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July, 1958

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On reaching the surface, the hoppits are tipped by the lazy-chain method into Aveling-Barford dumpers. The Aveling-Barford dumper is powered by a Fordson Major diesel or petrol engine, which delivers 42 b.h.p. at maximum r.p.m. Transmission is through a constant mesh gearbox giving six forward and two reverse speeds; the maximum forward speed is 11 m.p.h., and the maximum in reverse is 4-18.

One of the features of this dumper is the two-way steering which is achieved by a single steering wheel in the centre of the cab, and duplicate foot pedals. This allows the driver always to face the direction of travel and minimises the time lost by manoeuvring.

The weight of the machine, when fitted with a rock body, is 8,570 lb., and the maximum payload which can be handled is 8,270 lb. The load is tipped by gravity and power assistance derived from torque reaction of the brakes on the driving wheels.

Safety-glass windows fitted into the steel cab provide protection without impairing visibility.

Because of the proximity of certain cables at the shaft top, these standard SF 3-cubic yard machines had to be slightly modified; a narrower body was fitted and 12.00 instead of 13.00 tyres were used. These alterations, however, did not affect the payload capacity of the dumpers.

The dirt is transferred into railway wagons for removal into barges and then out to sea, or into lorries and taken to stone disposal sites adjacent to Wearmouth colliery.

Usually 8 men are employed in the shaft bottom during mucking.

Walling

The first 400 ft. of shaft is lined with 3 ft.-thick mass concrete and thereafter the thickness is reduced to 1 ft. 6 in. The concrete lining is not reinforced.

A batching plant incorporating a 30-ton cement silo, and a high-discharge cement mixer, both manufactured by Stothert and Pitt Limited, Bath, Somerset, are used to prepare the concrete, thus ensuring an accurate and easily controlled mix.

The aggregate and sand bins are loaded by means of the Rushworth crane which was previously used for the initial stages of the sinking, and the correct mix is run into a travelling weighing hopper fitted with special ball bearing travel wheels and a dial-type weigher. From the hopper, the mix is transferred to the one cubic yard capacity high discharge mixer where the correct amount of water is automatically added.

The concrete is fed from the mixer into two cubic yard-capacity Blaw Knox bottom roller gate hoppits and lowered down the shaft.

In order to keep the aggregates at a uniform temperature throughout the whole lining, the sand and gravel pits are heated by steam pipes; the average temperature of concrete entering the shaft is 70°F.

A strict watch on temperatures was maintained while lining the frozen portion of the shaft by placing maximum thermometers in the relief pipes. These showed that the heat generated by the concrete rose to over 100°F. and only after the concrete had been placed for four days did the temperature start to fall. This generated heat thaws the face of the excavation sufficiently to counteract any freezing of the concrete in the initial stages.

The type of walling scaffold in use at Wearmouth is shown in Fig. 26. The top deck consists of a rectangular scaffolding which forms a catwalk for the men pouring the concrete, and the lower, or working deck, is circular with six doors across the middle and two at the sides. This has a covering of 4 in. deals.

The concrete is lowered to the top deck of the scaffold, where it is emptied into a movable chute which guides it behind the shuttering. The concrete is then rammed with Blaw Knox pneumatic vibrators.

The shuttering, which is carried on angle rings. consists of 16 segments, 2 ft. 6 in. in width, and before being placed in position, the backs are coated with mould oil to facilitate easy withdrawal.

When a section of walling is completed, the scaffold is immediately lowered and while descending, the relief pipes which have been left in the concrete are drilled out. On reaching the bottom of the newly concreted length, cement grout is pumped through the relief pipes to fill any voids between the strata and the concrete. Grouting is continued in each relief hole until cement is observed to flow out of the hole above. This operation is controlled from the top deck, and as the scaffold is raised, the shuttering is stripped by men on the bottom deck.

It should be mentioned that the corrugated-iron back-sheeting is left in place and not removed with the temporary lining.

While working in the frozen ground, an average of 200 cubic yards of concrete per day was laid.

The mix was 4:2:1 with a water cement ratio of 0.5 and the average seven-day test cube result was above 3,000 lb. per sq. in.

The maximum length of 3 ft. walling concreted in one day was 21 ft. 6 in. and the largest depth of walling attempted was 93 ft., which was completed in 17 shifts.

While working through the frozen ground, the air temperature in the shaft was kept above strata temperature by the use of electrical heaters. These heaters were placed adjacent to the fan so that warm air is blown into the shaft.

Thawing

When the walling had been completed through the frozen ground and had been thoroughly grouted, it was decided to thaw out the strata. This is an extremely important operation and great care has to be taken to ensure that the ice wall is not thawed out too quickly, otherwise it may become very difficult to deal with the inevitable leakage water.

The freezing plant was disconnected from the circuit and brine was circulated as before but now the temperature was gradually increased from about -2°F. to about 30°F. by placing a steam, radiator in the brine circuit and by reducing the density of the brine. The complete thawing process took three months approximately.

When water appeared, leaking through the lining, a start was made to inject liquid cement into the feeders, and this process is normally carried out up and down the shaft as leaks appear. At the time of writing, which was after the shaft had been completely thawed, the water leakage for the entire shaft had been reduced to less than gal. per min.

While the shaft is being thawed out, the concrete tends to expand due to the heat being applied to the strata, and at the same time shrinkage occurs due to the concrete maturing. It is normal, however, for shrinkage cracks to appear which are sealed by cement injection.

The freezing tubes were not withdrawn at Wearmouth. They were perforated at intervals of about 10 ft. and cement was pumped into them to seal up the holes which would otherwise have caused a weakness in the strata round the shaft. In all, about 170 tons of cement was required for this purpose.

General

The final depth of the 24 ft. diameter shaft will be 2,100 ft. and at present it has reached a depth of over 1,400 ft. It is hoped that the sinking, including the construction of skip pockets and insets, will be completed by mid 1959.

Ventilation in the shaft is provided by two Aerofoil fans, manufactured by Woods of Colchester, working in tandem. The steel ventilation ducting terminates before the shaft bottom is reached, and the last length consists of canvas ducting which does not suffer a great deal of damage from shotfiring.

Illumination in the shaft is provided by a cluster of four lamps suspended from the lighting cable. The cable is raised and lowered by a lamp winch on the surface.

The decision to use a plain concrete lining instead of incorporating reinforcements was based mainly on the fact of the uncertainty concerning the bond between the concrete and the metal when subjected to fairly wide temperature changes. Concrete and steel have different coefficients of expansion and as there is also bound to be a concrete shrinkage, the general effect would be, in all probability, a loosening of the bond.. This, in turn, would result in leakage.

Acknowledgments

For permission to publish this article and for supplying photographs, we are indebted to Mr. S. J. Temperley, area general manager, No. 1 Area, N.C.B., Durham Division, and we would like to express our thanks to the Group and Colliery Management of Wearmouth Colliery, for permitting us to visit the site and take photographs, and also to Mr. W. I. Spike and Mr. W. M. Wild of the Foraky Boring and Shaft Sinking Co. Ltd., for supplying us with additional information.

References

Wood, A. F., M.B.E., A.R.I.C.S., "Placing of Mass Concrete Shaft Lining in Frozen Ground." Trans. Inst. Min. Engineers, 1956-57, 116, 713.

Durham Coalfield, Regional Survey Report, H.M.S.O.

Wild, W. M., and Spike, W. I., "Sinking at Wearmouth Colliery New Shaft 1955-57." Paper presented to the North of England Branch of the Royal Institute of Chartered Surveyors. May 1957.

Drawings and Photographs accompanying the article