Strengthening effect of cyclic load on siltstone and its macro--micro fracture mechanism

Cyclic loading affects the mechanic characteristics and long-term stability of geotechnical engineering. Although the fatigue failure and weakening effect of cyclic load have been extensively studied, the strengthening effect and its failure mechanism are somewhat unclear. In this study, a series of cyclic-to-monotonic loading tests with the upper limit in elastic deformation stages was conducted on siltstone. Results show that the siltstone in the test is a typical porous weakly cemented rock, and its characteristic stress can be determined by volume strain curve, AE counts and ultrasonic velocity. The strengthening and weakening effects of siltstone under cyclic loading depend on the upper limit of cyclic load. When the upper limit of cyclic load is in the elastic deformation stage, the strength, elastic modulus, and characteristic stress of siltstone were improved significantly. The microscopic mechanism of strengthening effect was analysed through AE b -value, sources, and electron microscopy scanning. Under the uniaxial compression, the evolution law of AE sources shows a specifically spatio-temporal zonal disintegration phenomenon. The essential reasons for the strengthening effect are mainly reflected that the micro fracture mode was changed. Under the cyclic load test, the spatial distribution of AE sources of siltstone is more uniform, which indicates the skeleton area of the specimen which can bear the axial load increased. However, the burst of AE signals is delayed, and the crack scale is small due to the strengthening effect. The difficulty of geotechnical engineering disaster prediction and prevention was greatly increase, and the attention should be paid to similar geotechnical engineering.