Pressure relief mechanism and experiment of directional hydraulic fracturing in reused coal pillar roadway

In order to solve the problem of reused coal pillar roadway support,a new method to control the deformation of coal pillar in hard thick roof is presented in this paper-pressure relief technology of directional hydraulic fracturing ( HF). The purpose is to transfer the high mining stress on the roadway pillars and improve the state of the reused coal pillar roadway. De-stressing mechanism by directional HF is presented. A case study was performed at the S1206 meth- ane roadway of the Yuwu Coal Mine. Determine the directional (HF) boreholes program,boreholes layout parameters and construction technology. It is found that directional slots can be successfully cut by using dedicated grooving bits. 21-28 m staged fracturing time 48% of the total time of fracturing,which are the key position of the entire drilling;the indirect method to determine the fracture extended range of about 20 m. Pillar stress,front abutment pressure and road- way convergence were monitored in different stations with and without HF treatment. Stress changes in three directions within the pillar where HF has been performed were also monitored. It is found that the wall-to-wall convergence and the roof-to-floor convergence were reduced 30. 5% and 50. 2% due to the HF treatment,respectively. Front abutment was detectable within 50 m ahead of the longwall face. The peak pressure occurred at a distance of 10 m out by the face. Failure extended out by the face for a distance of around 10 m. Vertical stress is a major contributor to the influ- ence of mining-induced stress. The direction of the induced-stress is consistent with the maximum principal stress. Mo- nitoring and evaluation showed that the pressure relief effect was obvious. It is demonstrated from the case study that the function of the used roadway can be preserved by using the directional HF technique.