Estimation of the BQ system and rock mass modulus based on the P-wave velocity of the rock mass: a case study from the Himalayas tunneling

Abstract

The classification of rock mass quality is essential for geotechnical and engineering applications. Commonly used classification methods include the Rock Mass Rating (RMR), the Q system, the Geological Strength Index (GSI), and the Basic Quality (BQ) system. Among these, the BQ system has been widely adopted as the standard for engineering classification of rock masses in China. However, its application and the correlation between rock mass elastic modulus (E_m) and rock mass P-wave velocity (V_pm) for the Himalayas rock mass remain unexplored. This study investigates the correlation between BQ and V_pm for rock masses along Himalayas. Existing empirical correlations of E_m with RMR, Q, and GSI were modified by incorporating V_pm, and their suitability was assessed statistically. The results indicate that while the correlation between V_pm and E_m using GSI-based input data lacks consistency, certain equations based on RMR and Q demonstrate a good agreement after modification. The results reveal that correlations using RMR₈₉ and RMR₁₄ yielded lower prediction errors compared to those using GSI. Specifically, the MAE ranged from 7.5% to 18.3% for RMR-based models, while GSI-based correlations exhibited MAE values between 12.8% and 24.6%. The coefficient of determination (R²) for most RMR-based equations exceeded 0.95, indicating strong predictive capability. The results contribute to a better understanding of rock mass behavior in the Himalayas and provide improved predictive models for estimating rock mass elastic modulus based on P-wave velocity.