Abstract: A deeply embedded tunnel in the western Sichuan crosses the worldfamous Xianshuihe tectonic fault zone, which has very complicated geological conditions, such as significant elevation differences in topography, extreme climatic conditions, strong plate movement, complicated and dense fault structure, weak and broken rock mass structure, complicated hydrogeology, extremely high ground stress and other adverse environments. Large deformation hazards induced by the soft and weak rock mass in the deeply embedded tunnel remain the most prominent geological engineering challenges faced in the railway construction. The accurate prediction of the large deformation has an important contribution to the tunnel construction. Based on the back analysis results from the 9-borehole in-situ stress tests, three methods were used to predict the large deformation, namely, compressive strength stress ratio method, geological analysis method and surrounding rock characteristic method. Several targeted measures for the treatment of a large deformation tunnel are proposed. The following conclusions can be obtained: (1) the measured maximum lateral principal stress of the tunnel is 42.74MPa at a test depth of 1036.4 meters. The back calculated in-situ stress of the tunnel is between 5 and 45.2 MPa, and the maximum in-situ stress is located at the maximum embedment depth of 1219 meters. (2) The prediction method of “calculating by multiple methods and determining the large deformation grade and section after synthetical analysis” proposed in this paper effectively ensure the objectivity of the predicted results. The grade of deformation mainly results in slight, medium and strong large deformation, the proportion of each part accounts for 21%, 6% and 7%, respectively. (3) Based on the predicted results, targeted support measures are proposed: the mitigation measures for tunnels with different deformation grade are given, as follows: Measures such as bolt support, shotcrete or shotcrete + hanging reinforcement mesh, setting steel arch frame and improving the strength of the shotcrete and so on can be adopted for the slight large deformation. Measures such as set up longitudinal joints in shotcrete layer, shotcrete + hanging reinforcement mesh or shotcrete with steel fiber, shrinkable steel frame and so on can be adopted for medium large deformation. Measures such as strengthening stratum Shotcrete + hanging reinforcement mesh or steel fiber shotcrete, grouting, longitudinal joint in shotcrete layer, shrinkable steel arch, long anchor bolt and so on can be adopted for strong large deformation. (4) The above research results can provide reference for the large deformation prediction and mitigation in complicated tectonic structural zone.

Key words: Xianshuihe Tectonic Structural Zone, Prediction of Large Deformation, Classification Method, Mitigation Measures