|
|
Tunnels:
A Public Radio Commentary
by Bill Hammack
Listen using RealAudio
When my wife and I visited Boston our hosts asked what we'd like
to see: Faneuil Hall? Harvard Square? New Hampshire? The Big Dig?
At the Big Dig my ears perked up.
Its the name for the huge tunnel and highway project around and
under Boston Harbor. This Big Dig had great appeal to me because
as an engineer a tunnel puts me in touch with my primal
engineering roots. Humankind's first engineering project was
tunnelling - enlarging a cave. Also, tunnels reflect our constant
struggle against nature.
For example, because water doesn't flow up hill, the Romans built
tunnels through mountains. And as our population grew we needed
to get places faster so we built huge tunnels to move ourselves
under rivers or through mountains. But building these large
tunnels underwater was a difficult task. For example, in 1800,
miners tried to burrow under the Thames river in England. As they
dug the walls of the huge tunnel broke and water rushed in so
quickly they didn't have enough time to reinforce the walls.
The French engineer Marc Isambard Brunel changed all this; he
revolutionized tunnelling in a way so obvious it seems too simple
not to have been invented before. The problems of digging under
the Thames fascinated Brunel; to solve these problems he 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 this worm, used to cut through wood, and 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.
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 drive
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 tourists attraction. People paid a
penny a piece to walk through the tunnel which was filled with
exhibitions by painters, tightrope walkers, puppets and conjurers.
But 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.
The tunnel failed, but Brunel's tunnel shield succeeded. His
"human worm" is still used to burrow underneath rivers and lakes
- including the tunnels in the Big Dig under Boston Harbor,
although I don't think there will be any puppets or conjurers in
Boston's new tunnel, just tons of traffic.
Copyright 1999 William S. Hammack Enterprises
|
|
