Influence of soft and hard backfill materials on failure modes of roadways with varied geometries under uniaxial compression

Filling the roadways of the abandoned mines can enhance the structural stability of surrounding ore bodies. However, existing studies on backfill reinforcement for different roadway shapes focus on mechanical properties and failure modes, with less emphasis on the specific mechanisms governed by crack-type evolution during the failure process. This study investigates the mechanical effects of three backfill conditions (unfilled, soft-filled, and hard-filled) on six common roadway geometries with scaled specimens subjected to uniaxial compression tests. Crack initiation and propagation were monitored with digital image correlation (DIC). Results show: (1) Roadway geometry strongly affects strength and fracture behavior. With identical effective apertures, the horseshoe-shaped specimen reaches a maximum strength of 23.5 MPa, compared to 16.7 MPa for the square specimen. Sharp-cornered geometries exhibit pronounced stress concentrations, leading to early cracking and faster failure. (2) Fillings improve strength and mitigate damage. Hard fillings (≈1/6 of the specimen strength) control cracks more effectively than soft ones (≈1/3 strength). For circular specimens, the crack initiation stress increases by 99.7% and 105.6% after filling with soft and hard filler, respectively, relative to the unfilled case (9.25 MPa), representing the greatest enhancement among all geometries. (3) DIC analysis identifies tensile, shear, and mixed-mode cracks. Unfilled specimens frequently shift between modes, whereas filled ones show more stable patterns with delayed crack initiation. (4) Energy analysis reveals higher peak dissipation in unfilled specimens. Filling improves energy dissipation performance across all roadway shapes, with both fillers showing the most significant suppression effect in triangular specimens—by 63.0% and 58.3% for soft and hard fillers, respectively—and the least effect in arched-wall specimens—12.9% and 17.6%, respectively. These results contribute to understanding the damaging effects caused by the dynamic evolution of crack types in backfilled roadways and surrounding mine bodies in underground rock engineering.