Over the recent decades, several authors have documented empirical associations among rock mass classification systems, which are derived from extensive measurements and observations under particular site conditions and in various geological regimes. This underscores how important the reliability of a correlation equations is, and how critical is the apprehension of each systems objective. Consequently, none of these formulas can claim universal applicability in the contemporary context. Further to this, to ensure effective design and evaluation of an underground opening, it is essential to possess a comprehensive understanding of the geological, structural geological, hydrogeological, and geotechnical attributes of the excavated rock. In the context of the Tanahu Hydropower Project in Nepal, diversion tunnels and auxiliary galleries are being constructed in fractured dolomite. 644 tunnel profiles were mapped, and rock mass was classified using three empirical systems; GSI, RMR and Q. After processing and plotting the in-situ data, correlation equations for the three indices were examined using regression modeling aiming to analyze the results and identify the most optimal equation. These relationships were then compared with those found in existing literature. The developed regression models reveal reliable correlations between RMR, Q, and GSI indices, enabling the engineering geological evaluation for a broad range of rock mass qualities. These mathematical formulations of geo-mechanical indices will serve as valuable tools for tunnelling professionals during decision-making processes, preliminary design phases, stability assessments, and estimates for support systems.