Causes of Man-Made Accidents in Gas-Dynamically and Geo-Dynamically Hazardous Coal Mining

The article reviews the largest accidents in underground coal mines in Russia for the last ten years. The spotlighted features of the events can be assumed the major causes of the man-made accidents. The ways of preventing them are discussed.

The growing coal production industry in Russia faces the challenge of deeper level mining, under difficult ground conditions, high gas content and considerable cuttability of coal, and high gas and coal outburst hazard (1, 2).

Actually, in most of mines in Kuzbass, the representative thickness of coal beds is from 1.5 to 10 m, gas content of coal is high – up to 25 m³/t on dry ash-free basis – and depth of mining is 200 to 600 m. In Vorkuta deposit mines in Pechora Coal Field, a typical mining flow chart covers a number of coal beds concurrently (from 1 to 34), coal bed thickness varies from 1 to 4 m, gas content is high – to 30 m³/t on dry ash-free basis – and mining depth ranges from 50 to 1000 m.

Such extremely difficult ground conditions affect mining profitability. Cost effectiveness of mining depends on ground conditions and technical conditions, and on coal quality that governs coal selling price and a fully mechanized face output (FMFO). Annual FMFO, i. e. is over 2 mt in highly profitable mines, 0.5 to 2 mt in profitable mines and less than 0.5 mt in non-profitable mines (3).

Thus, the higher coal production output per unit time makes a mine more profitable. In order to improve cost effectiveness, many mines have recently been using high-duty mining machinery of foreign manufacture as a rule. At the same time, the amount of accidents with big numbers of casualties grows in the mines. The accident in Yubileinaya Mine in May 2007 e. g. took lives of 38 miners. The accident was classified as explosion of air-and-methane mixture and coal dust. A similar accident took place in Ulyanovskaya Mine and caused death of 110 people. That was the largest accident in Russia’s coal mining industry for the last 80 years. By night time between 8th and 9th May 2010 two methane explosions took place in Russia’s largest coal mine Raspadskaya, resulting in severe damage, including destruction of air feed to face area. Nearly all roadways 300 km in overall length were destroyed. As a consequence, 91 people died, out of which 20 were mine rescue-men.

The three mentioned mines belong to the EVRAZ Group (4). EVRAZ is an international vertically integrated mining and metallurgical company with assets in Russia, Ukraine, USA, Canada, Czech Republic, Kazakhstan and South Africa. The head-quarter is in London. The company is among the top steel producers in the world (5).

Raspadskaya Mine accident is the most representative catastrophe from the viewpoint of unfavorable development of geomechanical, geo-dynamic and gas-dynamic processes the initiation mechanism of which is described below.

As a result of stoping, at the top of the mined-out area and in overlying rock mass, huge quantity of methane has accumulated, which is not recorded in drifts, stopes and other openings accessible for direct monitoring, since methane is lighter than air. Upon the collapse of the roof rocks (geomechanical process), accumulated methane discharges as blowout in the mine openings (gas-dynamic process) and methane content jumps in the air there. Methane release is, as rule, accompanied by a powerful shock wave (geodynamic process) that may cause failure and short-circuiting of electric network, and inflammation and explosion or combustion of gas and coal dust. Further on, the cloud of glowing gas-coal mixture caught by a powerful air flow swiftly spreads over the entire mine field. While flowing, the volume of explosive substances in the cloud grows owing to inleakage of gas and dust accumulated in old, poorly ventilated mine roadways, which is the explanation for the high destructive force of the second explosion. The main cause of the second explosion in Raspadskaya Mine is most likely dilution of methane content up to dangerously explosive concentration due to infeed of fresh air flow in the mentioned cloud. It is well known that methane explodes when its concentration reaches between 5 and 15 % in mine air and burns when its concentration exceeds 15 %.

Based on the aforesaid, it is possible to draw a conclusion that the major technical cause of the accident was the exceeded permissible volume and concentration of methane in mined-out voids and overlying rock mass, as well as the limited ventilation ability to dilute methane in mine air upon the stoping roof collapse down the safe limit. It is worthy of mentioning that there were most probably organizational causes, too, such as inefficient gas-dynamic, geodynamic and geomechanical control, in particular, over main and immediate roofs of the coal bed, poor scientifically based evaluation of the accepted engineering solutions on mining under off-standard conditions, lack of clear regulatory framework to administer mining operations in new complicated conditions, imperfect analysis and record-keeping of accidents that took place under similar conditions to the conditions in Raspadskaya Mine, e. g. in Ulyanovskaya and Yubileinaya Mines, incompetent expertise of project solutions and other minor causes.

The elimination of the listed aggravating factors resulting in emergency situations requires control over geomechanical, geodynamic and gas-dynamic processes in rocks, coupled with comprehensive consideration of the above described nature and mechanisms of these processes.

The technology to control the processes consists in setting a caving step for the main roof under specific conditions of mining and in exclusion of uncontrollable roof caving. To this effect, induced roof caving should be implemented as soon as the exposure span in the roof has reached a limiting value. In this case, stoping operations are stopped, power supply in the area of the main roof caving is cut-off, and people are evacuated from the face area to a safe place. After that, remote induced caving of the main roof is carried out, and, after airing of the face area and upon permissible concentration of methane-and-air mixture in the face air, production is recommenced.

In Severnaya Mine, Vorkutaugol, Severstal, in the day time on 25th February 2016, at a depth of 780 m, methane outburst and two explosions took place and caused rock fall and fire. In the first hours after the accident, it was succeeded to save 81 miners, four miners died, while the fate of 26 mine workers was unknown. In the night time on 28th February, the third explosion occurred during the search and rescue operations in the mine. Six people died, including five mine rescue-men and one miner. All in all, the victims of the mine accident were 36 people. In 2015 Severnaya Mine produced 1.5 mt of coking coal concentrate, i. e. the mine is more than profitable. Presently, it has been decided on closure of Severnaya Mine.

The most effective action to prevent instantaneous methane explosions is the advanced mining of protective beds. Undermining of a coal bed with high methane content initiates cleats that actively liberate occluded methane. Prompt removal of this methane will reduce methane content in roadways down to safe limit.

At the same time, methane drainage in outburst-hazardous coal beds is a complex, laborious and money-taking operation that is not always accomplished properly, which even more complicates the prevailing conditions. When rock mass and a coal bed with high methane content experience local jointing, methane is released in mine air faster than is removed, which elevates the risk of a gas-dynamic event. It is only possible to prevent or mitigate the hazard by the purposeful methane drainage hole drilling (6).

Based on the analysis of the causes of the man-induced accidents in underground mineral mines for the last ten years, it is necesscary to look for a science-based integrated approach to design and introduction of safety control and maintenance systems in mines. The Institute of Integrated Mineral Development (IPKON), Moscow/Russia, has developed the method of stress state control in rocks based on the studies into generation of particulates in dolomite specimen subjected to uniaxial compression in a sealed cell. Particulates split off the specimen faces under deformation were recorded using an aerosol particle counter. Later on, a series of tests of different kinds of rocks displayed this phenomenon in all tested specimens of rocks.

Figure 1 illustrates the physical model serving the background for the proposed stress control technique. A specimen with a through cylindrical hole in the center was subjected to uniaxial compression. Here, a is the length of the specimen side, r is the hole radius, θ is the angle relative to which actual stresses at the contour of the cylindrical hole will be analyzed.

In this case, the hole boundary experiences the maximum σ_θ when θ = 0 and θ = π, i. e. at the points n and m of the diameter:

(σ_θ)_max = 3 P,

where P is the compression of the specimen. Then, the maximum compression stress is three times higher than the constant pressure P applied to the faces of the specimen.

When θ = π/2 and θ = 3π/2, follows:

σθ = –P.

Consequently, the maximum tension stresses act at the points p and q. Thus, zones of maximum compression and tensions are generated at the boundary of the hole. These zones are the potential sources of particulates. This means that particulates may be sourced both by the zones of maximum compression stress (at the points m and n) and of maximum tension stress (at the points p and q).

The laboratory and production testing has yielded new quantitative and qualitative criteria for pre-failure state of rock masses, and the designed equipment enables the real-time analysis of rock mass condition and the estimate of the limiting pre-accident state of rocks.

Acknowledgement

This study has been supported by the Russian Science Foundation, Project No. 16-17-00066).