Tunneling underwater was considered impossible until the French-born engineer, Marc Brunel and his son, Isambard, built a tunnel beneath the Thames between Rotherhithe and Wapping in 1841. In the nine years it took to complete, it was flooded five times, killing at least 12 men. Water seeped through the riverbed and poured into the tunnel when the roof gave way.
The Brunels were successful only after they invented the shield, which was a 1 movable framework that supported the tunnel face and the earth immediately behind it. The workers would excavate a few feet of tunnel, then jack the shield forward against the new face. The newly exposed part of the tunnel was lined with bricks and the process would be repeated until the tunnel was finished.
Lining a tunnel right up to the shield meant that, in theory at least, the only place where a leak could occur was at the face. And shields were designed so they could be made watertight and stem a potential flood. In practice, there were still fatalities.
In 1908 the first attempt to drive the Lotschberg Tunnel through the Swiss Alps, under the Kander River, proved disastrous. Geologists had predicted that the tunnel would be going through safe bedrock, but the roof collapsed near the face and the tunnel quickly filled with soil and water, killing 25 men. The tunnel was rerouted and successfully completed upstream where the bedrock proved sounder.
Underwater tunneling is still one of the most difficult and dangerous feats of engineering. The problems vary enormously depending on the nature of the ground. Water only trickled into the English end of the Channel Tunnel, because the ground is virtually impervious to water, but at the French end engineers had to contend with the full sea pressure.
Various techniques have been used to reduce the risks of flooding. By filling the tunnel with compressed air, the flow of water can be held back until there is time to erect the lining lithe pressure inside the tunnel equals the pressure of water outside, no water can get in — just like a diving bell. This method was used for many of the tunnels built in the 19th century, but it has drawbacks. A large compressor plant is needed with standby equipment to insure
against a loss of pressure. It can also give workers ‘the bends’ when they emerge into normal atmospheric pressure.
At a pressure of three atmospheres (441b per square inch) a man can work only one hour a day and has to spend six hours in a decompression chamber. All the tools and spoil have to be passed through a complex system of air locks.
High-pressure tunneling is avoided whenever possible and other techniques are used. The ground ahead of a tunnel is often strengthened before digging by injecting it with a cement-like compound called grout. Grouting also makes the ground more resistant to water by filling and sealing small fissures.
The biggest aids to modern tunneling are moles — massive machines that not only burrow through the ground but also provide protection for workers by acting as a shield. Moles can also trail an erector arm that lifts heavy precast concrete segments which are used to line most tunnels.
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