Phase V West Ashley Sewer Tunnel and Influent Pump Station

Completed in 2020 by Southland Contracting, the Phase V West Ashley Sewer Tunnel and Influent Pump Station Project consisted of the replacement of an existing sewer tunnel, the construction of a new influent pump station (IPS), and the installation of associated near-surface and deep subsurface connections. The existing sewer tunnel extended from the Croghan Spur in West Ashley to the Plum Island Wastewater Treatment Plant (PIWWTP), and the new influent pump station was constructed at the PIWWTP. The most challenging aspect of this project was sinking a 60- foot diameter concrete caisson shaft 150 feet into mixed ground conditions for the 60 MGD IPS. Soil conditions and geography were challenging as the project took place in locations surrounded by water and marshes. Geotechnical conditions in the top 50 feet were not accurately described by the geotechnical information provided and impacted the time required to complete the work. Our team worked side by side with the design engineers to develop a redesign to maintain the intended service of the facility while adjusting the structure for the unforeseen conditions that impacted construction. Construction also included approximately 9,500 linear feet of 86-inch diameter soft ground tunnel for 54-inch carrier pipe between 125 and 130 feet below ground. The installation of force main to connect the new influent pump station to the existing plant headworks included 2,100 linear feet of 48-inch diameter pipe, of which 1,200 linear feet is supported above the marsh wetlands on pile bents. Modifications to the existing near-surface collection system included gravity main and force main trenchless slip lining, a new submersible grinder pump station, and rehabilitation of an existing 100 gallons per minute (GPM) suction lift pump station. This project phase also included building a new 3,200 square foot pump control building at the PIWWTP.