China’s National Coal Mine Safety Administration has introduced a production cap of 8 million tonnes (Mt) of coal per year for coal mines considered at risk from ‘bumps’, eruptions of rock from the sides of mine shafts caused by pressure from the rock overhead.

Bumps have become increasingly common in Chinese coal mines, with a 2017 study published in the Journal of Rock Mechanics and Geotechnical Engineering reporting that bumps had been recorded at 147 Chinese coal mines between 1985 and 2014, compared to 32 mines prior to 1985. Official data also reveals that more than 10% of the country’s coal capacity is at risk of bumps, with operations responsible for 400Mt of production likely to experience the eruptions.

While bumps can be caused by faults in the tectonic structures beneath coal mines, the report notes that the risk of bumps is increased by mining operations at greater depths, as these operations carve artificial underground tunnels, resulting in a difference in stiffness between the overhead rock and the rock that makes up the sides of the tunnels.

These differences create strains and stresses that are further exaggerated by mining processes, which can create gaps in rock structures, causing them to release what is termed “accumulated strain energy” in the report.

“In practice, as mining proceeds, high in-situ stress can result in instability and reactivation of faults, release of high stress and possible occurrence of coal bumps,” wrote the researchers in the report. “Modeling results suggested that immediately before fault reactivation, the normal and shear stresses on the fault surface increased rapidly and then the fault slip occurred instantaneously.”

The administration has also prohibited the affected mines from expanding their operations beyond the 8Mt cap, to improve operational safety in the long term.

In 2015, Chinese Government announced plans to close 6,000 non-coal mines by 2020, an initiative to improve operational safety, but one that left coal mines unchanged, until Wednesday’s introduction of a production cap.

Chinese industry has struggled with safety performance in recent years, with the explosion of the Yancheng chemical plant in March killing 78 people, and leading to tighter safety checks at the country’s coal mines.