Experimental Study on Shear Failure Characteristics and Instability Precursors of Sawtooth Granite Structural Planes

Shear slip along structural planes in jointed rock masses is the primary trigger for rock slope instability, threatening geotechnical engineering safety. Direct shear tests were conducted on prefabricated granite specimens with regular sawtooth structural planes (undulation angles: 15°, 30°, 45°; tooth spacing: 10 mm) under 2, 4 and 6 MPa normal stresses, with synchronous acquisition of acoustic emission (AE) and infrasonic signals to explore shear failure characteristics, acoustic spectral features and instability precursors. Results show (1) peak shear stress and stiffness rise significantly with increasing undulation angle and normal stress, and failure modes evolve from sliding friction-dominated to asperity shearing-dominated, finally to composite asperity shearing and compressive crushing. (2) The spectral characteristics of both acoustic emission (AE) and infrasonic signals are closely related to the shear fracture mechanism. (3) Approaching peak shear stress, dominant frequency ratio correlation dimension drops to a minimum and the ib-value rises to a pre-sudden-drop critical point; higher undulation angles align these values with stress closer to the peak, valid as instability precursors. (4) A two-level early warning model (early to imminent warning) is proposed via cross-frequency band AE-infrasonic monitoring, providing a fundamental basis for rock slope stability monitoring using these signals.