The build-up of methane gas behind the coal seam at the coal face, thereby causing “outbursts”, i.e. a sudden discharge of coal into the workplace, was an ever-present danger, particularly in the anthracite section of the South Wales Coalfield. The following are two reports from the NUM South Wales Area Safety Department the first is dated 1959, and the second is 1985.
“Outbursts” of Coal and Firedamp
This phenomenon in which high volumes of gas and finely powdered coal are ejected with violence from the coal face is fortunately limited to a small area of the anthracite coalfield. It is not experienced elsewhere in the British Isles but is a feature of some parts of the French and Belgian Coalfields, and is also a problem in some Russian mines.
The First Outburst
The occurrences at Great Mountain Colliery during 1958 have been of special importance because they have occasioned a changed approach to the problem of outbursts in South Wales. The first outburst of coal and firedamp took place on the afternoon of Tuesday, 25th February 1958. It occurred at the P Conveyor Face in the Big Vein Seam in No.2 Slant, Great Mountain Colliery. The outburst happened at about 5.50 p.m., causing three of our members to lose their lives. The deceased were young men, two 33 years of age and the third 28 years of age. Death in each case was by asphyxiation.
The P coal face which is advancing in a north-easterly direction is 45 yards in length. The supply road is about 8 yards from the top of the face and is flanked on the top rib side by an airway. The purpose of this form of development is to comply with the Code of Working laid down by the Joint Advisory Committee on Outbursts in Coal and Firedamp. This Code provides that no working less than 15 yards wide be permitted in this seam at this colliery.
It was proposed to work the two conveyors to the left and rise of this development the presence of the pack on the rise side of the supply road would probably preclude the necessity to leave pillars of coal and so reduce the abutment pressures, which is a necessary step to mitigate against outbursts. The face is advanced on a 48-hour cycle and the coal is hand got, but assisted with shotfiring at the gate road and occasionally for breaking in on the coal face. At the time of the outburst, the face was disturbed, the coal being of a ‘woody’ nature. The section of the seam varied from about 9 feet at the gate to 2 feet 10 inches in stent No.2, again increasing, and subsequently thinning to about 4 feet at the top end of the face. The immediate roof is a soft friable clift which is overlain by a thick sandstone. The distance from the top of the seam of coal to the sandstone varies and is down practically to coal level at the gate road end of the face. The floor is variable but generally of a soft nature, with a result that floor penetration by the supports takes place. The face was supported on notched timber, generally with six-foot collars, a six-yard pack on the roadsides and a centre pack. but for a few turnovers previous to the outburst there had been a six-yard gate side pack, with a twelve-yard pack from the supply road, the mid-face space being supported on chocks. The gate was carried forward on 14-foot arches and the supply road on 12 foot arches. The face equipment consisted of a H. S.5 scraper chain conveyor, delivering on to a 26-inch gate conveyor. Compressed air was used to power the face unit and electricity for the gate unit.
At the time of the outburst, there was a complement of 8 colliers on the face, two men on the rippings in the Supply Road and a collier and assistant in the gate. Coaling had been done during the day shift and there were two “bunches” and some small coal to be cleared for the face to be moved over on the night shift. The first bunch was about 4.5 yards long. John Rees and Harold Williams were to work up and Malcolm Donoghue and Glyn Jones (deceased) were to work down. There was a bunch at the top of stent 3 which was also being cleared by two men working up and two men working down. During this time, the deputy was on the face, and he, like many of the workmen, has had previous experience of outbursts, although on this occasion no one heard anything unusual.
At about 5.50 p.m., Michael Gibbon, the collier in the top stent, was getting coal near the ribside and was using a mandril. Whilst doing so, he heard some pounding in the coal, which he considered normal, but when he used the mandril again, he heard “rapping” and as the noise increased, he realised something was going to happen. He made a rush down the face, although he was caught by the blown-out coal; he recovered after receiving attention. All the men on the face ran for the gate end, which is also the intake end, this being the agreed procedure as the amount of firedamp given off usually fills the return airway. Just below the bunch of coal in No.2 Stent a chock had been erected on the face side of the conveyor, and immediately against this on the upper side was a fairly big stone. This combination restricted the room far travelling; normally one could proceed fairly easily, but in the emergency, the men seemed to have reached this point nearly together and in the confusion, failed to get clear and were caught by the ejected material. Meanwhile, the deputy made a roll call of his men and five were missing. Volunteers immediately went back to the face and soon dug out William Picton and Mike Gibbon, who, after first aid treatment, were sent out. During this time, management had been informed, all interested parties were notified, and a rescue party was taken underground. These men were in the face of the supply road in about an hour after the outburst and, strangely enough, reported that only a trace of firedamp was visible on the detector. A quick examination was made in the blown-out coal for any bodies then activity was directed to cleaning off the blown-out material which extended to within 10 yards of the gate. Soon one body was seen – that of Glyn Jones, but it needed considerable work to be done before it was released. After examination by two doctors who were on the face, life was pronounced extinct, and the body was taken to the surface. It was whilst moving Glyn Jones’ body that Mel Jenkins’ body was seen on top of the blown-out coal. After some coal had been cleared, this body was released and taken to the gate for examination. Whilst some of the coal was being turned down the face, two cap lamps were uncovered, and it became apparent that there was another body on the face. Renewed efforts were now being made to clear the coal. The ejected coal is similar to black flour and the dust created in recovery operations was considerable. Ieuan Lewis’ body was ultimately discovered; strangely enough, whilst the others were head foremost to the gate, Lewis seemed to have changed his mind and decided to get back to the supply road, but he was overcome. When Lewis’ body was taken to the gate for examination by the two doctors it was after 10 p.m.
About midnight, a meeting of the representatives of HMI, NCB and NUM who were present took place, and it was agreed to clear up all the blown-up material on the face side immediately, and all discharged timber etc., marked so that a joint inspection could be carried out as soon as possible. This work started, but due to the extreme dustiness, the number of persons shovelling had to be reduced to a minimum. On Thursday morning, a joint inspection of the face was carried out the coal had been cleared to the upper roadside of the supply road. Examination of the face showed no sign of a blow-out, nor was there any parting between roof and coal. After surveyors had picked up the details, it was decided to take off the remainder of the “bunch” of coal above the first stent, between shifts, so as to make it easier to handle the blown-out coal. It was agreed, also, that in view of the absence of any signs of the source of the outburst so far, that the coal from the supply road to the rib side of the airway be cleaned out. This was done during the afternoon and night shifts. On Friday morning, another joint inspection was made of the face, when it was seen that the outburst had come from the top corner of the face. It had blown out practically parallel with the face, hence the reason for the material being within 10 yards of the lower gate, whilst it had only blocked the entrance of the face end of the supply road. Again, the void left in the top corner made one wonder where the material came from. The hole was about 10 feet long, varying from 12 feet to 20 feet high and was about 4 feet at its widest part, yet the rough estimate of the amount of blown-out material is about 150 tons. Examination of the hole showed that the roof levelled off from the normal gradient of about 1 in 8, which suggests that there may be a disturbance in this region.
As a precaution, an advance hole had been bored to a depth of 27 feet in the face of a gate, under the supervision of Mr. Austen Thomas, B. Sc., who is the investigator for the Sub-Committee (Research) of the Main Outbursts of Coal and Firedamp Committee. (The NUM are represented on this Sub-Committee by Messrs. L.R. James and A.W. Jones). An advance hole had also been bored to a depth of 27 feet in the face near the top rib, but about three yards of this had been worked off. During and after boring nothing untoward was detected in any of the holes. Actually, the hole in the gate road had been bored on the Monday previous to the outburst. On Monday, 3rd March 1958, a meeting of the Research Sub-Committee was held at Great Mountain Colliery and the accident was discussed and certain possible improvements and safeguards were suggested. The disturbing feature of this outburst was that it occurred in narrow workings. The rules already promulgated referred to narrow work, but now a new condition had to be considered.
The suggestions made were as follows:
- To introduce a system of volley firing ax the corners made by the coal face and the ribs.
- To ensure adequate egress through the face at all times, it was considered that had the bunch of coal, the chock and the large stone not been present in close proximity one to the other, all three lives may have been saved.
- To equally distribute man density on the face on each shift.
- To provide complete and positive support all around the arches set in the gate road.
A subsequent inspection on March 5th showed that the recommendations had been put into practice. Four holes had been fired into the top corner and six in the lower corner. These holes were bored and fired between shifts; there was no undue issue of firedamp after the volleys, but one hefty pounce after the gate volley. The main purpose of these volleys is to release any pent-up abutment pressure and by doing so to induce an outburst if one is imminent. The two major outbursts of 20/11/56 and 12/12/56 in a heading in the same district were induced in this way. This was borne out by the nature of the blown-out coal, some of it was in lumps up to 1 cwt. These two outbursts produced 400 and 300 tons of coal respectively with a calculated emission of about a quarter of a million cubic feet of gas. The last outburst has differed from the usual – there are few instances of an outburst on longwall faces; there was very little gas given off as our inspector was within 8 yards of the seat within a little over an hour; and there was practically no gas cap in the general body. There was not the usual violence and noise, as the man on the gate road heard nothing and only the timber supports about 7 yards from the hole were displaced.
The manner of working in the seams and collieries in this area which are liable or may be liable to outbursts, are governed by the code of rules prepared by the Joint Advisory Committee on Outbursts. Whilst the rules have no legal significance other than to the extent that they represent a safe system of work in the opinion of those who promulgate them, the NCB does observe the code and no relaxation is allowed without the prior consent of the Committee. Research and interchange of information with other countries that experience this phenomenon has been undertaken for many years and is continuing.
The Second Outburst
On Wednesday, 16th April 1958, another outburst occurred on the same coal face, this time in the gate road at the other end of the face. There was, on this occasion, no personal injury, the outburst occurring between shifts immediately after firing a round of shots in the rib corner in accordance with the recommendations of the meeting on the 3rd of March. There was a sandstone roll in the roof of the gate road which extended about 10 yards up the face. The coal section varied from 8 feet on the right-hand side of the gate road to 4 feet 6 inches on the left-hand side. Since the previous outburst, volleys had been fired both at the ends and along the face in order to induce any impending outburst.
Two days before the incident, three advanced boreholes were drilled in the face of the gate road. Two were 27 feet 6 inches deep and the third 25 feet deep. The one in the corner was at an angle of 35 degrees with the face and the one at the centre and left side of the gate was at right angles to the face. The hole to the left gave off large volumes of firedamp and great difficulty was experienced with the volume of fines produced in drilling caused by miniature outbursts into the hole. In the other holes, the gas emission was moderate. At the end of the day shift on the 16th of April, five-shot holes were bored into the face of the road. The four holes in the rib were charged each with 20 ounces of Unigen EQS explosive and the remaining hole with 16 ounces. The holes were 5.5 feet and 6 feet deep. The shots were fired from a point 120 yards back along the intake from the face and the return airway was full. Two hours afterwards, the intake was clear, but the return was still full of gas. When it was possible to approach the face, the outburst coal was found 35 to 40 yards back from the face, and the gate road was virtually full of dust to a point 20 yards back. The estimated weight of the coal displaced was 400 tons. The heading was supported by 14 feet steel arches set on six-inch wood blocks and the quantity of air circulating was 14,000 cubic feet per minute. It is reasonable to assume that had this outburst been induced by firing it would have occurred subsequently. As it occurred at the intake end of the face and such a large volume of gas was produced, had men been on the face their escape would have been impossible and their asphyxiation almost inevitable. There can be little doubt that volley firing on this occasion prevented a major catastrophe.
A subsequent meeting of the Joint Advisor Committee on Outbursts reviewed these two outbursts and unanimously agreed to seek from the Chief Inspector of Mines the exemptions necessary to undertake volley firing in one operation along the whole face. Experience has shown that the liability to induce an outburst and to minimise the delay between the firing of the shots and the subsequent outburst increases with the shock given it is also considered that a larger number of smaller shots are more effective than a few heavier shots.
At present, the restriction imposed by the fact that no approved exploder exists for firing more than six shots simultaneously limits the area that can be treated in one operation, although some compensation is sought by making the charges as heavy as possible. There is a danger that firing shots at one part of a face could precipitate an outburst in an adjacent part of the face whilst it is being prepared for firing, it is therefore much safer that the face be fired in one operation.
The Third Outburst
Yet another outburst occurred at about 5.40 a.m. on Tuesday the 20th of May 1958. This one took place in the right-hand side of the loading road on the same face as the two previous ones. The third outburst, like the previous one, was induced by volley firing. On this morning, a round of six shots had been fired on the lower end of the face and forty minutes later a volley of holes bored in the agreed fan pattern was fired on the lower or right-hand side of the loading road. These volleys are fired, not for the purpose of getting coal, which on this face is very soft and of very poor quality, but to attempt to induce an outburst. This volley of six shots had a total of 108 ounces of explosives. The firing is done when all men are out of the district and the firing point is nearly 200 yards from the face and on the intake side. About one minute after firing the volley the shot firer heard the outburst, and all members of the Research Committee were immediately notified.
Later in the morning, a visit was made to the road, and it was soon seen that this one again was of a major character. 550 tons of very fine coal had been blown back along the road, tapering to a distance of over 50 yards and filling the 14-foot arched roadway practically to the roof. It was also extending to a distance of 15 yards up the face, and also filling a new airway 25 yards from the face. It appeared that the quantity of gas emitted was somewhat less than the previous one in which 500 tons of coal had been released. Examination of the face after clearing the coal showed that there was a sharp trough fault in the seam, at about 30 degrees off the heading to the right and rising at about 45 degrees. A hole was exposed, about 4 feet wide, 8 feet high and as far back as could be seen, about 25 yards long, but it was agreed that in all probability the actual hole had been smaller and the ground had fallen as coal was taken away. The face had advanced about 18 yards since the previous outburst of 500 tons which had happened on the 15th of April 1958.
Of the last five outbursts in this slant, four have been induced by volley firing and the knowledge gained by these may prove to be valuable when the new Cynheidre sinkings are developed. Cynheidre is in the area of the coalfield liable to outbursts. At about 12.30 p.m. on Monday 27th of October 1958 another outburst of coal and gas occurred at Great Mountain Colliery. This time in the Gras Seam in the No.11 West District The outburst which resulted in no injury to personnel, occurred at the face of a narrow heading. The Gras seam had been intersected by the cross-measure drivage 140 yards from its entry from the main slant. The purpose of the drivage was to develop the Gras seam, beyond a washout. Where the seam was intersected it was only 4 inches thick, but in the first 10 yards of the drivage in the seam, it increased to 10 inches. In the anthracite area of South Wales, 112 outbursts were recorded between 1907 and 1958. In the period 1947 to 1958, 11 have occurred, and 4 in 1958. 24 of these occurred in Great Mountain Colliery.
Cynheidre Colliery
At Cynheidre the situation became so serious that moves were made to monitor geological movements/conditions in Cynheidre via a computer at Swansea University. In September 1985 teething troubles were being experienced in this system, and the in-seam monitoring system was under development. It relied on a system of monitoring the changing conditions by measuring energy. Three non-fatal incidents occurred in April, June and July of 1985 and were reported as follows; “There had been an increase in the number of Seismic events in the three days before the outburst, in addition, desorption ratio 2.7.2.8. Because of these conditions, volley firing was resorted to with 18 holes fired in the stone area, i.e. from chock 1-18; 5 holes in chock 110-114; 6 holes in chock 50-75 on the night shift before the outburst. The outburst occurred at 11.56 a.m. on Wednesday, 3rd April 1985 in the B.23 face X.M.5. The shearer had cut out the gate road end, flitted to chock 22, started to sump in chock 23, 24, 25 when the outburst occurred. The coal heaved out from chock 10 to 24 to a position immediately in front of the forward leg of the powered support. The estimated CH4 released was in the region of 135,000 cubic feet. All alarm systems were operated 19 men were on the airlines in the supply road, 4 on the airlines on the face, and 23 men were in the gate end. These men except for those on the gate end remained in this position until they were instructed from the control room approximately an hour later, when it was safe to leave the station and proceed to the surface. The face was unit ventilated some 60,000 c.f.m. circulating.
Events Leading up to the Outburst: Tuesday 26th of April 1985 – Owing to thin seam section and desorption readings of 3.8 to 4.3 on borehole A 1 hole 15 feet in depth and with 2.7kg of powder was fired in the Gate Stable, also due to thin seam section 15 inches to 18 inches of roof was being fired down from chock 1 to 18 as well as volley firing in this area, during the week. Subsequent desorption ratios were 2.7 to 2.8. Monday 1st of April 1985 – Output 284 tonnes on Days – Advance 1 metre. Output 161 tonnes Afternoons – Advance 0.3 metre. Pronounced cracking chock 70 to 78 on the day shift Slushy Coal – chock 58 to 62 on the afternoon shift. Heavy Backslipping – chock 58 to 62 on the afternoon shift. Tuesday, 2nd of April 1985 – Output 298 tonnes on Days – Advance 1.1 metre. Output 44 tonnes on Afternoons – Advance 1.1 metre. At the end of the afternoon shift, 18 holes were fired in the stone area from chock 1 to 18 in two rounds with 10.8 Kg of powder. Wednesday, 3rd of April 1985 – Output 188 tonnes – Advance 0.6 metre. Five holes fired in roof roll chock 110 to 114, 5 Kg of powder. Six holes pulse fired chock 50 to 75. Chock 50, 55 and 60: 30 feet hole in coal. Chock 65: 27 feet in coal and 3 feet in rashings. Chock 70, 75: 30 feet in coal and heavy gas emission. 2.7 Kg of powder and 16.2 Kg total fired at 21.37. CH4% went up from 0.77 to 1.08 after firing, also 24 holes fired in Gate Stables using 18 Kg of powder.”
On the 24th of June 1985, there was an induced outburst in the intake drivage of the Pumpquart Development at 11.41 a.m. “Whilst shotfiring a round of 23 holes with a pull of approximately two feet, an induced outburst occurred yielding approximately 30 cubic metres of material and about 2,000 cubic metres of CH4. The shift had started by filling out eight mine cars of rubbish, trimming the rippings and putting the heading ready for boring. Due to the fractured nature of the ground, the practice is to only fire enough room for one arch. It was assessed that two-foot holes would be sufficient for this. 23 holes were bored after first boring a nine-foot probing hole in the middle of the heading and finding no coal. These were charged with 11.8 Kg of Driftex and half-second day detonators. The three heading men were deployed as sentries in the entrance of the return heading and the deputy and electrician went to the Panel 1 Gate Road Cross-Cut Firing Station. After warning the return side of the P.O. that he was firing, the round was detonated. The electrician said that immediately after firing he heard the distinctive noise of an outburst, and then the alarms went off. All men were accounted as safe and before crossing the district off at 1 p.m. a cursory visit was made by the Under Manager, Outburst Engineer and Deputy.” “A new situation arose on the night shift of the 4th of July 1985 in the BV 23 panel. The seismic probes indicated an increase in activity on the latter part of the night shift. The number of events increased from 50 at 5 a.m. to 112 at 7 a.m. monitored in the control centre at the surface. Prior to the installation of the seismic equipment, this information was not available. A further difficulty is the establishment of an event threshold since no previous information is available. However, Management deemed it necessary to introduce an outburst procedure at the start of the shift and to delay the cutting until the events returned to normal, a position achieved by 11 a.m. cutting continued normally for the rest of the shift and thereafter with a reduced number of events.
Information supplied by Ray Lawrence and used here with his permission.
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