应用断裂强度理论模拟基坑开挖过程及实测数据分析

SIMULATION ON FOUNDATION PIT EXCAVATION WITH FRACTURE STRENGTH THEORY AND INTERPRETATION OF OBSERVED DATA

基坑开挖过程中,在支护结构背后、土层表面或自然边坡的顶部,经常会观测到有裂缝存在,但以往的有限元方法对裂缝的存在和影响认识不足。在分析土体破坏机制的基础上,应用断裂强度理论作为土体的破坏准则,充分考虑裂缝的存在和不利影响,建立土体开裂后的本构关系,并开发了二维弹塑性有限元程序PLST。结合天津国际贸易大厦基坑工程的现场测试结果,应用该程序对其基坑开挖过程进行模拟分析,将计算结果与实测结果进行比较,得出了相关结论。

A large commercial center has been built in Tianjin, China, and the main building is 218 m high. To construct the building, a pit of 15.3 m deep, 60 m wide and 150 m long has to be excavated. During foundation pit excavation, the developed cracks will affect the stress and deformation of the retaining structure. The fracture strength theory is taken into account in the constitutive relation of cracked soils. An elastoplastic finite element computer program is developed in order to simulate foundation pit excavation procedure. The calculation results are compared with the observed data from a construction site. The analysis results show that during pit excavation, tensile stress zones are developed in the ground surface behind the retaining structure. Once the tensile stress becomes larger than the tensile strength of the soil, fracture will arise in the tensile stress zone. To some extent, the existence of these cracks may decrease the stress and stiffness of the retaining structure. At the same time, the passive earth pressure increases and the active earth pressure decreases. In the crack range the active earth pressure even decreases to zero. Under this condition, the fracture strength theory is more fit for the soils than the other conventional theories and the calculated results agree well with the test data.