| The pressure arch reflects the self-stabilizing ability of surrounding rock,and can bear the load and limit the displacement of the upper surrounding rock.Currently,in the design of subway tunnels,both the calculation of the critical buried depth and the determination of the earth pressure load on the tunnel need to be discussed based on the pressure arch theory.Pressure arch is an internal macroscopic structure of a medium caused by displacement of force transmission,and the formation process cannot be monitored,so it is impossible to accurately evaluate the range and thickness of the pressure arch through measurement in actual engineering.What factors affect the formation of pressure arches? When the factors such as tunnel burial depth change,what will happen to the thickness and shape of the pressure arch? Is there a correlation between pressure arch and surface subsidence? Can the relationship between the two be quantitatively expressed? There are relatively few studies on these issues.In this paper,PFC2 D particle flow software is used to simulate unsupported tunnel excavation in stratum,analyze the pressure arch effect in soil and the settlement law of the surface subsidence,find out the correlation between the two.By studying the deformation of soil within the pressure arch,establish the relationship between the pressure arch and the surface subsidence,and provide a reference for the formation mechanism of the pressure arch in unsupported tunnels and the calculation of surface subsidence.The main research results are as follows:(1)The relationship between the relative thickness of the pressure arch and the relative surface subsidence under different burial depths is established.The deformation of the surrounding rock after tunnel excavation was analyzed.It is found that the deformation of soil mass within the pressure arch is greater than that of other area,because the pressure arch plays a role in maintaining the stability of the upper soil mass and limiting the further development of the upper soil mass deformation.By analyzing the distribution of the relative thickness and relative surface subsidence of pressure arches under different burial depths,the correlation between the relative thickness of the pressure arch and the relative subsidence of the ground is found.When the tunnel burial depth increases,the relative width of subsidence trough increases,the subsidence becomes more dispersed,the maximum value of relative surface subsidence decreases,and the corresponding relative width and thickness of pressure arches become larger.(2)The relationship between the relative vertical deformation of the pressure arch structure and the relative surface subsidence under different burial depths is obtained.The relative vertical deformation and relative surface subsidence of pressure arch structures under different relative burial depths are analyzed,and it is found that the relationship between the two is approximately proportional.Combining the proportion of disturbed soil in the soil overlying the tunnel,the feasibility of solving relative surface subsidence based on the relative vertical deformation of pressure arch structures is proved.Based on the stress analysis of the pressure arch structure,the characteristic equation of the arch axis of the pressure arch structure is obtained.It is found that the characteristic equation of the arch axis of the pressure arch structure is elliptical,and its morphological characteristics are related to the tunnel span,height,and lateral pressure coefficient of the surrounding rock.(3)Based on the pressure arch theory,a calculation formula for surface subsidence of unsupported tunnel excavation under different burial depths is established.Based on the deformation of the pressure arch structure during unsupported tunnel excavation,the deformation of the pressure arch structure is divided into vertical deformation caused by the reduced thickness of the pressure arch and the shortening of the arch axis.Calculate the vertical deformation of pressure arch structures under different earth pressure theories,and establish a relationship with the maximum value of surface subsidence.By comparing with the model experiment,the calculation theory of surface subsidence for unsupported tunnel excavation based on the pressure arch theory was determined at different depths. |
