Surface Settlement Deformation Analysis Based On Pile-anchor Retaining Structure

The specificity of deep excavation is unloading, and the excavation process will cause structure deformation, settlement of surrounding soil and soil uplift in the pit. With the accelerated pace of urbanization, building deep foundation in a congested urban district has become a necessity, excavation is no longer the traditional simple question that only to ensure its own stability, we should pay more attention to deformation of surrounding soil, because it is related to the safety of adjacent buildings and the normal operation of nearby traffic. Considering that there are more and more pit accidents caused by excavation, study on deformation mechanism, changing laws and control measures of the pit deformation are very significant. Excavation unloading and foundation deformation have been studied by theoretical analysis and numerical simulation method, the main contents and conclusions are as follows:Firstly, analyze on deformation phenomenon and deformation mechanism of foundation pit, influencing factors of pit deformation and the prediction methods of surface settlement at present stage have been discussed emphatically. Laws of foundation pit deformation will be studied from geotechnical parameters, design factors and construction factors.Secondly, excavation unloading effect has been studied by theoretical analysis and finite element method based on pile-anchor system, it shows that:the static lateral soil pressure coefficient is variable when unloading, and with the increasing distance from the cutting face, it is close to normal static lateral soil pressure coefficient K0 continuously; we can obtain the unloading range of the active area soil that above the bottom of pit is 1.5L(L is the length of pile) by analyzing the variation of unloading static lateral soil pressure coefficient K0; we also can obstain the influence depth of unloading is 1.0～1.5H(H is the excavation depth in pit) by analyzing the lateral stress rate of the passive zone soil which under the bottom of foundation pit; the unloading stress path of the soil is not the same at different part of pit, according to the change of soil stress, foundation pit can be divided into six regions, when we determine the mechanical parameters of soil, the soil in different regions should be taken to a different laboratory test.Thirdly, Beijing railway command center phaseⅡexcavation is analyzed by FEM, and plane finite element software Plaxis is used to simulate the exvacation steps, different soil constitutive models have been analyzed and compared, the results show that Hardening-Soil model can reflect the characteristics of the relationship between soil strain and stress when unloading, for excavation that has the obvious nature of unloading, Hardening-Soil model is more suitable than Mohr-Coulomb model in the numerical simulation analysis.Fourthly, pile-anchor support foundation deformation has been studied from the soil parameters, design factors and construction factors, through the discussion of influencing factors, we get the sensitivity of pit deformation to different factors and find the main factors of pit deformation, such as soil elastic modulus, Poisson's ratio, internal friction angle, ground overload, overbreak, anchor obliquity and anchor prestressing force, from the above results, deformation control measures have also been given.