Target Stratum Determination of Surface Hydraulic Fracturing for Far-Field Hard Roof Control in Underground Extra-Thick Coal Extraction: A Case Study

Surface hydraulic fracturing of far-field hard roof is an effective mean to hard roof control, in which determination of target stratum is the key. However, groundless selection without scientific theories still remains, especially when multiple hard roof exist in the overburden. For this, a longwall mining numerical simulation method is developed, with which energy release and changes of the peak values of front abutment stress of working face after weakening far-field tight roof are analyzed. The results show there is significant impact by position, lithology and thickness of roof on the energy release. Other conditions being the same, the energy released by roof breaking shows a declining trend of L with the position rising. The energy released from far-field hard roof is remarkably larger than that of the neighboring strata. Its sudden surge always corresponds to the valley values of peak curve of working face front abutment stress, particularly true for low-position tight roof. The peak values decrease upon the weakening of the low-position hard roof. The roof's fracturing should be given priority for its greater impact on working face, when the tight roof are close in thicknesses. In this case, fracturing priority should firstly go to the lowest-position and then above. After an on-site fracturing of the determined strata, support pressure and deformation of advanced support segment decrease, testifying the determination. This provides a theoretical support for determining the target stratum for hydraulic fracturing and facilitates the wide application of hydraulic fracturing technology in hard roof control.