| With the continuous progress of the society,China’s urbanization rate is getting higher and higher.The city is getting bigger and bigger,and the population is constantly surging,which leads to more and more serious traffic pressure in the city.It is the general trend of the society to solve the increasingly serious traffic jam in the city and continuously develop the underground track project.The underground tunnel makes the city has the space,the underground rail transit carrying capacity is very strong,its increasing has become the city to accelerate the key point.However,in the process of tunnel excavation,uneven settlement of overlying soil due to formation loss often occurs,which seriously endangers the safety of underground space structure.Therefore,we often adopt composite support structure to support excavation soil.Combined support of grille arch and shotcrete is a very common support form in tunnel excavation under complex geological environment.During the support,the grille arch enhances the stiffness and strength of shotcrete,and the shotcrete also enhances the relative stability of arch support.The mechanical properties of the two are complementary and improved.In addition,in the process of tunnel excavation,the soil arch can withstand part of the pressure caused by the upper load,which brings the advantage that the load pressure of our combined support is reduced,and the design is optimized and materials are saved.In the analysis of soil arch effect,we found that the soil arch effect formed by different gradation of sandy soil is also different.This paper,by using elastic mechanics on the basis of the thin shell theory,tunnel bracing structure of simplified mechanical model is set up,using the theory of too sand and division deep shallow buried tunnel boundary type theory,computing Ⅱ ～ Ⅴ level of surrounding rock under the condition of vertical load and horizontal load,displacement and internal force was calculated by the matrix displacement method.In this paper,sand with different particle size as the screening condition,the soil is divided into coarse sand,medium sand,fine sand and silt.Midas-gts geotechnical software is used to establish the combined support model of tunnel excavation for the above four soils,and the process of excavation and support is numerically simulated.The vertical displacement and stress of overlying soil under the above soil conditions are studied to analyze theformation process of soil arch effect in soil and the mechanical characteristics of composite support.When the overburden ratio is 0.91,1.36,1.82,2.27 and 2.73,the stress and vertical settlement of soil are studied to analyze the formation process of soil arch effect in soil and the influence of conditions with or without arch effect on composite support.Using MIDAS GTS-geotechnical software for Ⅱ ～ Ⅴ level four conditions of surrounding soil tunnel excavation bracing structure model is established,and numerical simulation analysis was carried out on the process of the excavation support,research the combination support of vertical displacement deformation and stiffness,strength and stability analysis.Choose Ⅳ class surrounding rock conditions for numerical simulation analysis,using the method of control variables,the bolt diameter is 25 mm ～ 35 mm and arch under the condition of 22 mm ～ 26 mm diameter combined bolting of maximum principal stress.The horizontal and vertical displacements of bolt and arch support with different diameters are studied under the same steel quantity.Choose Ⅳ class surrounding rock conditions for numerical simulation analysis,establish,grille arch combination anchor supporting tunnel excavation supporting model.The changes of surface vault uplift and settlement,maximum and minimum principal stress and shear stress in the tunnel excavation under different support combinations are studied,and the redistribution of surrounding rock pressure is studied. |
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