| In recent years,with the rapid development of cities,the urban underground space in China has been expanding continuously,and the development and utilization of urban underground space can not be separated from the foundation pit engineering.Because the foundation pit engineering is a comprehensive and complex geotechnical engineering,a lot of problems are still under study.For example,during the construction of the construction of the foundation pit engineering,some significant design changes are often encountered,in order to make full use of the underground space.When the foundation pit engineering enclosure structure has been partially constructed,the existing construction foundation pit is deepened and excavated due to temporary increase of the number of basement floors and other reasons.The temporary change of the design depth of the foundation pit makes the foundation pit support construction more difficult,resulting in the original support design scheme can not meet the support intensity requirements.This paper takes an existing foundation pit project in Zhengzhou as the research object,studies the displacement,stress and strain laws of the joint support structure for deepening of the existing foundation pit and the surrounding soil buildings,and analyses the on-site monitoring data.The main research work is as follows:1)Using Beijing Lizheng 7.0,the supporting structures of existing foundation pits before and after deepening are calculated respectively.Firstly,the scheme design is carried out under the excavation depth of the original foundation pit,and the overall stability of the foundation pit is analyzed under the old support system;Then,when the existing foundation pit is deepened,the original support structure is changed in design.When calculating after the design change,due to the fact that it is not possible to calculate the soil nailing wall pile anchor composite support form,the original support structure and the upper soil mass are simplified to uniformly distributed loads for calculation.The calculation results of Lizheng show that the composite support design form of upper soil nailing wall+lower double row micro piles in section 1-1 and the composite support design form of upper micro pile composite soil nailing wall+lower pile anchor in section 7-7 can effectively ensure the overall stability of the foundation pit slope.2)Using Midas GTS NX finite element numerical simulation software,different excavation and support construction processes are simulated by establishing a foundation pit excavation and support model.Different cross sections were selected for comparison.In nine working conditions before and after the design change,the stress and strain displacement of the foundation pit soil,the axial force of the anchor rod,and the bending moment of the pile body were studied before and after the support design change.The stress characteristics of the foundation pit retaining structure and the deformation characteristics of the surrounding soil were obtained before and after the foundation pit was deepened;At the same time,the vertical displacement of soil mass with or without buildings in the same support section was compared and analyzed,and the displacement change curve of soil mass at the top of the foundation pit was obtained.3)The detailed modeling of the brick concrete structure building group on the north side of the foundation pit is conducted,and the deformation of the building group during the construction process before and after the foundation pit deepening is analyzed.The distribution rules of the maximum principal tensile stress,displacement,and shear stress of the brick concrete building group during the foundation pit construction are obtained.4)Based on the analysis and research of on-site monitoring data,it is found that the composite support schemes for the foundation pit meet the design requirements,and the deformation of the foundation pit is in a safe and controllable state. |
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