At the recent subsidence management conference organised by The Mine Subsidence Technological Society (MSTS), Daryl Kay, an associate at Mine Subsidence Engineering Consultants presented his paper Challenges for Assessment of Tilt Impacts due to Mining a Series of Longwalls.
According to Kay, an important question is to define what is an unserviceable, and therefore unacceptable level of tilt. Pre-mining tilts exist in structures but are difficult to measure.
A method to assess pre-mining tilt in buildings is being developed by Mine Subsidence Engineering Consultants, in conjunction with the NSW Department of Primary Industries and the Mine Subsidence Board. It is worth noting that current research into the impacts of subsidence on building structures has found few serviceability issues so far.
A number of features need to be considered when predicting tilts, and these are related to the shape of the observed subsidence profiles, Kay said. Key parameters related to surface subsidence include seam extraction height, panel width, pillar width and depth of cover.
“When studying incremental subsidence profiles over a series of longwalls, there are a number of major features that become apparent. The first feature is that the incremental subsidence and tilt above the second and subsequent longwalls in a series of sub-critical longwalls are generally greater in amplitude than the subsidence and tilt above the first longwall or a single panel,” Kay said in the paper.
In one example subsidence and tilt due to the first longwall were smaller than for subsequent longwalls. As the width-to-depth ratio increases the difference in magnitude between the first and subsequent longwalls increases.
A second feature is when the subsidence contributed by a panel in a series of longwalls is examined individually the shape of the incremental subsidence profile is not symmetrical. Typically, there is more subsidence over the tailgate side of the longwall than the maingate side.
A third feature is the point of maximum subsidence is not centred over the longwall panel.
A fourth feature is subsidence profiles for super-critical panels tend to flatten out at the centre of the panel, which is particularly important when developing predicted subsidence profiles. Kay said if a mathematical formula is adopted for a predicted subsidence profile, it must be flexible enough to allow for this feature.
A fifth factor is subsidence can be affected significantly by the geological characteristics of the overburden. In one example, the presence of sandstone channels inhibited subsidence with only 60% of subsidence occurring as the longwall was extracted. The remaining 40% occurred as the following longwall was mined.
In terms of predicting mining-induced tilts, several prediction methods exist, one of which is the Incremental Profile Method (IPM) developed by Mine Subsidence Engineering Consultants directors Arthur Waddington and Don Kay.
This method differs from many other methods in that it provides an incremental view of subsidence. Most other methods only provide a prediction of total subsidence which is fine if the subsidence profile is symmetrical. However, if conditions change – wider longwall panels or changing geology – the predictions have to be extrapolated from an empirical database.
Being able to identify incremental subsidence is crucial in relation to sensitive surface structures where more finetuned predictions are required than a total prediction. This is what the IPM has been proven to do. It can also be used to generate the total subsidence by adding together the incremental profiles.
Another advantage of the method is that it takes into account the variations in depth of cover and seam thickness across each panel, rather than simply taking average figures. It allows the unique geological characteristics of the overburden to be incorporated into the prediction.
The method has been shown to provide consistently good tilt predictions throughout the NSW Coalfields. It has been peer reviewed by engineers and adopted in several Commissions of Enquiry in NSW related to subsidence.
Kay said the IPM model can be applied quickly and, he believes, is one of the most accurate methods available.
In assessing tilt impacts, Kay said because of the potentially high costs it was important to assess the extent of such problems before mining took place as part of the Subsidence Management Plan process.
“The specific assessment of tilt impact on a structure should generally be approached on a case by case basis. Tilts that are related to water drainage, for example, may be compared to appropriate Australian Standards. This exercise has been carried out for a sewerage system in the Southern Coalfield,” Kay said.
In the case of buildings, a method for assessing tilt impacts was developed by Mine Subsidence Engineering Consultants and has been accepted by a number of Commissions of Inquiry in NSW.
