Mechanical properties and microstructure of layered cemented paste backfill under triaxial cyclic loading and unloading

In this paper, an experimental investigation was conducted to study mechanical properties and microstructural characteristic of layered cemented paste backfill (CPB) under triaxial cyclic loading and unloading condition. A number of CPB specimens were prepared with different number of layers (1, 2, 3, and 4). Four confining pressures (0, 0.2, 0.5, and 0.8 MPa) and three cycles were designed. The results indicated that cyclic loading and unloading had obvious influence on the mechanical properties of the CPB, which was closely related to the number of layers and confining pressure. Compared with the conventional triaxial compression test results, when the confining pressure is 0 and 0.2 MPa, the cyclic loading and unloading can increase the uniaxial compressive strength (UCS) of CPB, and when the confining pressure is 0.8 MPa, it decreased the UCS. When the confining pressure is 0.5 MPa, it increased the strength of the CPB 1 and 2 layers, but decreased the strength of CPB with 3 and 4 layers. The change of UCS had a positive linear correlation with the change of secant modulus of unloading curve caused by the cycle loading and unloading. The results of SEM and CT scanning showed that there were more pores in the post filling layers due to the difference of curing time which were compacted easily under small confining pressure. Under the action of cyclic loading and unloading, the CPB was repeatedly compacted, resulting in the increase of UCS. With the increase of confining pressure, the difficulty of pore compaction increases gradually and stress concentration was easy to form around the pores, which resulting in local fatigue damage and reduce the strength under cyclic loading and unloading.