Abstract: Aiming at the insufficiency of hard rock stress relaxation characteristics and rock burst research, we carried out cyclic loading and unloading tests on stress relaxation process of straight-wall-top-arch roadways (tunnels) under low, medium and high loads, derived the stress relaxation law, established the stress relaxation model, and explored the stress relaxation mechanism. The test results show that the stress relaxation process of the straight-wall-top-arch roadways (tunnels) subjected to cyclic loading and unloading under a low load presents three evolution stages: rapid decay, gradual decay and stability, and the stress relaxation trends are consistent;, the time required for stress relaxation increases, and the degree of stress attenuation loss decreases as the number of cycles increases; the stress relaxation process under low load is a process of simultaneous attenuation of tensile and compressive stresses in the surrounding rock of roadway(tunnel), with no internal damage and no overall rupture. Under a medium load, the stress relaxation process of straight-wall-top-arch roadway (tunnel) subjected to cyclic loading and unloading also presents three evolutionary stages similar to those under the low load; compared with low load scenario, the stress relaxation process lasts longer under the medium load and internal damage occurs, but subsequent cycles do not lead to surrounding rock failure. Under a high load, the surrounding rock of the straight-wall-top-arch roadway (tunnel) presents a continuous increase in tensile stress and a continuous decrease in compressive stress, the roadway (tunnel) continues to damage and rupture, and the side walls produce stratified burst, eventually forming a V-shaped burst crater, overall, two significant burst failures can be observed. The fractional-order calculus theory is used to describe the stress relaxation process under cyclic loading and unloading. A fractional-order model, namely the stress relaxation equation, is established, and the fitting effect is basically basically satisfactory. Furthermore, we define the degree of stress relaxation that fully can depict the whole process of stress relaxation and the degree of relaxation. Based on the findings of this paper, engineering measures should be taken to stop the deformation of the relaxation process of the rock explosion rock mass from developing to a critical state in engineering practice.

Key words: straight-wall-top-arch roadway(tunnel)；cyclic loading and unloading；stress relaxation, tensile stress, rockburst