| The Sichuan-Tibet Railway is a key construction project of the National "Thirteenth Five-Year Plan",and the Zheduoshan tunnel is a key control project of the Sichuan-Tibet Railway.The tunnel is located in the well-known “Y” shaped wedge of the western part of the country.The regional structure is complex and the neotectonic movement is active.A series of major engineering geological problems may occur during the construction of the project.How to tunnel through the Zheduo Mountain safely becomes a key issue restricting railway construction.Therefore,this paper takes Zheduo Mountain of Sichuan-Tibet Railway as the research object,by summing up the past geological route selection experience and lessons,combined with regional engineering geological conditions,the geological route selection principle applicable to Zheduoshan tunnel is proposed.Applying this principle and combining with the method of geological line selection,the optimal scheme is obtained by comparing different schemes of mountain tunnel.Applying this principle and combined with the geological route selection method,the different cross-hill crossing schemes of Zheduoshan tunnel are compared Select,get the most superior ridge plan.The main conclusions are as follows:(1)Zheduoshan tunnel is located in the "Y" shaped area where three deep and active faults,Xianshui River,Longmen Mountain and Anning River meet.The Zheduoshan tunnel passes through the area with high mountain valleys and deep valleys,huge terrain height differences,strong tectonic movements,frequent seismic activity,extensive geothermal distribution,and sensitive ecological environment.Choosing the best and feasible technology,saving investment,and controllable risks are the major challenges facing the Sichuan-Tibet Railway during the planning stage.(2)Railway geological route selection is mostly affected by regional engineering geological conditions,such as stratigraphic lithology,topography,geological structure,hydrology and earthquake,etc.The combination of these most basic conditions by weight can often cause various bad geological problems.These bad geology are not accidental and isolated.They often restrict the railway geological route selection.From these points of view,reviewing the literature and combining with previous engineering examples,the main controlling factors of geological route selection are summarized: active faults and other deep and large faults,high ground temperature and high-temperature-hot-water,rock burst under high ground stress conditions and the secondary disaster chain of "collapse and slip flow" caused by large deformation and high intensity earthquake.Zheduoshan tunnel is a typical difficult tunnel in complex mountainous area.The key point is that there are many undesirable geological types and the principle of universal line selection cannot be well applied.By summarizing the development and distribution characteristics of unfavorable geology along the study area,and combining with the main controlling factors of geological route selection,drawing on the principles of representative railway engineering geological route selection in complex mountainous areas in the past,the following four principles of route selection for various engineering geological problems are proposed:(1)Active faults and other deep faults: The line should try to avoid active faults and deep faults.When it is necessary to cross,choose a section with relatively good stability or a hard rock area to pass.The line should not be routed around the fault,and the project should be installed on the lower wall as much as possible.(2)High ground temperature and high temperature hot water: The line should bypass the geothermal anomaly area that may occur in a large area and choose to pass through the relatively low ground temperature zone;the line should pass through the high temperature area in the form of a bridge or roadbed and reduce the tunnel burial depth as much as possible;when it cannot be circumvented,the watershed and the replenishment area with the shortest high-temperature thermal damage section and relatively low temperature should be selected to pass,as much as possible to raise the line elevation,and pass through the hydrothermal structure at a large angle.(3)Rockburst and large deformation under high ground stress conditions: In high ground stress areas,the depth of the tunnel should be lowered through the extension of the line or through the mountain tunnel,and the direction of the line should be parallel or at a small angle to the maximum principal stress direction of the area to reduce the tangential stress of the tunnel after excavation;at the same time,the tunnel should be given priority to travel in hard rock areas.In soft rock areas,sections with undeveloped or weakly developed groundwater should be selected to pass through.Tunnels along the deep-cut valley should not be placed in areas where the hump stress is increased.Try to choose stress reduction zone or original rock stress zone.(4)High-intensity earthquakes and their secondary disasters: Hazardous rock geological development such as falling rocks,landslides,collapses,fallouts,and rock piles are along the line,especially in the canyons and steep slopes.The roads and bridges should not be used in these areas when the project passes.Tunnels should be adopted.The entrance and exit of the tunnel should try to avoid the development section of ultra-high dangerous rock falling rocks,and avoid geological hazards such as landslides,staggering,collapse,and rock piles.In the wide river valley area,the railway route selection is mainly to avoid the secondary disaster risk of earthquake on both sides of the river valley,and to avoid the route passing at the foot of the slope.The choice of the project is based on simple and easy-to-repair subgrades and simple bridges.(4)The traditional survey plan and route selection concept face problems such as difficult data acquisition,long time span,and blank areas in Zheduoshan area.Therefore,"Space-Air-Ground" integrated comprehensive survey methods are innovatively introducd in the survey.In the process of line selection,geophysical three-dimensional inversion technology,"Space-Air-Ground" multidimensional survey technology and BIM geological modeling technology are used to achieve geological disaster reduction line selection.The leap of technology from "geological plane line selection"-"geological cross-section line selection"-"3D geological line selection" has enriched the disaster reduction line selection theory.(5)By summarizing the geological route selection principles applicable to Zheduoshan tunnel,four crossing schemes were selected,and the optimal route scheme was finally determined,namely the 3750 m crossing scheme.Aiming at the pull-through scheme,combined with engineering geological conditions,some evaluation opinions are put forward,which provide reference and basis for the design and construction of the line.(6)Traversing the tectonic line at a large angle is the basic principle of railway route selection,but there has a technical obstacle in ridge crossing project of Zheduo-mountain.Existing technology of engineering unable to solve the water gushing from huge facture、coupling of high geothermal and large deformation in high crustal stress in deep.There was an enormous risk of traversing the tectonic line in straight way in consequence.This article improved the theory of railway route selection at traversing the tectonic line at a large angle: avoiding these engineering geological problems which existing engineering technology unable to solve.Getting through main fracture zone at a large angle as usual and paralleling the main fracture zone when there is a safety distance between the railway and the fracture zone.Reducing the depth of tunnel as far as is possible.Selecting area contains service gallery,make duration risk and engineering risk controllable.(7)According to the former disaster mitigation route selection experience of the high earthquake-intensity secondary disaster,the hot water and rock burst and large deformation at high geostress.Summarizing the principle of route selection when meet with former four problems,which was avoiding the secondary earthquake disaster effectively by using the tunneling to get through the highly seismic area,evading the peak stress region and reducing the tunnel depth by mountainside railway tunnel at the same time,placing the railway in cold groundwater recharge area to reduce the risk of hot water.Stick to principle of making the route at hard regions rather than the soft ones. |
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