Rock bursts are violent failures of rock that occur in high-stress mines, whereby internal mechanical forces exerted by weight and pressure, can cause seismic rock explosions. They can also trigger movement in nearby geological structures and are caused by both natural earthquakes and mining activity.
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These events make deep mining, such as for coal and copper extraction, extremely dangerous, with rock bursts estimated to cause around four mining deaths globally every year.
Mining Safety in the US
The US has the highest global coal reserves and produces over six million metric tons every year.
The Mine Safety and Health Administration (MSHA), a US Federal Government Agency, reported coal miner deaths reached an all-time low in 2020, with 29 fatalities.
These deaths were the result of different types of accidents, mainly from powered haulage and machinery.
Mining Safety in China
China is by far the biggest coal-producing nation and the biggest consumer.
China produced over 3.5 billion metric tons of hard coal in 2018, and with greater production, comes greater risk.
Chinese authorities acknowledge some 2,000 coal mining deaths occur every year, but this figure could be a lot higher.
Global Coal Mining Trends and Risk
Globally, mining deaths have reduced in line with a reduction in the number of miners overall, as international demand for coal slows, in line with economic trends towards cleaner, renewable energy technologies.
Nonetheless, deep coal mining still carries high risk wherever it occurs, and looks set to continue for some time, despite the decline.
Engineering scientists researching seismic activity in relation to deep mining have identified the characteristics of earthquakes in strata being mined, and also how mine towers behave.
It is hoped their work can lead to the prevention and control of rock bursts and reduce risk in deep mining, whether for coal or other minerals and metals that will still be required to power renewable energy technologies.
Mining Earthquakes in Multi-Coal Seams
Single-coal seam mining occurs near the surface of strata, whereas multi-coal seam mining occurs both near the surface and in underlying deeper layers of strata.
Near-surface layers of coal are almost completely depleted worldwide, so deep multi-coal seam mining has increased.
Historically, seismic research has been carried out on single coal seam mining using computer modeling.
However, the shift towards multi-seam mining due to higher economic benefits, improved technological factors, combined with a depletion of surface layers, has shifted scientific focus towards multi-coal seam safety.
New Research in China Focuses on Multi-Seam Coal Mining
Resource and environmental engineering researchers at Shandong University have been working with the Guizhou Institute of Technology in China.
They have studied the relationship between multi-coal mining seams, and the energy, fracture, and movement of strata.
Researchers discovered that fractures and movements of rock often occur in the uppermost coal seams. These comprise thick and hard strata, with high energy and huge thickness.
They claim more research on the dynamic mechanism under thick and hard strata in multi-seam coal mining is urgently needed to guide those involved with coal mining safety.
Seismological Observation System Technology
The researchers used Seismological Observation System (SOS) Microseismic Monitoring system technology, developed by the Polish Institute of Mining Research.
SOS detects vibration parameters to predict where and when disasters may occur during real-time coal-seam mining.
They studied data taken from three Chinese mines; the Baodian coal mine in Jinging, which comprises sedimentary fine sandstone, the Huafeng coal mine in Qufu, comprising conglomerate, and the intrusive igneous rock within Yangliu coal mine, Huaibei.
Microseismic energy data was collated using evolution law and temporal distribution, which measure seismic energy every day over a period of around six months.
SOS Research Findings
The SOS research identified that microseismic events with low energy occur more than in multi-coal seams being mined under thick strata, nearer to the ground’s surface.
Earthquake events occurred more frequently in lower-seam mining than in upper-seam mining, but without the obvious time intervals found in upper-seam mining.
In upper-seam mining, strong earthquakes with high energy often occurred on the working face, and below thick and hard strata.
This indicates that lower seam mining causes secondary movement of fractured rock contained in the higher surface seams.
Fracture and movement of thick and hard strata in the higher seams were found to be caused by movement in the overall strata. However, as it was fully fractured, it was considered stable.
Lower coal-seam mining appears to cause movement and space for settlement, causing a secondary movement of fractured rock, which then initiate dangerous rock bursts.
Seismic Rocking Effects on Mine Towers
In a separate study, civil and mechanical engineers at the Opole University of Technology in Poland studied the seismic rocking effects on a mine caused by both natural and man-made earthquakes.
The research considered rotational forces, which is the seismic movement of a mining tower in one or more of its x,y and z axis, as well as translational, which is the uniformed or continuous movement around an internal axis.
Mine towers are structural headframes, positioned above an underground mine shaft. They enable the hoisting of machinery, materials, as well as mining personnel, to and from the mine beneath.
Mine towers are typically made of steel or reinforced with concrete. They are thin and tall, reaching up to 87 meters in height, a little more if reinforced with concrete.
They are designed to hold considerable loads without buckling, tilting, and subsequently failing.
Seismic records were used to analyze the seismic response of selected steel mine towers for both horizontal seismic energy and horizontal rocking, in both natural and mining-induced earthquake events.
The higher the rotational influence from induced seismic activity compared to natural earthquakes suggested the rotational component may be more pronounced for events closer to the earthquake epicenter.
Results showed the rotational element either increased or sometimes even decreased the seismic response of the mine tower headframe.
This led to the conclusion that the rocking component should be included in the future design of tall towers and buildings, including mine towers used for deep mining, but that more research is needed.
Research to Conserve Historical Buildings May Improve Mining Tower Design
In a third study, researchers at Imperial College, London, used solid blocks to study the rocking response to seismic activity.
Using rotational mechanical inertia to apply equal and opposite forces, they mitigated the effects of earthquakes on rocking structures. This demonstrated how mechanical devices adopt resisting forces, proportional to a relative acceleration between terminals, which can be used to oppose the motion.
It results in lower seismic energy and enhanced stability of rocking behavior, without changing any geometry.
Although this research is being developed to protect and conserve historical buildings, the underlying principles can be applied to the engineering design of mining towers to make them more resilient to earthquake damage and rock bursts.
References and Further Reading
Seismic rocking effects on a mine tower under induced and natural earthquakes (02.04.2021) Bonkowski.P.A, Kus.J, Zembaty.Z in SpringerLink online (accessed 29.10.2021) https://link.springer.com/article/10.1007/s43452-021-00221-7
Study on Characteristics of Mining Earthquake in Multicoal Seam Mining under Thick and Hard Strata in High Position (04.03.2021) Zhu.G, Zhang.K, Yang.L in Hindawi journal (PDF) (accessed 29.10.2021) https://www.hindawi.com/journals/sv/2021/6675089/
Roadway surrounding Rock under multi-coal-seam mining: Deviatoric Stress Evolution and Control Technology. (24.11.2020) (Chen.D, He.F, Jiang.Z, Li.Y, Shi.S, Wang.E, Wang.L, Wu.X, Xie.S, Zhang.Q, in Hinsawi journal (pdf) (accessed 29.10.2021) https://www.hindawi.com/journals/ace/2020/9891825/
MSHA: Deaths among coal miners reach ‘historic low’ in 2020 (19.01.2021) in Safety and Health Magazine online (accessed 29.10.2021) https://www.safetyandhealthmagazine.com/articles/20741-msha-deaths-among-coal-miners-reach-historic-low-in-2020
Daily Fatalities Report (29.10.2021) in United States Department of Labor https://arlweb.msha.gov/stats/charts/combined.php
Coal Mining in the US industry trends (2016-2021) in IBIS World https://www.ibisworld.com/united-states/market-research-reports/coal-mining-industry/
Coal reserves by country (2016) in Worldometer https://www.worldometers.info/coal/coal-reserves-by-country/ (Accessed 1.11.2021)
Geomechanics of subsidence above single and multi-seam coal mining (June 2016) J.P Carter, J.P Hambleton, A.M, Suchowerska, in Journal of Rock Mechanics and Geotechnical Engineering https://www.sciencedirect.com/science/article/pii/S1674775516000159
Seismic protection of rocking structures with inerters (01.12.2018) C.Malaga-chuquitaype, R.Thiers-Moggia in Department of Civil and Environmental Engineering, Imperial College, London, UK (pdf) https://spiral.imperial.ac.uk/bitstream/10044/1/66647/6/J23_Rocking_protection_inerters.pdf
Leading Hard Coal producing countries worldwide in 2018 (2021) Garside.M in statista.com (online) (accessed 02.11.2021) https://www.statista.com/statistics/264775/top-10-countries-based-on-hard-coal-production/
Miner Deaths and Injuries in China (2008-2019) in factanddetails.com (online) (accessed 02.11.2021) https://factsanddetails.com/china/cat13/sub85/item321.html