Study on the mechanisms of time-dependent crack propagation and the gradual collapse of roadways in soft-hard composite strata

As mineral extraction extends to deeper strata, the creep deformation differences between soft and hard rocks are further amplified by high stress. This increases the demand for controlling the lifecycle deformation of soft-hard composite rock roadways (S-HRs). This paper investigates the time-dependent characteristics of soft-hard composite rock composed of mudstone and sandstone (M-SR) via laboratory experiments and the damage bond model previously proposed by the author. The results show that crack propagation in M-SR exhibits a clear time- dependent effect, with the creep damage of mudstone being greater than that of sandstone, and this trend increases over time. Over 90 % of microcracks during the first two creep stages develop and coalesce in the mudstone, whereas crack propagation in the sandstone is inhibited. This process plays a critical guiding role in the final failure mode of the M-SR. After excavation, a tensile stress zone forms around the S-HR, with more drastic changes occurring on the sidewalls. This zone expands over time, whereas the deep compressive stress in the roof and floor shifts toward the sidewalls, exacerbating the depth of sidewall failure and continuously inducing the flow of the rock mass into the roadway from the sidewalls. After 48 days, the deformation of the sidewalls increases by 281 %. Increasing the support of sidewalls is a viable approach to solve this issue, and the support range should extend beyond the stress concentration zone. Leveraging the feedback mechanism between the roof and the sidewalls helps reduce creep damage and deformation on the sidewalls.