From our April 2003 newsletter

Providence, Rhode Island. The Narragansett Bay Commission has earmarked $2 billion for a massive new waste water tunnel to be constructed far beneath the city over the next five years. The tunnel will be 30' in diameter, 3.5 miles in length, and will manage storm-caused overflow at the city’s waste water treatment plant. In November 2002, the San Francisco-based tunnel builders Shank/Balfour Beatty began excavating three vertical shafts which will eventually give both workers and equipment access to the new tunnel’s level 250' below ground. The project will comply with the federal Clean Water Act.

The access shaft excavation is being done by a Tramac V45 breaker (mounted on a Komatsu PC300 excavator), which was supplied on lease/purchase by New Hampshire Hydraulics of Merrimack, N.H. John Fallon is the dealer representative.

	Cramped quarters? This bird's eye view shows there's just enough room for the hammer and excavator, which are lifted out by crane every 2 feet to remove the dirt.

Freezing the Ground
Because the sedimentary soil in the Providence area is unstable, a special ground-freezing technique was used on the shaft excavation area before digging began. This preparation has taken about six months. Briny water, cooled to -30° by ammonia in one of two massive heat exchangers installed in 40’ trailers, was piped down through vertical pipes around the projected shaft area. The soil will remain frozen for the duration of the project, eliminating the risk of cave-ins and providing a firm base for excavation.

The V45/PC300 combination is currently working on the second, and largest, of the three shafts, which is 42' in diameter. This shaft will allow Shank/Balfour Beatty’s giant tunnel-boring machine to be lowered to the 250' work level; the other two, with diameters of 22' and 35', will accommodate workers, additional equipment, and ventilation. When completed, they will be lined with poured, reinforced gunnite walls, which will last for the life of the shafts, and the soil will be allowed to thaw.

Tight Quarters
Even in the largest shaft, there is little room for other equipment when the breaker and its excavator are working. The excavator/hammer is lowered into the hole by a crane and lifted out after two feet have been broken. Muck buckets are then used to remove the dirt. The V45/PC300 is returned to the new floor of the shaft to excavate the next 2'. In all, the shaft is being deepened by about 8' per day.

The Right Equipment for the Job!
Continuous hammering in frozen ground required a specially-made moil point tool – or “frost bit” – for the V45. The bit is machined to have four angles, which penetrate the frozen sediment easily. Shank/Balfour Beatty project supervisors tell us the tool has been performing very well.

The 7500-lb Impact Class V45 is able to break up the frozen soil efficiently, while its advanced Tool Protection Technology means little danger of downtime due to tool breakage. (As further insurance, a back-up V32 is equipped with a similar custom bit.) Reliability is enhanced by Tramac’s fully-hydraulic operating principle, with no gas assistance needed for the raising or lowering of the piston, no progressive leaking with loss of impact power, and no delays for gas recharging.

To date, the City of Providence has not given a blasting permit for the bedrock portions of these shafts (bedrock is reached at approximately 140'). But, according to operator Ray Riel, “The V45 broke through the bedrock layer in the first shaft almost as easily as it’s breaking the frozen ground.”