The case study focuses on the unique permitting and construction challenges presented by the pipeline rehabilitation project, including access, constructability within the salt marsh, potential hurricane/tropical storm winds and storm surge, and construction timing within highway right-of ways and on private properties.
Charleston Sewer District’s 54-inch Ashley Interceptor is one of the two branches of the District’s 22.2-mile interceptor system. It extends for 7.4 miles along the east side of the Ashley River before combining with the Cooper Interceptor for the remaining 1.8 miles to the Felix C. Davis WWTP.
The project included rehabilitation of 10,700 LF of reinforced-concrete-pipe sewer constructed during the late 1960’s. The interceptor was inspected in 2010 and found badly deteriorating. Phase 7 is part of the Sewer District’s ongoing interceptor rehabilitation program. The interceptor is laid within marsh associated with the Ashley River and is susceptible to inflow and infiltration. The sewer also parallels an interstate highway, I-26, and crosses beneath a major local highway. The location of the interceptor therefore added difficulty to the construction activities. The construction period during hurricane season and was affected by potential and actual impacts associated with passage of Hurricane Irma. The cured-in-place pipe (CIPP) rehabilitation was constructed using onsite wet-out methodology to achieve greater inversion lengths and fewer inversions.
The low bid in March 2017 yielded a contract of $4.95 million. Construction began in July and will be complete in January 2018. 10,700 LF of sewer was rehabilitated, including 120 LF of 36-inch sewer by centrifugally-casting geopolymer. Twenty-eight manholes and the receiving splitter box at the downstream V.C. Pump Station were rehabilitated. All rehabilitated manholes were fitted with new rings and covers.
The entire length of the project was bypassed, utilizing four (4) diesel bypass pumping units. The primary bypass piping consisted of two (2) 24-inch high-density polyethylene (HDPE) pipelines. The bypass capacity was 21.6 MGD, and on normal days bypassed flows of 7 MGD. The setup included 8 secondary bypasses for lateral sewers connected by manifold into the primary bypass. The primary bypass crossed beneath one major highway (US Hwy 7). To simplify the crossing the bypass was extended enough to lay the pipelines above-ground beneath the Hwy 7 “Northbridge” which crosses over the Ashley River.
Permits were required from the SC DOT, the US Army Corps of Engineers (ACOE), and the SC Department of Health and Environmental Control – Office of Ocean and Coastal Resource Management.
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