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, replaces 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.

On a complexity note, this project featured design and construction of a massive infrastructure, deep tunnel project in a sensitive coastal area surrounded by water and marshes with very poor soil conditions and geographic challenges.  The most challenging aspects of this project include sinking a 60 ft. diameter concrete caisson shaft 150 ft. deep in mixed ground conditions for a 15 MGD Influent Pump Station and (2) each 5’ diameter drilled piles 140 ft. deep.  The tunnel portion includes 8,300 LF of 86” tunnel for 54” fiberglass reinforced pipe (FRP) approximately 125 ft. below ground.  In addition, 1,200 LF of 48” pipe was installed above the marsh wetlands on pile bents.  Project is located on an estuary with fill, soils, and Cooper Marl.  Project required intensive full-time construction inspection and administration services to maintain integrity.

Hussey Gay Bell, in collaboration with Black & Veatch, has been involved with all design and construction administration phases of the project which is being 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.