Stability of base-exposed backfill roof considering interfaces between adjacent drifts in underhand drift-and-fill mining

Stability of base-exposed backfill roof in underhand drift-and-fill mining is crucial for the safety of those working beneath. Given the commonly used primary-and-secondary mining sequence, interfaces are formed between adjacent filled drifts, which can weaken the integrity of the backfill roof. These interfaces also lead to two common drift layouts: aligned drifts and staggered drifts. However, less attention has been paid to the interfaces and the two drift layouts were not adequately distinguished in previous studies. In this paper, the interfaces between filled drifts were firstly considered to investigate the stability of backfill roof. Failure modes and strength requirements of backfill roof in aligned and staggered drifts are comprehensively investigated by FLAC3D, with a focus on considerations of varied shear parameters of the interfaces. Results show that failure modes in aligned drifts transition from block sliding to top caving, bottom caving or sloughing as the interface cohesion increases from zero to at least half of the backfill cohesion. Further increases in interface cohesion allow aligned drifts to behave as if there are no interfaces between them. The critical stability conditions of backfill roof in aligned drifts were mostly determined by the interface strength instead of the backfill strength. However, the stability of backfill roof in staggered drifts is barely affected by the interface strength. The outcomes are expected to provide references for mining engineers to optimize drift layouts and perform cost-effective backfill roof strength design at mines using underhand drift-and-fill mining method.