Study On Stress Path And Time-Space Effect Of Deep Excavation

With the emergence of the high-rise construction and the underground project, the number of deep foundation pit project rapidly increases. The underground space is one kind of resources which does not arouse people’s widespread interest. As a result, the deep foundation pit design and the construction question become the important point of geotechnical engineering projects. There is a question about the research of deep foundation pit project that arises from the different issues because of choosing the different material constitutive model. The research on soil stress path around the deep foundation pit is not so systematic and perfect.This paper integrates some modern theories and technologies into the deep excavation in the process of soil in different regions of the stress path, the foundation pit in the process of excavation and seepage, coupled stress, soft soil foundation of the effects of time and space effects on a systematic theoretical analysis and numerical simulation. And the main achievements are as follows:1. Based on lots of national and international research results, the headway and the existent problems of stress path research and time-space effect of deep foundation pit are summarized systemically, thus the direction and content of the paper are advanced.2. The basic soil physical and mechanical properties in the deep foundation pit are analyzed systemically and several common characteristics of the constitutive model and the applicability on the deep foundation pit are compared carefully. The results show that improved plastic Mohr-Coulomb model, Drucker-Prager model and modified Drucker-Prager/Cap model are more suitable for excavation works in the stress and displacement analysis. In addition, the combination of commercial finite element software is used to explore the creep characteristics of the soil to consider the extended Drucker-Prager model and modified Drucker-Prager/Cap model in the deep foundation pit project.3. The change of the soil around the two-dimensional and three-dimensional deep foundation pit stress path is analyzed systemically. The results show that plane excavation pit can be divided into lateral unloading zone, axial unloading zone and the transitional region. Three-dimensional path of the deep foundation pit is due to the complex three-dimensional stress path. According to the changes in the three direction of stress, it can be divided into active region and passive region. The numerical simulation of the soil stress path around the deep foundation pit in the course of excavation has been completed by applying improved Mohr-Coulomb plastic model. The results show that, in the course of excavation of plane excavation pit, the horizontal stress reduces and the vertical stress changes marginally in the lateral unloading zone; the vertical stress reduces rapidly. At the same time, the horizontal stress has been correspondingly decreased in the axial unloading zone. The situation of stress in the transitional region is complex. Specific changes should be considered in the unloading volume of excavation as well as the lateral displacement of protecting structure. In the process of excavation of three-dimensional deep foundation pit, vertical stress is basically unchanged and changes of horizontal stress are closely related to the site in the active region. In the meantime, vertical stress and horizontal stress significantly reduce in the passive region, but the horizontal stress in the corner reduces more than many other regions.4. On the basis of summarizing the law of ground water seepage systemically, three-dimensional finite element model of seepage has been established that is applicable to the deep foundation pit dewatering. And the results show that:water level outside the foundation basically does not change if the diaphragm wall in the form of supporting for the foundation has been used to carry out excavation and construction of rainfall, and therefore it can reduce the effective precipitation excavation and foundation construction in the external environment. The underground continuous wall should be the first choice if excavation pit project has been carried out in downtown city.5. Based on the principle of effective stress of saturated-unsaturated soils and the continuity of the current law, a pore pressure and displacement coupled finite element model has been derived. Based on the coupling model, the three-dimensional seepage and stress of the foundation project has been coupled numerically simulated. And the simulation results show that:there is few effects on the of precipitation foundation; the of the excavation pit in the soil decreases and the of the foundation soil outside remains basically unchanged. Excavation in the process of setting diaphragm wall can effectively support the restrictions in a row as a whole wall of the lateral displacement, and at the same time, it can enable the wall of the largest central lateral to displace from the edges to both ends so as to avoid the constant shift in the middle.6. On systematically analyzing the performance and the reasons of space and time effects of the excavation project, the author combines a concrete foundation pit with Drucker-Prager/Cap numerical simulation model Calculation. The results show that:due to the effects of space envelope in different parts, the foundation pit has different structure and stiffness. Excavation of the time is due to the effects of the process of excavation of soil physical and mechanical properties and the soil itself is caused by the flow characteristics. During the excavation process, the envelope lateral horizontally displaces, land subsidence around the foundation and the outer envelope in the size of earth pressure in space are obvious differences. In addition, for obvious behavior of soft clay foundation, excavation of the exposure should be avoided waiting for a long time, and set up in a timely manner and effectively be supported to enhance envelope rigidity of the structure in order to reduce the effect of the foundation space and guarantee security and stability of the foundation.