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GIRDLENESS  SEWER OUTFALL

This is the main scheme of the Corporation Act of 1899. There was already a sewer from near Rubislaw Quarry, along Queen's Road, Carden Place, Carden Terrace, and Skene Street. By the new Act a sewer commences at the junction of Summer Street with Skene Street and goes along the following route:- Skene Street, Rosemount Viaduct, Skene Terrace, North Silver Street, Golden Square, South Silver Street, Crown Street, Portland Street, under the railway and across Old Ford Road to the Esplanade, the River side to Point Law, under the Dee, round Torry Harbour, along St Fittick's Road, along the north side of Nigg Bay, and terminates in the sea a little to the North of Girdleness.  From Summer Street to Silver Street the track of the sewer was laid open. From Skene Terrace to Windmill Brae in Crown Street, the sewer was made in a tunnel passing under Golden Square at a depth of 30 feet. Here granite rock was encountered, and blasting was necessary.  In Crown Street a great depth of fine stratified sand was found. Excavation was easy, but it was necessary to provide against caving in of the sides by driving planks deep into the sand on both sides. From Windmill Brae to Portland Street advantage was taken of the deep excavation to form above the sewer a subway for conveying electric cables from the Corporation Electric Works to the west end. In crossing Union Street there was found at a higher level than that of the sewer a large stone-built sewer, 5 feet 6 inches high, and 3 feet 6 inches wide, clean, and free of sediment. This was probably the 1st made sewer in Aberdeen. As the whole of Union Street belonged to the town the sewer had been made by the Town Council.  At Academy Street there was met with also the old intercepting sewer on its way from Great Western Road to the Irrigation Farm. It too was above the level of the new sewer. Both these were connected with the new sewer, and thus a great volume of sewage was diverted.  At Portland Street a connection was made with the Holburn Culvert, allowing an overflow in the event of the sewer becoming full in time of heavy rain. No difficulty was found in making the sewer along the River side. At Point Law the difficult task of crossing the Dee had to be faced. Care had also to be taken to place no obstacles in the way of carrying out a scheme projected by the Harbour Trustees.  The sewage outlet can be traced on the inset from the plume heading across Greyhope Bay towards the harbour entrance.

- and to think we ate aa' the Buckie's from the Bay o' Nigg rocks.

DOCKS IN THE DEE
From Sir Thomas Dick Lauder's account of the flood in the Dee in 1829, the whole estuary from Virginia Street to the South Esplanade must have been a continuous sheet of water at high tide on the day of the flood.  Apparently never having heard of this the Harbour Trustees are in mind to construct Quays and Docks along the River from Point Law as far up as Victoria Bridge, and to deepen the bed of the river so that ships can go out and in at any state of the tide. With this in prospect the Harbour Trustees demanded that the upper surface of the tunnel to be made under the River for the sewage should be 38 feet below the surface of the River at mean high water. As the rise of the tide at Aberdeen is about 18 feet this makes the top of the tunnel 25 feet below the surface of the river at low water, and it is thought that 4 feet between the bottom of the river and the top of the tunnel would be sufficient to protect it from injury caused by anything resting on the bottom of the river. At present the top of the tunnel is 20 feet below the bed of the river. The navigation channel therefore might be excavated so as to have a depth of 34 feet at high water and 21 at low.

THE RIVER DEE TUNNEL
The tunnel under the river Dee is 342 feet long and 8 feet in external diameter. It was driven from a vertical shaft on the Torry side, 13 feet in diameter, sunk to a depth of 62 feet below high water mark. The shaft is lined with a cast-iron cylinder built up of flanged segments bolted together. The lower edge of the first ring of segments was sharp, so that the cylinder cut its way down as the inside material was excavated. The bottom of the shaft was in glacial stony clay; above it was a layer of fine i-and ; above it 40 feet of grey laminated clay ; and at the surface there was six feet of red laminated clay. When the glacial clay was reached water began to come in, apparently quite fresh. The quantity increased with the depth, and when the bottom was reached the shaft could not be dried though 60,000 gallons were pumped out per hour. By letting the water rise to the surface and come to rest divers were able to lay a plug of concrete, six feet thick, on the bottom. This effectually stopped the water from coming in. The glacial clay had been the bottom moraine of a glacier descending the Dee valley, and probably it had extended far up the river, its base rising with the rise of the river bed. Hence, if the glacial clay were pierced to the bottom, water might rise in the bore and overflow at the top. Fresh water was found also in a coffer-dam made in the harbour when the new Regent Bridge was founded. The glacial clay had been formed under a glacier which melted as it entered the sea, and hundreds of tons of large rounded blocks brought down by the Dee glacier have been dredgd from the navigation channel.

EXCAVATING THE TUNNEL
It was thought necessary to use compressed air in making the tunnel to prevent water from oozing in. A lock was made in the Torry shaft by placing 2 floors across it with an interval of 6 feet between them.  The floors were made air-tight, and there was a door in the roof of the lock and another in the bottom. When a person entered the lock the doors were shut, and compressed air was let in till the air in the lock reached the same density as that in the tunnel; then the lower door was opened, and by descending a long ladder the tunnel was reached. It was lit by electricity, and the air was kept fresh by a current of compressed air passing through a pipe terminating at the working end, and the discharge of the same quantity of tunnel air at the shaft. The tunnel was pierced by means of a shield with cutting knives in front. It was of the same diameter as the tunnel externally, and it was pushed forward by hydraulic rams giving a pressure of 100 tons. At the same time the silt in the bed of the river was excavated and removed at a door in the front of the shield, and passed up the lock to the outside. As the shield advanced a cast-iron lining was built up of flanged segments bolted together. A grouting of cement and sand was injected between the lining and the surrounding silt by means of an air compressor, so that the cast-iron casing of the tunnel is protected externally from the action of water. When everything was finished the tunnel received a lining of cement concrete internally to protect it and to facilitate the flow of the sewage. The tunnel is divided longitudinally from end to end by a steel diaphragm bolted to flanges at the top and bottom, so that the sewage can be sent through both divisions simultaneously, or through either alternately when it is necessary to clean it out. At the bottom of the Torry shaft there is a pit to receive sludge, which can be pumped out when necessary. The vertical shaft is fitted internally with 2 cylinders connected with the 2 divisions in the tunnel. A similar shaft has been made at the Point Law end of the tunnel, only a sludge pit is not required at the north end.

The material excavated in making the tunnel was different from that found in sinking the vertical shaft on the Torry side. It was a homogeneous grey green silt, with occasional thin seams of sand. Old maps show that the site of the tunnel was in 1829 part of the channel of the Dee. It is probable that in the great flood in August of that year the bed of the river had been excavated to a great depth by the rush of water at the fall of the tide, and that, the hole had been gradually filled up afterwards by fine silt. Among the material excavated from the tunnel there was a fragment of a china plate. This was sent to the British Museum with a request for a statement of the probable age of the fragment. The reply was that it had been made in the early part of 19th century, so that it might have found its way into a deep hole in the River soon after the flood.

From Victoria Bridge to Point Law, and from the Torry end of the tunnel to the north end of St Fittick's Road, the sewer is only about six feet in diameter internally, which is sufficient for the present population and a considerable prospective increase. If necessary these parts of the sewer could be duplicated at little expense ; but the tunnels under the Dee and St Fittick's hill and some other parts have been made from the first sufficient for a population of 270,000, because the extra expense thus incurred was but trifling compared with what it would have been to double them afterwards.

ST FITTICK'S ROAD TUNNEL
From the Torry end of the tunnel to St Fittick's Road the track of the sewer was an open cut; but to pass under the hill a tunnel was made. Three shafts were sunk, one at each end about 32 feet deep, and one in the middle 90 feet deep. The tunnel was excavated in hard, stony, glacial clay, so compact and hard that it seemed as if the tunnel could have been made from end lo end without timbering; but to prevent accidents the roof was carefully supported till the sewer was formed. The internal diameter of the sewer is 7 feet, and it is formed of a tube of concrete, 15 inches thick, faced in the lower half with red brick. In going down the brae between the summit of the hill and the level ground at the head of Nigg Bay solid rock with glacial clay above was encountered, and loose blocks of gneiss were met with, lying upon one another. They were scarcely rounded at all, and it seemed as if they had been originally diffused through the body of a glacier, and that they had been brought together by the melting of the ice in which they had been embedded. There were many large vacant spaces between the blocks, into which an arm could be thrust at full length. Upon the upper side of several of the blocks there was a black powder, which dissolved in hot hydrochloric acid and was probably oxide of manganese. It had been in the mass of the glacier, and had been left where it was found when the glacier melted away. Its origin must have been in rocks containing oxide of manganese, which had been abraded by ice charged with sand. Among the stones in the glacial clay were some pieces of serpentine, which probably came from the Coyles of Muick, near Ballater

THE BAY OF NIGG CUTTING
On leaving the tunnel a raised sea beach was met with. Here, for the sake of saving a little expense, the sewer was reduced to 5 feet of internal diameter; but on coming close to the Bay of Nigg the diameter was increased to 7 feet all the way to Girdleness Lighthouse. The track was blasted out of solid gneiss rock to a depth rising from 10 to 36 feet. About 40,000 tons of rock were removed. Some bands of igneous rock were crossed, one of which of a chocolate brown colour contained carbonate of lime and effervesced with hydrochloric acid. At the place where the sewer proper ends and the outfall begins, the gneiss could have passed for granite, and an old quarry filled up with rubbish had yielded the stones used in building the tower of the Lighthouse. In this quarry was formed a large chamber in which are placed apparatus for regulating the outflow of the sewage. At the end of the sewer there is a sliding door called a penstock, whose rise or fall confines the sewage if necessary to the sewer, or admits it into the chamber. At the lower side of the chamber another penstock keeps in the sewage and excludes the sea waves when it is raised, and lets out the sewage when it falls. In the middle of the chamber between the 2 penstocks there is an automatically working valve at the mean level of the sea, which closes when the level of the sea is higher than that of the water in the sewer, but opens when the tide falls. A substantial house has been built over the valve and penstock chamber to protect them from the sea in storms.

THE SEA OUTLET
The outlet portion of the sewer extends 190 yards beyond the Penstock Valve House. It is constructed of 67 large pipes of cast-iron, averaging 7 tons in weight, bolted together by means of external flanges. The pipes are 9 and 6 feet long, and 7 feet in diameter. Being for the most part under water at high tide they are protected by a heavy mass of concrete at the sides and over the top. The mouth of the outlet is 21 feet below mean high water of ordinary stream tides. This excludes air from the pipes, which would be a source of danger if the open mouth of the outlet were struck by a heavy wave while the upper end was closed. The total length of the Girdlenesss main sewer exceeds 3 miles. The construction of the sewer was let in 2 contracts, one for the section extending from Skene Street to Point Law, and another for the section from Point Law to Girdleness - both sections being about the same length. The upper section was begun in 1900 and finished in 1902, no great difficulties being encountered in constructing it. The lower section was begun in 1902, and ii was completed in 1907. It comprehended difficult works, and their construction showed great ingenuity and engineering skill in their design, and splendid workmanship in their execution.

This once elegant, but now rather downtrodden building was constructed as part of the massively engineered sewage outfall system created for Aberdeen as the result of a scheme drawn up in the years 1896-98. Previously, a sewer built in the 1860s had been expanded bit by bit, but by the close of the 19th century, the growth of the City as well as changing standards of sanitation required more radical action. The new sewer was over 3 miles long, piping effluent from both north and south of the River Dee. It was designed to discharge up to 81 million gallons per day, a quantity representing a significant degree of future planning. This Penstock and Valve House provided an automatic flap that prevented the sea from backing up the sewer, but opened when required by pressure of sewage. The building itself was constructed by John Morgan. The line of the sewer tunnel, which was dug through substantial rock cuttings in places, can be clearly observed on the shore between the Valve House and Nigg Bay.


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Last modified: 01/09/2013