Energy, Burst Mechanics Required for Coal Bursts and Energy Release Mechanisms

Given the increasing incidence of coal bursts in Australian mines, it is necessary to develop mechanistically sound methods for assessing the risk they pose to personnel and equipment underground and methods of minimisation of their likelihood and the severity of any that occur.

This project was undertaken by University of New South Wales and Strata Control Technology and the outcomes are published as separate reports within this project's report.

University of New South Wales Report “ Definition and Quantification of the Energy, Burst Mechanics Required for Coal Bursts and Energy Release Mechanisms”

The objectives of the project were to:

• Define the energy required to cause a burst in a ribside or out of the roof of thick coal;
• Assess strain energy about roadways during development and longwall retreat;
• Assess gas potential energy about roadways during development and longwall retreat;
• Assess the effect of seismic energy on bursting about roadways during development and longwall retreat;
• Assess the energy stored in common geological features which are intersected by roadways;
• Assess energy release mechanisms by considering the post-failure behaviour, natural and mining-induced fracturing and time dependence.

The expectations are that the following outcomes will be achieved:

• Definition and quantification of energy required to cause a coal burst under a range of coal geological domains;
• A review and quantification of the magnitude of the energy available from various sources and mines;
• Quantification of energy release mechanisms;
• Determination of the effect of mine design and support on the risk of a coal burst; and
• Definition of the risk of a coal burst with various geological and mining domains.

The scope of the project is very broad. The report covers the following topics:

• What is a coal burst?
• What is the cause of a coal burst?
• What are the energy sources that cause coal bursts?
• When does a coal burst occur?
• What is peak particle velocity?
• What are the differences between coal bursts and outbursts?

Although this study successfully delivers an improvement in understanding of the energy release mechanism of a coal burst event, it has certain limitations. The study utilises a broad range of modelling packages to avoid any biased outcomes; however, the numerical models still incorporate certain assumptions. The accuracy of the analyses can be affected by error in laboratory and field data, and limited by lack of sufficient computing power and simplifications in the numerical idealisation of the actual problem.

Strata Control Technology Report “Definition and Quantification of Energy Bursts”

The research has been aimed to:

• Develop an improved understanding of the mechanisms of coal bursts;
• Improve prediction of burst prone ground; and
• Develop effective burst mitigation and control strategies for the mining industry.

There appear to be two overall approaches to the study of coal bursts; qualitative and quantitative.

• The qualitative approach is a descriptive analysis based on case histories looking for common factors such as depth, structures, rock type, coal or layer thickness and mine layout;
• The quantitative approach is based on an energy source analysis relating to the type, magnitude and availability of energy about excavations and structures to rapidly displace material.

This report provides a quantitative approach to understanding the mechanics of the processes and their likelihood under a range of mining and geological circumstances.  The rock failure modes and associated energy are discussed. The overall summary of the risk areas noted for Australian coal mining practice is presented.

A key factor for a burst is the magnitude of energy available during the mining stage (development or extraction) and the resistance to movement of the face or rib coal. Resistance to movement increases with depth into the coal rib / face. As such, deep bursts require a large energy source to overcome resistance. This tends to limit the range of potential mechanisms available for large / deep bursts. Low energy bursts occur in the skin region of the roadway and are noted as slumps or local tensile failure of small volumes of the skin material. The resistance to movement will be variable between seams due to the effect of seam thickness, boundary conditions to the seam, bands in the coal and structural features impacting on the integrity of the coal in-situ. Therefore, the magnitude of energy to cause a burst can be variable for different seams and also within the one seam depending on local features impacting the seam.

Prediction of the specific location of a burst during mining is outside of current capability, however general areas can be specified as risk zones. Mitigation methods must have the capability to mitigate the cause and mechanics of a burst risk. In general, mitigation requires either elimination / reduction of the energy source associated with the burst mechanism within the risk zone. Methods have been discussed, however in most instances they are also difficult to enact and typically required separation of personnel from the risk zone during mining operations. Other approaches of hard barriers to projected material are also used however more development is required in this regard.