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. 

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. 

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?”