Modified Q-index for prediction of rock mass quality around a tunnel excavated with a tunnel boring machine (TBM)

Rock mass quality is closely related to tunnel stability and supporting measures. The Q-system, based on the drilling and blasting method, is one of the most important methods for rock mass classification systems and provides reliable long-term protection for tunnel excavation and reinforcement. However, in comparison to the drilling and blasting method, tunnels excavated using the tunnel boring machine (TBM) method have smooth and integral walls. The number of structural planes in these tunnels, their extension lengths, opening widths, and other characteristics are significantly different from those excavated using the drilling and blasting method. These differences lead to prediction errors in rock mass quality when the Q-system is applied to tunnels excavated by a TBM, and the coincidence rate is less than 70%. In this study, a reduction factor R_Kv, based on the wave velocity test, is used to replace the RQD/Jn term in the Q-system to reflect the integrity of the rock mass. This replacement can overcome the shortcomings that result from the smooth walls in TBM tunnels by applying the wave velocity during tunnel construction. Based on multiple regression analysis of R_Kv, we established a Q_T method for rock classification of material surrounding TBM tunnels. This new method provides a prediction coincidence rate of more than 85%.