Study On Hydrogeological Conditions And Route Optimization Of Yigong Tunnel Of Sichuan-Tibet Railway

Along the Sichuan Tibet railway,the terrain is ups and downs,the mountains are high and the valleys are deep,the geological conditions are complex,the neotectonic movement is intense,the deep and large active faults are widely distributed,the internal and external dynamic geological actions are strong.The railway mostly passes through in the form of tunnels and bridges,and the proportion of tunnel engineering is high and the burial depth is large.Tunnel project accounted for a high proportion and the buried depth is large.Among them,the proposed Yigong tunnel is the longest tunnel in the whole line,with a total length of 42.468km and maximum burial depth of 1700m.The fracture development and hydrogeological conditions along the line are extremely complex.There are water inrush problems in the tunnel construction,which threaten the safety of construction workers and affect the surrounding groundwater environment.In view of the above situation,analyzing the hydro-geological conditions of the Yigong tunnel,selecting a reasonable tunnel route,and optimizing it will help reduce the probability of engineering hydrogeological problems caused by tunnel construction and ensure the safety of tunnel construction.Based on the in-depth analysis of the hydrogeological conditions of the Yigong tunnel,combined with the characteristics of the tunnel itself,this paper establishes a comprehensive evaluation model for the selection of the Yigong tunnel route scheme by combining the analytic hierarchy process and the fuzzy comprehensive evaluation method,and the four route schemes are comprehensively optimized to determine the best scheme.The main research results are as follows:1)The magmatic rocks and gneiss are relatively developed in the study area.The Yigong Tunnel mainly passes through the gneiss of A rock group of Nianqing Tanggula Group(Pt_2-3Nq^a),gneiss of B rock group of the Nianqing Tanggula Group?An Nq^b?,Devonian grey gneiss-like dolomite monzonite?D₁gn???,and the Jurassic dolomite hornblende diorite?J₁?????,Jurassic dolomite granodiorite gneiss?Jgn?.Jurassic gray fine-grained tonalite?J₁o??.The fault structure in this area is relatively developed.The whole is generally controlled by the Jiali-Yigong Zangbu fault zone?F3-6?and the Jiali-Polong Zangbu fault zone?F3-8?,with multiple branch faults,and the rock mass near the fault zone is relatively broken.2)The type of groundwater in the study area can be divided into two types:loose rock pore water and bedrock fissure water.The bedrock fissure water can be subdivided into weathered fissure water and tectonic fissure water.The main type of groundwater in the tunnel site area is bedrock fissure water,which main occurs in the pores,fissures and fracture zones of the weathering layer of magmatic rocks and metamorphic rocks.The broken zone runs off into the distance and discharges in low-lying areas.Atmospheric precipitation and replenishment of ice and snow melt in the area account for a large proportion of the recharge source of groundwater.3)The study area can be divided into six first-level hydrogeological units,namely,Jiali-Galongsi Arc unit?,Yigong Ancient Township unit?,Tongmai unit?,Bairenmuqu unit?,Dania Dezha unit?,Magula Qinduo unit?;six secondary hydrogeological units,namely,Galangcun subunit?-1,Zataduo subunit?-2,Jiaolong subunit?-3,Laiqu subunit?-4,Jialongbaqu subunit?-5,Chalong Longbaqu subunit?-6.The Yigong tunnel as a whole belongs to the first-level unit of Yigong Ancient Township,which mainly passes through six second-level hydrogeological subunits.4)Consider the engineering hydrogeological control factors that influence the tunnel route selection,determine the division criteria of each factor,and use the theoretical formula method to predict the water influx,influence radius and external water pressure of the four route schemes of the Yigong tunnel.Among them,the normal water inflow of CK line is 33196.49m³/d,the maximum water inflow is 96246.95m³/d,the maximum impact radius is 638.63m,and the maximum external water pressure is4.46MPa.On the basis of following the principle of constructing the index system,natural geographical factors,Geological-hydrological factors,and tunnel engineering factors were selected as the three first-level indicators for the rout selection of Yigong tunnel,and these three first-level indicators were further subdivided into twelve secondary indicators,including nine quantitative index and three qualitative indicators,so as to establish index system for the evaluation of the Yigong tunnel route scheme.5)Through the comprehensive comparison and selection of four line schemes of Yigong tunnel by using the established model,the maximum fuzzy comprehensive score of CK line scheme is 2.9131,the minimum fuzzy comprehensive score of IC1K line scheme is 2.7259,and the ratio of fuzzy comprehensive evaluation scores of IC11K line scheme and IC12K line scheme is close,which are 2.8247 and 2.8258 respectively.The result shows that the CK line is relatively superior.and it is a recommended line scheme,which is consistent with the route plan recommended in the feasibility report of the Design Institute,indicating that the established model can be used to assist the decision-making of line scheme and has certain practical significance.