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IV. - THE INVESTIGATION
Exploration of the Affected Area
The outbye stone dust barrier in the intake and the one in the return were found intact but the inbye barrier in the intake had been completely destroyed, although it was known from a report by the first rescue team, and later from Davies, that except for one cross member it had been undisturbed by the first explosion.
All the electrical apparatus was found switched off and there was no evidence that any of it had been otherwise at the time of the first explosion. Davies's partly burned waistcoat and broken parts of his cap lamp were found near the intake hoist, while a few yards further inbye were the burned out remains of a small sack which had contained bolts. Between the hoist and the face, two props and a bar had been blown outbye but falls had not occurred as a result. One coupled bar series had been displaced on the face, again without a fall of roof having taken place.
Blackburn's probable position at the time of the explosion was deduced, from the presence of tools and the length of conveyor not cleared of coal, to have been at a point 25 yards from the tailgate. The detector, extinguished, was recovered from a bar at No. 5 pack.
Tests made by flame safety lamp failed to detect the presence of inflammable gas in the general body of the air of the district, but the lamp was extinguished when held just inside the wastes adjoining Nos. 1, 2 and 3 packs. Moreover, samples drawn from every waste showed on analysis, with two exceptions, the presence of inflammable gas above the lower limit of inflammability. The results of this sampling are shown in detail in the inset on Plan No. l.
Although small deposits of coke were found on the face, coal dust was discounted as having played any material part in the propagation of the explosions.
There was no damage in the return but the two door frames in stenton 20 were found to have been forced back 1 6 inches and 4 inches respectively.
Flame, which had traversed the full length of the face, had apparently entered each waste and had died out at points 110 yards and 140 yards from the face in the intake and return respectively.
The amount of violence caused by both explosions was slight.
From the available evidence the indications were that both explosions were of firedamp and that the first was the bigger of the two. It originated on the face near Nos. 1 and 2 packs, travelled towards the return down which it continued, overtook Musgrove, damaged the doors in stenton 20 and then petered out. Its progress in the other direction from its point of origin was of a milder character, for although it knocked Davies over and set him alight it had then almost spent its force and indeed was unable to do more than disturb one of the bars of the inbye stone dust barrier.
The second explosion, which seems to have originated at about the same place, expended very little force in the direction of the return, as emphasised by the fact that a fire extinguisher left standing upright on the floor at the roadhead by a member of the first rescue team was not even disturbed. Its main force was towards and along the intake, for it destroyed the inbye dust barrier therein and still retained sufficient force to make its blast felt at the loading point. The part possibly played by this dust barrier in arresting the spread of the second explosion was investigated and considered by Dr. W. Murray of the Safety in Mines Research Establishment, whose findings are of such importance that I have deemed it desirable to include them as Appendix B to this Report.
Throughout the life of the Plough Face up to the time of the explosion, deputies had not reported the presence of inflammable gas either in the general body of the air or at the waste edges. The statutory determinations made monthly in the return airway had shown the inflammable gas contained in the general body of the air to vary between 015 per cent. and 035 percent., and in each case to be only slightly higher than that determined at the corresponding point in the intake airway. I must point out that these determinations would be unlikely to disclose any sharp fluctuations in the firedamp content such as would be caused by the emission of firedamp from the waste due to fracturing and collapse of the upper beds. The face had advanced 34 yards from the back rib and had reached that critical stage where the fracturing of the beds still spanning the waste area should have been anticipated. As Musgrove had just examined the face and, furthermore, had seen Blackburn's detector burning normally at No. 5 pack, evidently the general body of the air on the face was free from firedamp up to within a few minutes of the explosion taking place. The occurrence of a firedamp explosion so soon after Musgrove's inspection required the sudden expulsion of gas and such, in my opinion, followed the breakage of roof beds in the waste adjacent to Nos. 1 and 2 packs. It was in roof breaks in this area that the highest concentrations of firedamp were found during subsequent investigation. Furthermore, in my opinion, such collapse and expulsion were, undoubtedly, heard and felt by Davies immediately before the explosion. With a means of ignition there at the same time, all the conditions required to produce an explosion were present.
Contraband was not found in the workings or in the clothing of the men in the district, and smoking or the illegal use of a naked light can be discounted as a possible means of ignition.
All the safety lamps in use in the district were examined by the Safety in Mines Research Establishment and passed as safe with the exception of Davies's electric cap lamp, the damage to which, I am satisfied, was consequential to the explosions.
Electrical apparatus was not in use at the time of the explosion but as the face signalling system had been in use but a few minutes before, it was subjected to exhaustive tests by the Safety in Mines Research Establishment and found to be in full conformity with the approval specification and to be safe.
Shot firing had not been carried out for two days prior to the explosion and there was no evidence of roof breaks in which flame, if any, from such shot firing could have lingered.
There was no indication of spontaneous heating or that frictional heating in connection with the use of machinery had played any part.
Careful examination of all the compressed air equipment did not reveal any leakage where heating could have taken place.
There remained the possibility of incendive sparks being produced by impact or sliding friction between hard surfaces such as rock on rock, rock on metal, or metal on metal. To explore further this possibility, Dr. H. Titman of the Safety in Mines Research Establishment inspected the face and collected samples of the roof stone and equipment in use and subjected them to a series of laboratory tests. The results clearly revealed that there were present on the face at the time of the explosion, and particularly in the vicinity of Nos. 1 and 2 packs, a number of objects, any two of which as a result of impact or sliding friction, one with another, could have produced incendive sparks.
These results were not unexpected and served to supplement research into the hazards of incendive sparking, a subject which the Safety in Mines Research Establishment have been investigating for some considerable time (see S.M.R.E. Research Reports Nos. 75 and 90). Definite conclusions as to the full extent of the hazard involved have not been reached, but very recent research has shown that it is much more serious than was realised at the time of the explosion. Thus, aluminium itself, or aluminium-based alloys containing little or no magnesium, can give rise to incendive sparking on impact with rusty steel or iron. Moreover, the rubbing of such materials on rusty steel or iron leaves a smear of aluminium or alloy, from which incendive sparks can be obtained by scraping or light glancing blows. It is imperative that this information should be fully recognised throughout the industry, and for this reason I attach as Appendix A a report by Dr. Titman on his investigation at the colliery and on laboratory tests carried out by him in connection with it.
Having regard to the results obtained by Dr. Titman and to the absence of any other possible igniting means, [conclude that the first explosion probably occurred while the lowering of the roof beds and falls of roof in the waste were forcing firedamp from part of the waste much faster than the air current passing along the face was capable of diluting it to safe limit so that an inflammable mixture of firedamp and air in the vicinity of Nos. 1 and 2 packs was ignited by means of incendive sparks resulting from impact or sliding friction caused either by displacement of supports or falling rock. With regard to the second explosion, I conclude that it occurred when, during the process of restoring the ventilation, an inflammable mixture of firedamp and air was moved on to material undergoing combustion, such as smouldering coal dust or, more possibly, firedamp ignited by the first explosion and still burning in a roof break or cavity at the edge of the waste.
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