Look Back Friday - The Mersey Tunnel
Posted on 25th June 2021 at 01:25
Why was a tunnel created?
The Mersey Tunnel joins Liverpool with Birkenhead of a distance of nearly three miles. It is the worlds largest underwater tunnel that runs beneath the River Mersey. Who would have thought that the tunnel took nearly nine years to build with the construction project presenting unusual engineering problems.
The Port of Liverpool only exists because of the River Mersey, a broad stream wide enough to act as a highway for world shipping and trade. The communication between Liverpool and the towns on Cheshire banks was extremely poor. Ferries were established to try to overcome the problem however, the lack of physical connectivity continued to divide this economic powerhouse. The spectacular increase in road traffic also began to cause issues to the port, the vehicular ferries were not equipped to deal with the flow of motor traffic to and from a port of world importance.
In 1922, Sir Archbald Salvidge a prominent Merseyside leader at the time, put a proposal together for a committee to be appointed to examine the feasibility of a bridge or a tunnel to be built as a link across the river. Merseyside composed of four major area Liverpool and Bootle on the Lancashire bank and Birkenhead and Wallasey on the Cheshire bank. Each municipalities sent delegates to a Merseyside Co-ordination Committee, which commissioned engineers to report on the relative merits of a bridge or a tunnel. The engineers included the late Sir Maurice Fitzmaurice, Sir Basil Mott and John.A Brodie.
The possibility of a bridge was fully considered. It was decided that such a bridge would need to have a span of 2,200 feet, a headway of 185 feet and a width of 90 feet. The cost of a bridge between Liverpool and Birkenhead was estimated at £10,550,000. The estimate for a roadway tunnelled beneath the river, however, came out at £6,400,000. In addition to its economic advantages, the tunnel would have branches on either side of the river. Thus, the report pointed to the advisability of a tunnel larger than any subaqueous tunnel ever before conceived.
For two years the matter was discussed. The immense cost of the undertaking was not the only difficulty. It became apparent that the four corporations could not agree, and in the end Wallasey and Bootle dropped out, leaving the entire responsibility to Birkenhead and Liverpool.
Finally, in 1925, Parliament sanctioned the building of the Mersey Tunnel. Princess Mary, released the power which operated the first drills on December 16.
How the Mersey Tunnel was built
At first the excavation was done by pneumatic hammers, but the rate of excavation was increased by the use of explosives. In all, 147,000 lb of gelignite were used. The excavation alone was a mammoth achievement, as 1,200,000 tons of earth and rock were removed. The rate of excavation was more than half a ton of rock for every minute between June 1926 and August 1931. The haulage of the rock to Storeton Quarry, on the Cheshire side, and to Dingle and Otterspool on the Lancashire side, was all done at night.
The great danger that was expected in the undertaking of this project was the seepage of water into the workings, particularly in the under-river sections of the tunnel. Shafts were sunk on either side of the river, 21 ft 2½ inches in diameter and 190 feet deep. These shafts were lined with cast iron until the red triassic sandstone strata was reached; after this no lining was necessary. About halfway down these shafts two pilot headings - exploratory bores - were driven, one at the top and the other at the bottom of the cross-section of the proposed main tunnel. The pilot headings were 15 feet wide and 12 feet high. No lining was required for these headings except in the vicinity of faults in the rock.
To obviate the influx of water a special cementation process was used. Cement was injected under pressure through boreholes in the rock. The cement thus filled the local fissures through which water might pass into the working. When 850 feet of heading had been driven from the Liverpool shaft, and 1,170 feet from the Birkenhead shaft, it was found that the flow of water was not sufficient to warrant the continuation of this process. A considerable influx of water, however, had to be dealt with. The Liverpool heading was started first and a special drainage heading, 7 feet in diameter, was driven from the foot of the first shaft up a gradient of 1 in 500 to meet the lowest part of the tunnel under the river. Drainage shafts were driven vertically to connect the pilot headings with the drainage heading. The flow of water in this heading was found to be 2,410 gallons a minute.
On the Birkenhead side, however, the influx of water was found to be considerably less and could be dealt with by using of pumps. When the pilot tunnels met beneath the river, all the water drained by gravitation into the Liverpool drainage heading, from which the maximum quantity of water pumped was 4,300 gallons a minute.
The purpose of the pilot tunnels, each the size of a London Tube railway tunnel, was mainly exploratory. The lower tunnel was driven in advance of the upper heading and vertical boreholes were made to discover the nature of the rock through which the upper tunnel would have to be driven. Similarly, boreholes were driven in advance of the heading to give due warning of any difficulty that might have to be encountered.
Twenty-seven months after the start of the work the two headings met. This was an occasion of great importance. Sir Archibald Salvidge, whose efforts in favour of the enterprise broke through the thin wall of rock. Liverpool and Birkenhead Councils exchanged greetings 150 feet below the River Mersey.
Meanwhile, a full-scale experimental tunnel, 300 feet long, was being built on the Birkenhead side. As there was no precedent for many of the problems involved in building such a large subaqueous tunnel, the engineers decided that it was necessary to complete a section of the tunnel to find out which method would be most satisfactory.
The most suitable method of procedure was found to be that of excavating the upper half of the tunnel first. As the work progressed the cast-iron lining was fitted in segments of 24 inches. Then the lower half was excavated while the upper semicircle of the lining rested on the natural rock. The upper segments were then bolted together before the support of the rock was removed and the lower segments of the lining erected. Before the building of this experimental section of the tunnel it had been thought that the segments could not be made longer than 18 inches but experiment not only proved that it was safe to extend the length of each segment to 2 feet. This reduced the cost of the work but also increased the rate of progress.
The method of erecting the lining was of great importance. Although at certain places it was necessary for the segments, to be erected by hand, a special machine was designed to assist the process. Operated by compressed air, the machine was mounted on a truck which moved on rails laid in the tunnel. The erector had a telescopic arm which could be rotated and was moved as the segments were put in place.
The enlargement of the upper pilot heading to the full diameter was the first operation on the main tunnel. At several points in the upper tunnel rock was excavated into chambers in which the erectors were placed after the first segments of the lining had been fitted by hand.
Except when the rock surface was unusually broken or dangerous, enough space was excavated for two segments to be fitted at one operation. In the midstream portion of the tunnel there was a thickness of only 3 ft 6 inches of rock between the roof of the tunnel and the bed of the Mersey. Here it was necessary to support the arch with timbers reinforced by steel bars. Pneumatic hammers were used instead of explosives for excavating in this vicinity.
The removal of debris required careful organisation. An electric railway was already laid in the lower pilot tunnel, and it was decided to make use of this. Chutes were excavated from the upper to the lower tunnel. Down these chutes the debris was shot, and then shovelled into the wagons which were hauled by electric locomotives to either shaft. When the two tunnels were enlarged into one, the railway lines had to be moved, for they impeded the enlargement of the lower semicircle. A temporary roadway was then suspended from the arch, and the lines were re-laid on to a hanging roadway. Lifts at intervals handled the debris, and thus the remaining excavated matter was removed.
As lengths of the tunnel lining were finished work was begun on the roadway, which was built 1 ft 6 in below the horizontal diameter. Therefore, in the under-river section of the tunnel there is almost as much space beneath the roadway as there is above it. There is room, for a second roadway to be built at a lower level, should the volume of traffic ever make this a necessity.
The roadway is made of reinforced concrete, anchored at either side by the cast-iron lining. The roadway is supported in the circular section by two intermediate walls, 12 inches thick and 21 feet apart. In the shallow invert section of the tunnel the road is supported on columns spaced 7 feet apart.
“Gunite” rendering, which forms the strongest concrete composition was applied to the surface of the tunnel. A waterproof layer of bitumen emulsion was next applied, and this was covered with a special finish in a pale oatmeal colour. Finally a glass dado was built to a height of 6 ft 3 in throughout, the full length of the tunnel.
One of the greatest problems encountered in the design and building of the Mersey Tunnel was that of ventilation. The problem was unique in the history of engineering. For the first time in the annals of science a means had to be found of ventilating a tunnel more than two miles long and 150 feet deep, with internal combustion engines passing through at the rate of 4,150 an hour.
The problem was so important that extensive full-scale experiments were made before the choice of a ventilation system was made. As soon as a 1,000-feet section of tunnel and roadway had been completed on the Birkenhead side, this was used for the large-scale experiments. Temporary blowing and exhaust fans were installed, with a combined capacity of 600,000 cubic feet of air a minute. Brick bulkheads were built across either end of the experimental section of tunnelling.
In this section quantities of damp straw, which produced dense smoke, were set alight. A steam boiler which made heavy clouds of vapour was drawn through the tunnel, and the effect of various ventilating systems was calculated. Finally, it was decided to use the upward semi-transverse system.
By this system of ventilation pure air is blown through the ducts beneath the roadway. The air enters the upper half of the tunnel through orifices placed at intervals of 18 inches at roadway level. The orifices are graduated in size to ensure an even distribution of pure air throughout the entire tunnel. The impure air is drawn along the roof of the tunnel itself to the exhaust chambers at each of the six ventilating stations.
These stations, three on either side of the river, provided an architectural problem of no mean order. Air had to be taken in through the roofs of the buildings rather than from openings in the walls.
Duplicate sets of blower and exhaust fans, ranging up to 28 feet in diameter, are housed in each ventilating station. The buildings are divided into three airtight compartments, one for the blowers, one for the exhaust set and one for the switchgear room. The fans can deliver 2,500,000 cubic feet, of fresh air to the tunnel every minute and withdraw a similar volume. The largest of the fans, 23 feet in diameter, is housed in a casing 50 feet wide.
All this machinery is controlled from a central operating room in the George’s Dock, Liverpool, ventilation building. The instruments record the conditions of the ventilation system at all parts of the tunnel. Precise instruments record the visibility in the tunnel, measure the exact quantity of poisonous fumes and register the flow of air. Invisible rays across the tunnel record the number of vehicles that pass and an automatic counter shows how many vehicles are in the tunnel.
The width of the roadway in the main tunnel is 36 feet between the kerbs, which allows for four lanes of traffic. In the branch tunnels the roadway is 19 feet wide. The capacity of the Mersey Tunnel is 4,150 vehicles an hour, spaced at intervals of 100 feet apart and moving at a speed of 20 miles an hour. At this speed vehicles run through the tunnel in six and a half minutes.
For a length of more than two miles the tunnel has an internal diameter of 44 feet and the bottom of the tunnel at its lowest point is 170 feet below the level of the Mersey at high water. This enormous tunnel, burrowed beneath the mighty river, is a monumental feat of engineering skill.
On July 18, 1934, the Mersey Tunnel was opened to traffic by His Majesty King George V. The main tunnel has a length of 3,751 yards, from the Old Haymarket, Liverpool, to King’s Square, Birkenhead. The branch tunnels which lead to the docks on either side of the river bring the total length of roadway to 5,064 yards, or nearly three miles. The words of His Majesty King George V at the opening of the Mersey Tunnel sum up the Mersey Tunnel preject perfectly: “Who can reflect without awe that the will and power of man which in our own time have created the noble bridges of the Thames, the Forth. the Hudson and Sydney Harbour, can drive also tunnels such as this, wherein many streams of wheeled frame may run in light and safety below the depth and turbulence of a tidal water bearing the ships of the world?”
Tagged as: challenges, construction, look back at construction, look back Friday, mersey side tunnel
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