Research On Mechanical Effect And Support Of Tunnel Entrance Section With Small Spacing And Weak Surrounding Rock Under Unsymmetrical Pressure

With the advancement of national highway construction,the number of mountain highway tunnels began to increase rapidly.Small clear tunnels have been widely used in highway and railway construction due to their construction convenience and economic advantages.However,due to the poor geological conditions of the tunnel entrance section with a small clear distance,the strength of the surrounding rock is low,and often accompanied by topographic bias,the use of improper construction distance and support parameters may lead to excessive deformation of the surrounding rock during the tunnel construction process.Large or even collapse,causing heavy losses.Due to the joint distribution of the rock formations,the tunnel will often cause geological bias.In this case,the deformation of the surrounding rock and supporting structure is more complicated than that of the terrain biased tunnel.Especially for small-spaced tunnels,the effect is more obvious.Usually cause serious deformation and failure of the surrounding rock in the middle and the collapse of the tunnel face.How to choose a reasonable construction distance in topographical bias and reasonable tunnel spacing,construction method and support parameters in geological bias to ensure the stability of the surrounding rock at the entrance of the tunnel during the excavation process.It is a biased small clean tunnel The core issues faced in design and construction.In response to the appeal problem,this paper uses the Liangjiang Tunnel in Changshou District,Chongqing City as the engineering background,and uses the numerical simulation method to conduct a systematic study on it.The main research contents are as follows:(1)Analyzed the equivalent model,mechanical behavior and calculation method of tunnel surrounding rock pressure of topographically biased rock mass and geologically biased jointed rock mass.(2)The finite difference software FLAC3 D was used to simulate and study the surrounding rock of a small clear distance tunnel with five different face distances of 0B,0.5B,1.0B,1.0B,1.5B and 2.0B in the case of V-level surrounding rock.The degree of influence on stability is analyzed separately for the influence of surrounding rock displacement,stress,plastic zone and surrounding rock between different tunnel construction distances.According to the simulation results,it is confirmed that the optimal construction spacing of the tunnel faces of the first and subsequent tunnels is similar.The selection of the construction plan of the terrain biased tunnel provides a basis.(3)To study the stability of surrounding rock after excavation of five different tunnel spacings of 0.3B,0.5B,0.75 B,1.0B,and 1.5B in a small clear distance tunnel under the effect of joint bias.Comparing and analyzing the distribution of surrounding rock displacement,stress and plastic zone under each parameter,obtaining the variation law of surrounding rock stability with supporting parameters,and deriving the optimal distance between adjacent tunnel excavation,which is a similar geologically biased tunnel design plan Provide evidence.(4)On the basis of the tunnel spacing of 0.75 B,three construction methods,namely,the double-side-side piloting method,the single-side-side piloting method,and the reserved core soil method,are selected for research,and the excavation sequence under different construction methods is studied.The surrounding rock and structure are affected by the force,the maximum principal stress of the surrounding rock,the maximum displacement of the surrounding rock tunnel,the horizontal displacement and vertical stress of the sandwiched rock column are compared and analyzed.According to the analysis results,combined with economic and reasonable actual conditions,select the best Construction method,(5)When the tunnel is a small-distance tunnel,the use of prestressed anchor rods to reinforce the surrounding rock in the middle is an important reinforcement measure to improve stability.Establish numerical simulation of tunnel construction and excavation,study the influence of different sizes of pre-stressed bolt reinforcement(60KN,12 KN,180KN)on the stability of tunnel surrounding rock,and determine the optimal reinforcement measures for the middle rock column,which is the optimization of supporting parameters Provide evidence.