Hybrid assessment of pre-blasting weakening to horizontal section top coal caving (HSTCC) in steep and thick seams

Horizontal section top-coal (HSTCC) caving offers a powerful method to efficiently excavate rude coal in steep and thick seams, and pre-blasting weakening has a profound effect on pursuing great production, high efficiency and good benefit under particular conditions like a small-scale working face with large-scale sectional caving height. +564-level HSTCC working face in B_3–6 coal seams of Jiangou Colliery in Urumqi was taken as study case for in situ industrial experiment. Total thickness of seams in the study case is about 50.0 m and average angel here is over 83°. In the industrial experiments, at first we adopted continuous charge machine and emulsion matrix explosive to substitute for traditional blasting schemes for specific geological settings in the study case. Hybrid analyses and assessments with blasting crack propagation analysis, abutment pressure monitoring prediction and economical benefit assessment were attributed to be able to attest pre-blasting weakening effects practically. Meanwhile crack propagation analysis after pre-blasting weakening showed that in all triple monitoring bore holes rock masses of top-coal would be fallen into three stages from the bottom up: fracture zone, plastic zone and elastic zone generally, and fracture toughness respectively in correspondent zones was calculated by the analytical formula: 0.5616–0.8806, 0.6403–0.9541 and 0.7535–1.1900 MPa m^1/2 after pre-blasting weakening. Pressure monitoring prediction and economical benefit assessment also indicated that it was necessary to introduce the pre-blasting weakening with predominant blasting scheme from both views. For excavation in extremely steep and thick coal seams, relevant results would be a useful tool to study the mechanism of pre-blasting weakening both qualitatively and quantitatively.