CHARACTERISTICS OF MEASURED STRESS AND ROUTE SELECTION STRATEGY UNDER HIGH IN-SITU STRESS RISK CONTROL ALONG LALIN SECTION OF SICHUAN-TIBET RAILWAY

With the extension of China's railway to the western mountainous areas such as the Qinghai-Tibet Plateau, there are more and more deep buried tunnels. High in-situ stress has become a kind of complex geological condition which controls the route selection scheme. If it is possible to quantitatively assess the in-situ stress state of the tunnel site before lifting, then the risk control is the most active in the stage of route selection. We have measured a lot of in-situ stress data along the Lalin section of Sichuan-Tibet railway. The structure of in-situ stress in the study area is mainly reverse stress regime and strike-slip stress regime. The direction of in-situ stress is distributed between N19°W~N30°E. The average transition depth of in-situ stress state is about 500 m. Less than that depth, the maximum horizontal in-situ stress has disordered change with buried depth. But the maximum value does not exceed the maximum stress level. More than that depth, the maximum horizontal stress can be estimated approximately by Helm formula. This is the most important rule of in-situ stress with depth. Based on this: If the buried depth of the tunnel is less than the transition depth, the supporting structure and construction method should be dynamically adjusted during the construction period. Because this is conducive to control the risk of high in-situ stress. If the buried depth of the tunnel is more than the transition depth, the in-situ stress of tunnel site can be estimated by Helm formula. According to the understanding, we can use the method of route selection to choose the better scheme of economy and technology. Finally, the route selection program of high in-situ stress area is established. The paper gives useful ideas for solving the problem of lack of quantitative evaluation method of in-situ stress in railway scheme selection stage.