Geomechanical Factors Affecting Highwall Mining

CSIRO has developed highwall mining design techniques suitable for australian conditions.

Because highwall mining is reliant on the self-supported capacity of the rock mass, highwall stability, pillar design and stability of unsupported spans are major issues for highwall mining operators.

Project Objectives

The overall project objective is to provide the highwall mining industry with practical and reliable design techniques for span assessment and layout configuration for highwall mining under Australian conditions. Case studies were conducted at Oaky Creek, Moura, German Creek and Ulan highwall mining operations.

The project's specific objectives are to:

• Accurately characterise the immediate roof of the Oaky Creek and Moura highwall mining sites and monitor their response to operations.
• Use monitored data from Oaky Creek to test a range of predictive models for suitability and effectiveness.
• Propose a preliminary predictive model after initial work at Oaky Creek. Following work at Moura, test and modify the model accordingly.
• Fully report research findings and transfer these as effectively as possible to the mining industry.

As part of the project, researchers will:

• Review highwall mining experience in Australia with a view to identifying the key factors which lead to stability or instability.
• Verify the current span stability assessment models against mining experience under different roof conditions.
• Improve layout design procedures by identifying the key factors affecting design and their relative importance via parametric study.

Findings

The research found that three major factors affected design of highwall mining pillars:

• The average in situ mass coal strength.
• The pillar width to height ratio.
• The strength of the interfaces between the coal seam and the roof and floor when the whole thickness of the seam is mined.

The research also found the exact stiffness, strength or parting strength of relatively competent roof and floor material had little effect on the pillar strength.

A review of highwall mining experience at the four mines found that successful highwall mining operations required:

• Site investigation to obtain comprehensive geological information.
• Assessment of span stability in advance - if this is predicted to be poor, remedial measures, such as leaving coal in the roof of entries, need to be considered.
• Layout designs that take account of pillar strength including interaction with roof and floor. Factors of safety on layout design must be adequate to ensure long-term stability.
• Guidance of the continuous highwall mining system to preserve pillar and span integrity. The introduction of the Horta inertial navigation system has significantly improved highwall mining practice to the point where guidance has been virtually eliminated as a problem.
• Critical panel width with barriers needs to be considered to isolate problems. Massive panel failure occurs when a sufficient number of entries are mined, usually when a critical panel width is reached. A highwall mining panel narrower than its critical width is unlikely to fail in a catastrophic manner, since stress concentration caused by local failure can still be sufficiently bridged away to the abutments. In contrast, a panel greater than its critical width will be much more vulnerable to a catastrophic "domino" failure due to insufficient stress bridging once a local failure is initiated. The critical panel width varies, depending on the roof geology and overburden depth.
• Knowledge of in situ stress and stress concentration near the highwall, especially for benched highwalls.
• Constant monitoring of mining conditions during operations.

Where To From Here

A second ACARP project (C8033 Optimal Design and Monitoring for Highwall Mining) is under way to develop integrated pillar/panel stability assessment guidelines.

The aims of the project are to:

• Evaluate the likelihood of medium/long term stability and subsidence for highwall mining designs over a wide range of geological conditions, based on back-analysis of all pits in Australia that have been highwall mined.
• Carry out a quantitative assessment of possible 3D stress concentration near the highwall face and its effect on mine stability, particularly under benched highwalls that may have suffered significant blast damage.
• Integrate pillar and panel layout design guidelines that minimise the risk of catastrophic panel failure.
• Test and evaluate a seismic monitoring system to provide early warning of the onset of large-scale instability.

This project is scheduled for completion by June 2001.