The primary method of developing the mine workings of a coal seam is to adopt what is known as the bord and pillar system. This involves driving a group of development roadways, comprised of both headings and cut throughs to form pillars of coal nominally 60 to100 metres in size. See Fig.1.
The headings and cut throughs, also known as first workings, are driven to a height dictated by the thickness of the coal seam. The width of these openings is determined by a number of factors, but mainly to dimensions that provide for adequate support of the roof, whilst making allowance to accommodate the passage of the men, materials and machinery required to support the mining operations at the working face, and for the belt conveyor system used to deliver the coal mined to the surface.
The cut throughs’ main function is to provide a passage to control the flow of air between the headings, and the movement of the mining equipment in the immediate face area during the mining operation. As the mining operations advance, stoppings are erected in the cut-throughs to direct the course of the ventilating air from the surface, through the intake airways up to the working face/s and back through the return airways to the mine exhaust fan located at the surface.
As the main development headings advance, District development headings and cut throughs are set off at right angles to create access and ventilation roadways for the establishment of individual working Panels. Each Panel is set up with the necessary mining equipment, manpower and materials to work a particular area of the mine, and there will normally be a number of these working Panels each contributing to the overall daily production of the mine. Once a Panel has reached the predetermined limit of its first workings, pillar extraction can commence.
A background to the extraction of coal by machines in pillars.
The extraction of pillars is part and parcel of the bord and pillar system and has been practiced since the hand mining of seams commenced and continues to this day. When mechanised mining began in the 1930s the Miners’ Federation strenuously resisted the extraction of pillars using machines (machines in pillars) and it was not until 1954 that the Mines Department and the Federation gave the approval (in the former case) and agreement (in the latter case) to the use machines in pillars. As might be expected pillar extraction using machines had a shaky and tentative start industrially, and the tried and proven open ended lift system of extraction was adopted, it is assumed, as something of a compromise, acceptable at the time by both mine management and the miners.
The 1950s saw the arrival of the continuous mining machine in Australia and it is interesting to note that the first of these machines to be purchased by AIS Collieries and placed in regular production arrived at Wongawilli Colliery in 1952. The traditional open-ended lift system, practised throughout the hand-mining era, continued for some time after 1954, using continuous miners.
Pillar Extraction Systems
Over time a number of pillar extraction systems have been developed to meet the needs of particular seam conditions with varying degrees of success.
The principal objectives to be satisfied in the extraction of pillars are:
(a) to commence the extraction of the pillars as soon as possible after the first workings have been completed.
(b) to carry out the mining operations of extracting the pillars, with safety.
(c) to achieve the maximum recovery of the coal available in each pillar, at an acceptable cost/tonne.
A fundamental principle common to all pillar extraction systems is to remove as much of the pillar as rapidly as possible, leaving behind only remnants of coal in the stooks that can be expected to be “crushed out” when the goaf falls. Where the size of the remnant pillar or stook left unmined is large enough to prevent the goaf falling, that pocket of standing goaf leads to increased roof abutment stresses being projected from the goaf to the roof above the adjacent pillars. These stresses can result in premature failure of the fender, or the roof or both, in the surrounding workings.
A progressive and controlled collapsing of the exposed roof in the goaf area, created as the pillars are extracted, is an essential part of the extraction process. Failure to achieve this result can, in extreme circumstances, induce a “creep,” or create a “windblast”, when a large area of goaf suddenly collapses, creating a major displacement of air, with the potential to injure nearby workers and damage equipment.
The Open Ended Lift System
The open-ended lift pillar extraction system is what could be called, the “traditional method” of pillar extraction. See Fig.2.
The system involves the creation of open lifts, by mining along the goaf side of the pillar being extracted, using a continuous mining machine. Whilst the system is simple in concept, whenever localised poor roof or floor conditions are encountered, or the continuous miner operator departs from the planned extraction sequence for some reason, the mining of subsequent lifts, and in extreme cases, adjacent pillars, can be jeopardised. One of the advantages of the open ended lift system, claimed by the continuous miner operator and crew, was that it allowed them to observe the behaviour of the goaf at close quarters This enabled them to rapidly retreat from the lift, if the “sounds and signs” emanating from the supporting wooden props and bars, signalled an impending fall of roof
This system has a number of inherent disadvantages; These included, the overall safety of the operation, the opportunity for the operator to adopt a poorly ordered system of extraction, and the relatively low level of the % of coal recovered from the pillars.
Dealing with the safety aspects, the length of each lift meant that the operator and crew were “in the lift” for a considerable amount of time, with the roof above, cantilevered to the goaf edge, and dependent for roof support on props placed along the edge of the goaf. When the lift was completed, and open to the goaf over a large area, unpredictable roof behaviour could occur resulting in a possible roof fall, or roof material from the goaf, flushing into the lift.
Any of these conditions could give rise to the rapid abandonment of the lift as noted above along with the remains of that pillar in large or small stooks, or in an extreme case the development of a creep where crushing of the adjacent pillars by the roof could result in large areas of standing pillars being lost to any further extraction. The open-ended lift system realises an approximate 45% recovery of the coal available. For the above reasons the open-ended lift system of pillar extraction is no longer widely practiced.
The Split and Lift System
This system of pillar extraction was adopted to remove some of the problems encountered when using the open-ended lift system to extract pillars with Continuous Miners.
This system can reduce the length of each lift so that the continuous miner crew is only exposed to a reduced span of cantilevered roof and time spent in each lift. This system utilises fenders as they are called. These fenders are created when a split is driven through the pillar leaving a strip of coal of about 3 metres in width, and separating the split from the goaf edge. The use of the fender avoids exposing the miner operator and crew to the open side or goaf edge.
Difficulties can arise from time to time when an incorrect choice of fender width is made, and this can lead to the fender collapsing when the goaf caves and the goaf material and fender coal flushing into the split being mined; or to heavy roof conditions developing in the split as a result of the fender being unable to adequately support the roof leading to heavy roof conditions in the lift, and in some cases, the roof falling.
An equally serious shortcoming is that the presence of the fender can create a false sense of security in the minds of the operators by not exposing them to the goaf edge, thus preventing them from being able to observe the behaviour of the goaf at first hand. A final matter is that when a split is driven through the pillar, it is usually driven through an area in the pillar where it erodes the load-bearing capacity of the pillar, at its core. The roof stress levels are very high in this area and total failure of the roof can occur, in the split, or at the point where the second split is turned away, as a result of creating an increased area of exposed roof, to make the turnout.
This system could, therefore, demand a level of roof support equivalent to the first working of the pillars and as such was not considered an economical proposition, and included some unpredictable safety hazards. The split and lift system is no longer used in seams of similar thickness and cover, found on the South Coast of NSW.
The Wongawilli System
The disadvantages of the lift and fender and split and lift systems of pillar extraction led to the need to come up with a system that would minimise those problems, increase the fraction of coal recovered and improve the overall safety and economic aspects of the above extraction systems. See Fig.4.
Such a system was developed and adopted at Wongawilli Colliery in 1958. As a result of its success it has become widely known as the Wongawilli System and having proven very successful in the Illawarra area, the system is now widely used elsewhere in Australia and overseas.
This system is a variation of lift and fender with the following advantages as it
(a) offers significant improvements in the overall safety in the extraction of pillars.
(b) increases the % of the coal recovered from pillars from 45% to 90% where the system works well.
(c) minimises the solids development needed to create an area for extraction, and maximises the time spent in extracting the pillars
(d) requires a series of extraction sequences that are simple, repetitive and unable to be readily varied and
(e) reduces, and concentrates the activities required in the extraction process, and reduces the need for frequent movement of the continuous miner and exposure of the machine and crew to the exposed goaf area.
Details of the mining sequences in the Wongawilli System are as shown in Fig.4. The typical block to be extracted by the Wongawilli system is developed by driving two roadways, nominally 30 metres apart, to create pillars having centre-to-centre roadway dimensions of 30 x 60metres. These first workings are driven to the predetermined limits of the Panel, usually about 1000 metres. They are driven in virgin coal and are bounded on one side by a block of coal approximately 60 to 100 metres wide, and 1000 metres in length.
The choice of the width of the block to be mined takes into account the need to choose a width that will provide an optimum performance from the shuttle car, in terms of total tramming distance, and the standing time imposed on the continuous miner whilst the shuttle car travels to the conveyor to discharge its load and return to the miner.
The method chosen for ventilating the working faces during pillar extraction is a very important consideration. The final choice has to include factors such as the ability to effectively ventilate the working face with brattice under the influence of the main mine fan, or the need to employ an auxiliary fan operating in support of the main mine fan to provide adequate ventilation. The brattice system is preferred because of its simplicity, but it is not always suitable and the auxiliary fan used in the first workings of the Panel may have to remain in service.
The first workings of the Panel are ventilated using the intake and return headings, with an auxiliary fan supporting the main mine fan located on the surface. At the extreme end, or inbye point of those heading/s they are holed into an adjacent return airway that will serve to ventilate the goaf area during pillar extraction, by allowing ventilating air to bleed off the Panel ventilation system over the goaf and into the returns.
To facilitate this practice the inbye end pillars in the Panel are deliberately left standing. (Fig. 4). This is done to enable the headings and cut throughs associated with those pillars to remain standing and provide bleeder headings that will enable air to flow over the goaf and bleed off any gas, that can, and in most cases will, accumulate in the goaf. As the area of the goaf increases it is essential that this path is not restricted or become tight as it is called in mining terms.
Pillar extraction commences by driving a split in the pillar from a point about 10 metres outbye of the second last cut through, to the extremities of the virgin block to create a fender of approximately10 metres in thickness. Factors considered in determining the width of the fender include the strength of the coal, the height of the seam and the nature of the adjacent strata. The width of the fender, and the care that must be taken to ensure that the fender is not increased or decreased in width as the split is driven, are crucial to controlling the behaviour of the roof in the split, each lift and the goaf.
The extraction of the fender proceeds by systematically lifting off the coal in the fender and holing into the goaf at the inbye end of the split, to be followed by tramming the continuous miner back along the split and creating the next holing into the goaf, leaving behind as little coal as possible, in what are known as stooks. The coal as mined, is loaded into the shuttle car, and trammed to the main heading to be discharged on to the Panel belt conveyor.
Two shuttle cars are normally used in the development of multi heading Panels, however this is not possible in the single heading layout used in the pillar extraction phase. The use of one car does create a delay in mining, when compared to using two cars, as the continuous miner has to await the return of the shuttle car before mining of the split can continue. Fortunately the extent of this delay diminishes as the tramming distance of the shuttle car run reduces as the miner retreats back to main headings.
As the mining of the fender is carried out, wooden breaker props, or mobile hydraulically powered breaker line supports are set on the goaf edge of each lift to exercise control over the behaviour of the goaf and to provide ongoing roof support in the lift being mined.
It is essential for proper roof control that the goaf does collapse soon after the fender is mined. Where the goaf “hangs back” as it is called in mining jargon, and is reluctant to fall, and wooden props, as opposed to mobile hydraulically powered breaker line supports have been used as breaker props on the goaf edge, the deliberate removal of the wooden props is carried out to induce a goaf fall.
As a general rule the goaf roof should progressively collapse two to three lifts back from the lift being mined. As can be seen in Fig.4, when the extraction of the fender finds the continuous miner back at the pillars formed in creating the Panel entries, those pillars are extracted in the same orderly fashion before moving outbye to create the next split.
The Wongawilli system has many advantages when compared to the other systems of pillar extraction using continuous miners, and is adaptable to a wide range of seam conditions. As can be seen from Fig.4 the system provides a safe concentrated mining operation. In this the continuous miner because of its overall length only partly intrudes into the lift to complete the holing of the lift, and all of the ventilating air in the Panel is able to be directed along the fender, over the machine, and into the goaf.
The system has distinct economic benefits, in being able to limit the first working costs involved in developing the Panel, and providing a means of rapidly extracting the pillars at an acceptable cost/tonne. The % coal recovery from pillars mined using this system is as high as 90% under most conditions. In summary the system has many advantages and few disadvantages, as the repetitive and concentrated nature of the mining operation imposes a discipline that strictly limits the opportunity for major departures.
The approval by the Mines Department and acceptance by the Miners’ Federation in 1954, allowing the use of machines in pillars brought with it many advantages and as might be expected some problems. Not the least of these were the industrial implications of adopting a system that had been strenuously opposed for almost two decades by the Miners Federation and the need to continue to use the traditional system of pillar extraction used in hand mining.Whilst the traditional methods of developing bord and pillar workings required some changes to accommodate the continuous miner, the extraction of pillars using systems from the hand mining era soon proved to be not only unsuitable but also unsafe and uneconomical.
The development of the continuous miner and shuttle car in the USA and the eager acceptance of these machines by the mine operators in this country did however provide the opportunity to develop bord and pillar panels and extract pillars at an acceptable cost/tonne. As time went on, the wider entries required by the continuous miner and the increasing depths of cover over some seams, produced difficult roof conditions that demanded an improved system of pillar extraction. The development of the Wongawilli System answered that need in the South Coast mines and elsewhere, and effectively prolonged the use of the continuous miner in seams being worked under 5-600 metres of cover.
In the late 1960s the introduction of Longwall Retreat mining in seam conditions considered no longer suitable for mining with continuous miners, offered a means of carrying out pillar extraction on a grand scale, but at a considerable capital cost. Ultimately the inability to economically mine some areas with continuous miners lead to a much wider application of the Longwall Retreat system. Ironically, the Longwall system soon advanced from being the only system considered suitable for mining under difficult roof conditions, to being the preferred system of extraction under any conditions.
Whilst the Wongawilli System has not outlived its usefulness in the majority of mines, the extraction of pillars using continuous miners has been overwhelmed by the Longwall system. The emphasis now is on the development and application of machines that have the ability to develop and make ready, Longwall panels at a speed that matches the ability of the Longwall equipment to very quickly extract a Longwall Panel and move on to the next. It remains, however, that in those mines where pillar extraction using continuous miners is still an option the Wongawilli System is preferred.
The author has used information compiled from his own experience and by reference to technical papers prepared by the principal authors and their associates, in the following publications:
(1) Australasian Institute of Mining & Metallurgy Monograph 12 – Australasian Coal Mining Practice – 1993
(2) Australasian Institute of Mining & Metallurgy Monograph 21 – History of Coal Mining in Australia – 1993