Study On Mechanical Characteristics Of Conversion From Large Section Station To Small Section Tunnel In Mountainous City

With the increasing functionality of subway stations,the structural design of subway stations is also becoming more and more complicated.Not only do subway stations have higher requirements in terms of cross-sectional size,but they must also be connected to various channels.At present,the mechanical research on the construction conversion of large cross-section subway stations to small cross-section structures such as access passages and ventilation ducts has yet to be improved.Based on the above background significance and scientific research value,this article takes Lijiaping Station of Chongqing Rail Transit Line 9 Phase I project as the background,divides the crosssection types into four categories,and discusses the changes of the displacement and stress of the surrounding rock and lining of subway stations during construction.Firstly,taking the excavation of large section station to small section access channel as an example,the model test is carried out to study and select similar materials for model test,and the mechanical properties of model test and numerical simulation software at the cross section are compared and analyzed,including the stability of surrounding rock of large section station,the stability of surrounding rock of small section access channel and the mechanical characteristics of support of large section station;Finally,taking Li Jia Ping Station as an example,the three-dimensional numerical simulation software is used to conduct in-depth mechanical analysis of the other three types of intersection types,such as the conversion of large cross section stations to interval tunnels,the conversion of different section types of subway stations,and the transformation of large section stations to ventilation channels.The main research contents and work are as follows:(1)Taking Lijiaping station of Chongqing Rail Transit Line 9 phase I project as the background,according to the function and characteristics of each section,the conversion types of cross section are divided into the conversion of large section metro station and small section section section section,the conversion of different section types of main tunnel of metro station,the conversion of large section metro station and small section access channel,the conversion of large section metro station and small section air channel.(2)Based on the model test similarity theory,according to the characteristics of subway station excavation,similar parameters were selected to establish the model test similarity coefficient function relationship.According to the principle of use of the similarity theorem,through physical and mechanical tests,the best mix ratio of similar materials in the surrounding rock is 664,and the best mix ratio of lining similar materials is water:(gypsum + calcium carbonate)= 1:(0.8 + 0.4).(3)According to the characteristics of the excavation from the large section station to the small section access road,a test system of the excavation amount of the crossing part is developed.The test system consists of four parts: model box and lining mold system,surrounding rock and lining simulation system based on similar materials,measurement system formed by strain acquisition instrument and model test excavation system.Finally,a complete model test system is formed.(4)Through the comparative analysis of three-dimensional numerical simulation and model test,the trend of numerical analysis and model test is more consistent in the change of the intersection area and characteristic points of the excavation from the station to the small section access channel.Surrounding rock displacement and stress changes within twice the diameter of the tunnel(16m)outside the intersection are significantly affected by the existing structure,and the settlement of the vault outside the double diameter remains stable.The support stress change at the edge of the intersection between the large-section station and the small-section access passage is significantly greater than the point farther away from the intersection,,especially the pressure value of the numerical simulation of the arch foot position of the side wall of the small section access channel,that is,the large section station support structure,has reached 14.61 MPa,the tension value of the initial excavation invert position of the small section tunnel reaches3.59 MPa。The side wall area about 8 m away from the intersection and the bottom area within 4 m are the weak points that need to be strengthened in the structural design.(5)Through the establishment of three-dimensional numerical simulation,the other three types of intersections are analyzed in-depth construction mechanics.The axial displacement between the different cross-section types of the station and the intersection of the main body of the station to the tunnel is large,and the change in settlement is not obvious.The initial supporting tensile force of the station increases rapidly at the position of the Matou wall,and the maximum tensile stress reaches 3.22 MPa.The excavation of the No.4 air duct has a certain effect on the displacement of the surrounding rock at the cross section.After the excavation and support of the air duct is completed,the supporting stress concentration at the straight wall of the air duct and the position of the upward arch is obvious.The maximum tensile stress of the support reaches 12.54 MPa.