Abstract: By means of axial loading and unloading of confining pressure, a series of unloading triaxial tests with different unloading rates and different confining pressures is performed using MTS815 rock test system to understand exactly the influence of unloading rate on strength of Beishan granite. Unloading rate is 0.1, 1.5, 2.0 MPa/min,respectively axial loading rate is 10 times of unloading rate of confining pressure; initial confining pressure is set as 5, 10, 15, 20, 30 MPa, axial and circumferential deformation are recorded by axial and circumferential extensometer; influence coefficient is put forward by elastic energy. The results show that: Specimens all show brittle fracture properties; strength increase with unloading rate increasing; c increase with unloading rate increasing; but φ remain the same with unloading rate increasing. The effect of unloading rate on strength, c and φ is caused by stress adjustment hysteresis and extra tensile stress. Influence coefficient is put forward based on elastic energy to determine unloading rate on destroy severity, destroy severity increase with influence coefficient increasing; in the same confining pressure influence coefficient increase with unloading rate increasing, which indicate unloading rate promote destroy severity. but in the same unloading rate influence coefficient decrease with confining pressure increasing, which indicate confining pressure reduce destroy severity.

Key words: Beishan granite, unloading rate, rock strength, influence coefficient