The second invariant of deviatoric stress of the coal and rock mass is closely related to the distortion energy driving the deformation and failure of the surrounding rock. Based on the second invariant of deviatoric stress, this study built a global model of gob-side entry with different widths of the coal pillar through numerical analysis, and compared and analyzed the evolution law of the surrounding rock distortion energy, plastic location state, and roadway deformation with the width of the coal pillar. This study concluded that the peak distortion energy in the virgin coal rib and the roof and floor of the gob-side entry gradually increases with the reduction in the coal pillar width. When the coal pillar width is 5 m, the second invariant peak value of deviatoric stress in the virgin coal rib reaches the maximum of 294.8 MPa². When the width of the coal pillar is reduced from 30 m to 5 m, the second invariant of the deviatoric stress in the side of the coal pillar and the roof and floor of the side presents the law of first increasing and then decreasing. The greater the damage degree of the coal pillar, the smaller the distortion energy it contains. The distortion energy is the key factor in driving the deformation and failure of the surrounding rock. The greater the distortion energy, the greater the deformation degree of the surrounding rock, the more vulnerable it is to external mining stress disturbance, and the greater the difficulty in controlling the stability of the roadway-surrounding rock.

Double-roadway tunneling technology can effectively relieve the mining‐replacement pressure associated with the combined use of double roadways and gob-side entry driving in traditional longwall panels, but it also introduces difficulties in maintaining the stability of the surrounding rock. In this study, numerical simulation, similar simulation tests, and field applications with in-situ monitoring are jointly adopted to investigate, under the combined layout of roof-cutting gob-side entry retaining and gob-side entry driving, the influence of the basic-roof fracture position above the gob-side entry retaining and the width of the coal pillar between the two roadways on roadway stability. The comprehensive analysis indicates that a reasonable interval coal-pillar width is about 8 m, and the key construction parameters of the pre-splitting roof-cutting scheme for the basic roof are: a cutting angle of approximately 75°, a borehole depth of about 8.2 m, and a borehole spacing of about 1.5 m. Compared with the conventional gob-side entry retaining without roof cutting, the optimized scheme yields maximum roof subsidence of about 445 mm and 421 mm for the gob-side entry retaining and gob-side entry driving roadways, respectively, and maximum (peak) sidewall convergence of about 496 mm and 319 mm, indicating that the deformation of the surrounding rock of the two roadways remains within a controllable range. The results show that the combined layout of pre-splitting roof-cutting gob-side entry retaining and gob-side entry driving has good application prospects for alleviating mining-replacement tension and maintaining the stability of the surrounding rock of small-pillar double roadways.

To address the issue of sustained deformation in the main roadway surrounding rock triggered by intense movement of overlying strata following the reduction of width of the stopping pillar (WSP) in closely spaced double extra-thick coal seams (CSDECS). Analyze the evolution patterns of abutment pressure, principal stress vector lines, and zones of deviatoric stress concentration (ZDSC) of the main roadways using multi-method approaches. The findings are as follows: As the WSP is reduced, the maximum abutment pressure (MAP) on both sides of the gate roadways’ surrounding rock becomes significantly more asymmetric and intense. The deflection trajectory of the maximum principal stress line (MPSL) in the two coal seams, induced by the advancing underlying panel, follows an approximate inverted ︺ shape. The evolution of the ZDSC and the main roadways in the adjacent working faces all shows three-stage characteristics. For the upper coal seam, it is characterized by crescent-shaped symmetry → slow and asymmetric increase of the peak value and the offset of the ZDSC → the ZDSC on the non-mining side (NM-S) reaches the maximum while the mining side (M-S) shows the reverse trend. For the lower coal seam, it is characterized by crescent-shaped symmetry → quasi-annular distribution with a slight increase in the peak value → significant and asymmetric increase of the peak values. Based on the identification of the key control zones in the ZDSC, an asymmetric reinforcement segmented control method was proposed. The findings provide useful guidance for analogous engineering projects.

Addressing asymmetric deformation, failure in the withdrawal roadway, and support crushing caused by overburden structure at final mining under the goaf. The stress evolution pattern of the surrounding rock in the withdrawal roadway and the dynamic characteristics of the maximum deviatoric stress zones (MDSZ) during this period were analyzed. The effect mechanism of the lateral pressure coefficient (LPC) and the coal seam spacing (CSS) on the stress distribution of the surrounding rock was explained.The LPC and CSS have a nonlinear effect on the rock stress redistribution of the withdrawal roadway. As the LPC increases, the peak abutment pressure on the non-mining side initially rises then stabilizes. The change in principal stress ahead of the advance working face (AWF) shows an “N”-shaped trend, and the maximum deviatoric stress (MDS) on the non-mining side first decreases and then increases. As the CSS increases, the peak abutment pressure and deviatoric stress on the non-mining side initially decrease and then increase. When the CSS is 75 m is comparable to that in the absence of an overlying goaf. The evolution model of the MDSZ in the retreat channel surrounding rock comprises four stages: low values at both sidewalls → value growth at both sidewalls → maximum value and zone at the goaf-side rib (G-SR)→ maximum value and zone at the solid-side rib (S-SR). A collaborative control scheme of “anchor cable reinforcement support on the S-SR” and “pre-splitting roof cutting on the G-SR” was proposed. Field application shows the sidewall displacement is about 150 mm, and the roof displacement is about 200 mm, which ensures the safe withdrawal of equipment. The research results serve as a valuable reference for similar engineering practices.

The advantage of applying gob-side entries (GSEs) with narrow coal pillars in extra-thick coal seams to improve the recovery rate is particularly significant. However, the ground pressure is intense and overlying strata caving is severe under fully mechanized top-coal caving mining (FMTCM) in extra-thick coal seams. The ground pressure environment of the surrounding rock in GSE with narrow coal pillars is complex, leading to the phenomenon of reverse deformation increase (RDI) where the failure of the virgin coal rib is greater than that of the coal pillar rib. To reveal the mechanism of RDI, this study takes the GSE of the Panel 8211 under typical FMTCM in an extra-thick coal seam as the engineering background. Through on-site measurements of the surface convergence of the GSE ribs, damage of support structures, coal mass damage depth, and abutment stress, as well as numerical analysis of the supporting characteristics of gob gangue on coal pillars, the coal pillar width, and the excavation timing, the following findings are obtained. The support of gangue on the ribs of the coal pillar is the direct cause of RDI. RDI only occurs when the gangue’s contact height with the coal pillar exceeds 20 m, while the degree of RDI no longer increases when the contact height exceeds 30 m. On the premise that the gangue’s contact height exceeds 20 m, the smaller the coal pillar width or the shorter the lag time between the excavation of the GSE and the completion of mining in the upper panel, the more significant of the RDI. However, coal pillar width and GSE excavation timing only affect the degree of RDI, rather than being preconditions for RDI. To address RDI in GSE with narrow coal pillars under FMTCM in extra-thick coal seams, a support strategy is proposed: reinforcing coal pillars with anchor cables to share the ground pressure on the virgin coal rib, while simultaneously using anchor cables to strengthen the control of deformation in the virgin coal rib. This measure has achieved satisfactory results in practice, and the overall deformation of the GSE meets the requirements of safe production.