Research and application of grouting reinforcement technology for small coal pillar roadways along gob in extra-thick coal seams

Gob-side entry driving with small coal pillars in extra-thick seams improves resource recovery but is constrained by fractured, low-capacity surrounding rock and large asymmetric deformation. This study develops and validates a grouting-reinforcement technology that couples hollow high-pressure grouting cable bolts with a nano-modified, two-component microfine grout, and elucidates the evolution and control mechanisms governing stability. A process-based mechanical framework is first established, showing two stages of surrounding-rock evolution: excavation-induced redistribution/damage followed by retreat-mining-induced reactivation/restructuring. Mechanistic analysis indicates that grouting increases normal and tangential stiffness and shear strength along fracture surfaces, forms a consolidation skeleton, reduces the loosening zone, seals water–air pathways, and strengthens bolt anchorage to create a load-sharing rock-grout-bolt composite. Grout diffusion was examined using UDEC hydro-mechanical simulations with Bingham-type rheology across water-cement ratios, and by laboratory tests on a nano-modified grout (D90 < 10 µm) exhibiting rapid setting, early strength, and high adhesion. The technology was implemented in Roadway #5121, Tashan Coal Mine, using Φ22.4 mm hollow cables; field monitoring (surface convergence, bolt working resistance, anchorage tests, and borehole imaging) provided verification. Relative to the non-grouted control, floor heave decreased by 28%, solid-coal rib convergence by 43%, and coal-pillar rib convergence by 44%, while roof subsidence showed no notable change; bolt working resistance stabilized and the bolt failure rate declined markedly. The results demonstrate that stage-targeted grouting-prioritizing ribs and floor and integrating with cable-bolt systems-rebuilds the bearing ring and substantially enhances roadway reliability. The quantified reductions provide practical design benchmarks and calibrated inputs for future numerical and probabilistic evaluations in similar geological and mining settings.