Mechanical behavior, acoustic emission and principal strain field evolution properties of layered cemented paste backfill under unconfined compression

In order to save costs, the layered filling method is adopted by mine enterprise to fill the underground goafs. To understand the mechanical behavior, damage evolution and failure modes the layered cemented paste backfill (LCPB), this study investigated the strength properties, acoustic emission (AE) characteristics and principal strain field evolution of LCPB samples with cement-to-tailings (c/t) ratio of 1:4, 1:6, 1:8, 1:10, and the height (h) of middle layer of 30 mm, 35 mm, 40 mm, 45 mm, 50 mm. The obtained results indicate that: (1) UCS of LCPB decreases with the increases of h and with the decreases of c/t ratio. The influence of c/t ratio on UCS of LCPB is in a good agreement with logarithmic function, while the variation relationship between h and UCS of LCPB satisfies the exponential function. (2) The acoustic emission signals first appeared during the transition from the compression to the elastic stage, reaching a maximum approaching the peak stress, and AE ringing counts also appeared during the post-peak stage. Internal fracture type of LCPB under external loading is a tension-shear composite fracture dominated by shear fracture. (3) The damage of LCPB occurs mainly in the middle layer and then extends to the upper and lower layer with the formation of visible localized zones. The damage of LCPB developed from significant shear damage to tensile-shear damage with the decreased of the c/t ratio, and transitioned from tensile-shear damage to complex damage dominated by shear damage and supplemented by tensile damage as h increased. Ultimately, the findings of this work can provide a reference for the design of LCPB.