The geological setting explains why the problem is so severe at Chaigou. The mine’s principal No. 3–5 coal seam exceeds 20 meters in thickness, with borehole data from the 1519 working face area showing thicknesses ranging from 18.93 to 32.93 meters and a recorded column thickness of 28 meters. The seam dips at an average of just 3 degrees and is classified as stable and mineable throughout the area. Crucially, the first 100.28 meters of rock above the seam are dominated by hard strata, including medium-to fine-grained sandstone, medium-to coarse-grained sandstone, sandstone, and conglomeratic sandstone, with uniaxial compressive strengths ranging from roughly 38 to 90 MPa and a mean of about 74 MPa. When such a thick seam is extracted by fully mechanized top-coal caving, the overlying hard layers fracture in stages and form cantilever, articulated, or multilayer composite load-bearing structures along the gob edge, concentrating enormous loads on the coal pillar and adjacent entries.

Using key-strata theory and borehole penetration strength tests, the researchers identified five key hard strata, labeled M2, M4, M6, M9, and M12, within the 0–100.28 meter interval above the seam roof. Their cumulative thickness is 48.56 meters, and their thickness-weighted mean compressive strength is nearly 80 MPa. Together with the strata groups they control, these layers form a multilayer composite structure that governs how overburden load is transferred after mining. The mine had previously protected its entries with 50-meter-wide interpanel coal pillars, leaving more than two million tonnes of coal unextracted in each pillar. Reducing pillar width would improve resource recovery, but narrow pillars are prone to extensive plastic failure under the combined effects of lateral abutment pressure and excavation-induced unloading, which degrades both the coal’s integrity and the anchorage conditions for support elements.

To quantify the stress environment, the team built a three-dimensional numerical model in FLAC3D along the dip direction of the 1519 working face, measuring 520 by 1 by 240 meters, with mesh refined to 1 meter near the pillar, entry, and gob boundary. A Mohr-Coulomb elastoplastic model was adopted for all coal and rock strata, and the calculation sequence simulated initial stress equilibration, extraction of the 1519 face, post-mining stress re-equilibration, and excavation of the 1521 return airway. After mining, the vertical stress within the gob dropped sharply while load transferred into the surrounding coal, forming a lateral abutment-pressure concentration zone. Within 0 to 15 meters of the gob boundary the stress remained low, but a concentration zone developed between 15 and 30 meters, peaking at approximately 23 MPa about 20 to 25 meters from the gob edge, a stress concentration factor of roughly 2.56 relative to the pre-mining vertical stress of 9.0 MPa.

The simulations compared coal-pillar widths of 10, 15, 20, and 30 meters. For widths of 10 to 20 meters, the high-stress zone lay mainly deep within the solid-coal rib, while stress concentration in the coal-pillar rib was limited. At 30 meters, however, a pronounced high-stress zone developed deep in the coal-pillar rib and strong concentrations formed in both ribs. Peak vertical stresses in the solid-coal rib reached approximately 22.5, 29.6, and 31.8 MPa for the 15, 20, and 30 meter pillars, with concentration factors of 2.5, 3.3, and 3.5. Maximum stresses in the coal-pillar rib rose from about 10.0 MPa at 10 meters width to 26.7 MPa at 30 meters. The 10-meter pillar therefore placed the 1521 return airway in a relatively low-stress zone on the gob side of the pressure peak and was adopted in the field, although its low stress results partly from plastic yielding, meaning the fractured coal still needs reinforcement.

The central question was how roof-cutting height changes the pressure relief. In the model, hydraulic fracturing was represented by a vertical slit at the gob boundary extending 30 or 100 meters above the coal-seam roof. With no cutting, the peak lateral abutment pressure was 23.20 MPa about 25 meters from the gob. A 30-meter cut reduced the peak to 20.98 MPa, a 9.6 percent decrease, and shifted the peak to about 30 meters. A 100-meter cut reduced the peak to 15.03 MPa, a 35.2 percent decrease, shifting it to about 35 meters. The team also computed the integral of the lateral abutment-pressure increment over a 100-meter range: relative to the uncut case value of 604.8 MN·m−1, the 30-meter cut achieved only a 4.9 percent reduction, while the 100-meter cut achieved 49.1 percent. Low cutting mainly redistributes stress deeper into the solid coal; high cutting, which weakens the intermediate- and high-level key strata, genuinely removes cumulative load.

The stress response of the entry’s two ribs was markedly asymmetric. In the uncut case, the mean vertical stress over the first 10 meters from each rib was 16.34 MPa in the solid-coal rib and 8.85 MPa in the coal-pillar rib, a ratio of 1.85. At a 30-meter cut the means fell to 13.82 and 8.19 MPa, and at 100 meters to 11.45 and 8.16 MPa, bringing the ratio down to 1.40. The additional relief from raising the cutting height therefore concentrated almost entirely in the solid-coal rib, while the coal-pillar rib, already weakened by plastic failure, changed little. This asymmetry dictated the control philosophy: the solid-coal rib needs protection against high-stress fracture development, whereas the coal-pillar rib needs its fractured coal re-bonded to restore a competent anchorage foundation.

The field scheme combined far-field pressure relief with near-field reinforcement. Low-level short inclined boreholes, drilled with a ZDY4000LP(S) rig and fractured using a compact 160 kW pump, targeted the extra-thick top coal and hard strata within 30 meters of the roof. High-level long directional boreholes, drilled with a ZYL17000D rig and fractured with a BYW80/450 regional pump system, reached 100 meters above the seam. For reinforcement, conventional cable bolts of Ø21.8 mm and hollow high-pressure grouting cable bolts of Ø22.4 mm were installed in the roof and both ribs, with initial pretension of at least 200 kN, supplemented by W-straps, double-layer steel mesh, and C25 shotcrete. A nano-modified single-component grout with particle sizes below 10 micrometers, an adjustable setting time of 17 to 120 minutes, and a 28-day compressive strength of at least 90 MPa was injected 30 to 50 meters behind the entry face at termination pressures of 5 to 10 MPa on the pillar side and 10 to 20 MPa on the solid-coal side.

Pull-out tests in the fractured coal-pillar rib quantified the benefit of grouting. Before grouting, three cable bolts with 1.18-meter anchorage lengths failed at ultimate loads of 135 to 148 kN, averaging 140.7 kN, through slippage or complete pull-out of the anchored section. After seven days of grout curing, all three test bolts were loaded to 350 kN without anchorage failure or slippage, an increase of at least 148.8 percent over the pre-grouting mean. During extraction of the 1521 working face, monitoring at a station 125 meters from the open-off cut showed that deformation accelerated once the face came within about 110 meters and intensified sharply inside 50 meters. At a face-to-station distance of 5 meters, rib-to-rib convergence reached 643 mm and roof-to-floor convergence 314 mm, a ratio of about 2.05, with a cross-sectional area reduction of approximately 22.4 percent.

Cable-bolt loads rose steadily as the face approached. Roof loads climbed from 173 to 318 kN, solid-coal-rib loads from 113 to 354 kN, and coal-pillar-rib loads from 154 kN to a peak of 490 kN when the face was 20 meters away, settling afterward to 430 kN. Despite the high peak, no anchorage failure or abrupt post-peak load drop occurred, indicating that grouting had restored a load-bearing anchorage foundation in the fractured coal. The authors caution that the field measurements represent the combined response of all measures and do not separately quantify each contribution, and that the conclusions apply primarily to conditions of multiple hard strata, an extra-thick seam, and a 10-meter pillar. Under other geological settings, they note, the roof-cutting height should be matched to the key-strata distribution and the entry’s stress environment.

Subject of Research: Roof-cutting pressure relief and surrounding rock control for narrow-pillar gob-side entry driving in extra-thick coal seams with hard roofs

Article Title: Roof-cutting pressure-relief effects and surrounding rock control technology for narrow-pillar gob-side entry driving in extra-thick coal seams with hard roofs

Article References: Shi, Z., Cheng, P., & Zhao, Y. (2026). Roof-cutting pressure-relief effects and surrounding rock control technology for narrow-pillar gob-side entry driving in extra-thick coal seams with hard roofs. Frontiers in Earth Science, 14, Article 1989900. https://doi.org/10.3389/feart.2026.1989900

Image Credits: AI Generated

DOI: 10.3389/feart.2026.1989900

Keywords: coal mining, roof cutting, hydraulic fracturing, gob-side entry, coal pillar, hard roof, pressure relief, grouting reinforcement, cable bolts, numerical simulation, strata control, extra-thick coal seam