{"id":74770,"date":"2011-07-08T17:31:08","date_gmt":"2011-07-08T17:31:08","guid":{"rendered":"https:\/\/zonda-develop.go-vip.net\/concrete-construction\/article\/burrowing-beyond-a-consent-decree_o"},"modified":"2011-07-08T17:31:08","modified_gmt":"2011-07-08T17:31:08","slug":"burrowing-beyond-a-consent-decree_o","status":"publish","type":"post","link":"https:\/\/zonda-develop.go-vip.net\/concrete-construction\/projects\/infrastructure\/burrowing-beyond-a-consent-decree_o\/","title":{"rendered":"Burrowing beyond a consent decree"},"content":{"rendered":"
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By Stephanie Johnston<\/b><\/p>

COMBINED SEWER OVERFLOW TUNNEL<\/b> <\/p>

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

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.<\/p>

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.<\/p>

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.<\/p>

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

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.<\/p>

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. \u201cthe bends\u201d) 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.<\/p>

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.<\/p>

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.<\/p>

The additional storage space this design change provided meant the Water Pollution Control Department<\/a> 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.<\/p>

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.<\/p>

\u201cPeople were amazed this had all been done underground,\u201d says Wastewater Treatment Plant Superintendent Brad Talley<\/a>, who began getting tour requests when the tunneling machine (right) was being assembled.<\/p>

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.<\/p>

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