Shank - Sewer Overflow Project

After ten years of planning and a phase one price tag of $163.5 million, Rhode Island has begun a massive effort to return Narragansett Bay to a pristine condition. The Combined Sewer Overflow Project (or CSO), awarded to the partnership of ML Shank and Balfour Beatty, is one of the state’s largest projects ever undertaken. The estimated time for tunnel completion, which began in 2003, is six years. The first segment includes the mainspine tunnel located off Allens Avenue along with seven drop shafts designed to carry sewage to the tunnel. Two additional tunnels are scheduled to be completed over the next twenty years which will bring the total cost of this venture to an estimated $550 million.

The southern portion of the project starts on the eastern side of the Thurbers Avenue curve, moves north along the Providence River, travels under downtown Providence, by the State House and finally after 16,000 feet, ends near the Foundry. The northern end of the tunnel is actually pitched 17 feet higher than the southern end so that the wastewater will flow downward toward the sewer plant. The tunnel is designed to hold at least 62 million gallons of polluted water that would otherwise end up in the Woonasquatucket and Providence Rivers.

The main shaft measures 50 feet in diameter and reaches 252 feet below ground level. These dimensions are necessary to accommodate the massive 690 ton Tunnel Boring Machine. The TBM has sixty-three cutter discs resembling railroad wheels that fracture rock on contact grinding away a 30-foot tunnel face resulting in the removal of 750 million cubic yards of material.

Railroad tracks were installed on the tunnel floor to accommodate rail cars that carried rock
fragments back to the vertical main shaft. Returning cars carried 16,000 precast concrete
panels each measuring 22 feet long by 10 inches thick by 4 feet wide and containing
almost 3 yards of concrete. Each panel covered one quarter of the circumference of
the tunnel. After all four pieces were fitted into place, a jack spreads the upper two pieces
“snugging” them up. The TBM eventually pushed itself against the face of the precast as it continuously bored and installed new precast pieces.

The segment panels were produced by PRM Concrete Corporation and fabricated at their Cranston location. Consistency in concrete production was essential throughout the production of sixty-eight panels per day so that stripping of the forms could commence early
the next morning. This rapid stripping requirement demanded constant adjustments to the approved mix design in order to compensate for seasonal temperature changes. The precast panel specifications mandated no spalling or chipping of the semicircular pieces.

A crane removed the precast panels using a vacuum picker. A series of four vacuum panels, matching the contour of the precast panels, would lift almost three cubic yards of concrete without causing any damage or stress to the panel. The panels were poured at 7 a.m., stripped by 1p.m., and then repoured that afternoon to keep this fast track project on target.

The CSO project is just one recent example of the versatility of concrete as a building product. Proportioning, design and control of concrete mix designs can allow for a variety of
concrete applications where performance is specified. At PRM Concrete Corporation,
successful concrete production is performance based – one yard at time.