Abstract: Due to the insufficient research on the stress relaxation characteristics of the nonholonomic hard rock, the stress relaxation tests were carried out on the gabbro specimens with pressure relief holes and rock burst tendency. The tests included the modes of pre-peak loading, relaxation and reloading, and re-relaxation. The pre-peak stress relaxation characteristics, relaxation model and ultrasonic variation law were obtained, and the mechanism of pressure relief hole in the stress relaxation process was revealed. Results showed that the stress relaxation process of hard rock with pressure relief holes experienced three evolution stages of rapid attenuation, attenuation and stability. With the gradual increase of pre-peak loading, the stress relaxation degree generally increased, the energy dissipation increased, and the proportion of energy used for damage and fracture evolution of hard rock increased. The deformation, damage, and fracture of hard rock in stress relaxation process can be reflected by the comparative analysis of the wave velocity, wave amplitude and waveform. The Burgers model can better express the stress relaxation characteristics of hard rock with pressure relief holes. Compared with the stress relaxation testing results of intact gabbro specimens, the hard rock containing pressure relief holes had higher stress relaxation degree, greater energy dissipation, and faster dissipation velocity. The research results provide significant guidance and reference for the long-term stability analysis, rock burst prediction and solution of hard rock with or without pressure relief holes.

Key words: hard rock, pressure relief hole, stress relaxation, wave velocity, energy dissipation, rock burst