Mechanical properties and crack evolution of interbedded cemented tailings backfill

In the process of filling a large-scale goaf, due to the limitations in the capacity of the mixing tank, it is difficult to completely filling the goaf all once, but multiple fillings of a goaf can easily produce a layered structure in the cemented tailings backfill. This layered structure has a significant effect on the mechanical properties of the cemented tailings backfill. To analyze the influence of these structural characteristics on the mechanical properties and evolution of cracks in cemented tailings backfill, the layered cemented tailings backfill specimens with height ratios of 0.2, 0.4, 0.6 and 0.8, and cement-tailing ratios of 1∶4, 1∶6, 1∶8 and 1∶10 were made, and then the uniaxial compression test was carried out by using a GAW–2000 servo test system, and finally the crack distribution inside the cemented tailings backfill were analyzed by using 2D particle flow software(PFC-2D). The results show that: (1) the relationship between the uniaxial compressive strength and the height ratio of the layered backfill can be represented by an exponential function, and the relationship between the uniaxial compressive strength and the cement-tailing ratio can be represented by a polynomial function. The relationship between the elastic modulus and the height ratio and the cement-tailing ratio can be represented by a polynomial function. The uniaxial compressive strength and the elastic modulus are found to decrease with increase in the height ratio, and increase with increase in the cement-tailing ratio, with both being more sensitive to the cement-tailing ratio. (2) The evolution curve of cracks in the cemented tailings backfill increases gradually at first, and then rapidly increases to about 80% of the peak strength, whereby the larger is the cement-tailing ratio, the lager is the height ratio. Furthermore, the earlier the fast-rising inflection point occurs, the more easily is the backfill specimen damaged, and the curve begins to decline rapidly after exceeding the peak strength. (3) The layered backfill fails primarily by mainly shear failure, tensile failure and conjugate shear failure, and the failure is mainly concentrated in the middle weak layer. The larger is the height ratio, the denser are the cracks, the bigger is the cement-tailing ratio, and the more easily the cracks evolve to both ends.