Just mention the phrase “pre-driven recovery roadway” and most longwall mining engineers immediately start to fill out their annual leave forms.
Everyone knows the story of LW6 at Pacific (now Teralba) colliery in the mid?1980s and the months of hard graft that were required to recover the face equipment. It is legendary in the annals of the Australian coal industry and this, more than any other factor, has undoubtedly led to the perception that pre-driven recovery roadways are very high risk and hence, unacceptable in terms of business risk profile.
But is there more to it than meets the eye?
Up until recently, pre-driven roadway holings had disappeared from the coal industry when in 1995, LW5 at Newstan colliery had to be recovered approximately 20m short of its pre-driven roadway due to periodic weighting difficulties on the face. This was not classed as a failed holing by the mine but signalled the end of its use at Newstan which up until that time had pioneered the use of the method and successfully holed some 12 pre-driven roadways.
In more recent times, the use of rapid face bolters and Huesker type mesh during face recoveries have made pre-driven roadways possibly even less attractive from a business perspective. The use of rapid face bolters has significantly decreased the time taken to bolt-up the longwall face (which used to be a primary reason in favour of a pre-driven roadway).
Similarly, the use of Huesker type mesh has become an almost standard part of longwall face recoveries for safety reasons and at face value, is inconsistent with the use of a pre-driven roadway whereby maintaining normal production through to holing is essential.
Nonetheless, other potential applications of the method have recently become apparent with significant financial benefits being realised if successful holings are achieved. For example, both Cordeaux and Wambo collieries in New South Wales have successfully used the method to take the longwall face either through or around existing workings and another mine near Wollongong is currently considering its use in conjunction with a hard dyke traversing the next few longwall blocks.
It would appear then that even though the use of pre?driven roadways for longwall recoveries is less attractive than it was a decade ago, it still has a potential place in longwall mining, albeit in innovative one?off applications. Hence, it is worth considering our “state-of-the-art” knowledge of factors that lead to the success or failures of pre-driven roadway holings, and an informed assessment of the true risk levels associated with such holings.
One point to think about is the perception of pre?driven roadways as high risk. Remembering that risk equals probability times consequence, the term high risk is often mistakenly used for high consequence. Certainly, a failed pre-driven roadway holing is undoubtedly a high consequence outcome, but that does not necessarily render it high risk as the probability of failure occurring must also be factored in.
In this regard, it is noted that there are other high consequence hazards in coal mining (for example, explosions and fires) that are almost universally accepted based on risk minimisation through the use of controls and barriers. Pre-driven roadway holings are no different except that the conditions of success and the effectiveness of controls have been poorly documented in the past. Spectacular failed holings in the past need to be put into their full and proper context.
In 1997-98, Strata Engineering and the US Bureau of Mines compiled a worldwide database of attempted longwall holings into pre-driven roadways and analysed the outcomes in a statistical manner to assess what lessons could be learnt. The results were a surprise to all involved and were published in the US at a major strata control conference in 1998. The database contained details of some 130 attempted holings from 17 longwall mines in the USA, Australia and South Africa. Of the 130 cases, only 11 have been associated with problems during holing and consequent losses. As such, statistically, the use of a pre-driven recovery roadway is not pre?disposed to failure and the associated likelihood of success in a general sense is at least 92% (ie a failure rate of about 1 in 13) providing specific lessons from the failed cases are given due consideration as part of a site specific design study.
In terms of the failed cases, two distinct mechanisms are apparent, these being a major roof fall in the roadway itself, or a weighting failure whereby the overburden causes significant roof to floor convergence thus preventing the operation and advance of the longwall equipment. Of the 11 failed cases, five are roadway roof falls and six are weighting failures. In terms of the 11 failed cases, there are a number of common (but not unique) factors that can be identified and are therefore, potentially key considerations in assessing proposed pre-driven roadway holings. They are:
* The weighting failures had CMRR (Coal Mine Roof Rating for the bolted horizon) values of
* The weighting failures had installed standing support densities of
* The weighting failures had installed primary roof support capacities (as given by an RDI value which is a measure of the installed roof support) of 0.15-0.9MPam, with an average of 0.59MPam. This compares with the full database of a range of 0.09-1.83MPam, with an average of 0.44MPam.
* The roof fall failures had CMRR values in the range of 40-68, with an average of 56.
* The roof fall failures had installed standing support densities in the range of 0-1.2MPa, with an average of 0.38MPa.
* The roof fall failures had RDI values of between 0.15-0.33MPam with an average of 0.24MPam. This compares with the full database of a range of 0.09-1.83MPam, with an average of 0.44MPam.
* Three of the six weighting failures and two of the four roof fall failures (ie, 50% in both cases) were associated with slow mining leading up to holing. At face value, this could be taken to indicate that slow mining rate is an overriding factor in failed holings. However, upon a more detailed examination of the specific case histories, it is apparent that it may not necessarily be the case, according to the level of installed primary and secondary support.
Originally published in the March 2001 edition of Australia's Longwalls.
