Deep-hole pre-split blasting mechanism and its application for controlled roof caving in shallow depth seams

• First roof weighting length and its induced accidents of the shallow depth seams were analyzed.
• Deep-hole pre-split blasting technology was presented to avoid large area roof weighting.
• LS-DYNA3D and discrete element analysis uncover the deep-hole pre-split blasting mechanism for controlled roof caving.
• Deep-hole pre-split blasting was successfully experimented for mining safely.

The shallow depth coal seams mined in the Shendong Mining Area has led to large area roof weighting frequently due to large thickness, high tensile strength and small overlying load of the main roof. As a result, many accidents have happened including hydraulic support being iron-bound, water inrush through the penetrating cracks along coal wall, and potentially induce large area residual pillars instability and even wind blast damage particularly in the conditions beneath a room mining goaf. To prevent the large area roof weighting accidents, deep-hole pre-split blasting technology for controlled roof caving is a suitable method and has been widely applied in the mines, and many in situ trials have already achieved a sound performance. According to the field conditions of the shallow depth coal seams, this paper employs cylindrical cavity expansion theory to calculate the three blasting-induced divisions including break zone, fracture zone and elastic vibration zone. The software of LS-DYNA3D was applied to establish a deep-hole pre-split blasting model, which presented the rock stress field and the break scope affected by the high energy explosion stress wave. The simulation results revealed the blasting for controlled roof caving mechanism and also optimized the blasting parameters. The field observations showed that, with the application of deep-hole pre-split blasting technology in Shigetai Mine, Shendong Mining Area, the first main roof weighting length was about 17.4 m, and neither a hydraulic support being iron-bound nor a serious roof subsidence in the form of step convergence in the mining face happened during the first weighting period. The field application of deep-hole pre-split blasting for controlled roof caving has achieved the expected effects.