Home » Features » Innovative Thinking Brings Croton Water Tunnels in Early and Under Budget

Innovative Thinking Brings Croton Water Tunnels in Early and Under Budget

BY DEBRA WOOD

While golfers putted on the Mosholu Golf Course in the Bronx, motorists drove on the Mosholu parkway, and Dewitt Clinton and Bronx Science High School students studied, deep below crews drilled, blasted, bored and constructed three tunnels one-month ahead of schedule and under budget, earning Schiavone-Picone, Joint Venture a 2012 Alliant Build America Award for the $212 million Water Tunnels and Associated Works, Croton Water Treatment Plant project in New York.

“We approached the job differently than it had been envisioned by the New York City Department of Environmental Protection,” said Patrick Rooney, vice president of operations at Schiavone Construction Co. of Secaucus, N.J., an AGC of New Jersey member. “It allowed the project to complete ahead of schedule.”

The project entailed constructing a raw water tunnel from the New Croton Aqueduct to the Croton Water Treatment Plant and two treated water tunnels between the under-construction, 290-milliongallons- per-day plant and existing distribution lines adjacent to the Jerome Park Reservoir.

“Overall, the Croton project is an enormous construction effort,” said James Schaefer, project manager with AECOM/Hazen and Sawyer, Joint Venture, New York, the lead engineer for the Croton water plant. “The tunnel work is a part of it, and it was done within its time schedule and on a very small site and completed under budget. The contractor did a great job to fi nish it.”

Gerard Cox, portfolio manager for DEP, praised the cooperation and also recognized the work of the N.Y.C. Sandhogs Local 147.

“Everybody wanted to work together to get the job done,” Cox said. Parsons Brinckerhoff of New York served as subconsultant for the tunnel design of the project. Kyle Ott, project manager, said, “It was a win-win for everybody.”

CHANGING DIRECTIONS
Engineers originally envisioned the project using a drill-and-blast technique, but from the outset, Schiavone-Picone, a joint venture between Schiavone and John P. Picone of Lawrence, N.Y., identified difficulties with that methodology.

“For this project to be successful under the original drill-and-blast schedule, a lot of things had to happen optimistically,” Rooney said. “The odds on everything falling into place were not particularly high.”

Rooney explained that the tunnels pass through some difficult ground and rock conditions, including the seismic Mosholu Fault Zone, and cross aging city water tunnels.

“We thought we could come up with a more cost-effective, safer design of installation and means and methods,” Rooney said. “Ultimately, we implemented a tunnel-boring machine [TBM} to replace the drill-and-blast methodology [for the treated-water tunnels].”

With the TBM, the mining completed significantly ahead of schedule and allowed room for possible delays on other aspects of the job, such as poor weather conditions. “The level of cooperation between us and the owner to make changes was phenomenal,” said Bryan Diffley, project manager for Schiavone.

TUNNEL BORING
Ott indicated initially he and others were a little surprised that the contractor wanted to use a TBM.

“I wasn’t sure it would work as well as it came out,” Ott said.

Ott had considered a TBM when coming up with the original plans, but thought the tunnels too short to benefit from it. The lengths were 3,100 ft and 3,700 ft. However, Schiavone was just finishing up using a TBM on the city’s Water Tunnel #3 project and had bored through similar rock. With some modifications, that machine could be used at the Croton site.

“Because the contractor had a machine available, it worked out well for everybody,” Cox said.

Rooney added, “Under any other circumstances, it would have been determined not to be a cost-effective solution with the procurement of a tunnel boring machine to run a relatively short distance.”

The tunneling lasted approximately six months for each structure. In addition to saving time, using the TBM allowed for a more efficient and safer installation of the steel and concrete pipe. There were no major accidents, which Bernard Daly, associate project manager for DEP, partially attributes to the TBM. DEP also required the contractor develop a safety plan, have a certified safety professional on call, and have a safety representative present when work took place. DEP safety auditors also monitored the job.

The original plan called for a horseshoe- shaped annular space to place two 9-ft-diameter pipes, one over the other, with no ability to use a crane. Instead, Schiavone-Picone drove two adjacent tunnels.

BENEFITS OF USING ONE LARGE SHAFT
Rather than excavate multiple shafts to access the three tunnels, Schiavone-Picone opted for a 30-ft-by-50-ft oval shaft that encompassed the footprint of all three tunnels, which required less excavation. Its engineers modified the original designs to maintain structural integrity.

“It made access available to lower the tunnel-boring machine and to lower 40-ft lengths of steel pipe as opposed to shorter pipe, which made the schedule for steel-pipe installation better,” Rooney said.

“It also was able to reduce labor forces, because we didn’t have to attend to three separate shafts with three separate crews, and ventilation and lighting.”

With two adjacent tunnels, crews could simultaneously place the 9-ft diameter, 1-in thick, 40-ft-long, steel pipe in the high-pressure tunnel and 11-in-thick reinforced concrete cylindrical pipes, 16-ft-long and 35-ton per section in the low-pressure tunnel in two distinct operations.

“That also helped with the schedule,” Rooney said.

Schiavone developed a low-profile, flatbed- style pipe-carrier rail system to convey the 27-ton steel pipes almost a mile into the 13.5-ft-diameter tunnel.

“The point of contact on this carrier to the pipe allowed for the pipe to be jimbled about and rotated and leveled as required to maintain the correct alignment of the pipe,” Rooney said. “In order to mate it properly, we needed some sort of maneuvering device, and this carrier acted as a carrying and an alignment and leveling device.”

Daly called the use of precast and steel pipes unusual for this sort of project in which cast-in-place piping is usually used. “We could never have any leakage from the tunnels, so we had to have two underground water mains,” Daly said.

The pipes are water tight, with the steel welded from the inside and the concrete pipe with gasketed joints. Every connection was tested to ensure they were watertight, Daly said.

For the backfill, Schiavone-Picone designed a 4,500-psi mix using 3/8 stone with various admixtures to facilitate the pumping and retard set, since the pumping reached distances of up to 4,000 ft.

DEALING WITH THE SEISMIC FAULT
While the majority of the tunneling passed through the Fordham Gneiss, a formation more than 1 billion years old, the U.S. Geological Survey indicated a fault line existed in the path of the TBM tunnels and helped the team determine the area of concern. They used ground-penetrating radar to find the faults, Cox said.

“Crossing a fault zone with a TBM is more risky than drill and blast, so this contractor was taking somewhat of a risk,” said James Morris, deputy resident engineer with the construction manager URS/Malcolm Pirnie and an Arcadis Malcolm Pirnie employee in New York.

From the surface, Schiavone-Picone injected a cement grout into a 40-ft by 60-ft fault zone to solidify the rock. “It was to increase the integrity of the existing rock to ensure when we mined through it, we wouldn’t have any collapses above us,” Rooney said.

BLASTING FOR THE RAW WATER TUNNEL
While Schiavone-Picone used the TBM on the two long treated-water tunnels, crews followed the original drill-and-blast technology for the gravity-fl ow, 850-ft, cast-in-place concrete-lined, 12-ft internal diameter raw water tunnel.

To address the risk of excessive vibrations on existing city water tunnels, Schiavone- Picone followed round limitations of 6 ft as a 100-ft section passed 77-ft above 1917-era water tunnels, currently serving the city. No damage occurred.

“We didn’t want to cause any kind of undue vibrations that would subsequently lead to their early demise,” Rooney said. Following New York City noise-abatement requirements, the contractor designed and constructed a 20-ft wall using steel container boxes to reduce noise and errant golf balls from entering the site. Most of the work took place 100-ft below ground, which would produce little vibration for golfers and drivers.

Another “benefit for using a TBM vs. a drill and blast was that there was less perceived observation from the adjoining communities,” Rooney said.

“The plan for tunneling, including control of vibration, led to a project where we didn’t have any complaints about this construction activity,” Schaefer said. “It was a successful part of the project.”

TAPPING INTO THE NEW CROTON AQUEDUCT
Another interesting part of the project was tapping into the New Croton Aqueduct, built in the late 1800s, for the raw-water tunnel and the low-pressure treated water connection.

The city relies on that aqueduct for water and was only able to shut it down for a brief, approximately three-month period, for the new pipe to connect with it. Diffley described the process as shooting out the surrounding rock, mucking it and entering the existing aqueduct. No damage occurred to the original structure.

“The contractor before he broke into the New Croton Aqueduct installed ground reinforcement to help stabilize the mass around the brick liner,” Ott said. “The existing brick liner was harder to take down than a lot of people thought.”

That work, blasting into the brick-lined tunnel, progressed more smoothly than expected.

“It was in phenomenal condition,” Rooney said. “The quality of workmanship and attention to detail was phenomenal. It was quite an impressive structure.” Crews found the old rail lines and tools used to build it.

Altogether, Daly said he was “happy with the quality job.”