Investigations into early mechanical performance and failure of paste-powder mixed layered backfill

The paste-powder mixed backfill technique can address the engineering failures caused by the slow setting and insufficient early strength of cemented backfills. Taking the slurry mass concentration regulated by fly ash and the number of paste-powder layers as the main influencing factors, the authors investigated the early mechanical properties and failure mechanisms of layered backfills through a series of laboratory tests, including slump, bleeding rate, setting time, and compressive strength measurements. The results show that fly ash can improve the setting and hardening characteristics as well as the early mechanical properties of backfills. As the mass concentration of backfills is gradually increased from 74% to 82%, the slump decreases from 142 mm to 104 mm, the bleeding rate drops from 6.32% to 2.27%, the setting time is decreased from 11 to 3.5 h, and the 1-day compressive strength increases from 0.02 MPa to 0.31 MPa. The uniformity of mixing of paste and powder determines the mechanical properties of layered backfills. The weak cemented surfaces formed by layering lead to the preferential initiation of macroscopic cracks during compression, resulting in disordered crack distribution in the samples. As the number of backfill layers increases gradually from 7 to 55, the uniformity of mixing of paste and powder is significantly improved, the toughness index increases from 1.2154 to 1.8884, and both the setting and hardening performance and mechanical properties are enhanced. The results provide experimental evidence for the paste-powder mixed backfill technique and the design of early mechanical properties.