EXPERIMENTAL STUDY ON SHEAR STRENGTH AND FAILURE CHARACTERISTICS OF MICA QUARTZ SCHIST AT DIFFERENT SHEAR ANGLES

The shear resistance of schist is influenced not only by the dip angle of the schistosity planes but also by the angle between the direction of shear loading and the orientation of these planes. To investigate the effect of shear angle on the shear properties of schist, direct shear tests with varying shear angles were conducted on mica quartz schist samples with fixed schistosity inclinations. The variation in shear strength and failure characteristics with shear angle was obtained. The results show that the peak shear stress, internal friction angle, and cohesion of mica quartz schist initially increase, then decrease, and finally increase again with increasing shear angle. At a shear angle of 60°, the peak shear stress and cohesion reach their maximum values, while the internal friction angle reaches its minimum. For shear angles ranging from 0°to 45°, failure occurs either along or across the schistosity planes, with the degree of such failure diminishing as the shear angle and normal load increase. At shear angles of 60°to 75°, failure along or across the schistosity planes becomes more pronounced with increasing normal load. At a shear angle of 90°, the shear failure surface is perpendicular to the schistosity, and failure occurs without following the schistosity planes. Furthermore, for shear angles between 0°and 45°, the total sliding force along the schistosity planes decreases linearly with increasing normal load. For shear angles from 60°to 90°, the slip component of the shear force along the schistosity planes gradually approaches zero as the shear angle increases. The horizontal component perpendicular to the schistosity direction plays a key role in determining whether shear slip occurs along the schistosity planes. These findings provide a reference for engineering stability assessments of schist slopes.