After spending six months deep underground, a tunnel boring machine has finished drilling the new West Ashley Tunnel, a 1.6-mile tunnel that will soon carry millions of gallons of wastewater daily from West Ashley to the Plum Island Wastewater Treatment Plant on James Island. The tunnel will carry wastewater that comes from homes and businesses—not storm water.
Completion of the tunnel drilling marks an important milestone in the West Ashley Sewer Tunnel Replacement Project, which is the fifth and final phase of the $196 million Sewer Tunnel Replacement Project. The multi-phased project began in the 1990s and is one of the largest and most costly infrastructure projects in Charleston Water System’s 99-year history. It is funded by bond issues and sewer rates.
The new West Ashley tunnel will replace an existing tunnel that is in poor condition and too small for today’s peak wastewater flows. This creates a bottleneck that occasionally causes sewer overflows in low-lying areas of West Ashley.
“Think about how often you flush the toilet, wash your hands, or run the washing machine,” said Kin Hill, P.E., Charleston Water System CEO. “The new tunnel will make sure that wastewater from all the homes and businesses in West Ashley can flow to our treatment plant for many years to come, which is critical to public health, the environment, and our quality of life.”
Drilling the new West Ashley tunnel began in November when the 8-ft diameter tunnel boring machine was lowered into a deep shaft at the Plum Island Wastewater Treatment Plant. From there, it drilled under the Wappoo Cut to another deep shaft off Albemarle Road. The tunnel is 125 ft deep.
The contractor will now install a pipe in the tunnel, build a pump station at the treatment plant, and connect the new tunnel to the existing sewer system off Albemarle Road. Construction is expected to be complete and the tunnel put into operation in late 2017.
Sewer Tunnel Replacement Project Background
The original sewer tunnel system on the Charleston peninsula and West Ashley was built in the late 1960s at the same time as the Plum Island Wastewater Treatment Plant. At the time, sewer lines discharged directly into the Charleston Harbor. The tunnel system was designed to collect the raw sewage and carry it under the Harbor and Wappoo Cut to the plant for treatment.
Over the last 50 years, the highly corrosive nature of wastewater damaged the tunnel. In addition, population growth and the resulting increase in actual and projected wastewater flows exceeded the tunnel’s capacity.
Charleston Water System has already replaced three sections of the original tunnel system—a section on the east side of the Charleston peninsula, another on the west side, and a third under the Harbor. A fourth section was added to collect wastewater from Daniel Island. The West Ashley Tunnel is the last section of the original tunnel system to be replaced.
The design team includes Charleston Water System staff, Black & Veatch, Hussey Gay Bell, and Hazen and Sawyer. The construction contractor is Southland Renda, a joint venture between Southland Contracting and Oscar Renda Contracting, both based in Texas.