Laboratory investigation of rock pillar reinforcement against impact loading by using high-tensile-strength polyurea with different coating thicknesses

Thin spray-on liners (TSLs) have been widely adopted in underground mining as rock support materials owing to their notable tensile strength, elongation capability, and bond strength with rock surfaces. In this study, to evaluate the reinforcing capacity of a high-tensile-strength polyurea-based TSL on rock pillars subjected to dynamic loads, a series of compression, impact, and compression-after-impact tests were conducted on polyurea-coated coal and limestone samples, which represent soft and hard rock pillars in underground coal mining and stone mining, respectively. The strength, deformative modulus, energy evolution, and failure modes of the rock samples with coating thicknesses of 1, 2.5, 5, and 10 mm were examined. The results indicated that there is a critical thickness above which additional coating does not improve the reinforcing performance. Coated coal samples exhibited a residual stress plateau in the post-peak stage, which was not observed in uncoated samples. The failure mode of polyurea-coated rock pillars depends on the energy absorption threshold of polyurea, causing the coating to fracture and leading to the overall failure of the structure. During the impact, the polyurea coating absorbed excessive energy, thereby enhancing the dynamic strength of rock pillars. Surprisingly, moderate impact loading converted the effect of polyurea coating from passive to active confinement, resulting in a residual strength that surpassed the inherent strength of the rock. Based on laboratory observations, this study concludes that ductile TSLs such as polyurea are particularly advantageous for reinforcing soft rock pillars.