The Lotschberg Tunnel in the Swiss Alps, completed in 1911, had earlier suffered disaster. After penetrating two miles into a mountain of relatively hard rock, drillers emerged into a zone of rocky debris. Broken rock and water under enormous pressure poured into the tunnel, killing 25 men.

To avoid such accidents, many tunnel builders have resorted to compressed air and steel rings called shields. A bulkhead that closely fits the inside of the tunnel is installed near the rock face where workers are drilling, and the air inside is compressed to resist the tendency of water to leak through loose rock. Workers enter the compression chamber through an air lock, and the compression chamber is pushed along behind them.

In one common variation, compressed air is also used to push a ring of steel called a shield into the leading end of the tunnel as it is dug, thus protecting workers from collapse until a permanent tunnel lining is built. Safer System in Channel

A much safer system was devised for digging the Channel Tunnel.

The French and British drilling machines heading toward each other under the Channel did not require shields, because each machine formed its own seal between the dangerous rock face and the tunnel behind it. The leading part of the machine resembles a telescope, the back of which grips the tunnel wall while the front end, which does the grinding, is pushed out into the virgin rock. When the front end reaches its maximum extension, the rear part propels itself forward, using its movable grippers for traction.

The circular face of the machine, which is turned by electric power, is studded with tungsten carbide "picks" that claw away at the rock, hurling debris into a conveyor that moves the waste to the rear. As picks wear out they can be replaced from behind the drilling plate without workers having to enter the dangerous space in front.

As the boring machine crawls forward at 14.8 feet an hour, it is followed by a train some 200 yards long, the head of which is a "segment erector." This device lifts huge segments made of iron and concrete into place, to form the rings that are fitted together to line the tunnel walls and support the structure. Each complete ring consists of eight curving segments bolted together and packed with impermeable material at their seams. At normal speed, tunnel workers could complete a new ring every 13 minutes.

The progress of the drilling creates a zone several feet long where collapse might occur in an accident. To seal this zone -- and to seal the cylindrical machine to the tunnel wall when drilling is temporarily halted -- a wet clay mixture is forced into the space between the machine and the rock wall.

A special safety feature of the tunneling machinery is that it requires very little human assistance. Only a driver and two people to operate the lining machinery are needed, so loss of life would be relatively light in an accident.