Initial weighting strength control of thick and hard roof based on pre-splitting blasting of whole roadway roof: numerical model and engineering verification

• Through the theoretical analysis of the roof structure in the initial mining stage, the “shallow - hole dense blasting - deep - hole directional pressure relief” technology is introduced. That is, shallow - hole blasting creates weak planes in the shallow roof, and deep - hole blasting conducts directional weakening of the main roof, thus avoiding the overall collapse of the thick and hard roof and the occurrence of strong mining pressure behavior accidents. • It is proposed that after the full roadway pre-splitting of the roof, the central caving mode of the main roof above the goaf presents a “W”-shaped zonal caving pattern. Its progressive and controllable fracturing mode effectively avoids the strong mine pressure behavior caused by the overall cutting and caving of the thick and hard roof. • A three - dimensional finite element numerical model is constructed, and displacement, vertical stress, and Von Mises equivalent stress indicators are introduced. Through analysis, it is found that after adopting the full - roadway roof pre - splitting technology, the initial weighting intensity of the stope is reduced and the duration is shortened. • The engineering site practice shows that after adopting the full-roadway roof pre-splitting technology, the initial weighting interval is 68.7 m, realizing the concept of “active roof control”. It achieves the goal of changing the overall cutting and caving form of the roof to a zonal caving form and “breaking the whole into parts” for energy release, effectively avoiding engineering failure problems. Thick and hard roof poses safety hazards to initial mining and caving of the working face due to its good integrity and large modulus. Taking the initial mining of No. 24110 working face in Nanyangpo mine as the background, this study proposes the "shallow-hole intensive blasting-deep-hole directional pressure relief" technology based on the roof pre-splitting of the whole roadway pressure relief concept: shallow-hole blasting creates weak surfaces in the shallow roof to ensure the lower layer breaks first and the main roof caves later during initial weighting; deep-hole blasting directionally weakens the main roof, realizing zonal caving of the roof and "dividing the whole into parts" for energy release. Numerical simulations show that with roof pre-splitting of the whole roadway, the working face advance distance for the first roof caving is 20 m earlier than that without pre-splitting. Initial weighting occurs at 60 m advance, with the main roof completely caving. The peak stress concentration of the stope coal wall and roof is significantly reduced. The roof caving morphology changes from the "O-X" type without pre-splitting to a fracture with a "W" shape at its center, avoiding strong mining pressure caused by overall caving of thick and hard roof and reducing accidents like support crushing and coal wall spalling. Field measurements of No. 24110 working face show initial weighting occurs when the head advances 54.7 m and the tail 82.7 m, with a calculated initial weighting step distance of 68.7 m, consistent with simulation results. Weighting features low intensity and short duration, with no obvious spalling in roadways and no large deformation of surrounding rocks, ensuring safe and efficient passage through the initial caving stage. This study provides a new pressure relief method for initial mining under similar conditions, offering reference for reducing weighting intensity and optimizing initial weighting step distance in mining engineering.