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September, 1957

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Recently through the courtesy of the National Coal Board Opencast Executive and Costain Mining Limited, we were able to visit the Acorn Bank opencast coal site near Bedlington, Northumberland to see and report on the progress that has been made since the letting of the contract in 1955.

The site is programmed to produce more than five million tons of coal in seven years.

Interest in the project quickened with the opening of the initial box cut which was claimed to be the largest man-made hole in Europe, in June 1956, the formation of which necessitated the removal of nearly 10 million cu. yards of overburden. Other developments which followed were the building of a two-mile private road from the site to the screening plant Fig. 1, and the commissioning of a fleet of four coal haulers with eight 40-ton bottom-dumping trailers. A drawing showing the layout of the site is reproduced in Fig. 2.

In June 1957, the site moved into its period of peak production and this will be maintained until the bulk of the coal has been removed, In the spring of 1957 four high-speed derrick cranes were installed to hoist skips containing 10 tons of coal each and tip them into the haulers, and the site now has one of the heaviest concentrations of earth-moving equipment in Europe.

World record established

The highest week’s output at Acorn Bank was achieved in June 1957 with the production of 20,800 tons which is claimed to be a world record for a deep site of over 200 ft. depth. The highest day’s output yet achieved is 4,017 tons.

Plant engaged on the removal of overburden includes two 1,150-B, 25 yard draglines manufactured by the Bucyrus-Erie Company, South Milwaukee, Wisconsin, U.S.A.; three 5-yard electric shovels by Ruston-Bucyrus Limited, Lincoln and a fleet of 22-ton Euclid rear-dump trucks, with Rolls-Royce oil engines. A number of 2-yard and 1-yard shovels manufactured by Ruston-Bucyrus Limited, Newton Chambers & Co. Ltd., Thorncliffe, nr. Sheffield and the Lima Corporation, distributors for which in the United Kingdom and Eire are Jack Olding & Co. Ltd., Hatfield, Herts., are employed on coal excavation.

Four 13-ton electric derricks, used for hoisting the coal, were designed and manufactured by Butters Bros. & Co. Ltd., Glasgow, and can operate with either 10-ton capacity skips or 5-ton grabs.

Auxiliary plant includes both Caterpillar D-8 and Vickers VR-180 dozers and scrapers, and a Caterpillar 977 Traxcavator fitted with hydraulic back ripper is being successfully used to remove the hard bands of shale occurring in the coal seams.

A recent innovation is horizontal drilling by 24-in, coal augers manufactured by Joy Sullivan Limited, Greenock, at the southern limit of the site and to date over 700 tons of coal have been recovered by this method, which is in its experimental stage.

Method of working

The general method of working the site planned at the beginning of the job has been followed. Along the eastern boundary an initial box cut 3,200 ft. long, 100 ft. wide and 230 ft. deep was formed in mid-1956. Parallel cuts 80 ft. wide and running in a north-south direction are to be made progressively towards the west and the number of similar cuts necessary before the site is worked out will be about 65. In June 1957 the eighth cut was being worked.

After removal of 12 in. of topsoil and 3 ft. of subsoil, which are stored on the site, 5 cu. yard electric shovels are used for reduction of overburden down to about 140 ft. above the bottom coal.

Highway on the high wall

Working on the high-wall highway thus formed are the two 1,150-B draglines, four electric derricks and rotary air-blast drills, in addition to the coal-hauling equipment as seen in Fig. 3. The draglines, working one at either end of the cut, remove the overburden down to the coal seams and deposit the spoil in the previous cut, from which the coal has been removed as seen in Fig. 4. In the early stages of this work some six million cubic yards was removed by Euclid dump trucks to an old site at Ewart Hill one mile distant, in addition to nearly four million cubic yards which is lying in dumps around the site to await replacement in the final void.

Shift system

Work at Acorn Bank goes on throughout the whole week, night and day. The draglines work three 8-hour shifts each day, and the rest of the site work is in two 12-hour shifts, each shift being in charge of a works superintendent. Coaling is carried out during hours of daylight.

Maintenance of plant

The modern and well-equipped site maintenance shops enable most jobs to be tackled without delay, including complete engine overhauls; and a staff of fitters, welders, and riggers is on call throughout the 24 hours for emergency as well as routine work; the maintenance team have a highly developed system designed to keep down time to an absolute minimum.

High-speed electric derrick cranes

For hoisting the 10-ton loaded skips, Fig. 5, from the cut and tipping into the coal haulers, or the alternative method which is by 5-ton grab, four electric derrick cranes are used. Each crane is mounted on three bogies at 60 ft. centres which are fitted with four large diameter single-flanged rail wheels set for 5 ft. 6 in. rail gauge. The two end bogies carry 60-tons of ballast. Figs. 6 and 7 show these cranes on the high wall raising the 10-ton loaded skips and tipping the coal into the 40-ton bottom-dumping trailers.

To enable the direction of travel to be altered through 90 deg., four 25-ton hydraulic jacks are fitted to each bogie, allowing the bogies to be raised and the wheel housings to be rotated to the required position.

Visibility for the operator is a feature of the machine, as the cabin is located on the top section of the mast, with all controls placed well forward in the control cabin so as to allow the operator a clear and uninterrupted view of the skip in all working positions, his eye level being 36 ft. above the ground.

Skips are all of welded sheet-steel construction with open end. The hoist trunions are set slightly forward of centre of gravity so the loaded skip is balanced by the tip rope. The skip is emptied by lowering on the hoist rope while the tip rope is held by the brake. Fig. 8 shows a skip being loaded by a 2 yard coal shovel. Details of the derrick cranes are as follows: — Maximum load — 13 tons (i.e. 10 tons coal + 3 tons skip) the crane was tested on a 25 per cent. overload, 16¼ tons.

Maximum radius — of 140 ft. for raising full loads. Jib length 152 ft. tubular lattice all welded.

Speeds of operation under full load —

(1) Hoistings	120 ft. per min.
(2) Slewing	450 ft. per min.
(3) Luffing	60 ft. per min.

Current supply — 440 volt, 3 phase, 50 cycles.

Motors —

Hoist	180 b.h.p.
Slew	35 b.h.p.
Luff	75 b.h.p.

A small plate-laying gang is employed on the tracks, and when moved to another position the derrick is towed by a dozer.

Coal augers

Horizontal augering for coal is claimed to be something quite new on an opencast site in this country. At Acorn Bank in recent months, a Joy coal auger has been in use for experimental drilling into the seams at the northern and southern limits of the site, to recover extra coal beyond the area being worked by the method adopted for the site generally. The total amount of coal recovered so far is around 700 tons.

The auger is electrically powered, with a supply of 500 volts, 3 phase, 50 cycles.

It is mounted on a steel frame with small diameter single-flanged steel wheels set for a 3 ft. 6 in. rail gauge. Four hydraulic jacks are fitted, one to each corner, for levelling up the machine.

A barrel-type cutting bead fitted with 18 Tungsten Carbide tipped bits and having a pilot drill with eight finger bits set in the centre of the barrel head, is used. The outside diameter of the cutting head is 30 in. and the length of auger flights 48 in.

The bits are fed into the hole by a hydraulically-operated rotating ram which travels a distance of 4 ft. to 6 ft. To facilitate hoisting, fitting and removal of flights, a small derrick arm has been incorporated. Horizontal holes are drilled to a depth of 60 ft. at approximately 3 ft. centres and two drilling heads are available, enabling flights to be stored in the completed hole and extracted for use by means of a winch fitted to the machine.

The penetration rate averages 1 ft. per min., and each completed hole yields approximately 12 tons of coal which falls on to a small mobile conveyor and is loaded into road wagons.

The next stage is to increase the depth of each hole and thus step up the output per hole.

* See "Colliery Engineering," Vol. 33, Sept. 1956, pp. 360—367.

Drawings and Photographs accompanying the article