Abstract: The values of geological strength index (GSI) are always regional and have an influence on the application of Hoek-Brown criterion. In this article, back analysis was performed for geological strength index (GSI), disturbance factor D and Poisson ratio based on the Hoek-Brown criterion for the tunnel from Yongping Road to the North Station of Qingdao Metro Line 3. Uniform design table was employed to deal with the range of parameters and the selection of experimental points for back analysis to reduce the number of experiments and guarantee better experiment results. Furthermore, the GSI values of 10 different cross-sections were calculated by FLAC^3D software and the relationship between GSI and rock mass ultrasonic velocity (V_p) was obtained using linear fitting method. The formula of GSI-V_p was applied to estimate the GSI values for the tunnel from Middle Chongqing Road to Yongping Road of Qingdao Metro Line 3. The calculated result was in good agreement with monitoring data, which verifies the rationality of the GSI-V_p formula and provides a new approach for accurately obtaining the GSI values.

Key words: Hoek-Brown criterion, displacement back analysis, uniform design, geological strength index(GSI), rock mass ultrasonic velocity (V_p)