Geothermal Genetic Mechanism Of The Gaoligong Mountain Trans-ridge Section And The Quantitative Evaluation System For Tunnel Thermal Hazard Risk

Tunnel high-temperature thermal hazard is one of the major problems existed in railway construction of southwestern China,becoming the restriction for the major constructions.The Dali-Ruili railway is located in the southern extension of the Hengduan Mountains with the overall tectonic system in north-south direction.The Gaoligong Mountain trans-ridge section of the project traverses the hydrothermal activity area where heat-and water-conducted faults are developed,confronted with serious tunnel high-temperature thermal hazard.To assess the availability of the Gaoligong Mountain trans-ridge section,numerous scientific projects had been carried out from 2006.11 to 2015.01.The detailed works were accomplished as follows:engineering hydrogeological mapping of 4000 km²;more than 60 deep drilling holes with temperature measurement;the investigation for 500 total numbers of geothermal springs and cold springs with related hydrochemical and D-O isotopic analyses,radioactive analysis and magnetotelluric measurement of tunnel section（EH4、V8）.All the above works provide a robust support for the construction of the Gaoligong Mountain trans-ridge section.They protect the building of the world-scale construction and has been verified by field construction.This study focuses on the tunnel high-temperature thermal hazard in the Gaoligong Mountain trans-ridge section of the Dali-Ruili railway.The fundamental scientific points are resolved by deeply exploring a large amount of basic information and informative data,analyzing the mechanism of hydrothermal activity,and establishing a hydrothermal genetic model.Then,quantitative evaluation of high-temperature thermal hazard and tunnel cooling measurements are raised.The achievements would provide a vital reference for the construction of the Sichuan-Tibet,Yunnan-Tibet railways and road tunnel in the north extension of the Hengduan Mountains and strongly support national major engineering projects in future.The main results of this study include the following five points:（1）The study area is located in the Hengduan Mountains and belongs to the southeastern margin of the Tibetan Plateau.It has undergone multiple phases of the Tethys Ocean tectono-magmatic evolution during the earth history,forming a complex tectonic environment and abundant geothermal resources.Controlled by deep and large heat-and water-conducted faults,three hydrothermal activity zones have been divided from east to west,together with the topographic conditions and outcropping characteristics of geothermal springs:the Nujiang north-south structure hydrothermal activity zone（Ⅰ）,the Gaoligong Mountain-Santai Mountain arc structure hydrothermal activity zone（Ⅱ）and the Tengchong-Lianghe arc structure hydrothermal activity zone（Ⅲ）.（2）The trans-ridge section of the Gaoligong Mountain mainly traverses the Gaoligong Mountain-Santai Mountain arc structure hydrothermal activity zone,which is the remnant of Gaoligong Mountain.Seven hydrothermally active faults are developed in the region,including four faults with strong thermal conductivity.Based on the test data and statistical analysis,the cation concentrations follow the order as Na⁺>Ca²⁺>K⁺>Mg²⁺,and anion concentrations follow the order as HCO₃^->SO₄^2->Cl^-.The hydrochemical type of geothermal water is mainly HCO₃-Na type.Based on the Na-K-Mg trilinear diagram,Si O₂ dissolution curve method,mineral saturation indices,chalcedony and quartz geothermometers,the characterizes of geothermal reservoirs have been analyzed.The geothermal reservoir of gneisses has the temperature of90-150℃（belongs to medium temperature geothermal system）and depth of 2.5-6.0km in the Banglazhang-Huangcaoba hydrothermal active sub-area,expect the Banglazhang hydrothermal active area.The Caoyang-Pingda hydrothermal active sub-area possess the granitic reservoir with the temperature higher 90-123.7℃and depth of 3-5 km.It belongs to medium temperature geothermal system except the Luxi active sub-area.（3）By analyzing heat source,geothermal reservoir,flow channel and cover structure of geothermal waters in the cross-ridge section of the Gaoligong Mountain based on hydrochemical compositions and hydrogen and oxygen isotopes,the hydrothermal activity pattern in this area is characterized as fracture deep circulation type.Groundwater is recharged by atmospheric precipitation at a higher area.Then it infiltrates and is heated by deep circulation.It arises by the heat from conduction and convection,and finally emerges as hot springs along fractures and joints.There may be a pattern of local magmatic heat source in the Tengchong-Rehai area,especially in the Rehai and Langpu geothermal field.The hot springs in the Gaoligong Mountain transgressive section display significant differences in hydrothermal activity patterns compared with that of typical hot springs in the Tengchong area.（4）Based on the hazard-forming environment,geothermal activity characterization,heat flow transport channel,shallow cold water effect and tunnel engineering conditions are proposed as four elements to control tunnel thermal damage.10 evaluation indexes are selected to establish the risk evaluation system of tunnel geothermal hazard.Based on the influence and mechanism of the shallow cold water affecting hot water transport in the water storage basin,a formula for calculating the cold-water mixing intensity in the water storage basin was constructed.The natural breakpoint and expert method analysis is combined to determine the index value domain and divide the four risk levels.Based on the mutation theory,the mathematical algorithm is optimized and improved,and then a mutation-detection-cloud evaluation mathematical model is established.Taking Gaoligongshan tunnel as an example,the evaluation system application verification is carried out.The comparative analysis is consistent in 34 out of 38 segments,with an evaluation accuracy of 89.47%.The results show that the constructed tunnel thermal hazard risk evaluation system can effectively evaluate the tunnel thermal damage problem under the fracture deep circulation type geothermal genetic model.（5）Based on the quantitative evaluation system of thermal hazard risks applied to the engineering example,the thermal hazard risk area is selected.In the condition of the press-in ventilation,the ANSYS Fluent fluid simulation is used.For this area,the ambient temperature can be reduced to 28℃at a wind speed of 9m/s,as well as the minimum air volume.It is advisable to adopt the principle of"mainly blocking,limited emission,strengthening overrun,detecting and releasing pressure"to deal with heat conduction fracture in construction,and to adopt measures such as mechanical cooling,water spraying and ice cooling for the segments with unsatisfactory cooling effect.Meanwhile,it is encouraged to strengthen individual protection.