Numerical Simulation Research On Prestressed Anchor Support With Advanced Steel Pipe Pile In Deep Foundation Pit

As one of the support forms in deep foundation engineering, technology of prestressed anchor support with advanced steel pipe pile is now being widely used and spread in the field of geotechnical engineering for its significant advantages, such as economic, reliability, convenience, small deformation, large support depth,etc. However, the theoretical study of the technology lags far behind in engineering practice. By now, it is still in lack of practical calculation method of the deformation and displacement, and most related design standards expound the aspect of ensuring the anchor strength, while lacking necessary theoretical analysis of the stability calculation of foundation system itself. Moreover, the deep foundation pit engineering design and calculation methods provided by the current norms give no deformation data. Therefore, it is necessary to study the stability and deformation of the deep foundation engineering.As a three-dimensional explicit finite difference program, LFAC^3D, which use differential technology, induct the time factor and the slip-line technology and achieve the simulation and analysis from continuum, small deformation to large deformation, can simulate and calculate the stress and deformation patterns of the engineering structure in three-dimensional rock and soil and other media. It has been widely recognized and applied both at home and abroad. Based on Lagrangian element method, LFAC^3D can apply to the vast majority of the engineering mechanics problems, especially for elastic-plastic and large deformation analysis, rheological forecast and the numerical simulation of the geotechnical engineering construction.Therefore, using FLAC^3D finite difference numerical simulation software, technology of prestressed anchor support with advanced steel pipe pile in deep foundation pit is simulated and analyzed, so as to study the mechanism of supporting structure and discuss the stability and deformation factors of foundation pit excavation and support. It has a very important significance to provide a basis for the following engineering design and provide experiences for similar foundation engineering.This article focuses on the dynamic simulation of deep foundation excavation and support on basis of analysing the mechanism of the advanced steel pile and prestressed anchor support structure, using FLAC^3D as the simulation and calculation software and the RBF neural network -based displacements back analysis to abtain the appropriate soil parameters. The main factors to control the foundation stability and the deformation are explored from the simulation and analysis results of the displacement isogram of the pit soils, which provide this supporting technology of the wider use.In this paper, the following aspects will be studied:(1) Current situation of Engineering applications, theoretical research and existing problems of prestressed anchor support with advanced steel pipe pile are introduced. The structural characteristics and the mechanism and construction process of the structure are analyzed. Meanwhile, the advanced steel pipe and prestressed anchor support technology is compared with the advanced steel pile and soil nail wall technology to show its superiorities whether in controlling the foundation deformation or in improving the foundation stability.Therefore, in case of reasonable design, the technology can play a full role in advanced supporting of the steel pipe pile and in fasten and strengthen of pre-stressed anchor, and can solve the foundation engineering problems which can not be solved by using only soil nailed wall support or prestressed anchor support. It has a wide range of engineering applications and development prospects.(2)Two commonly used constitutive models of soil and the yield criteria are briefly introduced and compared; The calculating principles and the advantages and disadvantages of the internationally accepted geotechnical engineering numerical software FLAC^3D are introduced; The reasonable computing model in simulation of the excavation support, the support structural unit and boundary conditions are chosen; The stress state of excavated soil is analyzed, and the initial stress field and the basic processes of the numerical simulation in the construction of excavation support is briefly introduced.FLAC^3D can provide the deformation and displacement information of the foundation engineering, comprehensively consider the change of the excavation and support process and play a very useful role in guiding the engineering design and construction. It can pre-test the effect of the foundation design by selecting Mohr-Coulomb basic model, using the basic unit hexahedron, using pile units to simulate steel pipe piles, using bolt units to simulate of prestressed anchor units and making multi-condition simulation in accordance with the step-by-step excavation and support process of the foundation pit, so as to adjust and optimize the design according to feedback information.(3)Combined with deep foundation engineering example, using fish programming language in FLAC^3D, simulation of step-by-step excavation of the foundation is carried out. Through analyzing the displacement results of the simulation and comparing with the displacement monitoring data, the reasonability of the selected model, simulation software and simulation procedure is proved.Simulation results show that: the common role of section CD advanced steel pipe and the pre-stressed anchor has played a good role in controlling the displacement and deformation of foundation. Meanwhile, it meets the requirements of the safety and the normal use requirements of the surrounding buildings. Design safety factor of section BC is on the high side. The support can be optimized in accordance with the security, economic and rational principles. (4) Generally speaking, soil parameters by laboratory tests can not be directly used as the model parameters in numerical simulation. Compiled with the commonly used BP neural network method, the RBF neural network-based displacements back analysis has a high-speed advantage. Soil parameters of the foundation pit can be inversed by using construction monitoring data so as to obtain the optimal model calculating parameters.(5)The effecting factors controlling foundation deformation and stability are summarized and analyzed and the excavation support program is evaluated and optimized. Through numerical re-simulation after repeated changes of model parameters and the analysis of the displacement distribution curve, the effecting characteristics to stability safety of foundation support structure and displacement deformation control, which are imposed by the nature of the soil foundation, supporting structure and construction process, etc. are studied.The research results show that: the main parameters of the sensitivity of foundation stability and safety and of the maximum displacement value include:①rock and soil parameters: bulk densityγ, internal friction angleφ, cohesion c and the tensile strengthσ_t ;②pre-stressed anchor parameters: bolt pre-stress T, free length l_a, bolt inclinationδ;③steel pipe pile parameters: the length and location of steel pipe pile;④construction process parameters: the depth of step-by-step excavation and the load size of the ground construction.Innovation of this article:(1)In order to select reasonable physical and mechanical parameters of the foundation soil, RBF neural network-based anti-displacement analysis model is set up and model parameters inversion is done by using displacement monitor data of the foundation.(2) Combined the sensitivity analysis and numerical simulation method, multi-project status simulation of the deep foundation pit example is made, the control factors to the stability and deformation of the deep foundation pit has been summarized and researched.