The influence of borehole arrangement of soundless cracking demolition agents (SCDAs) on weakening the hard rock

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۴٫۲۷۶ در سال ۲۰۲۰

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۲۶ در سال ۲۰۲۱

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۰٫۹۰۱ در سال ۲۰۲۰

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Q1 در سال ۲۰۲۰

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The influence of borehole arrangement of soundless cracking demolition agents (SCDAs) on weakening the hard rock

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The influence of borehole arrangement of soundless cracking demolition agents (SCDAs) on weakening the hard rock


The  hard roof  difficult to collapse easily causes gas  accumulation, which threatens the production safety of coal  mine. Therefore, roof  pre-cracking is required. Although blasting and hydraulic fracturing can  also crack the roof,  blasting can  easily induce rock bursts, whereas hydraulic fracturing needs complex equip- ment. In contrast, soundless cracking demolition agents (SCDAs) with noise-free, dust-free, and safe char- acteristics have obvious advantages. The  main component of SCDA is calcium oxide, which reacts with water to produce higher expansion pressure. In  this paper, focused on  the angles of  the borehole, the effect of SCDA is analyzed by numerical simulation based on Pingdingshan coal  mine. The research results

showed that the azimuthal angle a (between borehole projection and the roadway direction) does not

significantly affect the efficacy of SCDAs, whereas the influence of borehole elevation angle b is far more significant than that of the azimuthal angle. Therefore, the angle b is a dominant factor influencing the effect of SCDAs. Based on  different effects of SCDAs at different angle of boreholes, the weakening unit was established, so  the SCDAs could give  full  play to roof  fracturing. Moreover, field tests validated the importance of borehole angle on  weakening the hard roofs.

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  1. Introduction

The  hard roof  with high strength is difficult to  collapse in coal mine, which threatens  production safety [1,2].  At  present, there are  various methods of weakening hard roofs of mining face  [۳–۷]. However, research on hard hanging roof at the corner of mining face is sparse. After the hard roof is caved, the arcuate triangular block is formed at the corner, as shown in Fig. 1 [8,9], which is in the state of stress reduction. Because of the high strength, the arcuate triangular block is extremely hard to cave  timeously. The presence of multiple arcuate triangular blocks prevents the coal  roadway in  goaf  from being completely backfilled with broken rock,  with the constant advance of  the mining face.  The  arcuate triangular blocks at the lower corner lead to air leakage of the goaf while those at the upper corner contribute to  gas  emissions. To prevent the formation of arcuate  triangular  blocks during roof   caving, it  is  necessary to weaken and fracture the hard roof at the corners in advance before driving mining face.  Therefore, the weakened triangular blocks at

corners can  be caved immediately after mining, which guarantees safe  production in such coal  mines.

Existing rock  fracturing methods mainly include explosive blasting, hydraulic fracturing, and  high-pressure  gas   fracturing [10–۱۴]. In terms of explosive blasting, rocks are  broken by  high- pressure gas  and shock waves generated during explosion. In this process, chemical reaction is strong, and it  is hard to  realise con- trollable  pre-cracking  and  weakening  of   roofs.   Moreover, the explosion flames are   likely   to  detonate gas  accumulated at the upper corner. For hydraulic fracturing, the rock  mass is fractured by  high-pressure water. It calls  for  huge equipment systems and complex technological processes. High-pressure gas blasting is applied to  fracture rock  mainly through liquid CO2  phase transi- tion. By using a fracturing device, liquid CO2  is gasified to generate high-pressure  gas,   realising the  fracturing of  rock   mass.  This method shows poor controllability and the instantaneous release of high-pressure gas poses a significant risk. Of course, many schol- ars  are  exploring new methods recently. For example, Huang et al. [15] and Shang et al. [16,17] have studied microwave-assisted frag- mentation; Wu  et al. [18]  and Qin et al. [19]  have studied method of freezing rock  with liquid nitrogen, and they all make important discoveries.

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