Abstract：Research purposes: In recent years， the sudden and catastrophic geological disasters of Karst tunnel happen continuously and they not only cause the loss of state property，the construction delay and the environmental disruption，but also cause serious personal casualty． The reasons for the disasters are the risk hazard has not been fully understood and the full measures have not been taken． Based on the study on the influence factors of the mountain karst tunnel risk，this paper tends to evaluate the risk degree and grade the risk for laying the solid foundation for tunnel construction．
Research conclusions: The analysis of causes of Karst tunnel risk is made to find out the influence of the intrinsic factors，external conditions and engineering technical conditions on tunnel risk，and delete the overlapped factors．Based on the study， it is aware of that the formation lithology and geological structure that promote the development of Karst and Karst water are the intrinsic factors of the karst tunnel risk and play the leading role in the risk while the external conditions and engineering technical conditions play the auxiliary role in the risk． According to this ，the basic grading standard and method are formulated and the scoring formula is used for grading the tunnel risk．As long as the intrinsic factors exist，there is a possibility that the karst tunnel has high risk． The high risk and extremely high risk Karst tunnel should be avoided as much as possible． If there is no way to avoid，the traditional idea should be changed and the risk should be under control．
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2011, Vol. 28 
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