Creep characteristics of backfill under different stress levels and initial bearing ages and their effects on strength and permeability

The initial bearing age of the backfill is mainly determined by the roof collapse time, which critically affects its performance during the hydration stage. In this study, through 96 h uniaxial compression creep and permeability tests were conducted to systematically investigate the creep behavior of cemented backfill at different stress levels and initial bearing ages and their effects on strength and permeability. The results indicated that with the increase of stress level, the cumulative axial strain of 28 d backfill increases from 0.962% to 1.647%. As the initial bearing age of the backfill increased, the axial strain curve in the creep deceleration stage gradually changed from a concave to a convex pattern. At the same time, the strain rate curve of the backfill in the creep deceleration stage was characterized by a sawtooth-like fluctuation, and the creep time in this interval was shortened from 15-25 h (3 d) to 0-10 h (28 d). In addition, there were no obvious macroscopic cracks on the surface of the backfill after the creep test, but its compressive strength and permeability showed significant differences. Static load exerted a significant strengthening effect on the backfill with an age of 3 d, with its compressive strength index increased by 19.95% and permeability index decreased by 30.90%. In contrast, static load caused damage to the backfill with an age of 28 d, leading to a 31% decrease in compressive strength and a 116.29% increase in permeability. Therefore, stress level and initial load-bearing age had important impacts on the mechanical properties of the backfill under static load. These findings provided valuable insights and practical references for on-site safety applications and strength design of the backfill.