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The 2,300-foot-long tunnel is 114 inches in diameter. Workers laid temporary liner plates behind the digging to hold the surrounding dirt in place as the tunnel digger shield (TDS) moved at the rate of 16 feet daily. The contractor then used cast-in-place forms to complete portions of the tunnel behind the machine. Photo: Greeley and Hansen

By Stephanie Johnston

COMBINED SEWER OVERFLOW TUNNEL

OWNER:8City of Lafayette, Ind., Water Pollution Control Department
LEAD DESIGN ENGINEER:8Greeley and Hansen LLC
GEOTECHNICAL AND TUNNELING/DESIGN:8Haley & Aldrich Inc.
GEOTECHNICAL AND TUNNELING/ CONSTRUCTION:8Jacobs Associates
CONSTRUCTION OVERSIGHT:8Christopher B. Burke Engineering Ltd.
CONTRACTOR: A joint venture of Triad Engineering Inc. and Frontier-Kemper Constructors Inc.
BUDGET: $21 million
FINAL COST: $18.8 million
PROJECT DELIVERY METHOD: Design-bid-build
COMPLETED: April 2010

Tunneling projects are scary, and for good reason: their 15%-to-25% change order rate and the potential to end up on the wrong end of a safety-related lawsuit. Yet a community of 67,000 residents safely managed risks with only a 2% change order.

One of the 900 or so communities nationwide that have a combined sanitary and stormwater sewer, Lafayette averaged 96 overflows in a typical year and was working to reduce that number over 466 miles of pipeline serving the 20-square-mile city. But because the alignment that would eliminate one of the city's 11 outfalls lay directly under the central business district and four sets of active railroad tracks, open-cut construction wasn't an option.

The city spent $60 million to expand its wastewater treatment plant from 22 mgd to 52 mgd. In 2009 it signed off on a $179-million, 20-year long-term control plan (LTCP) to reduce the volume of untreated sewage discharged into the river that separates Lafayette from its sister community (and home of Purdue University), West Lafayette.

Hired to develop and evaluate potential solutions, consultants Greeley and Hansen explained how a 2,300-foot tunnel with 1.2 million gallons of storage capacity was the most cost-effective option.

A geotechnical analysis revealed sandy soils, which often contain a lot of water. So instead of a boring or earth pressure balance machine, the contractor recommended using a tunnel digger shield (TDS) equipped with compressed air technology. The air stabilizes the tunnel as it's dug with a digger arm similar to a backhoe. The soil is collected in a hopper and passed from the compressed to the non-compressed side of the machine via a screw conveyer, which dumps it into mining cars that exit the tunnel on a system of rails. The method is often used for alignments that are particularly susceptible to collapsing.

The Occupational Safety & Health Administration limits how long someone can work in compressed-air conditions and requires project sites to have a decompression chamber available to prevent exposing workers to the same dangerous condition (a.k.a. “the bends”) that deep-sea divers face. But the contractor's $6 million machine has a bulkhead with a window that separates the digging area from workers, so additional safety measures weren't necessary.

Originally, the tunnel was going to stop at a trunk sewer that runs directly to the river and, five to 10 years later, a structure would be built around the trunk so the tunnel could be extended. A smaller pipe would then connect to the interceptor sewer near a lift station at a city parking lot.

Value-engineering by Greeley and Hansen and the contractor, however, determined that a slight alignment shift would enable extending the tunnel another 325 feet to connect directly to the interceptor sewer. This eliminated the need for a 42-inch sewer, but meant the new alignment would run through the trunk sewer. The solution: install an inverted siphon to route the trunk's flow over the tunnel.

The additional storage space this design change provided meant the Water Pollution Control Department could remove the 1960s lift station, which was at capacity and had become a bottleneck, from service and let the city use the space to enlarge its bus depot.

This was the only change order. It cost $290,000 and delayed completion from September 2009 to April 2010, but the project still beat regulators' deadline of July 1.

“People were amazed this had all been done underground,” says Wastewater Treatment Plant Superintendent Brad Talley, who began getting tour requests when the tunneling machine (right) was being assembled.

He recently told a local newspaper the tunnel's performing better than expected. The volume of untreated sewage being discharged has been reduced by 35%, better than the anticipated 30%. In addition, outfalls connected to the tunnel have experienced a 60% reduction in discharge.

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