At the beginning of the 19th century, London was one of the busiest river ports in the world, and the 600-year old stone bridge over Thames was long out of capacity. Like many medieval bridges, this too was crammed with buildings, some of which overhung the road creating a dark tunnel through traffic passed. Although the bridge was some 8 meters wide, only half of it was available for traffic. This roadway was shared by ox-carts, wagons, coaches and pedestrians coming from both directions. At peak hours, a crossing could take up to an hour.
It was clear that another crossing was needed, but this new crossing was not to be a bridge but a tunnel under the River.
Attempts to tunnel under the Thames had been made in the past. In 1799, engineer Ralph Dodd tried to build a tunnel between Gravesend and Tilbury and failed. Then, in 1805, a group of Cornish engineers tried to dig between Rotherhithe and Wapping, but the clay was soft and soaked with water, and the mining engineers, used to hard rock, was unable to keep water out of the tunnel. The project was abandoned and the engineers declared that a tunnel underneath the river was not feasible.
But Marc Brunel thought otherwise. A brilliant French engineer who had escaped his country during the French revolution and made a name for himself in Britain, Brunel believe that a tunnel under Thames was possible. He had already put some thought into the process when he tried to persuade Emperor Alexander I of Russia to build a tunnel under the river Neva in St Petersburg. But the Emperor turned down his proposal and built a bridge instead.
Brunel identified the problems the Cornish engineers faced, and devised a new tunneling technique that has been used in one form or another in almost every tunneling work for the past two centuries—the tunneling shield. It’s a temporary structure that is pressed against the tunnel face and which protects workers and the project itself from falling materials or a cave-in. It is said that Brunel was inspired by the burrowing efficiency of a shipworm on a piece of submerged timber which he had picked up while working in a shipyard.
Diagram of the tunneling shield used to construct the Thames Tunnel, London.
Brunel’s tunneling shield consisted of a large, rectangular, grid of iron frame with 36 chambers distributed into three levels. Each chamber was open to the rear, but closed in the front with moveable boards. The front was pressed firmly against the tunnel face, and the workers would remove the boards one at a time and excavate the earth behind it to a predetermined depth. Then the board would be pushed into the hole and screwed back into place before the next one was removed. The whole process was repeated until the earth behind all the boards were excavated. Then the entire iron frame was laboriously moved forward, and the newly excavated section was shored up with bricks and mortar.
The tunneling shield was revolutionary, but the work was slow, progressing at only 8 to 12 feet a week. And although the shield worked well in preventing cave-ins, seep-ins were another problem. The filthy, sewage-laden water from the Thames above dripped down from the roof of the tunnel and poisoned the poorly ventilated space. Many miners including Brunel himself fell ill to a wide range of affliction such diarrhea, headaches and temporary blindness. Pumps worked all round the clock removing water from the tunnel, and when they failed, the whole shaft would flood to a depth of several feet.
A scale model of Marc Brunel's tunneling shield in the Brunel Museum at Rotherhithe. Photo: Duncan Kimball
In 1827, the miners hit a cavity in the riverbed, and water gushed in with ferocity. Brunel’s son, Isambard Kingdom Brunel, had to lower himself in a diving bell from a boat to repair the hole at the bottom of the river, throwing bags filled with clay to plug the hole. Following another disastrous breach and the death of six men in 1828, Brunel ran out of funds. The tunnel’s face was walled off and abandoned.
It took another seven years for Brunel to raise enough money to restart tunneling. He also built a better tunneling shield capable of resisting the pressure of the Thames. Despite the replacement of the old tunneling shield with a better model, it took another six years of round the clock labor before the tunnel finally emerged at Wapping in 1841.
The Thames Tunnel was a triumph of civil engineering, but it was not a financial success. The tunnel cost more than half a million pounds to build, far exceeding its initial cost estimates. The proposal to build an entrance to accommodate wheeled vehicle also failed owing to cost, and in the beginning, it was only used by pedestrians. However, it did become a tourist hit, attracting about two million people a year, each paying a penny to pass through.
In 1865, the tunnel was purchased by the East London Railway Company and converted for rail use. It later became a part of the London Underground, and was in use until 1962.
Comments
Post a Comment