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November 30, 1999

A public radio commentary

Alert eBay users could bid last month for the tunnel boring machines used to burrow under the English Channel to create the so-called "Chunnel." Lucky bidders brought home a machine whose blades spit a half a million cubic meters of soil out its back side - a high tech machine, no doubt, yet the origins of this machine lie in a lowly worm!

These tunnel boring machine are the legacy of a French engineer, Marc Isambard Brunel. In the 19th century the problems of digging under the Thames River fascinated Brunel. In the first attempts, solve this problem Brunel took his inspiration from the greatest designer of all time: Nature.

He studied the tunnelling of the ship worm Teredo Navalis - a pest that ate the wooden hulls of ships. He noticed the tough shell on the end of the worm, which it used to cut through wood. And he also noted that the rest of the worm was a long tube used to dispose of the wood shavings. Brunel conceived of a "tunnel shield" that turned miners into a huge human worm, digging under the Thames.

This shield prevented the tunnel from collapsing as the miners dug. It was a 120 ton cast iron structure, twenty-two feet tall, nine feet wide and divided into nine areas - it looked a lot like an iron tic-tac-toe diagram, but on each of the squares Brunel attached iron sides three feet deep. A miner stood in each of these opening, which were closed with fourteen three inch thick boards. The miner removed a board, dug four and a half inches into the soil, replaced the board, then removed the board below and dug four and a half inches again. When the miners were finished, workers standing behind the shield turned huge screws and drove it forward four and one half inches. As the miners began digging, bricklayers covered the newly exposed earth, building a permanent tunnel. Brunel hoped his human worm would burrow three feet a day, but he had to settle for one foot a day and it took eighteen years to build the Thames tunnel.

When the tunnel opened in 1843 it became London's biggest tourist attraction. People paid a penny apiece to walk through the tunnel which was filled with exhibitions by painters, tightrope walkers, puppeteers and magicians. Ultimately the tunnel failed economically and the bankrupt company sold the tunnel to the London railway; it is still used today as part of London's underground system.

Although the tunnel failed, Brunel's tunnel shield succeeded. His "human worm" is still used to burrow underneath rivers and lakes - including the tunnel boring machine used to create the Chunnel. Although I don't think there were any puppets or magicians ever in the Chunnel, just millions of speeding Britons.

Copyright 1999 William S. Hammack Enterprises