Published in Australia's Longwalls

 

Mine sites use strata consultants in various ways, depending on the level of in-house expertise. Consultants are used to an increasing degree where mine site staff do not possess the necessary skills or experience, or when abnormal issues are encountered and consultants can provide specific solutions. In some instances the consultant is used regularly as a specialist advisor to the site geotechnical engineer or to review/audit strata designs or procedures.

 

No single strata control design method is superior to all others in all circumstances. Different consultants and site geotechnical engineers will likely use one or a combination of empirical, analytical or mathematical modelling methods.

 

The geotechnical engineer will employ their own judgement to check if the design makes sense. Thus, the quality of the judgement is a matter of experience.

 

Any effective approach to strata support design should provide support for a known range of geotechnical conditions. This should be backed up by ongoing auditing (monitoring) to quickly detect unplanned anomalies such as faults, high stress or changed roof strata. When changes are detected, alternative support can be installed.

 

Before deciding on the support plan the mine manager has a number of options – implement the design provided; seek clarification from the consultant or site engineer as to the level of design conservatism, or possible alternatives; install part of the design most suitable to the mining system; or disregard the plan and seek alternative advice or an independent audit of the proposal.

 

In the end there is only one choice, a support plan that represents the strata support design agreed with the design engineer who has used appropriate methods, data and calculations. The limitations of the design should be understood and mine personnel need to confirm the proposed support design can be installed.

 

The mine site operations team has the greatest leverage on the success of the strata support to prevent strata failure. How effective you are at “keeping the roof up” isn’t just about installing bolts (or other strata support); it’s also about the “soft” measures – following the support plan, checking installation quality, having a robust monitoring inspection system and being able to respond quickly if strata conditions are not as expected, and likely to cause geotechnical hazards.

 

Using the monitoring results to test the strata support design, or to quickly decide if the geotechnical conditions have changed is an ongoing challenge at all sites. This applies in both development and longwall panels. The number of longwall strata failures in recent times raises the question of how effective or correctly focused the strata monitoring programmes were.

 

The design basis for strata support plans has become a specialist and skilled task, provided most commonly by consultants. But unless each site takes care to understand the limitations and risks associated with their own strata support system, overconfidence in the installed support can lead to unexpected strata failure.

 

Operators have the solution in their own hands. Time taken to understand the strata support data and design limitations and to set an effective auditing program that continually tests the support design, and detects geotechnical anomalies, will give each site a clear understanding of geotechnical business risk, particularly longwall extraction.