Macro-mesoscopic mechanical properties and damage progression of cemented tailings backfill under cyclic static load disturbance

Frequent mining stress disturbances in deep-earth resource extraction lead to different degrees of initial damage within cemented tailings backfill (CTB), which in turn changes the structural characteristics of the matrix and significantly affects the stability of CTB. To simulate the stress disturbance process of CTB during mining activities more realistically, cyclic loading and unloading disturbances were performed on the CTB. The stress disturbance levels (SDL) were 20, 40, 60, and 80% uniaxial compressive strength (UCS), and the stress disturbance counts (SDC) were 5, 10, 15, and 20 times, respectively. Subsequently, a series of experimental studies were conducted on CTB and disturbed damage CTB (DCTB) using UCS, nuclear magnetic resonance, ultrasonic pulse velocity (UPV) testing, and acoustic emission (AE) monitoring. The results show that the UCS of the backfill decreased with increasing SDL and SDC, except for 20% SDL and 5th SDC. The elastic modulus exhibited a rise at 20%-40% SDL and a fall at 60%-80% SDL. Compared with CTB, the stress-strain curve of DCTB showed a left shift -overlap -right shift -accelerated right shift change in the compacting stage with increasing SDL. Moreover, the porosity and UPV reflected by the mesoscopic structure correspond to the trend of UCS. The correlation mechanism between mesoscopic structure and macroscopic strength was determined, and the initial damage degree of DCTB was defined in terms of the average variation characteristics of macro-mesoscopic parameters. According to the AE monitoring results, it was observed that increasing SDL and SDC led to a significant increase in the AE energy active stage during loading. The damage curve established from the AE energy considering the initial damage degree rose exponentially. As the initial damage degree intensified, a noticeable and wide-range drop in the b-value before the peak was observed, indicating the occurrence of frequent large-scale fracture activities within the DCTB. Also, the damage degree of DCTB intensified, the collapse zone increased, and the failure mode of the backfill transformed from tensile to mixed tensile-shear damage. The research results can provide theoretical support and a reference basis for the stability analysis and strength design of the backfill under the action of mining stress.