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Advanced Fracture Propagation Testing of Coal Measure Rocks under Dynamic Loading to Replicate Coal Burst Scenario

Underground » Coal Burst

Published: December 23Project Number: C28009

Get ReportAuthor: Babak Khadivi, Jing Li, Amin Heidarpour, Hossein Masoumi, Qianbing Zhang, Jian Zhao | Monash University

This project is aimed to address critical challenges regarding enhancing the health and safety standards in underground coal mining operation, with a particular focus on understanding of burst occurrence mechanism. The outcomes of previous ACARP projects have notably contributed to advancing understanding of coal burst mechanism. However, a notable gap has still remained related to the scarcity of comprehensive laboratory data on static and dynamic testing of coal measure rocks. Such a scarcity stemmed from a complex nature of coal testing and its associated costs.

The primary objective of this project is to bridge the significant gap in current coal burst research by generating a set of robust experimental data sets pertaining to coal measure rocks. To achieve this, a number of distinct and innovative experimental tests was conducted on various rock types including coal and sandstone under both conventional and dynamic loadings.

An extensive testing program was undertaken, adhering to the recommended methods by American Society for Testing and Materials (ASTM) and International Society for Rock Mechanics (ISRM). The findings underscored the sensitivity of coal's dynamic behaviour, including failure and bursting, to principal stresses, pre-stressing (confinement) and wave velocity. Notably, the dynamic strength of coal exhibited dependency on both confinement and the rate of loading resulting in varying failure modes under different loading scenarios.

The dynamic properties of coal under various multiaxial loads, including uniaxial, biaxial, and triaxial compression, were systematically studied. It was observed that the confinement has a pronounced effect on the failure mode compared to the wave velocity. For instance, under uniaxial compression with relatively low to intermediate impact velocity, coal samples turned into powder, whereas under triaxial pre-stressing condition with the maximum impact velocity, the samples remained visually intact.

Test results illustrated that the enhancement of rock strength depends on the confinement level under a constant impact velocity. Such a trend was well observed at low confinements but not under high confining pressures, potentially due to the presence of cleats in coal. The resulting failure modes of coal also exhibited confinement dependency at the constant impact velocity. Under uniaxial compression, rock can be pulverized into powder, while under dynamic loading, the rock ejection phenomenon occurs from the free surface.

The study proposed a couple of innovative testing programs, outlining potential avenues for future research endeavours regarding coal burst phenomenon. Additionally, the National Drop Weight Impact Testing Facility (NDWITF) was utilised for testing of large scale cylindrical sandstone samples with a diameter of 150mm and lengths of 300mm and 420mm under high impact loading energy up to 200kJ. Failure of samples subjected to impact loading was monitored using an advanced 3D photogrammetry system capable of capturing up to 200,000 frames per second.

A comprehensive research effort was also conducted on fragmentation, brittleness and brittle fracturing of various rock types including coal, sandstone and granite. Such an investigation led to the development of an experimental-analytical burst-brittleness ratio, which can serve as a practical tool for geotechnical engineers (particularly those who work on site) to assess the burst proneness of coal and coal measure rocks.

The results from this project provide critical insights into the energy release mechanisms and failure modes of various rock types (particularly coal) under true triaxial loading condition. It highlights the significance of static loading as a dominant factor for triggering rock burst which is hypothesized to be associated with high brittleness of coal while also acknowledges the notable impact of dynamic disturbance. It is also revealed that the static stress can be a dominant factor/triggering mechanism in rock burst solely owing to high brittleness level of coal while the dynamic disturbance stress can be a significant inducement factor for coal bursting.

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