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Challenges from A to Z–The Rear View Mirror for One of North Carolina's Largest Sewers

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As one of five wastewater treatment plants (WWTPs) owned and operated by Charlotte-Mecklenburg Utilities (CMU), the Sugar Creek WWTP receives and treats wastewater from a portion of downtown Charlotte and south Charlotte within the Little Sugar Creek and Briar Creek drainage basins. Rainfall-induced sanitary sewer overflows (SSOs) within the Briar Creek basin have been reported since the mid-1980s. The existing Briar Creek trunk sewer, constructed of vitrified clay pipe (VCP) in the late 1920s, does not provide the level of service required by CMU, new state legislation, or EPA. The objectives of the Briar Creek relief sewer are to reduce the risk of SSOs and to increase sanitary sewer capacity to meet current and future wastewater flows. The Briar Creek relief sewer project replaces most of the vintage 1920s sewer and conveys approximately 95 million gallons per day (mgd)— the projected peak flow from a 10-year storm event. Phase I of the three-phase project consists of more than four miles of 72-inch and 60-inch diameter sewer that terminate at a new 200-mgd pump station at the Sugar Creek WWTP.

From its inception, the construction of the Phase I project was envisioned to be challenging—and the construction has lived up to that billing. From purely a constructability perspective, conditions include a shallow design slope (0.10% to 0.17%), easement widths as narrow as 40 feet, and excavation depths approaching 45 feet. The relief sewer is also installed adjacent to, and sometimes within 15 feet of, the aged VCP sewer, therefore attended bypass pumping systems were employed to maintain uninterrupted service and prevent overflows. The construction was further complicated by limited access and mobility along the alignment. These restrictions mandated close coordination of even typical sewer installation activities—staging of pipe and transport of materials to/from work zones.

The Briar Creek basin is primarily residential and nearly built out—the creek travels though many wooded backyards and at times within 25 feet of homes. Emphasis was placed on site safety and security. Safety programs included competent person, trench safety, and confined space training. Security fencing with privacy screening was installed along the majority of the alignment.

The relief sewer alignment crosses Briar Creek seven times within the Phase I project, so specific attention was give to the maintenance of creek flows and creek water quality. The creek flow maintenance strategy also addressed crossings of the creek by heavy equipment during the clearing, pipe installation, and restoration phases of the construction.

The Piedmont geology plays an undeniable role in sub-surface construction, its fractured and weathered rock affecting both open trench and tunneling activities. Rock jointing created particular challenges during the mining of a 2,400-linear-foot tunnel under a three-school complex. The geology requires more robust pipeline construction techniques, especially for tunnels, and hampers the production rate or time for installation. The geology typically necessitates drilling and blasting, which elevates the safety, monitoring, and public relations components of the project.

Public relations efforts initiated during the design phase of the project were continued and re-directed to minimizing disruption—whether in the form of noise, light, dust, temporary loss of property use, or vibrations from blasting activities. Public relations activities ran the gamut–from quarterly newsletters to topic-specific neighborhood meetings to supporting the resolution of blasting claims.

In the latter stages of the project, activation and abandonment of the existing sewer required careful planning of both access and methods due to the aged VCP materials' lack of integrity. Sizable distances between existing manholes and limited access to the shallow sewer in the backyards of private residences and at the local golf course presented unique challenges.

Promoting a healthy and opportunistic bidding environment for the contracting community also resulted in the management of multiple construction contracts and interfaces at numerous stages of the project (initial clearing, pipe connection, activation and abandonment). Consistent and collaborative teamwork mitigated challenges presented by these interfaces.

The completed relief sewer now reliably serves thousands of Charlotte customers within the Briar Creek basin. Construction of the Phase II portion of the sewer is slated for autumn of 2011, while design of the final Phase III section is anticipated to begin in late 2011.

This paper will cover not only the challenges described above, but also the methods employed by the project team to address them successfully.
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Keywords: Community Acceptance; Design and Construction Challenges; Relief Sewer; Sanitary Sewer Overflows; trenchless pipe installation

Document Type: Research Article

Publication date: 2011-01-01

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