Evanston, Ill., part of Chicago's affluent North Shore region and home to Northwestern University, is directly north of the windy city and shares many common attributes, including its Lake Michigan waterfront. It was also the site of the longest, large-diameter sliplining project on record for the district.

Evanston's combined-sewer system conveys sanitary and storm sewage to the district's Evanston Intercepting Sewer system, from which the water is directed to their Northside Water Reclamation Plant for treatment. An important component of the intercepting sewer system is a 10-foot, semi-elliptic sewer on Lake Street.

One of the more recent projects of the district's sewer rehabilitation program, the Lake Street sewer rehab project, included 7,000 linear feet of 120-inch semi-elliptic, cast-in-place concrete sewer.

"A closed-circuit television inspection of the sewer conducted by the district revealed that the concrete sewer pipe had cracked at a number of places and had lime deposits at cracks and 'cold' joints, and the concrete had eroded due to the action of hydrogen sulfide and flowing water," said Amreek Paintal, engineer of sewer design at the district.

Bid documents for the project included various options: segmental sliplining, cured-in-place (CIPP) lining and insertion of panels. Kenny Construction of Northbrook, Ill., submitted the bid with the intent to slipline the sewer.

Jack Callahan, vice president of the underground group with Kenny, said, "We thought it would be the most economical option due to timing and the size. We do a lot of CIPP, but this was a little too large for that method, considering the water situation."

As with many projects, there were obstacles to overcome. "We have a lot of experience in rehab and have tackled some pretty tough projects. Sliplining pipe of this size is not like sliplining with smaller sizes. This was an uncommon project, the first time it was done. The project went well and we made good production putting the pipe in," said Tom Gillis, project superintendent with Kenny.

The sewer line under contract had a semi-elliptical cross section of approximately 10 feet by 10 feet. Kenny bid the project planning to rehabilitate all 7,000 linear feet with 110-inch diameter pipe. Upon closer analysis of the existing sewer, they found that a reduction in liner diameter in several locations was necessary because the existing sewer was not round or straight and had many old repairs.

Maximizing ID

"The first 2,000 linear feet of sliplining had been completed using ... 110-inch flush reline pipe, which has an O.D. of 114 inches," said Gillis. "Conditions we discovered when the job was started prompted a revaluation of the original design and resulted in the installation of 104-inch ... pipe with an O.D. of 108 inches to be used for the remainder of the rehabilitation work." The pipe manufacturer delivered a flush-joint sliplining pipe that had precisely the same diameter at the pipe barrel and the bell joint; this also facilitated installation.

Most sliplining projects are located in congested areas with limited access to the line. In this case, that was so true that immediately accessible pipe storage was not possible. The district provided a staging area about 1.5 miles from the site where 2,000 linear feet of 110-inch diameter sliplining pipe could be stored. "We opted to get the first run of pipe on the ground before we started," said Gillis.

Shaft eliminated

The pipes were quickly joined and pushed while the flow was maintained.

"Initially we thought we would need three installation shafts, but we were able to do this with just two shafts because the pushing went so easily," said Callahan. "Access to the sewer was limited because of the 5 to 26-foot cover depth under the existing infrastructure, including roads and a railway."

The installation lines were quite long for pipe of such a large diameter. To decrease the friction, it had to be kept as neutrally buoyant as possible. Said Gillis, "The only issue with installing was to control the water to the level that would make the pipe neutrally buoyant. Once we figured that out, we had no problem. Typically, we had 15 to 18 inches of flow, and we calculated that including the weight of the pipe, its contents and everything else, we needed 27 inches of water outside the sliplining pipe to make it neutrally buoyant. We made some modifications and the pipe stayed buoyant."

The construction company worked to find the best option for the grouting, which included several grout lifts in stages to prevent uplift. "The grouting wasn't as simple as it would be on smaller diameter projects since surface access was limited. Because this is an interceptor that is sensitive to rain, if you're got a 6- or 7-foot bulkhead in the sewer and rain comes, that sewer is ineffective," said Gillis.

Although the grouting took some critical thinking, the pipe has high stiffness and has been performing well. "We are continuing to grout and are looking at being done with everything by the end of July," stated Callahan. PE