to calculate the overall stability of the foundation pit,which meets the design requirements and forms a supporting design calculation method suitable for the soil-rock composite foundation pit of adjacent buildings.(2)The mesh deformations calculated by different soil constitutive models are quite different.The deformation trends of micro steel pipe piles calculated by MC model,HS model and HSS model are consistent.(3)The settlement model of surrounding environment caused by excavation in MC model is different from other three constitutive models.The maximum vertical displacement calculated by HS model and HSS model is 35.96 mm and 35.29 mm,respectively,which is about 19.94% and 19.57% of the calculated value of MCC model.Compared with MC model and MCC model,HS model and HSS model are more accurate in describing the vertical displacement of surrounding soil caused by excavation and unloading on one side of the building and overloading of the building.(4)In HS model and HSS model,the ground settlement caused by excavation of foundation pit is consistent with the change trend of measured value.Because HS model and HSS model can not only reflect the shear hardening and compression hardening of soil,but also reflect the difference of loading and unloading stiffness,which can better reflect the overload of adjacent buildings.Settlement variation law of surrounding environment caused by excavation unloading.(5)Under different constitutive models,the horizontal displacement pattern of the pile body is different.The maximum horizontal displacement of the pile body occurs at the surface,and the direction of the pile body is directed to the pit.Due to the restraint of anchor prestress,the displacement of the pile body at the first anchor(MG1)occurs outside the pit.HSS model,which can reflect the difference of small strain,compression hardening,shear hardening and loading-unloading stiffness of soil,can accurately describe the change law of horizontal displacement of pile body caused by overloading of adjacent buildings and excavation and unloading of foundation pits.(6)In the calculation results of HSS model,there is a tension crack zone extending from the surface to a certain depth.The distance between the tension crack zone and the excavation edge of foundation pit is about 1.3 m(0.08 h,h is the excavation depth of foundation pit),the width is about 0.4 m(0.03 h),the depth is about 1 m(0.07 h),the direction and the surface are about 45 degrees,which can be observed and strengthened in practical engineering. |
