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

The initial bearing age of a backfill is primarily determined by the roof collapse time, which critically affects its performance during the hydration stage. In this study, 96 h uniaxial compression creep and permeability tests were conducted to 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 an increase in the 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 concave to convex. Simultaneously, 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 (3 d) to 0 ‒ 10 h (28 d). Additionally, there were no evident macroscopic cracks on the surface of the backfill after the creep test, but its compressive strength and permeability exhibited significant differences. Static load exerted a significant strengthening effect on the 3-d-old backfill, with its compressive strength index increased by 19.95%, while the permeability index decreased by 30.90%. In contrast, static load caused damage to the backfill on the 28 d, leading to 31% decrease in compressive strength and 116.29% increase in permeability. Therefore, the stress level and initial load-bearing age had a significant impact on the mechanical properties of the backfill under static load. These findings provide valuable insights and practical references for on-site safety applications and strength design of backfills.