Calculation Method Of Surrounding Rock Pressure Of Highway Tunnel With High Geostress In Squeezing Fold Structure Area

In recent years,with the tunnel project occupying an increasingly important position in transportation,it is not uncommon for tunnels built in China’s extruded surrounding rock.Especially in the western region of China,due to the continuous extrusion of the Qinghai-Tibet Plateau,an arc-shaped high-soil compression structural belt was formed on the northeastern margin of the Qinghai-Tibet Plateau.The problem of high geo-stress faced by tunnels in the area is more prominent,especially the squeezing fold structure activity,which poses a major safety hazard to tunnel construction and operation.At present,the calculation method of surrounding rock pressure of high geostress tunnel is not sufficient for the consideration of squeezing fold structure.In order to better solve the problem of high ground stress tunnel support in the squeezing fold structure area,in this paper,the stress state and the tunnel surrounding rock pressure of the extruded fold structure are analyzed by the combination of theoretical derivation and numerical simulation.And the specific research contents and conclusions are as follows:(1)The critical load calculation formula for generating a fold structure with one half wave under unconstrained conditions is used as the boundary condition for the formation of squeezed fold structure in rock formation.And the stress state of a pleated structure with “a half wave,vertical unconstrained” was derived.The analysis of the example shows that: The horizontal deformation of the rock caused by the extrusion structure in the horizontal direction causes the horizontal compression stress in the local area to be substantially consistent with the development direction of the fold.Further,a large amount of strain energy accumulated in the rock body causes high ground stress.As the depth increases,the average horizontal stress in the pleated body first appears as a gradually decreasing tensile stress,below a certain depth,and then appears as a gradually increasing compressive stress.The ratio of the average horizontal stress to the vertical stress varies with depth and is in line with the actual situation.(2)Based on the stress state in the squeezing pleated structure,the calculation domain is selected,and the stress on the boundary of the calculation domain as the boundary condition for the surrounding rock pressure of the tunnel.The analytical continuation method used to solve the displacement on the circular surface of the circular tunnel,and according to the relationship between support resistance and lining stiffness,the formula for calculating the surrounding rock pressure of circular tunnel in squeezed folds is derived.Through comparative analysis,it shows that: The surrounding rock pressure in the horizontal direction is the largest,and as the buried depth increases,the surrounding rock pressure also increases.According to the comparative analysis between different theoretical calculation methods,the calculation method of surrounding rock pressure provided in this paper can not only reflect the horizontal high geostress characteristics of the squeezing fold structure,but also reflect the interaction between the lining structure and the surrounding rock.(3)In order to further verify the rationality of the calculation method of the circular tunnel surrounding rock pressure given in(2),a finite element model for calculating the pressure of the tail of a circular tunnel is established,the stress state derived from(1)is introduced into the finite element model as the initial stress field for calculation and analysis.The comparison between the finite element simulation results and the calculation method of the surrounding rock pressure given in this paper shows that: the calculation method of this paper fits well with the surrounding rock pressure obtained by numerical simulation.Under the influence of the squeezed structure,the surrounding rock pressure of the side wall of both sides of the tunnel is the largest.According to the comparative analysis of different excavation methods of tunnel project,it shows that: During the excavation of high-stress tunnels in the squeezing pleated structure,the first part of the excavation has the greatest influence on the stress and displacement of the surrounding rock,which is the most critical part of tunnel excavation,and the single-wall guide pit excavation has the best control effect on surrounding rock deformation.Due to the effect of horizontal high ground stress,the surrounding rock pressure values on both sides of the tunnel are large.After the lining construction is completed,there is stress concentration in the surrounding rock on both sides of the side wall.The research conclusion has reference significance for similar tunnel engineering design and construction.