Monitoring and Stability Assessment of a Punch Longwall Roadway Developed using Highwall Mining Technology

This report describes the results of ACARP research project C9007. The project was established to assist an innovative roadway development trial at Kenmare 2, South Blackwater Mine. A continuous highwall mining system (CHM) was used to develop the roadways for a proposed punch longwall operation at Kenmare 2. The aim of the research project was to monitor, evaluate and model the roadway performance during the initial roadway driveage and subsequent widening using CHM.

The roadway development trial was conducted in January 2000. Significant roof falls were experienced, which contributed to much slower mining progress than anticipated and the operation was terminated prematurely. Despite this, most of the tasks originally proposed within the ACARP project have been completed. They comprise:

• Monitoring of roof deformation of a punch longwall roadway.
• Laboratory investigation of roof rock properties.
  
• Analysis and modelling of roof behaviour of punch longwall roadways.

The main outcomes of the ACARP research project are the observations and conclusions listed below:

• Extensive roof falls, mostly in a range from 0.3 m to 0.7 m, were observed to have occurred in a large portion of the roadway.
• The roof rocks within 1.5 m from the coal seam were tested to be weak.
• An improvement in roof stability with penetration distance was observed and it could be due to (1) a general increase in the strength of the roof rocks and bedding planes; (2) an increase in horizontal to vertical stress ratio and thus the magnitude of the horizontal stress.
• The deep-hole surface extensometers successfully recorded data associated with the development of an unsupported roof fall at 41 m inbye. The stand-up time of the roof was measured to be 10 hours at this location after driveage of the roadway.
• The numerical model and the analytical formula designed to analyse the span stability were demonstrated to be effective in predicting the roof falls.

The experience at Kenmare 2 highlights the significance of geological variation and the need for improved understanding of the roof behaviour. Despite the site investigation and geotechnical assessment performed before highwall mining commenced, it is clear from this experience that future highwall mining operations of this nature require more specific information from the area to be mined. This is especially important when mining near to the highwall and in proximity to pre-blasted slopes or in other areas of doubtful roof rock quality.

Despite the early termination of the mining trial, the ACARP research project as a whole is considered to be successful, and the outcomes will be useful for future punch longwall and highwall mining operations. The instabilities of the unsupported roadway span experienced during mining were able to be observed and monitored. This provided a unique opportunity to study the failure mechanisms of unsupported spans at shallow depths and to verify the existing prediction tools.

The innovative method of using a CHM system to develop roadways for a punch longwall operation could well be successful at a site with conditions more favourable to highwall mining.