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May, 1958

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In order to exploit the under-sea reserves of coal lying off the Durham coast, a considerable amount of development work is being carried out at several of the coastal collieries situated in this region. At Wearmouth, in the No. 1 Area, N.C.B. Durham Division, a new shaft 24 ft. in diameter and 2,100 ft. deep, is being sunk which will be the main winding and upcast shaft for the under-sea workings of this colliery.

Geological considerations

The coal measures of Durham are part of the North-Eastern coalfield which includes both Durham and Northumberland. As in the Scottish coalfields, coal seams are found in the carboniferous limestone as well as in the coal measures but they are of little economic significance. The East Durham coal measures are some 2,300 ft. thick and dip gently towards the coast and under the sea where they show signs of rising again to form a slightly folded syncline. The trough of the syncline occurs approximately below the mouth of the river Wear. The eastern limit of the field has not been definitely established, but undoubtedly valuable reserves of coal lie in this under-sea area.

At Wearmouth, the coal measures lie beneath a thick unconformable layer of Permian rocks, the main member of which is the magnesium limestone. This limestone, according to a Regional Survey Report, "is one of the heaviest water bearers in Britain and, for this reason, troublesome in shaft sinking. The running sands (yellow sands) at the bottom of the limestone are a particularly serious obstacle; fortunately, impervious strata seal off the running sands from the coal and the workings below these sands remain comparatively dry."

The yellow sands and magnesium limestone form a natural reservoir from which the Sunderland and South Shields Water Boards draw their supplies, and a proviso made by them laid down that no water should be drawn from the limestone or be polluted during the sinking.

The development plan

Old records of County Durham show that the sinking of Wearmouth Colliery "A" shaft was commenced in May 1826. On June 13, 1835, the first cargo of coal was shipped and the annual output for that year was 45,000 tons. On April 4, 1846, the Hutton Seam was reached at 286 fathoms and the shaft was then said to be "the deepest mine in the world."

The "C" pit shaft was sunk in 1906 using the freezing process and was lined with cast iron tubbing. The fact that this shaft was sunk successfully and the proviso made by the Water Board, decided the National Coal Board to adopt the freezing process for the new shaft. Accordingly a contract for the freezing and sinking was awarded to the Foraky Boring and Shaft Sinking Co. Ltd., Nottingham, at the end of April 1955.

The seams now being worked at Wearmouth, and which will be worked as part of the development scheme, are the Main Seam and the Maudlin Seam. The Maudlin Seam is at present split and worked as Top and Bottom but these eventually unite to form one seam reaching a thickness of 12 ft.

In an Easterly and North-Easterly direction, the method of working to be employed will be principally mechanised longwall advancing, In the South-East area, where there is full-section Maudlin (12 ft. in thickness), it is planned to adopt mechanised room and pillar mining.

The freezing process

The principle of circulating brine through the water-bearing strata has changed little since 1883 but many improvements in the technique of the process have resulted in the successful sinking of many shafts through difficult ground. Since nationalisation, nine shafts have been dealt with by freezing and their completion to timetable brought about without any setback.

As mentioned earlier, "C" pit at Wearmouth was frozen in 1906 and the contractors state that the price of this new shaft now being sunk shows only an increase in cost of approximately 20 per cent. per cubic yard of frozen ground. This remarkable fact has been made possible, they say, by improved technique and more frequent applications of the process.

The aim of the freezing process is to form a pillar of frozen ground by circulating brine at a temperature of about -4°F. through the strata. The shaft is then sunk through the pillar protected by an ice wall of sufficient thickness to withstand the hydrostatic pressure outside the wall.

The ice pillar is formed by inserting freezing tubes in a number of boreholes spaced uniformly on a suitable diameter around the proposed excavation zone. Freezing tubes have an outside diameter of about five inches and are connected together by internal threading giving a flush joint. The lower end at the bottom of the hole is blanked off to ensure that none of the brine is lost.

An open-ended pipe, 1½-in. in diameter is placed inside each freezing tube and both are connected to their respective circulation ring mains at the surface. The brine flows down the inside pipes, extracts heat from the strata, and returns via the outside tubes to the brine tank where it is cooled by the ammonia refrigerator or evaporator.

In some cases, especially when freezing to great depths, it is necessary to drill two concentric rings of freezing holes to ensure that an ice wall of sufficient thickness has been formed.

Preparatory work

One of the advantages of the freezing method of shaft sinking is that work can be started on a selected site for a shaft or shafts with little or no preparation. That is, drilling of the boreholes can be carried out whilst foundations for the winding engine and scaffold winch are being constructed.

At Wearmouth, a level terrace was formed at 80 ft. above O.D. to accommodate the various buildings and plant required during the sinking. To do this, about 70,000 cu. ft. of pit dirt had to be removed.

Before the drilling of the freezing holes could commence, it was necessary to construct a fore-shaft to provide a drilling platform and to house the brine piping circuits. A section showing the fore-shaft construction can be seen in Fig. 2.

Drilling the freezing holes

The first hole drilled was the pilot hole, situated 3 ft. from the centre of the shaft. The purpose of this hole is to verify the nature of the strata and to determine the depth of freezing by carrying out a series of water tests. The pilot hole was sunk by the coring method to a depth of 550 ft., i.e., from the top 80 ft. above O.D., and water tests were carried out which showed that the depth of the freezing holes should be 351 ft. Testing is carried out by casing the hole and plugging the bottom with cement. After making sure that there is no leakage, the cement is bored out and a further length of strata is drilled; the make of water is then determined by pumping or baling tests.

It was decided that 34 freezing holes on a 40-ft. diameter to a depth of 355 ft. would provide an ice wall of sufficient thickness to withstand the hydrostatic pressure. The holes were sunk using two standard Foraky XXII winches with 60 ft.-high derricks. The first 23 ft. of each hole, through the existing dirt tip, was bored by percussive drilling and thereafter rotary drilling was employed. In all, 12,620 ft. was drilled.

Drawings and Photographs accompanying the article