Theory and technical system of control of far-near field hard roofs from ground and underground in a large space stope

Abstract

The stope space, after mining an extra-thick coal seam with hard roofs, is large, and its law of mining pressure is complex. The mechanism of strong mine pressure action for this stope is unclear, and there is a lack of multi-point control technology system for different layers of hard roof, so it is necessary to develop a new theory and new technology system for hard roof control in a larger range of stope space. For this reason, the author's team has developed a theory and technical system for hard roof control in the large space stopes after nearly 15 years’ industry-university joint research. In view of limited space covered in the past study of strata control, a concept of large space stope is proposed. Based on the roof break characteristics and the mining pressure laws, the far-field hard roof and near-field hard roof are defined. Considering the gangue compression in goaf and the coordinated movement of overlying rock strata, a mechanical model of the break structures for far-filed hard roofs in the large space stope is established, and the control mechanism of the strong mining pressure, due to breakage and instability of the hard roof in the far-field, is revealed. Based on the elastic thin plate theory, the mechanical models of first and periodic breakage for the near-field hard roof with nine boundary conditions for different roof-cutting methods are constructed, and the pressure relief mechanism of directional cutting near-field hard roof is revealed. Combining the basic indexes such as working face parameters, mining pressure effect from the near-field, and the strengthening conditions such as mining gob-side working faces and double coal seams, a classification prediction and evaluation system of strong mining pressure is established, and the quantitative indexes and corresponding control technologies of four different levels of mining pressure are given. It has developed the ground volume fracturing technology by a horizontal well on hard roof (GVFTHWHR) and the ground staged fracturing technology by a vertical well on hard roof, and the crack expansion monitoring technology for far-field hard roofs, and effectively solved the problem of strong mining pressure from the far-field hard roof. It has pioneered a technology of directional and precise seam making for hard roof by the composite blasting, developed a technology of deep-hole blasting for hard roofs with liquid explosives (TDHBHRLE), and a technology of continuous and precise hard roof cutting with a chainsaw machine, and realized the precise control of the collapse shape and breaking characteristics of near-field hard roofs. A technology, the collaborative control of hard roof with ground fracturing +X, is proposed to realize a multi-point collaborative control of hard roof in the far and near-fields. Based on the above research results, a field test of the collaborative control with the GVFTHWHR and TDHBHRLE is carried out for the hard roof at the 8204 fully mechanized caving working face, an extra-thick coal seam workface, at the Tashan Coal Mine. The results show that after entering the collaborative control area of fracturing and blasting, the displacement of the roof and floor, and the two sidewalls of the gob-side roadway are less than 388 mm. The maximum resistance of the support in the middle of the working face reaches 13 295 kN, which is less than 15 000 kN, a rated working resistance of hydraulic supports. The strong mining pressure in the working face and roadway has been effectively controlled.

Keywords

• large space stope
• hard roof
• far and near field
• control theory
• control technology