Experimental investigation on filtration-viscosity coupling and reinforcement mechanisms of high-pressure splitting grouting in soft coal

• Filtration-viscosity coupling governs high breakdown pressure in soft coal grouting. • Pressure buildup is highly sensitive to injection rate, w/c ratio, and moisture. • A modified model incorporating grout rheology explains rate-dependent pressure. • Grout skeleton formation dominates reinforcement rather than matrix compaction. . High-pressure splitting grouting is a critical technique for reinforcing deep soft coal roadways, yet its application is frequently hindered by abnormally high breakdown pressures and limited penetration. This study investigates these challenges through laboratory experiments and theoretical analysis using synthetic soft coal materials. The findings elucidate that the pressure response is governed by a "filtration-viscosity coupling" mechanism rather than pure elastic fracture. Rapid fluid loss near the borehole leads to localized particle accumulation and the formation of a stagnant grout plug with high yield stress, which dissipates over 98% of the injection energy. This explains the failure of classical Hubbert-Willis elastic models, which underestimate the breakdown pressure by 12-67 times. A unified breakdown pressure model incorporating filtration-rheology coupling was developed, accurately capturing exponential pressure variation and predicting breakdown pressure with 1.8%-10.5% error in the filtration-dominated regime. Sensitivity analyses indicate that parameters such as injection rate, water-cement ratio, and coal moisture content should be treated as a coupled design problem rather than independent variables. Mechanical tests post-grouting confirm that strength enhancement is primarily driven by a rigid grout skeleton rather than matrix compaction, with the unconfined compressive strength increasing by over 60% and the dynamic elastic modulus nearly doubling. From an engineering perspective, effective grouting should prioritize filtration control via an optimal injection rate window and pre-conditioning of coal moisture to ensure efficient pressure transmission. These results provide a mechanistic and quantitative framework for optimizing high-pressure grouting parameters in deep underground engineering.