Study On Typical Engineering Geological Problems Of Tunnels For Sichuan-Tibet Railway In High Altitude And Large Altitude-difference Areas

The Sichuan-Tibet Railway Construction Corridor is located in the southeast of the Tibetan Plateau formed by the collision and uplifting of the Eurasian Plate and the Indian Plate.It belongs to the transitional zone of China’s topography from the first ladder to the second ladder where the terrain is ups and downs.The Sichuan-Tibet Railway line passes through the Hengduan Mountains where three rivers flow concurrently.The lithology of stratum is complex and changeable,the Neotectonic movement is intense and the deep-seated large-scale active faults are widespread.The strong earthquakes occur frequently,glaciers,glacial lakes and mountain hazards groups（chains）are developed,the stability of valley slope is poor and the climate changes dramatically.The Sichuan-Tibet Railway has four major environmental characteristics,namely"significant terrain elevation difference","intense plate activities","frequent mountain disasters"and"sensitive ecological environment",which is the most complicated super engineering in the history of railroad construction so far.The main geohazards along the railway include highly collapsed（perilous rock）rockfall,landslide,scattered rockfall and talus;large-scale rainfall caused debris flow,glacial debris flow and mixed debris flow;talus slope（cold weathered deposits）;high-steep unloading bank slope in growth period;active fault and high-intensity earthquake;high geothermal temperature and high temperature water;high geostress（rockburst and large deformation）;a series of engineering geological problems such as icing disaster,avalanche,seasonal frozen soil.Taking a series of typical engineering geological problems with high altitude and large altitude-difference characteristics in the Duozhe Mountain Tunnel which will be constructed in the Yakang section of Sichuan-Tibet Railway as the main line,the control problems in the survey of tunnel engineering which is the largest portion in Sichuan-Tibet Railway were investigated.According to the topography,climate,and structural features of high altitude and large altitude-difference areas,two major geological problems,namely extra-high position perilous rock and talus slope,which are the most threatening to tunnel portals,were concluded.Three aspects,namely exploration technique innovation,formation mechanism,and stability analysis are studied in the present paper.Meanwhile,based on the experience of engineering construction in Sichuan-Tibet Plateau,the stress distribution characteristics under different burial depth and topography condition of tunnel cavern are analyzed,and estimation method of stress in caves and correction method of surrounding rock based on stress estimation in the early period of geological route selection are studied.The research results are ultimately converted into production to guide the survey and design and geological route selection in high altitude and large altitude-difference areas of the Sichuan-Tibet Railway.Based on the studies on the typical engineering problems of tunnels in Sichuan-Tibet Railway for high altitude and large altitude-difference areas,the major research results are as follows:（1）Taking the failure mechanism of the high-hanging falling type perilous rock and the stress intensity factorK_J at the moment of crack instability expansion equal to the fracture toughnessK_JC（J（28）Ⅰ、Ⅱ）as the premiss,and considering the mechanical damage of the rock mass due to the freeze-thaw cycle effect,the stability coefficient _sF of the high-hanging type perilous rock is deduced.When only considering the effect of self-gravity,the transformation of the gravity point and the stress state of the perilous rock mass are simplified.According to the linear superposition principle of the stress intensity factor,the stress intensity factor at the end of the main controlled structural surface of perilous rock mass is calculated,moreover,the stability of typical perilous rock is evaluated.（2）The advantages of the integration of UAV（Unmanned Aerial Vehicle）technique and three-dimensional（3D）laser scanning technique are discussed,as well as the data acquisition process,data processing method,point cloud coordinate correction and color matching of the point-cloud fusion 3D space imaging technique.The fusion technique is used to identify the occurrence of structural planes and the distribution characteristics of joints in perilous rocks,and a method for estimating the volume of perilous rock based on the combination of structural planes and mathematical statistics is proposed.Meanwhile,it is innovative to propose the use of fusion technique to conduct data analysis of rock fragmentation and morphological statistics of block rock.（3）The formation mechanisms and failure modes of the talus slope are clarified,and the mechanical criteria of particle initiation on the talus slope surfaceand the stability evaluation method of the debris accumulation body based on the actual terrain and structure are established.Through the UAV and 3D laser scanning technique,terrain and geomorphology that are more in line with the actual situation are obtained,and the block structure obtained from the 3D point cloud data are modeled,and then a more reliable stability evaluation result is obtained.（4）Combining the method of on-site geostress testing and numerical simulation methods,the geostress characteristics of deep river valleys in large altitude-difference areas were obtained.By a large number of data fitting calculations,the functional relationship between the hump stress region in the river valley stress field and the burial depth H,the terrain Z and the gradient i is obtained,the relationship between the depth of the river valley and the depth of the hump stress distribution is obtained as well.（5）In view of the lack of basic data in the survey and design and in the early geological routine selection period in Sichuan-Tibet Railway,taking use of the existing research and analysis of the data,a formula suitable for estimating the geostress of tunnels in the Sichuan-Tibet Railway was obtained as well as a method for grading revision of tunnel surrounding rock based on geostress estimation.It is used to guide the grading revision of surrounding rock in different topographic areas of tunnels under the influence of high geostress.（6）Based on the study of the present paper,advanced techniques,survey procedures and test methods that can be used for surveying caverns during the survey and design period in dangerous mountainous areas are proposed,as well as suggestions for selecting the geostress testing area in deep river valley field.Finally,the principle of selecting geological routes of tunnels in high altitude and large altitude-difference areas for the Sichuan-Tibet Railway is summarized.The research results can solve the problems in the early geological routes selection and the survey and design of railway tunnels in complex and dangerous mountainous areas.Based on the combination of theoretical derivation and engineering practice,and the integration of traditional technique and advanced technique,the accuracy of tunnel surveys in high altitude and large altitude-difference areas is improved,and comprehensive application is carried out in the geological mapping of the Sichuan-Tibet railway,which has important academic value and Social value.