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Sequoyah Hills, one of Knoxville, Tennessee's historic and distinguished communities, has an older infrastructure requiring periodic attention. However, the unusual placement of some utilities, restricted access caused by bordering waterways, heavily wooded terrain, and meandering roads often complicate design and construction.

Many properties in the southern part of Sequoyah Hills abut the Tennessee River. Houses on most of these sites are set deep within their waterfront lots with boathouses, gazebos, and walkways extending to the water's edge. This carefully nurtured section of the community, renowned for its forested hills and stone shoreline bluffs, became an on-going source of concern when groundwater infiltrating into its older sanitary sewer overloaded a downstream pump station during wet weather. The infiltration was caused by deteriorated 50-year old masonry joints.

Although residents recognized the need to rehabilitate the aging sewer system, they were concerned about the impact this would have on their individual property and the community, as a whole, because of the disruption normally associated with pipeline replacement. However, engineers for the Knoxville Utilities Board (KUB) recognized the benefits that trenchless technology offered for projects involving such a variety of physical constraints.

"Working space and access restrictions eliminated some usual rehabilitation methods from consideration," said Julie Childers, P.E., project engineer with KUB. "After weighing all remaining options, we decided that segmental sliplining was the best process for this project since it permits small insertion pits and needs less equipment. Also, since individual liner pipe sections can be supplied in short lengths, they are easy to handle in tight quarters." The sliplining process involves inserting new pipe into a deteriorated sewer to restore hydraulic properties, structural characteristics, or to stop infiltration/exfiltration.

In spring and summer 1999 KUB employed Nashville-based Consoer Townsend Envirodyne Engineers, Inc. for site analysis, planning, and design. This first phase focused on a section involving 2,380 ft of 36-in. and 1,079 ft of 33-in. concrete pipe, plus several 6-in, service connections. Material and installation specifications permitted alternative types of segmental liner pipe: fiberglass reinforced pipe (FRP) and several profile-wall PVC pipes, all with belland-gasket joints. The specified nominal diameters were 30-in, and 27-in, liner pipes for the existing 36in. and 33-in, pipes, respectively. The rehabilitation contract was awarded to American Excavators (Columbia, Tennessee), whose winning bid was based on A2 PVC liner pipe manufactured by Contech Construction Products Inc. (Middletown, Ohio).


"Another benefit of segmental sliplining is that it need not interfere with flow in the existing line while work progresses. With most other relining or rehabilitation processes, service must be interrupted and bypass lines are required for the duration of the project. Uninterrupted flow also aids installation of the slipliner pipe by reducing friction," said Paul Stonecipher, P.E., design engineer for Consoer Townsend Envirodyne Engineers.

In urban environments, where manhole spacing is usually several hundred feet, the distance between manholes usually represents separate portions of the relining project; segmental sliplining can proceed from manhole to manhole. On the Sequoyah Hills project, however, space constraints hindered access to the sewer via most existing manholes and the largest section of the existing line--approximately 2,045 ft long--required sliplining in one single, continuous operation. This 36in. pipeline section contained five manholes, each having horizontal direction changes between inlet and outlet pipes ranging from less than one degree to more than six degrees. Such an unusually long "push," especially in the presence of significant line deflections, requires careful selection of materials and installation techniques.

Guy Anderson, project manager for American Excavators, has installed other types of liner pipes and was involved in final materials selection for the Sequoyah Hills project. Regarding the selection of A2 PVC liner pipe and relining the longest section, Anderson says: "Our biggest concern was making deflections at the liner pipe joints without disengaging the joint gaskets and without restricting our ability to push the liner into the existing sewer. A2 liner provided the strongest joint, even when deflected up to five degrees, which was a must for this installation. Alternative segmental liner pipes did not offer as muchjoint deflection, which would require shorter liner pipe lengths and additional joints to make the bends. Reducing the number of joints was another issue, since the liner would be air tested at 3.5 psi for nearly two hours following placement."

American Excavators successfully completed the long push and passed the air test without incident. A2 liner pipe's special joint design, which can manage pushing forces even when fully deflected, contributed to this success. Sections of the liner running through manholes were cut and removed after testing was completed. The annular space between the new liner and the existing pipe was filled with a non-shrinking grout and additional sealer applied at manhole entries.

"While we were not very concerned about relining the shorter sections on this project, we thought that the unusually long 2,000 ft section would be fairly difficult," comments Larry Pittler, resident inspector for Consoer Townsend Envirodyne Engineers. "However, the contractor used the right techniques, picked the most appropriate slipliner pipe, and relining went much smoother than expected."

The project, which began in fall 1999, was completed in June 2000. Despite the initial concerns of the residents of Sequoyah Hills, they were relieved to have the project completed with a minimum amount of property disruption. Inserting the A2 PVC liner pipe required only four excavations totaling 160 linear ft along the 3,459-ft long existing line. The benefits of trenchless technology and segmental sliplining resulted in a timesaving, low-disruption, cost-efficient solution to a difficult rehabilitation problem.
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Title Annotation:maintenance and repair
Publication:Public Works
Article Type:Brief Article
Geographic Code:1USA
Date:Mar 1, 2001
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