The art of digging a hole

Digging tunnels has become easier and safer thanks to custom-designed tunnel-boring machines. The 18.5-ft-diameter TBM nicknamed Sandie breaks through on one of the London Ring Main water tunnels, a project that consists of almost 50 miles of tunnels. The 48-ft-diameter Groene Hart TBM is being built by NFM Technologies. A Lovat hard-rock TBM uses an expanding thrust ring to anchor the machine and to provide a base from which thrust cylinders can push off of and drive the cutting head forward. Richard and Rick P. Lovat, president and vice president of Lovat Inc., stand in front of the machine their company built to dig a subway line under Lisbon, Portugal. The 1,200-ton machine uses 1,800 kW to turn the cutting head. A technician from Wirth Tunneling Ltd. in Scotland examines the grooves in hard rock made by a TBMs cutting discs. Tunnel-boring machines (TBMs) could be the ultimate power tool. They're monstrous, weighing millions of pounds and exerting forces measured in thousands of tons, and they can dig through some of the hardest rock on earth. Plus, most come with an entire railroad and train as ancillary equipment. What more could power-loving handymen want? In some cases, such as the English Channel Tunnel, the machines are so large and built to such unique specs that once they finish the job, they are stripped of usable equipment and abandoned underground. Companies also refurbish and resell some TBMs, usually the more standard sized machines with between 6.5 and 13-ft diameters. But for most projects, TBMs are custom built. As one TBM manufacturer says, "Every one of our machines is a prototype." One of the most important criteria in designing a machine is determining what it will be digging through. Hard-rock TBMs are simpler than those designed for soft ground because the tunnel, once dug, supports itself. There's no need for liners to prevent the tunnel from caving in, and the TBM can push off the walls to supply forward pressure. Of course there are limits. Rock with compressive strengths greater than 30,000 psi, such as dense quartzites that have strengths to 60,000 psi, are still more economical to drill and blast through than to excavate with TBMs. At the front of the TBM is the cutting head, a rotating drum with free-wheeling cutter discs mounted on the flat face. Cutter discs, an invention of The Robbins Co., Solon, Ohio, have a tungstencarbide bit or rim that circles a hardened tool-steel wheel. Discs are spaced about 3 in. apart and in a pattern that covers the tunnel face. The wheels are turned by friction as the cutting head rotates and hydraulic cylinders push from behind. This thrust is a function of the number and size of the cutting discs. The thrust a TBM can place on a disc goes up with the square of its diameter, according to John Turner, chief engineer at Robbins. "And while the largest discs were once only 12 in., we now use mostly 17 and 19-in. discs," The 17-in. discs are rated at 60,000 lb each, while 19-in. discs can handle 70,000 lb. "But individual cutters can see impulse loads 10 times their nominal ratings," notes Turner. "How the discs actually work is not well understood," adds Turner.