A New Classification Method of Surrounding Rock Quality for Phyllite Tunnels Under the Condition of Layer Orientation Parallel to the Orientation of Tunnel Axis

ABSTRACT The HC method for hydropower is a commonly used rock mass quality classification technique in China's hydropower industry. Due to the anisotropic nature of the layered schist in the study area, and the varying angles between different tunnel layers and the tunnel axis, significant discrepancies arise between the HC method's classification results and actual rock mass classifications when these angles are parallel. This study employs uniaxial compression tests on schist to reveal its anisotropic characteristics under loading directions at 0°, 45°, and 90° angles relative to the bedding planes. The compressive strength exhibits a V‐shaped variation with changes in angle between loading direction and schistosity plane, while the elasticity modulus shows a linear decrease as this angle varies. Numerical simulation experiments were conducted to monitor deformations of surrounding rock masses around tunnels. The findings indicate that as the angle between bedding orientation and tunnel axis decreases, both wall and roof deformations increase progressively. Under conditions of 0°, 30°, 45°, 60°, and 90° angles, the ratios of wall deformation values are approximately 1:3.73:4.74:5.44:7.7; whereas for roof deformation values, they are about 1:1.3:1.94:4.7:6.7. When applying traditional HC methods for classifying surrounding rock quality in parallel schist tunnels, a low agreement rate of only 13.33% was observed. However, by incorporating adjustments based on scoring criteria related to major structural plane orientations into numerical simulation results—specifically modifying weights assigned to structural planes—the agreement rate improved significantly to an impressive 100%. These research outcomes effectively enhance both accuracy and applicability in classifying layered rock masses, providing reliable foundations for tunneling construction practices.