Violent failures of coal pillars, known in practice as coal mine bumps, have long been a subject of investigation. Many field investigations have considered geological conditions that create high stress in the pillar as the main causative factor leading to bumps. In recent years, stress and constraint have been shown to be necessary in producing coal burst.

This research investigated the influence of interface friction and w/h ratio on the violence of coal specimen failure. In satisfying this objective, three violent failure parameters such as peak SPL, core zone failure, and ultimate stress were used to assess the violence of failure. The degree of the violence were investigated at three different interface frictions (high: μ = 0.40, medium: μ = 0.22, and low: μ = 0.13) and six w/h ratios (w/h = 3, 4, 6, 8, 12, and 16). The specimens were axially loaded in a universal testing machine equipped with a microphone to record sound pressure waves for peak SPL calculation. The failed specimens were also photographed after the failure for the measurement of core zones.

The violence of coal specimen failure was found to decrease with increasing w/h ratio and decreasing interface friction. In terms of peak SPL and ultimate stress, the influence of interface friction in reducing the violence was more significant for coal specimens at high w/h ratios than it was at low w/h ratios.

The high peak SPL region ≥ 124 dB was shown to be the most reliable parameter for assessing the violence of coal specimen failure, while the application of core zone failure depends on w/h ratio. Violent failure was also found to be independent of high ultimate stress. Stress merely contributes to the failure of the specimen, but it is the interface friction that greatly influences the degree of the violence.

There are four friction zones on the top end-surface of a coal specimen that may indicate the variation of confinement within the specimen. By the increase of w/h ratio, the zone of confinement was found to increase. Moreover, at the same interface friction, the increase of the confinement may also reduce the violence of failure.

Keywords: Coal specimen, violent failure, interface friction, w/h ratio, coal mine bumps, core zone, confinement