1 Calculation method of earth pressure
Classical earth pressure theory 1. 1
Rankine earth pressure method and Coulomb earth pressure method are classical earth pressure algorithms. 1857, Rankine put forward Rankine earth pressure theory. He took a point on the soil around the retaining wall, assumed it was in a state of limit equilibrium, and analyzed its stress, thus calculating the distribution and magnitude of active earth pressure and passive earth pressure acting on the retaining wall [1]. Rankine earth pressure theory assumes that the top soil surface of the wall is horizontal, the back of the wall is vertical, and the contact surface between the wall and the soil is smooth. In fact, there is friction between the back of retaining wall and soil, which will lead to shear stress. In this case, the active earth pressure will be larger and the passive earth pressure will be smaller, which is generally safe. Coulomb starts with the sliding wedge behind the retaining wall, assumes that it is in a state of limit equilibrium and studies it, thus putting forward Coulomb's earth pressure theory. Coulomb earth pressure assumes that the soil is homogeneous and isotropic, the failure sliding surface of the soil is plane, and the wedge is rigid, which only translates or rotates. In fact, the failure surface is a curved surface, and the damaged soil wedge will also be deformed. A large number of measured data show that there is a certain deviation between the earth pressure calculated by the classical earth pressure theory and the actual earth pressure.
1.2 improvement of traditional earth pressure
Rankine earth pressure calculation method and Coulomb earth pressure calculation method are both obtained under the condition of limit equilibrium, but in fact, because the high-rise buildings around the deep foundation pit have strict requirements for supporting displacement, the displacement in reality often cannot reach the displacement required by the classical earth pressure theory, so it is necessary to consider the influence of supporting displacement on earth pressure calculation. In view of the influence of displacement on earth pressure calculation, some researchers have also carried out monitoring and research. According to the field monitoring data, Yao [2] introduced two kinds of displacements of a point on the support and a point in the soil into the original earth pressure calculation model, and confirmed the reliability of the new calculation model in an engineering example; Yang Yongxin et al. [3] introduced the active earth pressure reduction factor to consider the influence of soil displacement on earth pressure calculation, and compared it with the foundation pit calculation example; Cao Jing et al. [4] fully considered the coordination conditions of supporting deformation and soil deformation, and put forward a new calculation method of earth pressure of supporting and retaining structure; Wang Wenjie et al. [5] started with the earth pressure calculation of limited soil, studied the earth pressure between adjacent foundation pits, simply deduced the calculation method and analyzed and demonstrated it; WU GANG and others [6] think that the included angle between pit walls will have certain influence on the calculation of earth pressure, but the specific influence mechanism needs further study. Because of the particularity of soil, many scholars consider the calculation of earth pressure from the perspective of space-time effect. Considering the creep and secondary consolidation of soil, Zhang et al. [7] improved the original calculation method of earth pressure and can be effectively applied to practical projects. Zhu Rong and others [8] respectively considered the influence modes of time and space on earth pressure calculation, and then coupled them to obtain a new earth pressure calculation model, which was compared with the actual project. Chen Ziwen [9] mainly studied the main areas where the calculation of earth pressure is affected by the time-space effect and the horizontal deformation of underground continuous wall structure. Gao Wenhua et al. [10] wrote a simple program to correct the influence of space-time effect on earth pressure, which provided guidance for the selection of relevant parameters of soil rheological model.
2 Support structure design
2. 1 Calculation method of supporting structure
There are three commonly used calculation methods for the design of foundation pit supporting structure: limit equilibrium method, elastic resistance method and finite element method. Although the limit equilibrium method has many limitations, it is still the most familiar calculation method for foundation pit support designers in China. The limit equilibrium method assumes that the supporting point is a fixed point, and the statically indeterminate problem is simplified into statically indeterminate problem through some assumptions, which will undoubtedly make the calculation much simpler, but it is obviously different from the actual situation. The elastic resistance method regards the soil around the bearing as a horizontal spring, and calculates the shear force, bending moment and deformation of the bearing through the approximate equation of deflection curve [1 1]. Whether it is consistent with the actual situation is usually related to the selection of subgrade coefficient. At present, M method is commonly used to calculate, and M is the proportional coefficient of subgrade coefficient increasing with depth. In recent years, the finite element method has been widely used, which can consider some complex problems, such as layered distribution of soil, layered excavation and continuous erection of support, and even simulate whether the soil on both sides of support is symmetrical. In principle, the problems existing in the first two methods can be solved by finite element method, but the constitutive model and correlation coefficient of soil are not easy to be accurately determined, so the finite element method is generally used to assist conventional design.
2.2 Selection of supporting structure
The supporting system consists of retaining structure and supporting tie rod structure, in which retaining structure can be divided into water-stopping structure and permeable structure [12]. The water stop retaining wall is provided with dewatering wells in the foundation pit, and the permeable retaining wall is provided with drainage dewatering wells inside and outside the foundation pit. The supporting tie rod structure includes various anchorages, horizontal supports and diagonal braces of steel pipes and plates. There are many factors to be considered in the selection of supporting structure type, which are mainly influenced by the depth of foundation pit excavation, the height of groundwater level, the slope design gradient and the allowable deformation of supporting, and of course, the surrounding environment and construction factors need to be considered. Different types of support have their own functions. Steel sheet pile is suitable for excavating soft soil and muddy soil which is not too deep, and can keep moisture to a certain extent, so it can be reused. However, the steel sheet itself is a flexible material, which is easy to deform under complex underground conditions, and the displacement of the supporting structure is difficult to control. H-beam is suitable for clay and sand with excavation depth less than 25m. Its construction is simple, and the beam can be reused, but the water seal performance is poor and the price is high. Diaphragm wall has the advantages of good water tightness and small deformation, and can be used as a permanent structure. At present, it is widely used in foundation pit support, but it also has some shortcomings such as high cost and difficult rock construction [13]. There are many other types of support, such as cast-in-place pile row support, internal support pile wall support, composite support structure and so on. There are many kinds of supporting structures, and the best supporting type should be selected from the aspects of environmental protection, cost saving and convenient construction on the premise of meeting the construction requirements [14]. Zhao Shougang et al. [15] according to the engineering requirements, give different weights to various factors that affect the selection of stents, and then compare them horizontally, and the scheme with the highest score is the best scheme. He Manchao et al. [16] mapped and analyzed the influencing factors through neuron processing function, and found the optimal support scheme through a large number of training samples. Xiao Wuquan et al. [17] used Matlab software to find the minimum value of each influencing factor assignment, and then found the most economical and reasonable scheme according to the optimization algorithm. Wang Cuiying et al. [18] put forward information construction monitoring, which can monitor all data in real time, count all parameters repeatedly, and adjust constantly to achieve the goal of optimization.
3 outlook
At present, there is no scientific and perfect guidance for the design, calculation and selection of deep foundation pit support in China. How to save the cost as much as possible under the premise of meeting the needs of the project is also the focus of the actual project, and avoiding the waste caused by conservative design is the problem we need to solve now. With the development of computer technology, finite element software such as PLAXIS, ANASYS, MATLAB, etc. It has been more and more used in today's foundation pit design, but there are also many problems in the analysis of finite element software, such as inaccurate parameters, lax layered distribution of soil, and difficult selection of soil constitutive model. It is hoped that scholars and engineering practitioners will sum up experience, innovate and establish a complete design system for foundation pit support.
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