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• W173746345 abstract "The fundamentals concepts of roof support are similar between coal mining, metal mining and tunnelling and yet there are different design approaches, different hardware, and different levels of site investigation. This should not be the case and this paper discusses the key geotechnical issues that should underlie all roof support design -the need to reinforce defects. A design method to build a beam in laminated rock under high horizontal stresses is presented. The method allows for changing rock strengths, defect properties and varying horizontal stresses. At one level the design can be implemented with design charts and tables. Lessons learnt during the implementation of the designs at 5 different mine sites are discussed. MINING ISN'T DIFFERENT If mining is thought to be different -then coal mining must be different again! Why is it that underground metalliferous miners cannot believe the amount of roof support that coal miners install ? Why is coal mine roof support hardware different from that sold into metalliferous mines ? What do civil engineers mean when they talk about dowels and bolts ? The aim of this paper is to tackle some of these questions while, at the same time, putting forward the case for a new (and at the same time a very old) approach to specifying roof support in coal mines. Coal mine engineers use the same theories of soil and rock mechanics as their colleagues in civil engineering -be they foundation, slope stability , or tunnelling engineers. The materials in which they design are similar, the laws of physics are the same and the demands for safe and cost-efficient outcomes are the same. However there is a difference between the way coal mine engineers and civil engineers practise their professions. The differences do not relate to the different geological materials but appear to be steeped in tradition and history .Civil engineers are taught soil mechanics with its strong focus on elastic theory and the use of failure models and factors of safety (limit equilibrium), mining engineers are taught rock mechanics with a focus on empirical methods and back analysis. Civil structures are capital intensive, and typically of a scale of tens to several hundreds of metres. The designs lack flexibility (restricted to a specific site) and have extremely tight specifications regarding stability, settlement, and serviceability .Even a domestic dwelling which may cost $100,000 to build requires a geotechnical assessment which costs in the order of $500. A major city development can cost $50-$100 million and require $30,000-$450,000 in 1 Partners International Pty .Ltd. U nanderra COAL98 Conference WoUongong 18 .20 February 1998 188 geotechnical design. In the civil arena, the owners/operators are rarely the builder so they rely on project managers, consultants, and contractors. As a design exercise, a modern longwall mine is no different. It is certainly capital intensive (say greater than $250 million), and has tight specifications -but this time on productivity and rate of return on capital. It does have a greater amount of flexibility in the design which allows it to develop with~)ut the areal density (holes/km2) of site investigation. The areal density of the site investigations can also be less as a result of the homogenisation that rock diagenesis tends to put over the complexity of sediments and soils. In contrast to th(~ civil venture, the mining company is typically the designer, builder, operator and owner. There is no reason why the same relative level of expenditure on geotechnical design should not apply to longwall mines -how many new mining ventures spent $150,000-$1 million specifically on geotechnical issues ? And if they did, did they get good value ? One of the considerations that appear to be lacking in coal mine geolmechanics is the appreciation of the role of defects in the behaviour of rocks and rock masses. Defects are the natural vleaknesses in rocks; in coal measures the defects are bedding surfaces, joints, coal cleat, greasy-backs etc. Given the relatively high horizontal stresses that characterise mining excavations in Australia and elsewhere, the most important defects are bedding. For example, mudstones, sandstones, and conglomerates all have a similar range of unconfined compressive stJengths but it is known that sandstones give better roof than mudstones, and conglomerates can span longwall panels and delay caving -the difference is in the frequency of bedding defects. In most cases, rock failure near an underground opening or even 'on a highwall is expressed as movement on existing defects. Until recently, coal mine geotechnical engineers did not have access to design tools to assess the role of bedding defects in controlling roof reinforcement. The previous design tools based on finite element and finite difference computer codes (such as FLAC) assumed the rock is a continuum (no open defects and the measured rock strengths reduced to account for most of the closed defects) and as a result the focus was on estimating rock strengths from laboratory testing and geophysical logs such as the sonic tool. Rock defects are m4~elled explicitly in discrete elements codes such as UDEC and these codes allow the defects to open and close. Discrete element codes are extremely numerically intensive and have not yet become commonplace in the industry. Barton (1996) gives a very good example of how UDEC and FLAC differ in the results of analysis of jointed rock masses. If defects are to be included in a design method, the key parameter is their location in the rock mass and their shear strength. Since 1993 Coffey Partners International Pty Ltd has approached the design of roof reinforcement in bedded strata from an almost traditional civil engineering viewpoint (Seedsman and Lo~~an, 1996). Building on the results of a number of excavations in the Hawkesbury Sandstone (Pells, Poulos & Best, 1991), a design approach based on beam building has been developed. The method recognises the role of bedding defects and seeks to install roof bolts so as to prevent the onset of movements along the defects. In this way the impact of the defects on the rock mass performance is negated. Analytical techniques, instead of numerical techniques, have been used so that the design focus can remain on the critical role of variability of the rock strata -the analytical techniques allow rapid redesigns and sensitivity studies. It is these design tools that have allowed new insights into coal mine roof reinforcement and underwritten the safety , productivity and cost improvements that are being achieved. COAL98 Conference Wollongong 1820 February 1998 189 BmLDING A BEAM IN COAL N[EASURE ROCKS Introduction There are specific steps in the application of beam theory to specify rock reinforcement in bedded strata. The general concept is not new but the ability to include the consideration of horil:ontal stresses has not been readily available in the past. Field of application The method is applicable to the building of a structural beam that can span a given opening. It is stressed that in thinly bedded roof there may be failure of scats between the roof bolts -this may mean that a strap or mesh is needed. The method does not specify strap or mesh requirements -this is done through a qualitative assessment of the immediate roof skin. Note that horizontal stresses in the immediate roof may approach or ex(;eed the compressive strength of the roof beam. In such a circumstance, beam building theory does not have application. The possible onset of failure can be recognised by comparing the measured unconfined compressive strength with the pre:sumed or measured horizontal stress acting across the roadway. The formation of a development roadway can increase the horizontal stress acting in the immediate roof by 20% to 30%. There is an additional concentration of stress on retre~lt of a longwall face (Mathews, Nemcik, & Gale, 1992). If compressive failure is not indicated, then the design approach is to maintain the pre-failure (ie elastic) behaviour of the roof as long as possible by preventing delamination." @default.
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• W173746345 title "A safe way to reduce roof support costs and improve safety and productivity" @default.
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