基坑支挡结构设计若干问题探讨

根据基坑开挖应力路径的室内试验和现场实测数据,从土工试验方法、土体强度指标、地基土水平反力系数等方面分析了现在基坑工程支挡结构设计存在的一些问题。研究成果如下:支挡结构设计参数的获取应考虑主动区及被动区地基土的应力路径,并恢复地基土的固结状态。基坑开挖后主动区和浅部被动区土体抵抗剪切破坏的能力大于以常规CU强度指标的计算结果,这有利于工程安全;深部被动区土体抵抗剪切破坏的能力小于以常规CU强度指标的计算结果,对于地基土被动抗力发挥较大的工程,需验算嵌固深度,确保被动抗力小于极限被动土压力。定义了被动区基坑开挖"强影响区",并给出了该区域深度的计算方法。以相应应力路径下的土体割线模量值作为地基土水平反力系数,讨论推导了作用于支挡结构上的分布土反力与支挡结构水平位移的计算公式,其计算结果更接近工程实际。本次研究对以变形控制为主的基坑设计具有较好的指导意义。

Discussion on several problems in design of retaining structures of deep excavation

Based on the laboratory tests on excavation stress paths and field test data,some problems in the design of the supporting structures of foundation pits are analyzed.The idea is proposed that the consolidation state of the foundation soil should be recovered and the stress paths corresponding to the active and passive areas of foundation soils should also be considered.It is found that the capability of soil resistance to shear failure in active and shallow passive areas is greater than the calculated results using the conventional CU strength index,and it is beneficial to engineering safety.However,the capability of the soils in the deep passive zone to resist shear failure is less than the calculated results using the conventional CU strength index.For the projects requiring a large degree of resistance capability of the passive foundation soils,the embedment depth of the supporting structures needs to be checked to ensure that the passive resistance is less than the limit passive earth pressure.The‘strong impact zone’of excavations in the passive zone is proposed,and the method for calculating its depth is developed.The value of secant modulus of soils under the corresponding stress paths is employed as the horizontal reaction coefficient of the foundation soils,and the formula for calculating the reaction force of distributed soils acting on the supporting structures and the horizontal displacement of the supporting structures is deduced.It can reflect the characteristics of load deformation of foundation soils under the action of the excavation stress path,and the calculated results are closer to those of engineering reality.This study has a good guiding significance for the design of foundation pits mainly dominated by deformation control.