Who: Middlesex County Utilities Authority, N.J.
Design: Hatch Mott MacDonald, Freehold, N.J.
General contractor: Kenny Construction Co., Northbrook, Ill.
Tunnel subcontractor: Bencor Corp. of America, Dallas
Cost: $75 million
Funding: New Jersey Environmental Infrastructure Trust, which leverages state revolving funds to provide low-interest loans to local governments
A recent project undertaken by New Jersey's Middlesex County Utilities Authority (MCUA) was designed to provide a redundant means for sewage conveyance from its 85-mgd Edison pump station to its 400-mgd central wastewater treatment plant.
MCUA was founded in 1950 in part to protect and enhance the marine habitat in the Raritan River and Bay by collecting and treating wastewater from nearly 800,000 people. Similar to many utilities, MCUA is charged with the maintenance of its aging infrastructure with dwindling dollars.
A 3,940-linear-foot tunnel was constructed under the 3,000-foot-wide river. The 15-foot, 6-inch tunnel has several uses, namely to connect the Edison pump station — one of five contributing pump stations — to the central wastewater treatment plant. Within the primary tunnel are two parallel force mains that will replace MCUA's existing 60-inch Arsenal force main, which was installed in 1969.
The existing line, made of prestressed concete cylinder pipe, is supported on piles and is located several feet below the river-bed. The Arsenal force main accepts continuous flow of wastewater from the northern part of the county serving more than 200,000 people. Since the pipeline has been in continuous service for 40 years with no ability to be inspected internally or externally, there was concern that the prestressing wire could fail, causing a catstrophic pipe failure and an uncontrolled discharge of raw sewage into the river.
To avoid the risk of a system failure, the NJDEP, through an administrative consent order, required MCUA to construct a new redundant pipeline under the river.
WEIGHING THE OPTIONS
The first design concept was to build an 8-foot-diameter tunnel in lieu of open-cutting below the water surface and installing a second 60-inch pipe and then rehabilitate the existing pipeline. This would create a dual-pipeline system allowing operators to shut down one line for inspection or repair without disrupting flows. The projected cost would be about $29 million if the old pipe couldn't be rehabilitated; a replacement tunnel and new force main would cost an estimated $30 million to $35 million.
But after researching techniques and the costs and risks associated with rehabilitating 40-year-old pipe, engineers decided the most practical and economical solution was to enlarge the tunnel diameter and install two new pipelines instead. In the end, enlarging the tunnel represented potential savings of more than $20 million.
The authority was concerned about the installation and long-term performance of the new pipelines. Because they were being assembled belowground within a confined space, they had to be light enough to be transported and lifted to be set to grade. And they must resist sulfuric acid, the byproduct of hydrogen sulfide gas generated by sewage.
“They wanted a pipe that was corrosion-resistant both inside and out,” says Angelo Bufaino, senior project engineer for Hatch Mott MacDonald, the firm that designed the new force main. “The pipe also was to be installed within a damp tunnel and partially encased in concrete, and there would be no way to perform external repairs if external corrosion occurred.”
Centrifugally cast, fiberglass-reinforced, polymer mortar (CCFRPM) and high-density polyethylene (HDPE) were the only materials that met the criteria. Ultimately, HDPE's tendency to expand and contract with varying wastewater temperatures led authority managers to specify CCFRPM.
“It was 15% lighter (211 pounds/foot) than HDPE (244 pounds/foot) and jointed, making installation easier,” Bufaino says.
SOFT GROUND CONDITIONS
General contractor Kenny Construction Co. brought in deep-foundation specialists Bencor Corp. of America to build access shafts on each side of the river. Bencor used slurry wall construction methods to create the 36-foot-diameter, 87-foot-deep launch shaft and the 28-foot-diameter receiving shaft.
To prevent soil heaving at the receiving shaft, Kenny Construction chose to excavate the final 15 feet of the southeast shaft underwater instead of using the specified jet-grouted plug below the base slab. Upon reaching an excavation depth of 70 feet, the shaft was flooded and the remaining 15 feet of soil excavated underwater. Divers then verified the depth, set the final rebar mat, and placed the tremie concrete slab.
Authority managers decided to leave the tunnel partially open to permit inspection and allow utilities to use it. In the final construction phase, two 16-inch HDPE pipelines were installed to convey methane from the county's landfill, which is also operated by the authority, to power the wastewater treatment plant 7 miles away. The existing gas pipeline has leaked in recent years, so the authority decided to augment it with a second pipeline using the tunnel.
Once the shafts were in place, a Lovat Inc. earth-pressure-balance tunnel-boring machine mined through clay, sand, silt, and gravel. Crews followed behind, placing the 9-inch-thick precast gasketed concrete segments. The annulus void of approximately 3 5/8 inches was grouted as the machine moved forward.
“The contract documents specified the use of a pressurized-face tunnel-boring machine due to the presence of soft ground deposits and potential flowing soils under atmospheric conditions,” says Julian Prada, resident engineer for Hatch Mott Mac-Donald. But because of the soft ground conditions, Kenny Construction used an earth-pressure-balance tunnel-boring machine. Soil was mined with a rotating cutter head, and a screw conveyor allowed the company to control the volume of soil excavated. The machine supported the ground during excavation, which helped to balance the earth pressure.
Preserving the in situ soil was especially critical because the existing force main alignment was within 15 feet at one point. “The 3,940-foot tunnel hit its target for both line and grade,” Prada says.
Once the primary liner was in place, installation of the carrier pipe began.
“The entire line [twin force mains] was installed from the southeast shaft,” says Bob Rautenberg, project manager for Kenny Construction. “The pipe was installed on two shifts; on our best day we installed 20 pipes.”
The next phase included grouting the dual pipelines into place. A blocking scheme was designed and installed to anchor the pipe and resist uplift.
“With the blocking in place, the backfill plan called for the first lift of material to be a lightweight cellular grout with a density of 35 pcf,” Rautenberg says. “The cellular grout encased the pipe to 8 inches below the spring line. Once the cellular grout was in place, both of the pipes were filled with water for ballast and then encased to a point approximately 4 inches below the outside crown of the pipe with 4,000-psi structural concrete.”
This design allowed for a 6-foot-high walkway along the center of the tunnel.
The pipe within the tunnel is connected to 60-inch ductile iron pipe risers, which are in turn connected to the surface piping at the northwest and southeast shafts.