Optimisation of Longwall Mining Layouts under Massive Strata Conditions and Management of the Associated Safety and Ground Control Problems

Industry concern regarding the potential hazards of longwall extraction beneath massive strata units is not just a recent phenomenon. It can be traced back to the initial attempts at longwall extraction in Australia in the early 1960's and has been in part linked to the continued increase in powered support rating and set:yield ratios since that time.

The potential operational consequences of massive strata have been clearly demonstrated more recently by the "catastrophic" series of face line roof falls that occurred during the extraction of LW5 at Newstan as well as windblast problems at several longwall mines in the Newcastle area.

At the time of writing, it is estimated that of the order of two-thirds of the established or planned longwall mines in Australia are either currently influenced to some degree by massive strata related hazards or have the potential to be similarly affected in future mining areas. Clearly, the likely behaviour and operational influence of massive strata units during longwall extraction should be a primary consideration of both mine planners and operators alike.

During the past 10 years, the outcomes of a number of intensive monitoring studies relating to overburden behaviour above longwall extraction panels have been published, some of which relate directly to massive strata behaviour and associated mining hazards. This is a valuable resource.

This project has utilised the findings of those research type monitoring studies and linked them to general mine site experiences from other longwall mines in Australia in an attempt to define a model for massive strata that deals with both technical behavioural aspects as well as operational outcomes. In this sense, it should be viewed as being similar in nature to the various Holla subsidence curves or the ALPS/ALTS chain pillar design methods, for example. Neither define the technical aspects of the problem in minute detail, but simply ensure that the essential or primary technical elements at mine site scale are catered for (eg w/H ratio in subsidence, Coal Mine Roof Rating in chain pillar design etc.) and summarily linked to the operational issue of significance. Providing that such models are based on a credible database of relevant information, the use of empirically derived techniques in mine design and operational management has a proven track-record world-wide.

It is considered that the database of information upon which the project outcomes are based is credible, recognising the fact that all such mining databases contain relatively few data points (as compared to medical statistics for example) such that engineering judgement is required in dealing with residual uncertainties.

The specific outcomes of this project are as follows :

• the establishment of a massive strata weighting model and empirical classification relating strata unit thickness and panel width to a wide variety of operational outcomes (good and bad) - see Figure A.
• a technical explanation and justification for the weighting model based on the outcomes of intensive monitoring studies from various mine sites which have been undertaken and reported by several different groups in addition to the authors of this report. This includes the major outcome that the key behavioural aspect of massive strata is not just whether it will cave or not, but the specific manner in which it caves. The uncontrolled rotation of caving massive strata units about the longwall face has been identified as a key influence on ground stability on the face itself. The other major finding is the positive linking of chain pillar cut-throughs with surges in overburden weighting effects and the occurrence of several different mining hazards (ie face roof falls / rapid convergence, windblasts, floor fracturing and gas migration). This latter aspect was a totally unexpected outcome of the project.
• a statement of the current knowledge as to the occurrences of windblasts during longwall extraction and the ability of real-time monitoring systems to predict their occurrence in advance. The project has concentrated on the use of specific micro-seismic waveform characteristics in improving prediction reliability, as well as an evaluation of the use of real-time displacement monitoring within the mine itself. Both resulted in positive outcomes with the potential for more rigorous testing within an operational windblast management plan. The recent application of hydro-fracturing to windblast management by being able to produce goaf falls "on-demand" and the resulting improvements in safety is fully recognised.
• the potential ground control hazards resulting from differential compaction effects around "channel edges" has again been recognised and stated, based on its continued occurrence at various mine sites. Mine site strata management should be aware of its potential and accommodate it accordingly.
• the key role of massive strata (in both a caved and bridging state) on pillar loading (chain and barrier pillars) adjacent to longwall extraction areas has been better defined, based largely on the recently published findings of the "ALPS/ALTS" ACARP Project (C6036). Similarly, a limitation within the associated monitoring database has been identified relating to the full caving of massive strata in close proximity to the extraction horizon and its impact upon gate road serviceability. Recent adverse and unexpected experiences in longwall gate roads at more than one mine site would suggest that this is a subject area worthy of further research.

The project outcomes are by no means a fully comprehensive model for massive strata issues and should not be taken as such. At best, they can be stated to give a reasonable approximation of some of the likely mining outcomes and associated hazards should longwall extraction take place in a range of massive strata conditions. Nevertheless, the ability to make such approximate predictions is invaluable and can be combined with risk management to good effect in such applications as mine feasibility studies, detailed mine planning, the development of mine site strata management plans and also as a reality check of more complex numerical simulation outcomes.