## 5 Study on the Pressure Relief and Control Effect of Roof Hydraulic Fracturing in Return Airways  
To verify the pressure relief effectiveness and practical feasibility of Scheme 9 under actual engineering conditions, a field experiment of roof hydraulic fracturing was conducted in the 21,103 Return Airway of the 2-1 CS at the Hulusu Coal Mine. Through comprehensive monitoring and analysis of water pressure evolution, water flow distribution characteristics, and acoustic emissions from roof fracturing, the stress regulation and fracture connectivity performance of the fracturing scheme were systematically evaluated. The results provide both a theoretical basis and practical guidance for the pressure relief and control of hard, thick roof structures in similar deep return airways.  
(1) Monitoring of Water Pressure Evolution  
During the fracturing process, a specialized high-pressure recorder was used to monitor the pressure variation in the hydraulic fracturing boreholes along the roof of the 21,103 return airway. Data were recorded every 2 s. Borehole A (depth: 35 m) and Borehole B (depth: 15 m) were selected as representative cases. The pressure variation curves during the fracturing process are shown in Figure 13.  
As shown in Figure 13a, the pressure in Borehole A exhibits a clear pattern of \"pressurization-plateau-sudden drop\". In the initial pressurization phase, water pressure steadily increased, reaching a local peak at around 15 min. This was followed by a plateau phase, maintaining approximately 35 MPa for 15 min. Subsequently, the pressure dropped abruptly to about 2 MPa, indicating that the fracturing fluid broke through pre-existing fractures or bedding planes, resulting in sudden localized pressure release. Further pressurization led to gradual pressure recovery and an increase to 33 MPa, continuing until fractures connected with the surrounding free surface or adjacent boreholes, thereby establishing a fracture network.  
Figure 13b presents the pressure curve of Borehole B, where water pressure rose rapidly and reached around 30 MPa at 12 min. It remained relatively stable, fluctuating slightly around 30 MPa over the next 22 min. This suggests that no violent rupture or pressure release occurred during the process, and the fractures extended in a controlled and stable manner, forming a continuous and effective water-conducting fracture network. Such behavior reflects the high structural integrity of the rock mass and the stable nature of the fracturing process, favorable for uniform stress release and energy dissipation.  
(2) Monitoring of Water Flow Diffusion Range  
Figure 13: Water pressure monitoring curves during the hydraulic fracturing process: (**a**) Pressure variation in Borehole A; (**b**) Pressure variation in Borehole B.  
The water outflow from Boreholes A and B, as well as surrounding anchor cable boreholes, was observed to assess the fracture propagation range induced by hydraulic fracturing. Results show that the fracture extension range from deep Borehole A was approximately 10-15 m, whereas the shallow Borehole B exhibited a broader range of 15-30 m. This indicates a stronger water-conducting capacity in shallow boreholes and demonstrates good connectivity and pressure-transmitting capability of the fracture network, further confirming the applicability and effectiveness of Scheme 9 in practical engineering scenarios.  
(3) Monitoring of Acoustic Emissions from Roof Fracturing  
During the hydraulic fracturing process, acoustic emissions from roof fracturing were automatically collected every 30 min using an acoustic fingerprint monitoring system. Monitoring results indicate that 10 to 26 roof fracture events were recorded every 30 min on average. This suggests that the flexural elastic energy accumulated in the roof was effectively released through the generated fractures, confirming the effectiveness of roof hydraulic fracturing as a pressure relief measure.