The Measurement Of In-situ Stress In The Typical Project Area Of The Muzhailing Tunnel Of Lanzhou-Chongqing Railway And Its Engineering Geological Significance

Lanyu railway,which is being constructed,will form an important large-scale transport line connecting northwest and southwest areas.As one of uncompleted parts of Lanyu railway,the Muzhailing tunnel is located in MinxianCounty,Gansu Province,Dingxi City,The tunnel runs almost from south to north,whose total length is nearly 19 km.In view of poor geological conditions and high construction difficulty,the tunnel is known as the most high-risk railway tunnel in China.In the Muzhailing tunnel project area,strata are mainly soft rock such as slate,carbonaceous slateand so on,as well as some hard rock,including sandstoneand so forth.However,under the strong influence of tectonic activities,fault and fold developed,and rock mass crushed.In consequence,with excavation of the tunnel,surrounding rock deformed to a large degree,and the tunnel lining deformed or was even destroyed,which resulted in slowness of the progress of the project.Therefore,it is urgent to find out the present in-situ stress state of the project area,and to analyze and predict the large deformation of soft rock and other possible engineering geological problems so as to ensure smooth construction of the tunnel.Relying on the project “In-situ Stress Measurement of the Muzhailing tunnel in Lanyu railway” undertaken by the Institute of Geomechanics,Chinese Academy of Geological Sciences.By measuring the in-situ stress of these areas,the present in-situ stress state is obtained.Then in accordance with the stratum lithology of the areas,the study analyzes and investigates the deformation of surrounding rock and other possible engineering geological problems,and then provides relative conclusions.The main research work is as follows:(1)The study systematically collects,analyzes and summarizes theses with reference to the in-situ stress state of Muzhailing.(2)The study carries out hydraulic fracturing in-situ stress tests and obtains the present in-situ stress state of Muzhailing;(3)The study investigates thelarge deformation of surrounding rock and other possible engineering geological problems.The main research findings are as follows:(1)The study shows that in the Muzhailing tunnel project areathe mean of maximum horizontal principal stress is 32.03 MPa and that of the minimum one is 18.73MPa;as the measurement goes deeper,the in-situ stress increases correspondingly;the direction of in-situ stress is N35°~55°E and the area belongs to the high in-situ stress area;(2)According to the direction of the tunnel and the direction of the present in-situ stress,the study finds that the angle between the direction of in-situ stress and the tunnel axis is not big,which is favorable for tunnel excavation;(3)In the uncompleted areas of the tunnel.In the slate area,excavation may result in slight to medium strength compressional deformation;and in the sandstone area,medium to high strength rock burst may occur.