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August, 1931

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The difficulties of removing the chocks or cogs from a seam with a heavy roof and soft floor have to be experienced to be fully realised, but even under the best of conditions the dislodgment of chocks is liable to be troublesome and to cause serious delays. At Dinnington, one of the collieries owned by the Seaton Burn Coal Co. Ltd., the conditions in the Main Coal seam are exceptionally unfavourable to easy removal, and chocks had frequently to be left in position on account of the danger of removing them. This seam has a normal section of about 4 ft. 6 in. and contains a dirt band, from one to four inches thick, situated about 2 ft. from the top. Both the top and bottom coal is of good quality, and, apart from a few inches in the immediate neighbourhood of the dirt band, is of a hard nature. Near the dirt band it is inclined to be friable, especially when the face is worked on line with the cleat. This fact is of some importance, as practical advantage is taken of it in working the coal. The soft section near the band lends itself to easy holing, while the top and bottom sections remain hard and comparatively uncrushed, though they can easily be mined without recourse to blasting.

The seam is situated at a depth of 54½ fathoms and is overlain by 14 fathoms of strong post rock (sandstone), the lowermost five fathoms of which are highly conglomerated and intensely false bedded. Moreover, the conglomerate follows no definite system of jointing and breaks off at all angles, so that great difficulty is experienced in inducing a line of break behind the main roof supports. There is a tendency for large areas of "goaf" to remain open as the face advances, and when the roof movement does take place an unusually strong system of support is necessary to prevent the total collapse of the face. These conditions are further complicated by a soft seggar floor, 2 ft. in thickness, which is liable to heave.

Hardwood skeleton chocks, built up with "nogs" 22 in. long by 4½ in. square section, are relied upon as the main support, and are erected in line at 4-ft. 6-in. centres. Each chock is erected on two oak pieces (from old wagon frames), 9 in. by 6 in. by 2 ft. 6 in. long, the introduction of this solid foundation having been followed by excellent results. Due no doubt to the smaller section pieces above taking up the crush, the foundation blocks are practically indestructible and have a very desirable steadying effect on the chock as a whole.

The seam is laid out in 33- by 26-yard pillars, both the bords and headways being from 4 to 5 yards in width. So excellent have been the results obtained by the automatic chock release now in use that it has been possible to extract the pillars on a longwall retreating system. The advantages of this system had previously been proved so far as the output was concerned, as it had been possible for the hewers to produce from 10 to 12 tons per shift against only 5 tons per shift with the usual methods of pillar extraction, But the heavy timber cost and the excessive wages paid in endeavouring to retrieve the chocks rendered the system uneconomic, particularly as the output could only be obtained over intermittent periods, when the roof conditions were suitable. The conditions were particularly bad when an old headways place was encountered, the whole of the roof supports being frequently lost without possibility of retrieval. An attempt was made to overcome this difficulty by leaving a strip of pillar to be extracted in a direction at right-angles to the cleat, the face of the next pillar being supported by chocks. Usually, however, the face closed, and not only was the timber lost, but the whole output of the district was seriously interrupted.

The system of working now adopted is shown in the sketch plan in Fig. 1. It will be seen that the cleavage planes do not run exactly parallel with the old headways places, and the first consideration is to work the coal on the lower side, gradually extending operations to the top end of the second pillar so as to have a single line of face following the direction of the cleat. This line is maintained until the lower side of the bottom pillar is within 5 yards of the next old road. The whole strip is then reduced to this width, all the supports being withdrawn from the lower side and the next pillars chocked up as before. Spaces are, however, left between them every 6 yards to enable a tubway to be laid down for the removal of the strip in eight sections. The double set of numerals on the sketch plan (Fig. 1) indicates the areas that are worked simultaneously during these operations.

The number of chocks in use on the double-pillar face is never less than 38, so that the importance of having them moved forward quickly and without trouble will be apparent. Indeed, the success of the whole system of working depends upon this factor, and has only been possible since the automatic release was introduced. The construction of this appliance will be understood from the drawings in Fig. 3. In its essentials the appliance consists of a double-webbed steel chock piece, capable of being built in with the wooden members and fitted with collapsible ends. The collapsing action is due to the slidable wedge members aa which are normally held in position by the catches bb. When built into the chock with the catches set the whole structure is absolutely solid, as it is not necessary to use the usual "tippler" or "tripping" piece on the cantilever principle nor is it necessary to stand the foundation pieces on a bed of loose material. All the members are in vertical alignment and the roof is resisted to the full crushing strength of the timber. With the tippler arrangement the cantilever pieces are ruptured by a combination of bending and shearing stresses, the latter predominating. As the resistance of oak to shearing stress is only from 700 to 1,000 lb. per sq. in, and to crushing stress is from 4,400, 8,900 lb. per sq. in. (Morley), the advantage of having the chock pieces in vertical alignment is obvious, an increase in strength up to 800 per cent. being gained, When it is desired to remove the chock the ends of the hooks b are knocked so as to clear the abutment on a, thus allowing the wedge member to drop out. Curiously enough the chock remains in position until the fourth and final catch has been released, when it suddenly collapses. Fig. 5 shows a photographic reproduction of a chock with the automatic release appliance in position, while Figs. 6 and 7 show the catches being raised and the final collapse of the chock.

Three men are employed to advance the chocks and tubway each night and to set the face timber, as shown in the diagram in Fig. 2. The chocks are set at 4-ft. 9-in. centres and are moved forward about 4 ft. each day. Before the automatic release was used four men were required to do the work on a face of only one pillar width, and, in spite of their being given occasional help, the results left much to be desired. It is further claimed that the C.Y. release, as it is called, adds substantially to the security of the operations, as no economic restriction need be placed on the number of the chocks used, and sufficient can thus be erected to give a solid line of support.

To summarise the advantages of the release :—

(a) The chock structure can be built on a solid foundation, with all the components in vertical alignment. The use of a "trip" or "tippler" member is obviated, so that the resistance offered to the roof is that of the crushing stress of the timber and not a combination of shear and tension.

(b) The appliance is simple in construction, has no loose parts and is practically immune from mechanical damage it is indestructible between oak pieces.

(c) It makes a very definite contribution to the safety with which chocks may be withdrawn, as the last catch can easily be released by a 6- or 8-ft. length of timber.

With regard to the first of these advantages, Dr. Winstanley, of the Safety in Mines Research Board, has frequently condemned the method of building chocks on thick layers of dirt to enable them to be withdrawn easily. He considers that chocks so built fail to offer enough resistance to be of service in protecting the face line, whereas with mechanical provision for withdrawal the chocks can be built tightly from roof to floor so that they are effective immediately. We understand that samples of the C.Y. Chock Release have been submitted to Dr. Winstanley, who is having them tested on longwall faces in some Scottish collieries.

The C.Y. Release is manufactured by Thos. L. Hale (Tipton) Ltd., of Staffs, and is an excellently turned-out job. Mr. J. D. Young, of Newcastle-upon-Tyne, holds the agency for Northumberland, Durham, Cumberland and Lancashire, while Messrs. Barnes & Bell Ltd., of Glasgow, are the distributors for the rest of the country. The invention is due to Mr. F. Cookson, who has taken out patent rights in all coalmining countries.

Safety Guards for Machinery.

Before leaving Dinnington we should like to draw attention to the safety guards which have been erected around all the moving parts of the various machines in use, and which, we understand, have been the subject of favourable comment from H.M. Inspectors. Two examples are illustrated in Figs. 8 and 9 and show the guards surrounding a 3,000 cu. ft. per min. Walker compressor. and the Robey engines driving a 22-ft. diameter Waddle fan. The Robey engines are unusually arranged, in that the cylinders, each of which is capable of driving the fan, are diametrically opposed, and can be connected or disconnected at will to a common crank. Thus, one engine only is in operation at any particular time, the other acting as standby. The guards are constructed of expanded metal fixed in a 1-in, by 1-in, angle-iron framework, hinged or sliding doors being provided to give ready access where necessary for lubrication, &c. The design and workmanship are due to the colliery staff, who are to be congratulated on the effective measures they have taken to avoid accidents, due to men being caught in moving machinery. This method of protection is extended to all the stationary mechanical apparatus at the colliery, and it is inconceivable that any accident should occur excepting through sheer foolhardiness.

In concluding our notes, we offer our thanks to Mr. R. S. Barrett, the agent of the Seaton Burn Coal Co. Ltd., for enabling us to visit the colliery, and also express our indebtedness to various members of the staff for supplying the data incorporated.

Drawings and Photographs accompanying the article