A new high-risk hazard has been identified in China’s western mining regions—particularly in the Ningdong and Huanglong coal bases—where compound roof water-sand inrush disasters are occurring alongside intense ground pressure. Unlike traditional non-dynamic water-sand inrush events in shallow/unconsolidated strata, this dynamic hazard occurs in deep, weakly cemented bedrock strata, making it uniquely destructive.
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The simultaneous occurrence of water-sand inrush and intense mine pressure greatly increases the risk of catastrophic failures, leading to severe economic losses and casualties. It poses a major threat to mining safety and severely limits the efficient, secure extraction of coal resources in these strategic mining regions.1
Water-Sand Inrush in Thick Bedrock Stope
Due to their abundant reserves, areas such as Huanglong and Ningdong have become key coal-producing hubs in China. However, the geology of these regions presents challenges, with thick coal-bearing strata formed during the relatively late Early to Middle Jurassic period.
The sandstone and mudstone layers from this era are weakly cemented, possessing low strength and prone to disintegration. This geological setting presents significant engineering challenges, particularly regarding roof stability in deep mining stopes. These stopes are hundreds to over a thousand meters deep and overlain by thick Jurassic formations like the Yan’an, Zhiluo, and Anding-Zhiluo, with overlying Cretaceous sandstones and conglomerates.
Such formations are common in coal mines such as Xinshanghai No.1, Zhaoxian, and Cuimu. Under mining disturbances, these weakly cemented roofs become highly susceptible to a severe form of dynamic sand inrush disaster—a phenomenon markedly different from the conventional non-dynamic water-sand inrush events seen in shallow-buried/unconsolidated strata.1
Dynamic sand inrush in these regions is driven by complex mine pressure behavior. It occurs when the overlying hard rock fractures, destabilizing the already weak roof strata and causing violent sand and water ejections.
A notable example is the 2016 roof debris flow disaster in a Huanglong coal mine. Unlike traditional inrush events, where material originates from loose surface layers and is powered by water pressure and gravity, these deep dynamic events draw material from within the bedrock itself and are directly influenced by overburden-induced mine pressure.
Measured data from Huanglong and Ningdong show that the water-flowing fractured zone from mining does not reach the surface/shallow layers, ruling out traditional sources of sand. Instead, the energy and materials involved are internal to the deeper rock mass, making this a unique, more dangerous hazard.1
Disaster Mechanism
Seepage system stability analysis is essential for predicting and preventing water inrush. Dynamic systems naturally face disturbances from environmental/internal fluctuations. A stable system returns to its original state after such perturbations, while an unstable one deviates significantly.
Lyapunov stability ensures that small changes in initial conditions do not lead to large deviations. If a system is asymptotically stable, it eventually returns to equilibrium after the disturbance. However, instability means even minor disturbances can lead to uncontrolled deviation from the expected behavior.1
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The degradation of weakly cemented formations in water is influenced by clay mineral content, cement composition, and pore structure. Studies indicate that plastic deformation destroying cement is a key mechanism behind the weakening of such sandstones. Clay minerals, especially at low moisture levels, significantly impact rock properties.
With increasing saturation, mechanical strength and fracture toughness decrease. In limestone, strength loss correlates linearly with adsorption capacity, largely determined by montmorillonite layers with high specific surface areas.1
Weakly Cemented Rock Burst’s Dynamic Characteristics
Strata instability and breakage are key drivers of disasters in coal seam stopes, with dynamic sand inrush within weakly cemented strata primarily powered by mine pressure from such breakage. The deterioration of these strata influences strata behavior as both underlying support conditions and overlying load distribution affect the breaking patterns and instability of surrounding rock layers, ultimately intensifying ground pressure and structural instability.1
Finite element analysis shows that after mining, the stress distribution in the upper rock strata is uneven, with vertical stress around the working face displaying an uplift pattern. This distribution is primarily influenced by the weakly cemented interlayer’s thickness and strength; thicker/softer layers reduce the peak load and shift the abutment pressure peak further from the coal wall.
Rock properties, mining depth, and hanging roof length affect abutment pressure, while main roof fracture is impacted by load, tensile strength, roof thickness, and cushion elasticity. The foundation type (damaged/softened) impacts rock beam behavior, including bending moment, shear force, and pressure distribution as mining progresses.1
When the upper hard rock becomes unstable and fractures, it releases kinetic energy and shifts the load to the underlying strata, potentially triggering instability and dynamic water inrush. These hard roof breaks typically occur at the point of maximum bending near the coal wall, where the resulting rebound and compression act as impact sources.
Thick, hard strata store and release more elastic energy than ordinary rocks, producing strong microseismic activity and intense strata pressure during breaking and movement, increasing disaster risk.1
An inclined block-bearing area above deep coal seam stopes transfers overlying pressure and elastic energy to lower strata. Simulation tests show that this dynamic load transfer can sharply increase support resistance and generate ultra-high water pressure in nearby aquifers. This may create sudden water channels and negative pressure zones, triggering dynamic water inrush between the aquifer and mining face.1
Industry Impact
Water–sand mixture inrush poses operational risks in western China’s coalfields, including equipment loss, mine downtime, and severe safety hazards.
In several cases, accumulated water and sand in low-lying areas have destroyed electrical equipment, emphasizing the urgent need to understand and prevent such disasters.
High-intensity, large-scale underground mining in regions with weakly cemented overlying strata increases the risk of these inrush events, leading to extensive deformation and failure of strata, with serious implications for mine safety and environmental sustainability.1,2
To address these challenges, an integrated proactive prevention strategy has been proposed. It includes advanced techniques such as hard rock pre-splitting and grouting reinforcement of weak formations, eliminating the fundamental conditions that trigger inrush disasters. The formation of these events depends on objective factors, material and power sources, and inducements such as aquifer conditions, joint distribution, and mining parameters.1
Comprehensive detection through drilling, lab testing, and structural analysis identifies material sources, while energy-based analysis of key stratum failure provides insights into dynamic load impacts. Prevention involves controlling aquifer drainage, optimizing support resistance, managing mining height and speed, and sequencing operations to prevent stress concentration.1
Future Directions
The combined disaster of water–sand inrush and strong ground pressure in thick bedrock stopes with weakly cemented roofs poses greater risks than previous events, necessitating targeted control measures based on understanding its formation mechanisms.
Future research must focus on analyzing how clay content, porosity, and loading affect the mechanical degradation of weakly cemented strata; understanding stress variation in upper hard rock and main roof due to water-induced weakening; and examining how hard rock instability triggers bursting forces and sand inrush channel evolution.1
References and Further Reading
- Liu, Z., Dong, S., Guo, X., Li, X., Chen, D. (2025). Research status of roof water and sand bursting disaster in thick thick-bedrock stope of coal seam. Scientific Reports, 15(1), 1-17. DOI: 10.1038/s41598-025-14746-x, https://www.nature.com/articles/s41598-025-14746-x
- Zhu, J., Li, W., Teng, B., Lu, Q., Li, D., & Li, L. (2024). Overburden failure and water–sand mixture outburst conditions of weakly consolidated overlying strata in Dananhu No.7 coal mine. Scientific Reports, 14(1), 1-14. https://doi.org/10.1038/s41598-024-59240-y
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