1556 challenges. Regarding mining depth, Chinese coal mines are extending deeper at a rate of 10–25 m per year. There are 47 mines with depths exceeding 1000 m, and the maximum mining depth has reached 1510 m. Deep mining introduces high stress, high ground temperature, high seepage pressure, and intense mining disturbance, leading to disasters such as large surrounding rock deformation, roof collapse, and rockburst. Regarding mining intensity, a number of ten-million-tonnage mines and faces have emerged, with some mines exceeding 30 million t/a and face production exceeding 15 million t/a. Mining faces are developing towards super-height and super-length: the mining height of ultra-large fully mechanized faces reaches 10 m, and the total mining-cutting height of fully mechanized top-coal caving faces reaches 20 m; the length of super-long faces in medium-thick seams reaches 450 m. High-intensity mining creates ultra-large excavated spaces, resulting in extensive overburden movement and more intense strata pressure behavior, imposing higher requirements on face strata control. Strata control is divided into face strata control and roadway surrounding rock control. Under conditions of hard or fractured roofs, hydraulic supports alone cannot fully resolve face strata control issues; measures such as destressing and reinforcement are required. In China, coal seams with hard roofs account for over 30%, covering more than 50% of mining areas. Hard roofs are characterized by great thickness, underdeveloped fractures, good integrity, high strength, and strong self-stability. After face extraction, hard roofs do not cave timely, easily causing large-area roof hanging. Sudden collapse of such roofs releases elastic energy instantaneously, forming intense impact loads leading to disasters like large-area weighting, rockburst, and air blasts. For roadway surrounding rock control, the synergistic control technology of "support-modification-destressing" has been proposed, where destressing employs techniques like hydraulic fracturing and blasting to reduce or transfer high stress in the surrounding rock, mitigating mining disturbance effects , thereby controlling deformation and failure. Hydraulic fracturing has become an effective means for controlling strong strata pressure in coal faces, managing large deformations in high-stress and strongly disturbed roadways, and preventing rockburst. This paper takes the hard roof hydraulic fracturing project in a 10m ultra-large mining height face as an example to introduce the research and application of underground coal mine hydraulic fracturing technology for strong strata pressure control. 2. CHALLENGES IN STRONG STRATA PRESSURE CONTROL FOR THICK-HARD ROOF FACES 2.1 Geological Conditions of the 10m Large Mining Height Face 2.1.1 Face Overview Caojiatan Coal Mine is located in the Yushen Mining Area, north of Yulin City, Shaanxi Province. The mining area covers about 2735 km² with resource reserves of about 48 billion tons. The current approved capacity has been increased to 25 million t/a. The mine primarily extracts the 2-2 coal seam, mined in eastern and western panels. The face length is 300 m, with an advance length of 5977 m. The seam burial depth ranges from 268 to 356 m, with an extractable thickness of 7.2–12.3 m, averaging 10.5 m. The seam dip angle is 0°–6°, and the Protodyakonov hardness coefficient is 2–3. 2.1.2 Occurrence Characteristics of Multi-layer Thick-Hard Roofs To fully understand the occurrence state of multi-layer thick-hard roof strata in Caojiatan Coal Mine faces, an underground surrounding rock strength penetration testing device and an electronic borehole camera were used. Twenty-two detection holes were arranged in the auxiliary haulage roadway and return airway to conduct comprehensive testing of the strength and joint/fracture conditions of roof strata within 100 m above the seam.
RkJQdWJsaXNoZXIy MTM0Mzk2