| The Sichuan-Tibet Railway is the second"sky road"to Tibet after the Qinghai-Tibet Railway.The Sichuan-Tibet Railway starts from Ya’an City in the east and Nyingchi in the west.Among them,the proposed Sejila Mountain Tunnel is a key control project of the Sichuan-Tibet Railway from Changdu to Nyingchi.The entrance of the tunnel is located at Lulangbing Station,and the exit is near the village of Nichi,Nyingchi Town.The tunnel runs through the Sejila Mountain Range,with a total length of 37896m.The maximum buried depth of the tunnel is 1673m.The hydrogeological conditions along the route are complex,crossing faults and densely jointed zones,which will cause water inrush problems during the construction process and impact on the surrounding environment.In view of this,the hydrogeological condition analysis of Sejila Mountain Tunnel,the study of the rock mass permeability characteristics of the tunnel site,the tunnel water inrush prediction and the analysis of the water environment impact are provided in this paper to provide a certain basis for the tunnel construction.In this paper,the hydrogeological conditions of Sejila Mountain Tunnel are analyzed and the rock mass permeability characteristics of the tunnel area are analyzed by referring to geological background data,on-site hydrogeological survey,water chemical composition analysis,borehole pumping test,crack measurement method,numerical simulation,etc.On this basis,the analytical method and numerical method are used to predict the characteristics of tunnel water inflow and seepage field changes,and analyze the impact of tunnel construction on the water environment.The main research results are as follows:（1）The type of groundwater in the tunnel site is mainly bedrock fissure water,which can be divided into metamorphic rock fissure water and magmatic rock fissure water according to its lithology,and can be divided into weathered network fissure water and structure according to the type of water medium Fissure water;groundwater in the tunnel site is dominated by atmospheric rainfall and ice and snow melt water replenishment,and through surface runoff and fissure migration,under the control of terrain,it will eventually converge to the lowest discharge datum in the area.According to the analysis of water chemistry,the types of water chemistry are SO4-Ca·Na type and HCO3-Ca type,which are weakly alkaline and have a low degree of mineralization.They are both fresh water and have good water quality.The area within the tunnel site is bordered by Sejila Mountain Ridge,which belongs to the Lulang River hydrogeological unit to the east and the Niyang River hydrogeological unit to the west.（2）Based on the spatial combination characteristics of the development of rock fractures in the tunnel site,the type of rock permeability structure is determined as a network-shaped permeation structure mainly composed of cracks with different strikes and a band-shaped permeability composed of fault fracture zones and dense joint zones The structure is divided into grades of permeable structure according to the geometric parameters of the fracture.The area is dominated by grades I-III.Based on the pumping test and calculation and analysis of the permeability tensor in the tunnel site,the permeability of the rock mass is generalized and layered:fracture The calculation results of the permeability tensor indicate that the average permeability coefficient of the full-strongly weathered layer on the surface is 0.241 m/d.The mixed hole pumping test characterizes the permeability of the weakly weathered layer in the area,and the average permeability coefficient is 0.046 m/d.（3）Use Visual Modflow software to simulate the dynamic change of groundwater seepage field under different working conditions in the tunnel site.The simulated groundwater seepage field is basically consistent with the practical situation.The model results show that the groundwater level of the gully and shallow buried section changes greatly when the tunnel construction is completely drained.The water level of the borehole in the gully is reduced to 149.56 m within half a year of the tunnel construction,The rate can reach 0.82m/d,then the descent curve tends to be flat,and the descent is 147.33 m after 5 years;the water level of the borehole passing through the tunnel through the mountain gradually increases with time,and it drops by 52.19 m one year later m,but compared with drilling in the gully,the downward trend is more stable,maintaining a relatively constant rate,and the depth is 54.37 m after five years.After five years of tunnel construction,the plugging treatment continues to operate.One year after completion,the groundwater level of the borehole near the fault impact zone that the tunnel traversed has recovered from 4122.5 m to 4270.4 m,and the rate can reach 0.539 m/d for 3 years.After the water level is restored to the pre-construction state;the groundwater level of the hole at the mountain through the tunnel continues to recover at the same rate,which is slower than that of the borehole at the valley,and recovers to 3452.9m after 3 years.State before construction.（4）Prediction of tunnel water influx and influence range based on analytical method.The atmospheric rainfall infiltration method,groundwater runoff modulus method,groundwater dynamics method（Koskoff formula,empirical formula in railway survey codes）are used to predict the tunnel water inflow,and compared with the numerical simulation method to predict the normal tunnel water inflow Between88011.03-96626.27 m3/d,the average value is 92318.65 m3/d;the maximum water inflow is between 165111.22-190198.77 m3/d,the average value is 178575.33 m3/d.Analytical method is used to predict the influence radius of tunnel construction,and combined with numerical analysis,the influence radius of the deep buried section of the tunnel is about 600 m,which has slight impact on the water environment;while the shallow buried section of the tunnel passing through the fault valley,the scope of the water environment impact is significantly expanded Up to 2.5 km,the groundwater level along the fault valleys and the intersecting faults and its broken zone caused a significant drop. |
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