In South Carolina, Charleston Water System is replacing its network of deep tunnels that carry sewage to the Plum Island Wastewater Treatment Plant. The project began in 1999 and is now in the fifth phase.

“The current infrastructure in Charleston is nearly 50 years old,” Project Manager Enrique Baez says. “This will be a new, more reliable way to discharge the sewer in the West Ashley area.” The tunnel replacement backs up the existing system and adds redundancy and additional capacity to the sewage system, he says.

The original tunnel system was created in the late 1960s to deliver wastewater to the treatment plant. Over time, the corrosive wastewater spoiled the tunnel. This deterioration left the tunnel at risk of failure, which could block flow in the tunnel and cause sanitary sewer overflows, a serious threat to public health and water quality in the Charleston Harbor on the oceanfront. 

“Sewer infrastructure is something most of us never think about but it's absolutely essential to public health, environmental protection and ultimately our quality of life," Charleston Water System CEO Kin Hill says. "Completing this project will make sure we can continue to provide these essential services."

Phase Five

Three sections of the original tunnel system have been replaced and a new fourth section added. The fifth and possibly final phase, the West Ashley Tunnel, is the final section of the original tunnel system to be replaced. Construction of the new tunnel calls for drilling a 1.6-mile, 135-foot-deep tunnel to the plant. 

Replacing three sections of the original tunnel system cost $135 million for the section on the east side of the Charleston peninsula, another on the west side, and a third underneath the Charleston 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. This fifth phase will bring the total project cost for all phases to approximately $186 million. The West Ashley Tunnel is funded by a 2010 bond issue and supported by current sewer rates. A sixth phase of the project is possible.

Phase five also includes building a new 3,200-square-foot pump control building at the treatment plant and other sewer system improvements. Construction began in March and is slated to take approximately 30 months to finish in November 2015.

“It will prevent sewer from backing up upstream in areas and reduce an odor issue near the Croghan Shaft location,” Baez explains. 

Soil conditions and geography have been challenges. The project takes place in locations surrounded by water and marshes. Soil conditions in the top 50 feet of ground were difficult to work with.

This “slows down the construction process and we proceeded very carefully,” he says.

Caisson Drop

Tunnel work is relatively routine for Baez and his team. What makes this job unique is that it used the caisson method to sink the pump station shaft. It built the shaft walls at grade level and the shaft sinks under its own weight until it reached a depth of below 140 feet. 

The project is extremely unique for the 60-foot-diameter caisson and Baez says it might be the largest caisson sunk in this area. The project engineer is Black & Veatch and the contractor is a joint venture of Fort Worth, Texas-based Southland Contracting Inc. and Roanoke, Texas-based Oscar Renda Contracting Inc.