As bord and pillar workings are developed roof support becomes a major consideration and operating cost item.
Support requirements vary considerably and are dependent on the physical conditions in the seam, which are often depth dependent, and the length of time that the workings are required to be serviceable.
The most common support problem in bord and pillar workings is at the intersections of headings and cutthroughs. The exposed roof spans are greatest at these points and therefore the intensity of roof support must also be greatest there if roof failures are to be avoided.
In difficult ground conditions where the workings’ sole purpose is to develop main or auxiliary access roads for later extraction operations such as longwalls, a method of minimising the intersection problem is to extend the length of the pillars to reduce the number of intersections and/or to stagger the cutthroughs to make all intersections threeway rather than full four-way holings
To ensure long term stability of the strata overlying the bord and pillar workings, if this is required, pillar dimensions are critical. The size of a stable pillar sufficient to resist creep or failure and to support the general strata above it is dependent on the strength and nature of the coal and surrounding strata, the height of the pillar, discontinuties such as cleats and mining induced fracture planes in the pillar, the pillar’s width and length, the width of the headings defining its boundary and the three dimensional stress field existent at the pillar.
Overseas, particularly in RSA, empirical formulae have been developed to estimate the size of stable pillars. It is not generally appropriate to apply these to Australian conditions because of differences in the many parameters listed above from seam to seam and district to district. If doubt exists about pillar stability then a full rock mechanics investigation and analysis for the particular site conditions is required.
In some Australian states legislation dictates the size of pillars and width of headings as a function of depth. This legislation is not firmly based on rock mechanics principles and may also be quite wrong.
In very general terms it would appear that where headings are driven 6 m wide and no higher than 4 m, pillar centre dimensions of around 50 m x 50 m are usually stable to depths of 600 m in Australian practice.
During 1991-92 investigations took place through an Australian Mineral Industry Research Association (AMIRA) project to attempt to develop more appropriate criteria for the design of pillars.
For the support of the roof immediately above the headings in bord and pillar workings legislation generally requires that some form of systematic support is to be installed. The intensity of the support varies from almost nothing in the case of shallow mines having hard conglomerate or massive sandstone roof to closely spaced steel cross supports or arches supplemented with bolts where the workings are deep and the roof is weak laminite, mudstone or coal.
Timber supports both as props and cross bars, either full round or split, are still in use as a means of roof support but the trend to wider use of roof bolts, with or without steel straps is general.
The advantages of all-timber support lies largely in its yielding capacity which gives warning of convergence but its ultimate load carrying capacity is much less than that of steel supports or correctly designed roof bolting patterns. Additionally timber is becoming scarcer and more expensive to obtain. It is very bulky material to convey to and handle at the face and it is flammable and subject to fungal and insect attack. The extensive use of timber in main long term roadways inevitably involves on-going maintenance costs to replace defective supports and creates the possibility of widespread roof failures. As such it is rapidly falling from favour amongst operators.
Steel sections such as rolled steel beams, universal sections, reject rail sections, box beams and more elaborate pressed sections have been widely used as cross beam supports where roof conditions require more intensive support than timber alone can afford or bolting is ineffective. These are held to the roof either by timber props or steel legs and because of their weight they are usually provided with a safety bolt and saddle system to prevent them from falling should the props be dislodged accidentally. While they offer greater support than timber cross beams they are often very heavy to handle manually and increase roof support delay times and they do not always offer audible warning of impending failure. They are also more expensive than timber.
Steel arches are used only in the most difficult of conditions for permanent roadways because of their cost and difficulty of erection. The actual cost of an arch and its associated parts may be up to $2000 and may take two hours or more to properly install so that support costs are very high and production rates are low when they are used. Moreover they are difficult to match with continuous miners which cut out for squared working face sections rather than profiled cross sections. Arches are better matched to boom type heading machines which have profiling capability and then only in critical headings. In general arches have no place in normal bord and pillar workings.
With the improvement in roof bolting technology arising from speedier and more reliable bolting machines, either hand held or integrally mounted on continuous miners, and from resin anchoring in place of mechanical anchoring of the bolts there has been a general move towards the wider use of roof bolts as the main support for bord and pillar headings. Where the roof is friable or liable to weathering deterioration, the bolts are usually placed through light steel straps or perhaps steel mesh sheets to give a measure of cross support coverage. A popular bolt length in use is 1.8 m but depending on particular roof strata longer or shorter bolts may be more suitable for the conditions.Most bolting patterns have been derived by trial and error experiments but initial design guidance is possible from mathematical or physical modelling of the strata and working layout. The advantages of support by bolts is that it provides positive support and it is usually permanent. Quantities of support materials are much reduced, clearances and airway resistance characteristics of the resultant roadways are improved, and support time delays may be significantly reduced. There is usually no need to augment bolting patterns with side prop supports but timber props are sometimes retained to enable the erection of ventilation brattice and cables and to give warning of convergence, for the one defect with bolting is that little warning may be given if roof failure is imminent. Nevertheless roof support by means of bolt patterns only with no auxiliary timber props is becoming commonplace.