South Potomac Supply Improvement Project
Temple Hills, Md.
Owner: Washington Suburban Sanitary Commission
Lead Design Firm: Black & Veatch Corp.
General Contractor: Garney Cos.
Subcontractors: Eureka Concrete Construction Inc.; Kroeger Electric Co.
To combat failing prestressed concrete cylinder pipe (PCCP) in its system, the Washington Suburban Sanitary Commission has placed a high priority on replacing sections of existing PCCP with ductile iron or steel pipe. The $14.7-million South Potomac Supply Improvement Project addressed a critical WSSC water-supply line that affects about 180,000 customers in Prince George’s County, Md. The main serves as a primary feed to the National Harbor hotel-retail-casino development.
The team installed approximately 10,000 linear ft of 42-in. CL-56 ductile iron pipe with PRITEC coating along Henson Creek, portions of which disturbed and disrupted an existing hiking and bike trail through a park. Crews also removed and disposed of approximately 5,000 linear ft of existing 42-in. PCCP water main pipe. The project also included installing 10 precast concrete entry-port/air-release vaults; demolishing an existing flow-control vault; and constructing a new cast-in-place, flow-control vault structure, featuring remote flow adjustment using SCADA technology.
Construction of four 60-in.-dia jack-and-bore crossings, totaling 725 linear ft, presented some of the project’s greatest challenges, says Garney Cos., the general contractor. During construction of jack-and-bore sections below Henson Creek, Garney’s jack-and-bore subcontractor encountered an extremely hard layer of what turned out to be an ancient ocean floor oyster bed.
When traditional auger boring to get through that hard layer proved futile, the team implemented pipe ramming. That method—known to cause vibration—was marginally successful and the resulting vibration ultimately led to the collapse of Henson Creek and flooding of the tunnel and tunnel shaft. After immediately notifying the appropriate parties and stabilizing the situation by damming the creek and installing bypass pumping, the team received expeditious approval from environmental regulatory agencies to install the remaining portion of the trenchless crossing by open cut across Henson Creek. Quickly mitigating the emergency over one weekend avoided a negative impact to the project’s overall schedule and budget.
Before constructing a jack-and-bore section below the road, crews identified a federal government fiber optic communication line at the site of the trenchless crossing. After multiple attempts, the fiber optic utility could not be located. After establishing that the fiber optic line was not in the path of the jack and bore, Garney directed the jack-and-bore subcontractor to proceed with the tunnel. But after installing 40 linear ft of 60-in.-dia casing, the subcontractor encountered the abandoned 42-in. PCCP water main in direct conflict with the scope. Limited by the existing fiber optic utility and right-of-way constraints, Garney was forced to abandon the trenchless crossing and install the 42-in. ductile iron water main by open-cut excavation.
Over the course of six months, the project team went through multiple iterations of plan preparation, review and comments with Prince George’s County before the open-cut plan was approved. Garney implemented a substantial temporary traffic control plan and executed the open-cut crossing in two days, as planned.
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