West Ashley Sewer Tunnel, Charleston, SC

120’ Below Earth’s Surface:  A Journey to Fix Charleston’s Deep Tunnel System

From 2002 to 2008, Hussey Gay Bell was involved in Phases II-IV leading up to the Phase V West Ashley Sewer Tunnel & Influent Pump Station.  Phase II consisted of the design and construction of the $38.9 million Ashley Tunnel; Phase III consisted of the design and construction of the $48.6 million Cooper Tunnel; and Phase IV consisted of the design and construction of the $29.4 million Daniel Island Tunnel Extension.

The $50.8 million fifth and final phase of Charleston Water System’s 15-year+ sewer tunnel improvements program, replaced an existing deep tunnel from the Croghan Spur to the Plum Island Wastewater Treatment Plant (PIWWTP) located off of Harborview Road in Charleston with a new tunnel 120-foot-deep, 1.6 mile long tunnel from Albemarle Road, under the Wappoo Cut, to the Plum Island Wastewater Treatment Plant. Other improvements consisted of the construction of a new 60 MGD influent pump station, new pile-supported 48-inch force main, new 3,200-square-foot pump control building at the treatment plant, installation of near surface connection at the PIWWTP, two shaft sites and surface collection improvements in the Country Club of Charleston and along Harborview Road.  The shafts consist of a 60-ft ID at PIWWTP and 20-ft ID at Croghan, both built using the caisson sinking method, and 30-in. drop pipe at Porter Gaud. The 8,300 ft tunnel was excavated with an 86-inch diameter single shield tunnel boring machine, manufactured by Southland Contracting, and supported with ribs and lagging boards.

In tandem with the design and construction of the tunnel project, Hussey Gay Bell was contracted to design the Near Surface Facilities project which addressed the redirection of “near-surface” wastewater flows from a drop shaft on the failing sewer tunnel back into the existing sewer collection system. Existing flows entering the Country Club Drop Shaft were permanently diverted back into the near-surface collection system and conveyed to the Plum Island WWTP.  Gravity sewer and force main upgrades were made to accommodate the modified flow regime.

Hussey Gay Bell, in collaboration with Black & Veatch, was involved with all design and construction administration phases of the project which was constructed by the joint venture of Fort Worth, Texas-based Southland Contracting, Inc. and Roanoke, Texas-based Oscar Renda Contracting, Inc.

Phase V West Ashley Sewer Tunnel & Influent Pump Station
Charleston, SC

In the late 1990s, Charleston Water System embarked on a journey to fix its deteriorating system of deep tunnels originally constructed in the late 1960s to collect wastewater from shallow sewer lines and deliver it to the Plum Island Wastewater Treatment Plant off of Harborview Road in Charleston. Over time, the highly corrosive nature of wastewater damaged the carrier pipe inside the tunnel and the tunnel structure itself. This deterioration left the tunnel system at risk of serious failure, which would block flow in the tunnel, notably during heavy rain events, and cause sanitary sewer overflows a potentially serious threat to public health and water quality in the Charleston Harbor.