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September, 1956

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By using some of the larger types of American excavating machinery in the working of opencast coal sites in this country, sites that were previously considered to be uneconomical have been opened up. One of the largest of these, is the Acorn Bank site in Northumberland.

Acorn Bank is situated about one mile south of Bedlington in Northumberland, occupies 440 acres of agricultural land, and will involve the diversion of a road and water and electricity mains which at present cross the site.

Costain Mining Limited, to whom the contract was awarded, are programmed to recover over five million tons of coal in the next seven years, and at an average output of 16,500 tons per week.

The site

There are three coal seams on the site; the High Main, a fairly consistent seam averaging 3 ft. 3 in. thick and containing a thin band of shale about 1 in. thick; the Top Grey consisting of two leaves averaging 2 ft. 7 in. thick and with a 2½ in. band, and the Bottom Grey having four leaves, the top and bottom being of consistent unbanded coal 3 ft. 1 in. and 2 ft. 10 in. thick and the middle two each 8 in. thick with bands varying between 6 and 9 in.

The spacing between the seams varies considerably over the site. Between the High Main and Top Grey the interval is 40 ft. in the west, decreasing rapidly to a minimum of 6 in, in the east. The interval between the Top Grey and the Bottom Grey varies in a north-south direction, being about 15 ft. along the northern boundary and reaching a maximum of 120 ft. in the south-east corner of the site. The maximum depth of the Bottom Grey occurs at the north-east corner of the site where the cover is 230 ft. This reduces to around 100 ft. at the western end, where the High Main outcrops below the drift.

Several faults occur within the area, mostly in a north-south direction; the amounts of throw vary considerably, but average between ten and twenty feet. Drift, which covers the area, consists of boulder clay and rarely exceeds 30 ft. in thickness, except in the south-east where there is up to 50 ft. Strata over the coal seams are normal shales and sandstones, mostly fairly soft and usually broken.

There are old deep-mine workings in the seams around the site, and other deeper seams below the site area are still being worked by local deep mines.

Method of working

The mining contract stipulates that restoration of the ground to agricultural use shall take place progressively. This precludes the use of a permanent access ramp into the cuts, as it would mean occupying a large area of land for the duration of the job, alternative schemes had therefore to be considered. One possibility was to construct a vehicle tunnel at right angles to the cut and in the middle of the low wall. This would be formed progressively and covered by the spoil as the cuts moved across the site. The difficulties of working the several seams by this method and the possibility of subsidence due to the deep-mine workings beneath the site led to the rejection of this scheme. The somewhat unusual method finally decided on was to employ a number of high-speed electric derrick cranes travelling along the high wall. These would hoist the coal in skips which would be shovel-loaded in the usual way and would be discharged into road transport on the cut top.

The general method of working the site is as follows. An initial box-cut is being formed along the eastern boundary. This will be approximately 3,200 ft. long, 100 ft. wide on the bottom coal seam, and having a maximum depth of 230 ft. Of the eight million cubic yards of material to be removed from this cut a large proportion will be hauled a distance of about a mile to the previously worked-out Ewart Hill Site, where it will be used to restore the area and improve drainage by the formation of a low hill. The remainder will be stored on the Acorn Bank Site for use in filling the final cut.

Parallel cuts, 80 ft. wide, will then be made, running in a north-south direction and progressing towards the west.

Equipment

Initially, 12 in. of top soil and 3 ft. of underlying subsoil are removed and either taken to tip in the early stages or taken to direct restoration later on in the job. A small fleet consisting of five Super C Tournapulls of 12 yards struck capacity and one Euclid twin-power scraper, 18 yards struck capacity, is being used for this work.

For top reduction of the drift and rock down to about 140 ft. above the bottom coal, three Ruston-Bucyrus 120RB electric shovels of 5 cubic yards capacity will be used. Two Bucyrus-Erie 1150B draglines, one with a 180 ft. boom, the other a 215 ft. boom, will handle the remaining overburden. Final details of the method of working these are not yet settled, but it is likely that the two machines will be worked independently from opposite ends of the cut and will each cross on to the low wall side for rehandling and cross-dragging.

Three Joy-Sullivan 58BH rotary airblast drills are in use for overburden preparation.

During the initial excavation to form the box-cut, both the draglines and the three shovels are being used to load a fleet of sixty 22-ton and 15-ton Euclid rear-dump trucks. Direct loading of a 22-ton Euclid by the 1 150B is assisted by the use of a specially designed skid-mounted hopper which offers some protection to the vehicle. By using a 22-yard Hendrix bucket on one of these machines, 22-ton Euclids are being loaded direct, without a hopper.

Payloads average 11½ cubic yards solid for the larger Euclids and cycles per hour are between 4 and 5 on a 10,000 foot round haul. Great importance is attached to the quality of site roads which are formed of three feet of sandstone rock and kept well graded. Traffic "keeps right" during the day shift and "keeps left" at night, in order to spread the wear. The traffic passes over a public road by means of a temporary double-track bridge 100 ft. long.

Loading and hauling the coal

The coal will be dug by 1½-cubic yard electric shovels which will load direct into the 10-ton capacity derrick skips. These will also be electrically operated will have 152 ft. jibs and will be rail-mounted to facilitate movement along the top of the cut. They will load direct into the transport vehicles which will consist of 40-ton bottom dumping semi-trailers.

For hauling the coal to the screens eight bottom-dump semi-trailers are to be used, with four prime movers. They are British-made and are believed to be the largest yet made in England. The capacity of each is 60 cubic yards which allows for a load of 40 tons of coal.

The overall dimensions of the complete unit are such that the final assembly had to be done in the open under a mobile canopy as it would not have been possible to manoeuvre the tractor with the semi-trailer within the workshops.

The machines were designed by the Eagle Engineering Co. Ltd., Warwick, and the first was produced and delivered in 19 weeks. The bottom-dump doors are robustly constructed, and are opened by gravity. They arc held in the closed position by a series of cams, the release locking gear for the cams being operated by a hand lever, and the doors are closed by handwheel-operated cable winch.

The body is mainly constructed from in. and in. plates reinforced with channel and angle sections. The rear axle unit is a single unsprung axle with four earthmover tyres 1800 in. by 25 in. which are 5 ft. 2 in. overall diameter and are capable of carrying a load of 30 tons. They are interchangeable with those on the tractor unit.

Two hydraulic jack legs at the front corners of the body are operated from the mechanical hydraulic pump on the tractor so that the trailer can be uncoupled from the prime mover during loading operations. The approximate gross weight of the complete vehicle is 66 tons, and the overall dimensions are-length 50 ft.; width 11 ft. 3 in.; height 12 ft.

A quick-acting coupler is fitted to give economy in men and machines, and the prime mover has a 270 h.p. Rolls-Royce diesel engine.

Drilling and blasting

It has so far been found necessary to blast all material below the drift. There are restrictions on blasting due to the proximity of buildings and public services, and these have in some cases prevented the most economical loading of the holes.

Edeco tricone bits 6 in. diameter are in general use and have a useful life of over 2,000 feet in the shales. The drilling pattern varies but is averaging 18 ft. by 18 ft, in the box-cut area.

Explosives used are opencast gelignite and 999 (an ammonium nitrate type explosive) both of which are manufactured by Imperial Chemical Industries Limited, and used in the ratio of about 20 per cent. Gelignite and 80 per cent. 999.

The 999 explosive has been used in both polythene bags and in tins, the latter being for use in wet holes, which forms a large proportion of the total. Blasting ratios in the box-cut are heavier than normal, due to the restricted movement of the rock, and average four cubic yards per pound of explosive. This yield should be considerably increased when open-wall blasting commences.

Drawings and Photographs accompanying the article