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51. With the exception of the Nos. 1 and 2 Rise Districts, Skelly's Heading and the No. 1 Dip Shortwall Face off No. 3 Trunk Road, the explosion traversed almost all other inbye working faces and roadways and continued outbye along the main return airway for a distance of 60 yards beyond the main air-crossing over No. 3 Trunk Road and along the main intake haulage road for a distance of about half-a-mile from its junction with No. 3 Trunk Road. In all, not less than 2,000 yards of roadway were traversed by the flame of the explosion.

(b) Violence

52. The explosion left a trail of death in its path and many heavy falls of ground. But apart from the badly wrecked main return air-crossing over No. 3 Trunk Road, there were few marked signs of extreme violence. Many empty tubs were left standing, signal wire supports were not bent, while roadway conveyors. although disturbed and distorted here and there, were not seriously damaged. This absence of signs of extreme violence suggests an explosion in a combustible mixture not far removed from the lower limit of inflammability. The numerous falls gave a deceptive appearance of violence. It is probable that these falls were largely due to the pressure of the explosion blast causing a slight lifting of the lower detached roof resting on the supports, thus easing the displacement of the supports.

(c) Source of the Combustible Material

53. As might be expected with so many interconnected open roadways, it was not easy always to determine the exact direction of the passage of flame in certain of the inbye roads; but this is of little moment. It is important, however, to determine the source and nature of the combustible matter which enabled the explosion to spread as far as it did. The initial explosion in the waste of No. 2 Dip Right Face was almost certainly of firedamp only, and was essentially one of firedamp so far as the major part of the No. 2 Dip area was concerned. Thereafter, the spread of the explosion outbye was no doubt greatly assisted by the firedamp released or pressed out from the waste towards the outer end of the mothergate, especially at a point where the explosion had dislodged a stopping and a small roadside pack (see Plan No. 3). This firedamp would readily assist the explosion in its passage in both directions along No. 2 Dip Trunk Road and also into the face of Allen's Drift where firedamp had previously been reported and which remained fouled with firedamp for some days afterwards. But the fact of the comparatively short spread of the flame along the main return airway does not suggest any extensive accumulation of firedamp in the district as a whole. Indeed, the weight of evidence suggested that it was highly improbable that enough firedamp was available to propagate the flame of the explosion over the distance it was agreed the explosion extended. On the other hand, the great extension of the flame along the main intake suggests strongly that coal dust played no small part in propagating the explosion there.

54. Against the view that coal dust played an important part, the results of analyses of numerous dust samples taken after the explosion showed that a high standard of stone-dusting had been reached. Moreover, following the explosion, there did not seem to the ordinary observer much evidence of coking. Only on the New Conveyor Road leading from No. 2 Dip Trunk Road to the Back End Shortwall was coking at all obvious. It must not be assumed from this, however, that the part played by coal dust must necessarily have been small. On all roadways there was a thin dust deposits such as might have been left by the passage of an explosion through a dust cloud containing an admixture of coal and stone dust. This thin deposit contained fine coke dust which was not obvious to the eye. The quantity of coked dust that is found depends on the quantity of coal dust exposed to flame, and if this dust is disseminated in a cloud, then most of the coal dust may be burned completely and the quantity of coke dust or coke deposit may be small.

55. After careful consideration of the evidence on this point, I am driven to the conclusion that it is highly improbable that enough firedamp was available to propagate the flame of the explosion along the 2,000 yards of roadway including, most strikingly, the half-mile along the main intake. Since the only other possible fuel was coal dust, and bearing in mind the high standard reached in stone-dusting, it is necessary to inquire in some detail for the possible source of the coal dust.

56. The most likely source was coal dust from the inbye conveyor roads. It is well known that in dry and dusty mines, coal dust collects rapidly when conveyors are in use and that this dust is notoriously difficult to keep constantly neutralized because it tends to accumulate insidiously, not only as a surface layer on the sides and roof, but also in quantity around loading and transfer points and under and along the whole length of the conveyor structures. In such places it is difficult of access for brush and shovel and the coal dust is, therefore, not removed by the ordinary cleaning-up process which normally takes place daily, or even several times a day, in the immediate vicinity of a loading point. This dust is, nevertheless, still readily accessible to a blast of air, such as would be created in the early stages of a firedamp explosion at or near the working face. Clearly, so long as any dust remained to be raised, such a blast could raise this dust continuously as it travelled outbye along the conveyor roads and might thus produce a long cloud of coal dust moving ahead of the flame but ready to be inflamed when the flame of the firedamp explosion caught it up. If, of course, the roadways had been recently treated with stone dust, then the blast would also raise some stone dust as well. But, even although some cleaning-up, and perhaps even stone-dusting, is done during a coal loading shift in and around conveyor loading points, it is almost certain that, in general, the top layer of dust throughout the length of any conveyor roadway toward the end of a loading shift is almost entirely composed of coal dust. This top layer of coal dust would not be so marked in an outbye main haulage road where the rate of deposition of coal dust is normally much less than that on a conveyor road.

57. It seems to me that something of that nature must have happened in this case. The explosion, as already pointed out, was not of a violent nature. I consider, therefore, that as the blast from the initial moderate firedamp explosion travelled along the inbye conveyor roads, it raised a continuous dust cloud, composed mainly of coal dust from the recently deposited top layer and from beneath the conveyor structures; that this dust cloud burned as the flame from the firedamp explosion caught up with it; and that the blast also raised a cloud of dust from the main intake haulage road containing a much larger proportion of stone dust, but that as this dust was ahead of the coal dust blown from the inbye conveyor roadways, it did not contribute to the extinction of the explosion until the flame had travelled a long way outbye. I can see no other way of explaining why this explosion travelled for nearly half-a-mile outbye along an apparently well stone-dusted intake roadway.

58. If the explanation I have given is the right one then it stresses once again, and very forcibly, the danger of a thin top layer of coal dust on any roadway and, in particular, the need for unremitting and continuous attention to the neutralizing of the dust deposited in conveyor roadways, especially under the whole length of the conveyor structures and in and around loading and transfer points. It is more than thirty years ago since the Explosions in Mines Committee proved experimentally that coal dust lying on top of stone dust would propagate an explosion. Because of this knowledge, the original General Regulations of 1920 relating to Precautions Against Coal Dust required that the dust on the roof, floor and sides of roadways should consist of an intimate mixture of coal and stone dust containing not less than the prescribed percentage of incombustible matter. These Regulations were later amended in 1939 to require that such measures shall be taken as will ensure that on the floor, roof and sides, respectively, of every road or part of a road which is accessible, the dust which can be raised into the air shall contain not less than the prescribed percentage of incombustible matter. To maintain this dust-which may comprise only the thin top layer-in compliance at all times with the Regulations on any roadway, and especially on a conveyor roadway during a loading shift in a dry and dusty mine, is admittedly extremely difficult, if not quite impracticable. Clearly, the practice of scattering stone dust on such roadways by hand shovel in large quantity at infrequent intervals will not ensure compliance, because it does not neutralize the dangerous top-layer of coal dust which may be formed in a relatively short time and it does not ensure an intimate mixture. Much research has been done on this aspect of the dust problem in recent years by the Safety in Mines Research Board and other investigators. The danger presented by thin surface layers of coal dust is stressed in the 24th and 25th Annual Reports of the Board for the years 1945 and 1946, while details of the experimental work and suggestions for combating the danger are given in S.M.R.B. Paper No. 105 (published 1947). The problem was also referred to in a Paper,* read in 1937, of which I was a joint author. I am satisfied that, in relation to the propagation of an explosion, the full significance of the danger of a thin top layer of coal dust, such as may easily be deposited in some mines during a coal loading shift, and which may well form the bulk of the dust which can be raised into the air, is not yet sufficiently realized by the majority of mining men. It has been estimated that if the quantity of coal dust (of average inflammability) -sufficient to form a layer, a few 1/1,000ths of an inch thick on the roof, floor and sides of a roadway 50 sq. ft. in cross-section-were raised into suspension in that roadway, it would produce a cloud of sufficient density to propagate flame. I strongly urge, therefore, that in stone-dusting practice there should be strict adherence to the principle of "little and often ", i.e. the application of smaller quantities at more frequent intervals or, where practicable, the trial of some device which will ensure that sufficient (and no more) stone dust is disseminated at frequent intervals in the atmosphere of the roadways to mix and deposit along with the coal dust present, thus ensuring as near an approach as is practicable to an intimate mixture throughout. It occurs to me also, that if some practicable means could be devised for rendering and maintaining a roadway surface, and especially that part of the floor under conveyor structures, "dust intolerant", i.e. treated in such a way as would ensure that any coal dust deposited on it was rendered indispersable, the dust danger on conveyor roadways would be considerably lessened. It may be that the present research work on the consolidation of roadway dusts will lead to the development of a means to that end. Finally, I should like to press the point very strongly that if compliance with the dust regulations at all times in dry and dusty mines is to be reasonably assured, and the conditions in such mines are to be improved in respect of health and comfort, then all practicable steps must be taken in the first instance to prevent or reduce dust formation at its source, and to suppress by water infusion, sprays, wet-cutting, wet-drilling or other appropriate means, any dust that is inevitably made.

* Smellie, J. and Bryan, A. M. – "On the Efficiency of Incombustible Dust in the Prevention of Coal Dust Explosions" – Trans. Inst. Min. E.1937-38, XCIV.