Like most tunnels, the Anglo-French Channel Tunnel is being drilled from both ends. But if one of the tunneling machines digging it should stray even one degree off course, after 15 miles (24km) it would miss the other half of the tunnel by over a quarter of a mile (400m).
To keep the machines on line a laser guidance system has been installed at both ends of the Channel Tunnel. A thin pencil of light shines down the tunnel hitting the target on the back of the boring machine. The guidance system has a computer that is designed to measure how far the machine has gone, whether it has moved up or down and whether it has rolled or turned to the right or left.
All tunneling machines, or ‘moles’, have a tendency to deviate in one direction or another. As soon as the computer detects any deviation from the correct line it automatically steers the machine back on course. It sends signals to the steering `shoes’ which are hydraulic pads that control the machine’s direction by pushing off the ends of the tunnel lining. The signals sent to each shoe will adjust the direction of the mole so that the tunnel can be built in a curve or on rising or falling gradients, or straight, as the engineers require.
Since the laser can only operate in a straight line it has to be moved to a new site as the tunnel changes direction. The laser has to be positioned with great accuracy — any mistake will mean that it could be guiding the mole in the wrong direction. Usually the laser is positioned near the top of the tunnel where it is less likely to be disrupted by the clouds of chalk dust caused by tunneling.
In the past engineers have had to rely on conventional surveying methods. Theodolites are still used to measure horizontal and vertical angles. They are like a small telescope mounted on a secure base. The surveyor looks through the theodolites and focuses on a target farther down the tunnel. He can then calculate by how much the tunnel is rising or falling, or veering to the left or right, and make the appropriate corrections.
One of the most convincing demonstrations of the effectiveness of these traditional methods was the two tunnels bored by the Canadian Pacific Railroad through the Rockies between 1907 and 1909.
The purpose of the two tunnels was to bypass the steepest stretch of track in North America, the ‘Big Hills between Kicking Horse Pass and Mount Stephen, just west of the border between Alberta and British Columbia. It used to take tour engines to haul a train up. Coming down was even worse. Men were posted along the line ready to operate points to divert the trains travelling too fast up specially built spurs running steeply uphill.
The hill was bypassed by drilling two spiral tunnels with low gradients into the mountains. Both curved steadily round, rising slowly for more than 3000ft (914m), and both were built by drilling from each end. When the two halves of the first tunnel met they were only 2in (50mm) out.
Picture Credit : Google
