* Typical roadway widths in the database are in the order of 5-6m. There is a notable exception where several 11m wide roadways have been successfully holed (Mettiki Mine in Maryland, USA) although an installed standing support density of around 4.6MPa was used.
* There is no clear relationship between depth of cover and the failed cases as they occur across the entire range of the database (70-610m depth of cover).
* There are no failed cases for powered support ratings of >750 tonnes. However, only five out of 130 cases (ie 4%) incorporate a powered support rating >750t so this may be a function of the database contents rather than a technical outcome.
* Longwall face widths range from 76-305m which essentially covers the full range of the database.
* Of the 14 cases whereby soft floor has been recognised as a factor (through observed floor heave leading up to holing), only three relate to failed cases. Similarly, of the 11 successful cases, eight are also linked to the use of standing support densities of
* Conversely, many of the successful cases are linked to nonsoft floor.
* In terms of seam thickness (or extraction height), the failed cases cover the range 2.1-3m (average 2.6m), whereas the full database ranges from 1.5-3.4m (average 2.5m).
A number of key assessment and design conclusions can be drawn, starting with the observation that standing support density is a critical factor in minimising the likelihood of a weighting failure during holing (as would be expected). That is not to say that standing support is absolutely required (as some holings have been successful with no standing support) but a weighting failure becomes significantly more likely once the standing support density is reduced below 0.3MPa (all other factors being equal) for CMRR values 0.3MPa.
A further assessment is that it is not reasonable to use installed roof support as an effective substitute for standing support as the database shows no ability of an increased roof support density to prevent weighting failures (as would be expected).
Installed roof support RDI is a critical factor in minimising the likelihood of a roof fall failure (as would also be expected) in that all the examples of such failures occur at values 0.5MPam installed support RDI. CMRR would appear to play no major role in this regard (ie, being a significant predictor of the likelihood of a roof fall failure occurring).
Slow extraction rate in the lead-up to holing is a potential significant factor as the database indicates that around 50% of the failed cases are accompanied by slow mining leading up to holing. Therefore, it must be considered as part of an assessment and design study.
The depth of cover can be discounted as a significant variable, although logically the lower the depth of cover, the lower the general face loading environment (all other factors being equal). Powered support rating is also not a significant predictor of failed cases with one weighting failure having occurred with a theoretical rating as high as 726t. However, this may be masking a highly significant issue in relation to the actual hydraulic health of the powered supports, which is known to have been a factor in the example of the 726t supports and is not immediately apparent in the database, if taken at face value.
The existence of a soft floor environment that will undergo heave ahead of holing promotes the likelihood of a successful holing being achieved, but only in conjunction with adequate standing support. It is judged that this both protects the integrity of any high capacity standing support in the roadway and also delays the loss of structural integrity within the remaining coal fender as it is incrementally removed by mining.
Longwall panel width is not a significant predictor of failed cases and is therefore, not a major consideration apart from stating that in general terms, commonsense dictates that the wider the panel, the greater the potential exposure to major ground instability (all other factors being equal). Extraction height is also not a significant consideration in determining the probability of a holing failure occurring, although one would expect fender stability to increase as the extraction height is reduced (all other factors being equal).
Three other relevant factors that cannot be evaluated by the worldwide database (due to insufficient detail contained within the database) are overburden weighting, the alignment of major geological structures with the longwall face, and the orientation of the roadway with respect to the face. Nonetheless, these can still be considered as part of a design exercise based on the use of other relevant industry databases and risk based engineering commonsense.
The overall conclusion to be drawn from the worldwide database is that the holing of a longwall face into a pre-driven roadway is not necessarily a high risk venture, providing lessons of the past are considered and applied to future attempted holings. The key factors that have had a significant influence on the success or failure of holings in the past have been clearly identified from the database and hence can then be used pro-actively to maximise the likelihood of successful holings in future such attempts.
Based on the current success rate within the database combined with applying the strata control lessons associated with both successful and failed cases, it is judged that any future holings can be assessed and designed with at least a 95% confidence as to the outcome.
As a recent example, at the end of 2000 a longwall holing at Wambo colliery, in NSW, into an existing main headings roadway was designed and undertaken based on both an engineering and risk-based application of the worldwide holing database. The mine and seam in question had no history of previous attempted holings and there was a provision to install Huesker-type mesh across a portion of the face length leading up to holing.
The actual holing was controlled by a specific management plan and was completed without incident or any undue concern during the process as to the outcome. Essentially, the knowledge with the database allowed the decision to proceed to be made with confidence and ensured that residual business risks associated with the holing were kept within acceptable levels. As a result, the mine site gained significant economic benefit and maintained a prudent business risk profile throughout the entire process.
Originally published in the March 2001 edition of Australia's Longwalls.