A risk analysis method in rock mass quality classification based on Q system was studied

by using Monte-Carlo simulation technique in this article.All factors in Q system are treated as random variables in this method.By using the probabilistic models of these factors

the probability of the case that the Q value of the rock mass is poorer than that used in the design is adopted. The risk probability of each Q value

the grade of rock mass quality which may be appeared in rock mass and the percent of each grade can be got by analyzing Q value cumulative cure and rock mass quality distribution chart

which was proposed in this article.This method acquired good results in application to the Qingdao Jiao Zhou Bay cross-harbor tunnel project.