World's deepest cutoff wall reaches 430 ft

Two thin concrete cutoff walls were sunk more than 400 ft deep to seal the foundation of Manicouagan 3 Dam, a 350- ft- high earthfill located about 55 mi north of the river's confluence with the St. Lawrence River at Baie Comeau. The technique was refined by switching from a hollow drill stem to cables. This eliminated the time- consuming job of uncoupling the stem when changing bits. A hollow drill bit was devised that functioned like a sleeve around a pilot bit. This 10- in. pilot bit was mounted on a 8- in. drill stem that also delivered the slurry to the bottom of the hole.