REPORT. Mines Department, Dean Stanley Street, 21st June, 1927. To Colonel The Rt. Hon. G. R. LANE Fox, M.P. SIR, ON 4th November, 1925, there was a sudden and extensive upheaval of the floor along a length of working face in the Rams Seam at Pendleton Colliery, Lancashire, whereby five men lost their lives. Similar upheavals of the floor had occurred previously in this seam and, having regard to their strange and disastrous nature, you appointed us on 11th January, 1926, to be a Committee to make a special investigation into the causes of the accident on 4th November, 1925, and to suggest means of preventing such accidents in future. We now beg to report as follows: General Description of the Colliery.-The Pendleton Colliery has two shafts, each 510 yards deep, to the Rams Seam. Coal has been wrought from the Rams Seam from the present shafts for more than 75 years, and the distance to the working faces to the dip is now more than 3,000 yards. There is one main engine brow, which starts in the seam near the bottom of the downcast shaft and continues to the full dip for a distance of a mile. Near this point two levels are turned, one to the East and the other to the West. From points some 40 to 50 yards along these East and West Levels, known as No. 12 East and No. 12 West Levels respectively, slants have been driven to the dip-that from the No. 12 West Level, known as the West Slant, for a distance of 475 yards and that from the No. 12 East Level, known as the East Slant, for a distance of 640 yards. Off the West Slant has been driven No. 14 West Level and it was in a working on the dip side of this level that the accident on 4th November, 1925, occurred. The general plan of the relevant workings is shown on Plate 1. Description of place of Accident.-The site of the accident on 4th November is shown in detail on Plates 2 and 3. On referring to the former plate it will be seen that a brow, known as Morris's Brow, extends from No. 14 West Level to the dip for a distance of 60 yards and that off this brow two levels, known respectively as Morris's Level and Bramwell's Level, had been turned away and a face of coal, some 35-40 yards in length, and bounded on 62816 A 2 the dip by a downthrow fault, was in course of being worked in an easterly direction. The seam at this face was six feet thick and at a depth from the surface of some 3,550 feet. Conditions existing prior to the Accident.-On the day of the accident the mine, as a whole, was idle, but a shift of 17 men was engaged in and about No. 14 West Level. This shift consisted of a fireman (Thomas Barnes), 3 repairers, 7 colliers, a filler and 5 haulage hands. The district had been inspected by the fireman on the night shift prior to these men commencing work and the day shift fireman (Barnes) made inspection of the face and levels of Morris's Brow about 8.30 a.m. Barnes, in evidence, in reference to this inspection, said he found everything in good condition" The timber was set all right; there was no gas anywhere; the ventilation was going all right, and the roof and sides were safe." Occurrence of the Accident.-About 10 a.m., when the fireman (Barnes) was on No. 14 West Level, returning towards Morris's Brow from Clement's Brow, where he had been to visit 3 men whom he had sent to do repairs there, the accident occurred. Barnes said " I heard a loud report; then I felt the dust and a sudden gush of air.” Two men, David Seabridge and George Hassell, who were at work in the district when the upheaval occurred, gave evidence as follows: Seabridge said he was on Bramwell's Level about 5 yards from the coal face, when without the slightest warning, there was a loud report and he was thrown several yards in an outbye direction along the level, his body being hit by small pieces of dirt. The floor lifted and he saw a big red flash." All the lamps were extinguished, Hassell said he was in the act of putting a tally on a tub which was standing on the plate at the face of Morris's Level when the upheaval occurred. He was standing on the outbye side of the tub and would be about 2 yards from the coal face, with one arm over the end of the tub. Everything was quite quiet when suddenly he was picked off his feet and thrown outbye. He explained the expression "picked off his feet " picked off his feet" by saying just as though you were stood on a plank and someone snatched the plank from underneath you.' -- Condition of workings after the Accident. The condition of the area of workings affected by the upheaval is shown in plan on Plate 2 and in section on Plate 3. From these plates it will be seen that the accident had consisted of an upheaval of the floor along the whole length of the face from the downthrow fault at the dip end to the level airway at the rise end. The upheaval, except at the inner end of Bramwell's Level, where it extended into the level for a distance of some 10 feet, was confined to the area between the goaf and the coal face. Conditions as to firedamp.-When the upheaval occurred firedamp was given off and was encountered by the fireman Barnes on No. 14 Level some 70 yards on the inbye side of Morris's Brow. His lamp was extinguished, but he was able to continue on his way and to dilute and disperse the gas by breaking a pipe supplying compressed air to a small hauling engine placed at the top of Morris's Brow and allowing this air to issue freely. Later a lamp, still alight, was found near the bottom of Morris's Brow, which is a clear proof that the firedamp given off had not been present in any quantity in the neighbourhood of the lower (Bramwell's) level. Mr. David Coatesworth, H.M. Inspector of Mines, who had made 15 visits to the colliery between 25th January and 30th September, 1925, informed us that on no occasion prior to this accident had he found with a safety lamp any trace of firedamp in the mine. From this evidence and the results of our own investigations, we are quite satisfied that firedamp was very rarely present in the workings. Similar accidents at the Colliery.-Mr. J. T. Browne, General Manager for Messrs. Andrew Knowles and Sons, Ltd., gave us particulars, with sketches, of four occurrences similar in effect to that of 4th November, 1925, though happily not all accompanied by injury or loss of life. The sites of the occurrences are shown on Plate 1 and are numbered (1), (2), (3) and (5). In the area about the point marked (6) on Plate 1, we were informed by Mr. Browne that occurrences of this character became so frequent that all further working of the coal in that neighbourhood was abandoned. The site of the occurrence of 4th November, 1925, is marked (4) on Plate 1. Having set out the facts, we now proceed to give our views as to the causes of this accident and the steps to be taken in future to prevent such accidents or, at least, to mitigate their consequences. The Nature and Origin of Crumps.-The Crumps which occur in the mines of Lancashire are related to disturbances met with in other coalfields, which are referred to as Bumps,' Thrusts,' 'Creeps,' Squeezes,' etc., and to those shock movements experienced sometimes in metal mining, known as "Air-blasts and "Rock-bursts." In all these cases, the rocks, with their contained minerals which are being mined, are under considerable compressive stress, due, generally speaking, to the depth of working, and consequently to the great weight of overlying rocks. In some cases there is evidence that the strata mined are under enormous lateral or tangential stress due to structural or tectonic folds and thrusts. For example, after a detailed investigation of the Crumps which occur in the Lignite deposits of the Fuveau Basin of Provence, which are of Upper Cretaceous age, the conclusion was reached that these Crumps are not caused by the weight of overlying rocks in deep workings, but by great cumulative stress set up by the folding and thrusting of the strata containing the seams of Lignite, which on sudden release, causes violent roof-falls and up-thrusting of the floor of the mine.* In the case of the Pendleton Colliery there is no evidence that such structural or tectonic stresses play an important part in producing crumps. The pressure at successive depths below the surface of the earth may be assumed to be the weight of a column of rock equal in height to the depth in question, an average specific gravity of 2.8 being taken.† This assumes that the tangential compressive or tensile stresses are negligible and in any case they are extremely difficult, if not impossible, to estimate. Thus at the Pendleton Colliery, where the strata in that part of the mine under investigation are of the order of 3,500 feet in depth, the pressure due to the weight of overlying rocks must be about 4,250 lbs. per square inch. The limiting compressive stress, or crushing stress, for most hard rocks at the surface is much greater than this, and when such rocks are under cubical or hydraulic pressure, adequately supported on all sides, as when they are deeply buried below the surface, their crushing strength is greatly increased. Experiments have shown that under the latter condition the rocks will stand from three to seven times the compressive stress without fracture or flow, as compared with the same rocks under the ordinary laboratory tests for compression or crush with all sides unsupported. " During the extraction of minerals and the consequent opening up of underground roads and working places, the great stresses to which the rocks are naturally subject in virtue of their depth, are interfered with. The static equilibrium is disturbed, and movements supervene. Not only so but the static pressure or "pressure difference at some places underground is progressively and enormously increased by underground workings, so that a point is reached when masses of rock exposed in the workings suddenly break away, or, if relatively soft and yielding, may slowly squeeze out into open spaces. Any rapid change in the distribution of stresses consequent upon (or following upon) the working of a coal seam may produce sudden movements of rock masses, resulting in crumps, with accompanying falls of roof and sides, spalling or " rating" from the coal face, Annales des Mines, Tome VIII, 10° Livraison de 1925, "Vennes d'eau et coups de toit dans le bassin de Fuveau," par M. Jarlier, Ingénieur au Corps des Mines. "An Experimental Contribution to the Question of the Depth of the Zone of Flow in the Earth's Crust." Journ. of Geol. Vol. xx, No. 2, Feb.Mar., 1912, p. 103. |