PLYMOUTH |
The Encyclopaedia of Plymouth History |
|
|
2 - PRELIMINARY WORK All the strain and pressure of this massive structure would rest on the centre pier and it was imperative that it should be built on solid rock. But this was no easy task as the river Tamar was some 70 feet deep at the selected point, with another 20 feet of mud and gravel beneath that. The nature of the rock was in itself an unknown quantity. Again, Brunel used his past experience. When working on the tunnel under the river Thames in London, his father, Marc Brunel, had used a diving bell with compressed air in connection with the sinking of shafts for the tunnel. For the main pier of the Chepstow Bridge he had used an iron cylinder and compressed air for a similar purpose. So that was what he decided to do on the river Tamar. In order to explore the river bed he built a trial cylinder 6 feet in diameter and 85 feet long. This was suspended in a framework attached to the hulls of two old gun brigs and lowered through the mud to the rock like a diving bell. Five trial borings were made and the cylinder was then raised. It was moved thirty-five times until 175 borings had been carried out within the area that Brunel wanted to raise the masonry for the central pier. Finally, in January 1849 the water was pumped out of the cylinder and the mud was excavated down to the rock, which was found to be very dunstone or greenstone trap. This was then excavated down for three feet and at that depth it was pronounced cable of supporting any weight. To prove this, a short piece of masonry was built, in the centre of which was placed a copper tablet in an oak box with the inscription "Cornwall Railway, Saltash Bridge, Trial foundation of Central Pier, January 1849, I K Brunel, Engineer. William Glennie, Resident Engineer." The Bridge was originally designed to take a double line of railway track but when the Cornwall Railway encountered financial problems and decided to construct only a single line, the Bridge was redesigned for single track also. It is claimed that this saved £100,000. Construction could now begin.
|
