预留反压土对基坑开挖效应数值分析

针对某基坑开挖工程采用地下连续墙、坑边预留反压土可行性问题，以该工程预留反压土关键设计思路与施工技术安全经济为目标，首先对基坑开挖过程挡土结构位移土压力曲线进行分析，并基于 Midas /GTS 建立了该基坑开挖工程三维相互作用体系有限元模型，对有/无设置反压土两种基坑工况进行非线性数值比对。结果表明，反压土能有效阻止周围土层和基坑基底的位移，并可对地连墙的不均匀沉降位移起到一定的控制作用，减小对冠梁产生附加次生的弯矩及剪力而导致的结构开裂破坏作用。基坑隆起与是否预加反压土关系不大，若所处地质环境良好，适当的采用反压土支护形式，可有效降低挡土结构位移与内力。变化反压土体积( 包括顶宽、高度、坡度) 对地连墙支护结构最大位移反应具有显著影响，当反压土具有适当体积时，地连墙最大位移反应可降低至 5 cm 以下，采用最优反压土体积参数可大幅降低基坑开挖时的支护费用。

Numerical analysis on effect from reserved earth berm on excavation of foundation pit

Aiming at the feasibility of adopting both underground diaphragm wall and pit-side reserved earth berm for the excavation of a foundation pit and taking the key design idea and the safe and economic construction technology of the reserved earth berm for the project as the study objective，the earth pressure curve of the displacement of the retaining structure during the excavation of the foundation pit is analyzed at first，and then a finite element model for the 3 － D interaction system of the excavation of the foundation pit is established with Midas /GTS，furthermore a non-linear comparison is made between both the working conditions with or without earth berm. The result shows that earth berm can not only effectively prevent the displacements of the surrounding soil layer and the bottom of the foundation pit，but also can play a certain control role on the uneven settlement and displacement of the underground diaphragm wall and reduce the effect from structure cracking damage caused by the additional secondary bending moment and shearing force induced from top beam. The heave of foundation pit is not so related to whether earth berm is reserved or not; while the appropriate adoption of the earth berm supporting pattern can effectively decrease the displacement and internal force of the retaining structure，if it is located within a better geological environment. Changing the volume ( including top width，height and slope) of earth berm can significantly influence the response of the maximum displacement of the underground diaphragm wall supporting system，for which the response of the maximum displacement of underground diaphragm wall can be decreased to less than 5 cm，if appropriate volume of earth berm is adopted; thus adopting optimal parameter of the volume for the earth berm can largely decrease the supporting cost for the excavation.