Improving the Efficiency of Longwall Face Recoveries by Managing the Geotechnical Threats

Longwall face recovery is the most involved recurring geotechnical problem faced by operators, with major loss potential should problems occur. However, unlike most geotechnical aspects of coal mining, very little research has been undertaken on the issue of conventional longwall recoveries. The project undertook a wide-ranging study aimed at defining and minimising the geotechnical threats during take-off.

The stated project objective was to develop credible industry guidelines for the specification of effective ground control strategies, so as to minimise the likely geotechnical threats relating to the safety, operational costs and production delays associated with the recovery and relocation of a longwall face.

The project commenced with an industry survey of longwall relocation practice. The resulting database covers issues such as the geotechnical environment, support practices and ground control experiences, including any difficulties encountered. Fieldwork aimed at geotechnical characterisation covered a range of environments in NSW and Queensland, drawing also on existing data from a number of mines. Longwall take-off monitoring data was obtained from 24 face recoveries across all the major coalfields. The fieldwork identified a number of critical features of the geotechnical environment, support design and mining geometry that have a pronounced impact on ground control during take-off. A cantilever model of roof behaviour at the take-off point was developed and validated by the data collected. The roof cantilever acts to transfer load to the solid abutment, the primary support element.
Four parameters were identified as the main geotechnical hazards, namely:

• low roof competency (ie weak roof),
• an adverse weighting environment,
• geological structure and
• horizontal stress concentrations at the gate ends (generally the maingate).
  

All of these are identifiable either at the support design stage or, at worst, prior to the start of powered support removal, which is the critical stage of the take-off process. 

Four aspects of the geometry and process were seen to be particularly significant, in terms of their impact on roof stability and the success of the overall operation, namely:

• the ability to maintain powered support resistance during bolt-up and take-off,
• the direction in which the powered supports are removed,
• the impact of take-off chutes and
• the speed of the powered support removal process.

Several distinct ground control difficulties were identified, namely goaf tightening / ingress, roof sag and cavity formation in the tip-to-face area. Although often related, the means of addressing these issues can vary.

A number of issues remain, which are likely to increase in significance over the next decade and therefore deserve further investigation. These include:

• The industry is likely to become dominated by 300m to 400m wide faces and it is unlikely that the support regimes and processes applied currently will suffice as face width increases beyond 300m.
• There has been renewed interest regarding the use of pre-driven recovery roads in recent years (Thomas, 2006).
• Certain aspects of current mainstream longwall recovery thinking warrant review. An example is the use of buttress chocks / BLSs.

A number of innovations being pursued by individual mines have the potential to improve recovery conditions in specific circumstances. These include “hydrofracing” to control weighting on the approach to the stop line, as well as improved characterisation of roof composition and behaviour, as part of a formal longwall take-off management process.