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It is mandatory to procure user consent prior to running these cookies on your website. 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.
Full- Scale Experimental Tunnel 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 had such great effect, broke through the thin wall of rock.
Meanwhile, a full- scale experimental tunnel, feet long, was being built on the Birkenhead side. Because 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. Two pilot headings, 15 feet wide and 12 feet high, were driven along the line of the main tunnel for exploratory purposes.
Exploratory boreholes were driven ahead of the pilot tunnels to give warning of possible difficulties.
The method of erecting the lining was of great importance. Although at certain places it was necessary for the segments, which weighed 17 cwt, 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 in 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 organization. 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 this hanging roadway. Lifts at intervals handled the debris, and thus the remaining excavated matter was removed. When the main tunnel had been fully excavated and the lining completed, over 1,, segments had been used, exceeding 80, tons in weight.
The greatest care was taken to see that no water leaked into the lining before the segments were backed with cement. The empty space behind the lining was packed by hand with pieces of rock. In every segment two holes were provided through which thin cement, known as grout, could be injected under pressure. The grout injected was composed of water and neat cement.
This was injected at pressures up to lb a square inch, and thus could be forced into neighbouring fissures in the rock as well as filling the interstices in the packing. A further possible source of seepage was through the joints in the lining. These were caulked systematically with lead wire. Each building contains duplicate sets of blower and exhaust fans which range up to 28 feet in diameter.
The fans can deliver 2,, cubic feet of fresh air into the tunnel every minute and an equivalent volume of air is withdrawn at the same time. A different method was used for building the branch tunnels and the sections of the main tunnel that ran under the land.
The branch tunnels were built to provide direct entry to the Mersey Tunnel from the dock systems on cither side of the river. The Birkenhead branch comes to the surface at Rondel Street, at a strategic point in the Birkenhead Docks. The Liverpool branch leaves the main tunnel about a quarter of a mile from the entrance and curves back to the river front, emerging at New Quay, a short distance north of Pier Head. The branch tunnels have an internal diameter of 26 feet, and the full circle was not excavated.
Above the roadway they are semicircular, but the rock beneath has been excavated only to form a shallow invert inverted arch with its lowest point 10 feet below the diameter of the semicircle. The invert was built of concrete, waterproofed where necessary. When the tunnel, on its approach to the surface, passed above the water level in the rock, steel rib and concrete lining was used. When the tunnel passed into soft ground nearer the surface, however, the cast- iron lining was reverted to, to give added strength to take the weight of the surface buildings.
Except for the Liverpool entrance to the main tunnel, the ends of the tunnel were built by cutting a trench from the surface. The tunnel was built in this trench and the covering was replaced. In Liverpool, however, the end of the tunnel ran beneath Dale Street, one of the busiest streets in the heart of the city. A different method of procedure was suggested - the use of a hydraulic shield. The design of this shield involved complex problems.
Shields for tunnelling purposes are generally circular hut in this instance it hail to be semicircular. Again, a shield of such dimensions had never before been built. Eventually a suitable shield was built in the tunnel feet from the entrance. It ran on steel rollers at either side. The rollers ran on concrete beds, which were laid in small headings driven in advance of the main tunnel. The running costs of the Tunnel were to come from the rates.
About this time the proposals for the tramway were put on hold. There were various possible reasons for this.
One was opposition from Birkenhead who wanted to protect their Ferries, another was that the government had said that they would reduce their contribution if a tramway was laid. The Tunnel would however still be excavated and built for most of it's length with a massive space under the roadway designed for the Tramway.
A further Act was needed in mainly because the siting of the Birkenhead entrance was changed, which led to an increase in costs.
The Tolls were now to apply for up to 25 years. In there was a further Act to again change the Birkenhead entrance and also to move the Liverpool entrance from Whitechapel to the Old Haymarket. But the overall cost and Toll period was the same. This seems to have been mainly due to an incident in an American road tunnel, and a decision that there had to be a massive improvement to ventilation. This of course wouldn't have happened with a bridge!
As the government would not give any more money, the Tolls were now to last for up to 40 years. While all these Acts were being passed the actual construction started at end of It was a mammoth undertaking involving thousands of workers. The main tunnel there were branch tunnels at either end would be 2 miles yards long, and it would be wide enough for 4 lanes of traffic with a total interior diameter of 44 feet. You can get a bigger version by clicking here:- Workers picture.
The tunnel is not very deep, with the lowest point being only feet below high water level in the river. At one point, mid river, there is only 4 feet of solid rock above the tunnel. Perhaps they were lucky that they didn't get very very wet!
The pilot tunnel between Liverpool and Birkenhead was completed on 3 April when Sir Archibald Salvidge broke through the last rock he hadn't done any of the rest of the digging! There were in fact 2 pilot tunnels, with the one for the roadway above that for the tramway.
As the bottom pilot tunnel was dug first, did the dignitaries actually shake hands in what would have been the tramway, rather than the roadway? The construction continued through various difficulties with excavation and drainage, and the unfortunate deaths of workers. The general technique used was to excavate and line the top road half of the tunnel, and then to excavate and line the bottom tramway half of the tunnel.
You can get a bigger version by clicking here:- Road deck. The main Mersey Tunnel has a circular section for most of it's length, presumably due to the tramway. There is a branch tunnel on each side of the river. The branches are semi circles with just a shallow space beneath the roadway.
Part of the land sections of the "tunnel" were not tunnelled; they were built using cut and fill techniques; this includes the part which runs below Dale Street down towards the river. The construction of the tunnel was massive in itself, but it also involved construction of gigantic ventilating machinery, shafts and buildings. The Tunnel was eventually brought into use on 17 December , with an official opening on 18 July You can get a bigger version by clicking here: Official opening at Liverpool end of Birkenhead Tunnel.
The King and Queen went on to open a new Central Library in Birkenhead, the original Carnegie Library having been chosen as the final site of the Birkenhead entrance to the Tunnel! At the opening the King spoke the words " 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 traffic may run in light and safety below the depth and turbulence of a tidal water bearing the ships of the world.
The result was that by about , Birkenhead and Liverpool were not having to pay anything for the upkeep or debt on the tunnel. And by about the Birkenhead Tunnel should have become Toll free. Photos at "Old Haymarket" of Liverpool entrance to Birkenhead Tunnel about They were taken by John Bythell who has kindly allowed us to reproduce them. You can see more of his pictures at More photos of Liverpool area.
With increasing levels of car use, the Birkenhead Tunnel could not cope, and by s there was severe congestion on the roads around the Tunnel at peak periods.
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