THE objective of the three-year ACARP research project by Strata Engineering was to develop credible industry guidelines for effective ground control strategies, which would 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 began with a mining industry survey, which collated information on different longwall mines’ geotechnical environment, support practices and ground control experiences, including the specific difficulties encountered.

Fieldwork aimed at geotechnical characterisation covered a range of environments in both New South Wales and Queensland. The response of the industry was very positive and monitoring data was obtained from 24 face recoveries across all the major Australian coal fields.

Strata Engineering principal geotechnical engineer David Hill said the four main geotechnical hazards identified were weak immediate roof, adverse overburden weighting, geological structure and horizontal stress concentrations at the gate ends (generally the maingate).

“The important thing to note is that all of these are identifiable at the design stage or, at worst prior to the start of chock removal, the critical stage in the process,” Hill said.

The survey also identified four key aspects of the geometry and process of longwall face recovery.

“Firstly, maintaining the powered support resistance during bolt-up and take-off is the foundation for success. This requires specific attention, as the positive set is turned off for bolt-up.

“Secondly, during chock removal, the roof develops a ‘momentum’ in the direction of recovery. In Australia, chocks are generally pulled towards the maingate and ground behaviour tends to be more aggressive over the maingate half of the face.

“Thirdly, take-off chutes have become the norm over the last decade, primarily to allow rapid face bolters to be moved on and off the face. Although there are obvious overall benefits, chutes require specific attention in terms of support design and geometry to avoid difficulties during holing and subsequent chock removal.

“Fourthly, once chock removal starts, everything reasonably possible should be done to maintain the continuity and speed of the process,” Hill said.

Time is a major factor in the longwall recovery, and according to Hill, preparation is the key.

He said the question often arises as to the relative merits of installing more tendon support, as opposed to the earlier commencement of chock removal.

“The general rule is that the preparation done prior to the start of chock removal should result in a high degree of reliability in terms of roof control, minimising the likelihood of subsequent difficulty and delay.

“Quantified experience strongly indicates that the benefits of a relatively modest amount of preparation (e.g.: floor repairs or concreting, roof cabling, cavity remediation and powered support canopy contact) greatly outweigh the time and costs involved related to any remedial action that inadequate preparation might subsequently necessitate (never mind the associated delays).”

Several distinct ground control difficulties were also identified by the survey, namely goaf tightening/ingress, roof sag and cavity formation in the tip-to-face area.

Although often related, the means of addressing the issues can vary, for example, with regard to the chock removal sequence, Hill said.

“The rational application of cables is now common in reactively managing areas of deterioration, as well as proactively reinforcing zones of identified potential difficulty; one of the main findings is simply this: ‘if in doubt, cable’.”

Hill said visual and quantitative triggers can also be used to guide strata management and improve the likelihood of success prior to chock removal, rather than modifying the support and/or process in response to subsequent difficulties.

Visual indicators include face breaks, cavities, guttering, rib spall, powered support loading, bolt/cable loading, as well as issues with tendon installation or tension.

“Quantified triggers relate primarily to creep rates and evidence of roof beam breakdown; the project produced guidelines regarding the likely impact of creep rates in the critical period prior to pulling the chocks,” Hill said.

The project also developed a support design process that provides a framework for managing the geotechnical threats in a proactive manner, including a mechanism for quantifying roof support (bolt and cable) requirements. Aided by software, it enables alternatives to be identified and rationalised and is a global first for the longwall industry.

Although the study identified and developed strategies to minimise geotechnical threats during longwall face recovery, Hill said a new raft of issues will need to be addressed as longwall faces get longer and require different treatment.

“A number of issues remain still need to be investigated, and are likely to increase in significance over the next decade,” he said.

As more longwall mines expand to 300m or 400m wide faces, Hill said it is unlikely that the processes applied presently will suffice. He said the chock removal sequence will need to be reviewed (including the potential to pull chocks from mid-face towards both gate ends, as has been applied in the US), as well as improving the use of chutes, which currently are often only used to move the rapid face bolters.

The survey found there has also been renewed interest in the use of pre-driven recovery roads in recent years. “Some of the issues often flagged as hazards for pre-driven roadways were also identified by the project as factors influencing the success or failure of conventional recoveries,” Hill said. “The relativities of the risks associated with the two methodologies therefore deserves re-appraisal.”

Hill said it had become evident that certain aspects of mainstream recovery thinking warrant review, such as the use of trailing buttress chocks or mobile breaker lines.

“The most common operational difficulty related to ground control on a take-off face is an inability to advance the buttress chocks, due to roof deformation,” he said. “Often, without the buttress chocks, the practical impacts of ground movement would be reduced, if not insignificant. This is particularly the case given trends over the last decade regarding improved support products, including standing support, stronger grids and higher tendon densities.”

Meanwhile, Hill said some industry trends, such as an increase in powered support width to 2m, could have both positive and negative implications. While 2m-wide supports reduce the number of chocks to move (everything else being equal), they also require a re-assessment of support practices and patterns.

Innovations in the longwall recovery process are also being made at a number of individual mines, which have 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 take-off management process,” Hill said.

Already the project has attracted considerable interest from longwall operators, particularly those operating wider faces in more aggressive environments.