Evaluating Mohr–Coulomb and Hoek–Brown Strength Criteria for Rock Masses Using Probabilistic Assessment

The Hoek–Brown (H-B) criterion is widely recognized as a standard in geotechnical engineering for assessing rock mass strength across various rock mass qualities. However, challenges arise in explicitly defining the Mohr failure envelope, particularly when the strength parameter “a” deviates from the conventional value of 0.5. This study investigates the compressive strength of rock masses in the Himalayas, particularly in the context of deep tunneling and slope stability, using the H-B and Mohr–Coulomb (MC) criteria. Initially, the MC and H-B criteria were combined while varying the angle of internal friction, revealing an inconsistent trend in friction angles regarding rock mass compressive strength. The relationship between tunnel depth, slope height, and rock mass compressive strength was then examined by combining equations involving RMR, RQD, and modified H-B criteria. The combination of H-B and MC resulted in lower rock mass compressive strength values, while noncombined equations yielded higher values. Incorporating the geological strength index (GSI) provided higher and more suitable compressive strength values. For the Himalayas, the suggested H-B equations with GSI are recommended for both surface and subsurface excavations.