Hidden Tunnel Networks Beneath Mine Roofs Trigger Dangerous Pillar Collapse, New Study Finds

Beneath some of China’s largest coal mines lies a hidden hazard that engineers have struggled to tame: dense networks of abandoned roadways left behind by decades of irregular mining. When operators attempt to extract the narrow coal pillars that separate these ghost tunnels from active workings, the consequences can be catastrophic. A new study published in Results in Engineering by Dongdong Chen, Zitao Chen, and colleagues reveals for the first time how clusters of these abandoned passages destabilize the slender pillars and the massive roof blocks above them, and demonstrates a remarkably practical fix that turns excavated coal itself into structural backfill.

The research focuses on what mining engineers call the goaf-side narrow coal pillar, or GSNCP, a thin ribbon of coal left standing between an active tunnel and a mined-out void. In extra-thick coal seams averaging eleven meters, these pillars are routinely riddled with legacy roadways from earlier, less regulated mining eras. When a new roadway, known as a goaf-side entry, is driven alongside such a compromised pillar, the results on site have been alarming: severe rib heaving, roof convergence, rib collapse, and even complete roof caving in zones where abandoned roadways cluster together. Traditional support methods such as bolting and grouting, the authors note, fail entirely under these conditions.

To understand why, the team built a large three-dimensional numerical model using FLAC3D 7.0, spanning 500 by 500 meters laterally and 300 meters in depth, discretized into half-meter cubic elements with more than 7.6 million cells in the critical study area alone. The model reproduced the actual excavation sequence of the mine, including dynamic compaction of the adjacent goaf using double-yield elements and stepwise extraction of the working face at one-meter cycles. The coal was governed by a strain-softening criterion while the overlying strata followed the Mohr-Coulomb failure criterion, allowing the researchers to track how stress redistributes as abandoned roadways slice the pillar into segments.

The simulations revealed a striking sensitivity to the spacing between abandoned roadways. When intervening coal ribs between two roadways were narrower than five meters, the segments entered a low-strength bearing state and simply shed their load onto neighboring pillars, triggering severe rib heaving. At a width of seven meters, the internal abutment pressure peaked at a dangerous 27.9 megapascals in a single sharp spike, a condition the authors describe as an ultra-high strength bearing state in which the pillar risks internal plasticization and fracturing. Only when spacing exceeded nine meters did the stress curve split into a healthier bimodal shape, with the peak dropping to 23.1 megapascals and the load distributed across a broad high-strength bearing zone.

The second half of the puzzle lies overhead. Using the classical OX break theory of main roof behavior, the team developed a theoretical model linking the arc-shaped triangular roof block that forms above the goaf edge to the abandoned roadways hollowing out the pillar beneath it. This triangular block, whose dimensions follow the periodic weighting span of the main roof, rotates and subsides around a fracture line after the roof breaks. The researchers identified nine possible relative positions among the solid coal rib, the triangular block, and the abandoned roadways, and derived moment-balance equations for each configuration to determine when the block remains in equilibrium and when it fails.

The analysis produced two instability coefficients with clear physical meaning. When the coefficient K2 exceeds one, the triangular block undergoes rotational deformation instability, crushing the rock at its corners. When the coefficient K1 falls between zero and one, the block slides instead. Crucially, the calculations showed that as more abandoned roadways accumulate beneath a single triangular block, both coefficients decrease simultaneously, meaning the weakened pillar can no longer supply the support force needed to prevent sliding. Because at most three abandoned roadways can fit beneath one block, the team showed that the failure mode shifts from rotation to sliding as roadway density increases, with the block’s gravitational load transferring through the roadway roof to the solid coal rib and producing uncontrollable deformation, roof fracturing, or even roof cutting.

These theoretical predictions were tested in a 1:100 scale physical analogue model built from layered sand, lime, and gypsum mixtures matched to the site’s borehole geology, with hydraulic jacks applying 34.6 kilopascals to reproduce the overburden load at the mine’s 250-meter depth. The experiments confirmed the mechanism in all three fracture-line scenarios: with backfill in place, the pillar and roof stayed intact, but removing the backfill reproduced the full disaster pattern of pillar crushing and severe roof collapse observed in the field. The observed diagonal shear fractures, angled at roughly 55 to 60 degrees, matched the theoretical shear angle predicted by the Mohr-Coulomb criterion for coal with internal friction angles between 20 and 30 degrees.

Having diagnosed the disease, the researchers prescribed a cure that is as economical as it is effective: backfill the abandoned roadways with the very coal excavated during roadway driving. Laboratory tests determined the optimal recipe, mixing crushed coal, P425 Portland cement, and water in a ratio of 6 to 1 to 0.7. With a particle gradation of roughly 60 percent fines under five millimeters, the solidified mixture achieved a uniaxial compressive strength of 9.8 megapascals, about 80 percent of the raw coal strength of 12.3 megapascals, while still meeting the pumping requirements of mining concrete pumps. The mixture is pumped into mining-purpose backfill bags that prevent leakage into the goaf, with roof bolts fitted with end caps to avoid puncturing the bags.

Numerical simulations of the backfilled system showed a clear strength threshold. At just 20 percent of coal strength, the backfill carried little load and stress remained dangerously concentrated in the pillar. At 60 percent, the backfill began sharing roof load, though stress zones remained isolated. Only at 80 percent of coal strength did the stresses within the pillar and backfill merge into a single continuous high-strength bearing zone without peak eccentricity. The backfill also exerts lateral confinement on the flanking coal segments, creating what the authors call a strong-weak-strong configuration that boosts the pillar’s effective bearing capacity by approximately 27.9 percent when a single roadway among three is filled.

Field deployment at the 110203 longwall face validated the approach under production conditions. Borehole peeping through the pillar rib in the abandoned roadway concentration zone showed high internal integrity after treatment, and convergence monitoring recorded roughly 16 centimeters of rib-to-rib closure and 14 centimeters of roof-to-floor deformation at backfilled locations, figures notably smaller than in untreated areas. The study’s broader message for the industry is twofold: roadway density in protective pillars should be carefully controlled during mine planning, and where legacy roadways already compromise pillar integrity, solidified excavated coal offers a low-cost, locally sourced route to restoring load-bearing capacity and keeping goaf-side entries safe for mining.

Subject of Research: Instability of goaf-side narrow coal pillars beneath concentrated abandoned roadways in extra-thick coal seams and solidification-backfilling control

Article Title: Instability mechanism of narrow coal pillars in extra-thick coal seam within concentrated abandoned roadway areas and solidification-backfilling control of roadway excavation coal mass

Article References: Instability mechanism of narrow coal pillars in extra-thick coal seam within concentrated abandoned roadway areas and solidification-backfilling control of roadway excavation coal mass. (n.d.). https://doi.org/10.1016/j.rineng.2026.112978

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.112978

Keywords: coal mining, narrow coal pillar, abandoned roadways, goaf-side entry, roof stability, triangular block, numerical simulation, backfilling, Portland cement, strata control, FLAC3D, mine safety