To sink a pair of 6.5-ft-diameter open shafts 250 ft., mostly through rock, a specialty contractor is using a compact dry boring rig originally designed for cast-in-place piles.
Raito Inc., Geosystems Construction Specialists, the American arm of a Japan-based construction firm, is installing two concrete-lined vent and drop shafts for the new Combined Sewer Overflow tunnel in Providence, RI. Raito’s U.S. headquarters are in San Leandro, CA, with branches in Crofton, MD, and Waterbury, VT.
Raito’s equipment spread is nothing if not economical: the two main units are the above-ground drilling unit, known as SuperTop I RT 300, and a Link-Belt LS-308H II, a 110-ton (100 t) capacity lattice boom crawler crane equipped with an angle boom. The Link-Belt unit was selected for providing the right combination of compact size and necessary power/capacity.
Raito’s job is part of the first phase of the Rhode Island Narragansett Bay Commission’s massive Combined Sewer Overflow (CSO) project, a 20-years-plus, $700-million job designed to improve water quality in Narragansett Bay. Program manager for the project is the Louis Berger Group, East Orange, NJ; construction manager is a joint venture of Gilbane Building Co., Providence, and Jacobs Associates, San Francisco, CA.
At the heart of the project, which started in earnest last year (CEG Sept. 10, 2003, NE 19), is a 3.5-mi (5.6 km) long, 30-ft (9.1 m) diameter chamber, known as the main spine tunnel, approximately 250 ft. (75 m) below ground, which will hold up to 62 million gal. (235 million L) of sewage and rainwater from the older combined sewers of the Providence/Narragansett Bay area. Currently, during dry weather, wastewater and runoff water goes to the commission’s treatment plants. But during periods of any rainfall more than half an inch in 24 hours, flow quickly overwhelms the plant and it is diverted directly to the Bay. In addition to being a nuisance, this shuts down much the area’s shellfish harvesting industry for at least a week, as well as public beaches.
The tunnel will store effluent until it can be treated.
A joint venture of ML Shank Company Inc., Highland, CA, and Balfour Beatty Construction Inc., Atlanta, (parent company located in Great Britain) is constructing the main spine tunnel with a Hitachi tunnel boring machine. The machine started full operation this spring after the main access shaft and the head of the tunnel were excavated and blasted out.
In addition to the tunnel, this phase of the Narragansett Bay CSO project also involves the construction of a pump station and a series of shafts and chambers. Raito is building two of the drop and vent shafts at the north end of the tunnel, in downtown Providence near the State Capital. (Excavation of the main spine tunnel started at the Bay, or south, end of the job approximately three miles away, next to the commission’s treatment plant.)
According to Bill Allen, project manager of Raito, when completed the shafts will be 235 ft (71.7 m) deep with a 6 ft., 6.75 in. (200 cm) diameter. The first 59 ft. (18 m) of the shaft were sunk through fine silt and sand, which then abruptly gave way to the underlying bedrock, mainly stratified metamorphic formations.
For drilling holes like these, through both silt and rock, there are a number of options:
• Kelly bar or rotating table, which drives the drill bit from the surface as penetrates the ground or rock. The weight of the drill string drives the bit downward. This type of unit is most commonly used in drilling oil wells, but also caissons and drilled shafts.
• Pile-top drilling rig with a reverse-circulation drilling unit, which uses water or other fluid to flush rock bits and carry away cuttings.
• Downhole cutting wheel unit, which uses hydraulic power from ground level and a circulation system.
• The Raito method, which essentially bores through material dry, by rotating a casing string equipped with drill bits, then breaking up and removing the material inside the casing. As the drilling continues, crews add casing sections to the drill string, much like oil well drilling. The SuperTop I is designed for cased cast-in-place concrete piles, but it can also be used to drill hollow shafts without concrete fill.
The SuperTop I RT 300, manufactured by Nippon Sharyo Ltd., Nagoya, Japan, has a driving force of 132 tons (120 t) and an extraction force of 418 tons (380 t) and a stroke of 29.5 in. (75 cm). It rotates between .9 and 1.5 rpm. A diesel-driven power unit supplies the hydraulic force for the chuck, which holds the drill string and rotates it.
The advantage of the Raito system over the other methods is its simplicity and footprint: it doesn’t require systems to handle fluid used in drilling, since it can work dry.
With counterweights, the base RT 300 drilling unit occupies a space of 25 ft. by 18 ft. (770 by 550 cm); it weighs 53 tons (48 t). A typical job-site layout need only be 115 by 49 ft. (35 by 15 m) in area. Height of the unit before adding guardrails and taking into consideration the height of the pile sections is only 11.5 ft. (3.5 m), considerably less than taller pile-top rigs. The small footprint and minimum equipment requirements work to Raito’s advantage for the Providence job, because it’s located downtown, next to the state capitol, the Amtrak rail line, a canal and several major roads.
The key to the SuperTop I RT 300 operation is a wedge-shaped chuck, similar in principal to the chuck in a hand drill, which holds the casing in place. Only the chuck on the SuperTop I is hydraulically opened and closed.
In Providence, Raito first spotted the RT 300 atop steel H-beams on top of the hole, then bored a seven-eighth in. (2.1 cm) thick outer casing 8 ft. 2 in. (2.5 cm) in diameter through the silt overburden and into 23 ft. (7 m) of rock. This helps protect the adjacent embankment, over the nearby Amtrak rail line, from being disturbed. After sinking the outer casing, crews switched to the 200 cm inner casing, using a drill section with 36 replaceable cutting bits. The Link-Belt crane, from dealer Woods CRW Corp., Worcester, MA, spots casing sections on top of one another as the string progresses downward.
Then, after a section of shaft has been drilled, the Raito crew uses the crane to drop a 20,000 lbs. (9,000 kg) hammer down the hole, breaking up the rock. After it’s broken sufficiently, they use a proprietary Sharyo clamshell, designed to fit snugly inside the casing sections, to muck out the hole. Trucks haul spoil away after it has accumulated.
According to Allen, the Link-Belt model is the only one that meets Raito’s space and power requirements for this job. It has a track length of 20 ft. 11 in. (6.4 m) and an extended width of 17 ft. 6 in. (5.3 m). Its diesel power unit develops 332 hp (248 kW).
When the drill bits wear down and need replacing, the crane, with the assistance of the RT 300 in the extraction mode, pulls the entire drill string out. Here the hydraulic inching control on the crane helps considerably.
In the overburden, the team averaged 73 minutes per meter drilling speed; in bedrock, the rate has dropped to 231 minutes per meter for the inner casing. (This compares very favorably with Raito’s experience drilling deep shafts in Hong Kong and Japan.) While the weight of the casing string helps drive the teeth into the rock, too much weight can break the carbide teeth, which must be used in rock. Therefore, the RT 300 incorporates a load balancing system to apply just the right amount of pressure on the bottom casing section.
To date, the only problem Raito has encountered was the sudden appearance downhole of an underground stream, following a heavy rain, which almost halted drilling and then stalled extraction efforts. Crews took corrective measures and the flow also decreased.
According to information on the SuperTop unit from Sharyo, this depth is about the maximum any of its units have achieved in ultra-deep boring.
As far as alignment is concerned, initial tests showed the SuperTop I RT 300 unit is holding to its specified perpendicularity of 1 in 500 maximum deviation.
Crews are placing concrete liner at the rate of 20 ft./day (6 m).
The Raito system also is being used to drill test piles on a seepage control project in a dam near Waterbury, CT. When the Providence project is completed, Raito expects to move the unit to a project at the Atlanta airport.