Cool Crossings

May 2010 Vol. 65 No. 5

The machine, available in diameters from 54 to 78 inches, consists of a circular cutterhead mounted with single disc cutters much like those on larger diameter tunnel boring machines. These disc cutters are used to excavate hard rock ranging from 4,000 to over 25,000 psi UCS. In mixed ground, two-row tungsten carbide insert cutters and carbide bits can also be used.

As the cutterhead rotates, disc cutters ranging from 6.5 to 15 inches in diameter penetrate the rock face and create a “crush zone” through which fractures propagate. Material between adjacent crush zones is then chipped from the rock face. Muck scrapers scoop the muck into openings on the cutterhead called muck buckets, which transfer the material to a machine belt conveyor. Muck removal from the site is by either a belt conveyor or muck cars.

The machine is typically launched from a pit, and can be either self-propelled using a gripper system or pushed forward with a separate pipe jacking system. During excavation, the tunnel is lined with a primary liner of the contractor’s choosing, including steel casing, ring beam and board, reinforced concrete pipe and four-flange liner plate.

Robbins, (440) 248-3303,
In-Line Contracting Ltd, 780-448-9638