开挖卸荷状态下深基坑变形特性研究

深基坑开挖时因为卸荷作用会引起土体强度一定程度的降低，土体强度的降低会引起支护结构上土压力的变化，利用卸荷前土体强度指标进行土压力计算势必会小于实际情况，从而导致土体及支护结构变形计算值与实测值有一定的偏差。在深基坑开挖时，仅采用施工监测等手段进行事中控制是不够的，必须预先对变形值的发展规律做出模拟和预测。为探索深基坑开挖时，卸荷作用对基底土体和侧向土体强度特性和变形特性的影响规律，结合天津富力响锣湾大型基坑开挖，对开挖过程中土体基底回弹情况、支护结构以及周边土体的变形情况进行了全过程监测，利用试验结果数据计算出天津市富力响锣湾大型基坑开挖项目的卸荷强度参数，为后续数值模拟计算参数的选取提供了依据；利用ABAQUS有限元软件建立了三维数值模型，分别采用原状土的强度参数和卸荷参数对开挖过程中土体基底回弹、支护结构以及周边土体的变形情况进行了模拟，研究了两种情况下土体基底回弹、支护结构以及周边土体的变形规律，并将有限元模拟结果与监测结果进行了对比。研究发现，本工程土体卸荷后的扰动区在基底以下3～4 m范围，强度折减可达到20%～35%，根据卸荷比确定了周边土体强度折减为10%～15%；有限元模拟结果与实测值基本一致。通过分析可以看到，考虑基坑开挖卸荷作用是符合工程实际情况的，因此，建议在深基坑开挖设计时考虑土体的卸荷效应。该分析方法可为同类深基坑设计提供参考。

Analysis of deformation of deep foundation pit under excavation unloading condition

The soil strength will decrease to a certain extent due to the unloading effect induced by deep foundation pit excavation. The decrease of soil strength will then change the lateral earth pressure exerted on the supporting structure system. So the actual lateral pressure will underestimated using soil strength parameters prior to unloading, which will result in a difference between the calculated deformation and the measured results of the supporting system. It is not sufficient to just monitor the deformation of support system during deep foundation pit excavation. In order to precisely predict the results, it is critical to understand the pattern of support system deformation under unloading effect. To investigate the influence of unloading effect on the soil strength and soil deformation on the bottom and sides of the excavated area, a case study is conducted on an excavation site of Xiangluowan Project of Fuli in Tianjin by monitoring the deformation of bottom rebound, supporting system and surrounding soil deformation during the entire excavation progress. Reduced soil strength of Xiangluowan project was calculated from measured results, which is used as the basic perimeters for the numerical simulation in later phase of research. ABAQUS, a commercial FEM software, is used to established a 3D model of the foundation pit excavation. By the model, the deformations of bottom rebound, supporting system and surrounding soil are simulated using in-situ soil strength and reduced soil strength as perimeters respectively. The deformation patterns of bottom rebound, supporting system and surrounding soil under two different conditions are studied; then the results from the finite element model analysis is compared with the actual in-situ monitored results. It is found that the disturbed zone imposed by unloading effect is located with 3-4 meter below the bottom of foundation, the percentage of strength reduction of the disturbed zone induced by unloading is up to 20%-35%, the percentage of strength reduction of the surrounding soil is 10%-15%, the results derived from FEM analysis is proved to be consistent with the field test results. It shows, through this paper, that it is practical and reasonable to consider the unloading effect during deep foundation pit excavation, thus the same method and philosophy is suggested to be applied in future deep foundation pit design and engineering.