TUNNELLING UNDER THE THAMES

Dominion, Volume 32, Issue 94, 14 January 1939, Page 6 (Supplement)

TUNNELLING UNDER THE THAMES

How Men Work In Enormous Pressures

A wonderful tunnel that will alter the traffic situation of a large part of England is being scooped out under the bed of the Thames, says C. A. Lyon in the “Sunday Express.” Its building is a great romance of engineering. It pierces the river bed well below London between Dartford and Purfleet. When it is finished all the traffic between eastern and south-eastern England and the north will be able to take a short-cut and relieve the congestion of London's traffic. Evefly shovelful of the tunnel has been dug out by men working in a kind of gigantic underwater compressed air cylinder, where the pressure was two and a half times normal. Every minute a strict watch and a skilled manipulation of controls was necessary to keep the water out.

Extraordinary precautions had to be taken to enable men to work in this tremendous air pressure. The tunnel was started simultaneously on opposite banks where the Thames is half a mile wide, and when Hie two halves met in the middle the join was accurate to three-eighths of an inch.

The work will not be completed for another three years, but the driving of a pilot or testing tunnel, twelve feet wide, was successfully completed by 250 men. most of whom were working under the bed of the river. Extraordinary difficulties were overcome by the engineers in making this pilot tunnel. Their achievement is tr tribute to British engineering.

First they were faced with the problem that, when they dug. the water from the river above would seep through the chalk and flood .he tunnel. The only thing to do was to make the tunnel into a sealed cylinder and blow it up so hard with air that the incoming water would be pushed out. Air-compressing engines costing tens of thousands of pounds were built at the entrances to the tunnel site on Thames Marshes,

Air locks were constructed so that men and materials could go in and out without for a moment releasing the pressure.

In this way the 250 men began digding. It was soon seen that to keep the

water out and the air in involved a perpetual vigilance. The price of a mistake might be the destruction of the work.

Miners working with light pneumatic drills and shovels would be at the advanced end of the tunnel cutting away the chalk under the protection of a cylindrical movable shield.

Suddenly a hissing begins. A patch of chalk witli fissures in it has been struck, and Hie air is escaping into the wafer above.

If this went on it might bring the roof of the tunnel down, imprisoning the workers and stopping work for months.

The engineer on duty quickly telephones to the top. The pressure is reduced so that the air pressure exactly balances the pressure of water, seeping through from above. The work goes on again. Sometimes Hie opposite happens. Water conies spurting out into the faces of the workers. That means that the pressure is not great enough to keep the water out and there is danger of flooding. The engineer telephones again, and so the game goes on.