关键词: 深基坑开挖, 土体卸荷, 变形监测, 有限元, 对比分析

Abstract: 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.

Key words: deep foundation pit excavation, clay under unloading state, deformation monitoring, finite element, comparative analysis