Innovative Solution: A Combined Tunnel Option for Wastewater Applications

The City of Baton Rouge and Parish of East Baton Rouge (C/P) has entered into a mandated consent decree with the United States Environmental Protection Agency (EPA) and Louisiana Department of Environmental Quality (LDEQ) for violations against the Clean Water Act (CWA). As a result, the C/P implemented a Sanitary Sewer Overflow Corrective Action Plan (SSO CAP) to develop a program to reduce sanitary sewer overflows (SSO) within the C/P collection system. The primary method to reduce the SSO's of the program included increasing capacity, storage, deep gravity tunnel systems, and ballasted flocculation units. CDM is responsible for designing the Central Service Area Trunk Tunnel (CSATT) project, which collects flow from 12 pump stations and transfers those flows to a central pump station through a deep gravity tunnel network. The design of the central pump station and forcemain is the responsibility of another design team. A major challenge for both design teams entailed collecting flows from the east, delivering them to the central pump station and back west to wastewater treatment plant. Compounding the difficulty was that the alignment would either have to pass through several highly traveled roadways, along an Interstate 10, or adjacent to a railroad tracks. After performing a route alternative analysis and several design meetings with the client and pump station design team, a combined tunnel approach was recommended to minimize the surface disruptions and reduce the overall costs of the entire program. The new tunnel will be enlarged to house the gravity line leading into the pump station in addition the twin forcemains carrying the flows back to the wastewater treatment plant. Currently the tunnel is estimated to be approximately 3048 mm (120 inches) in internal diameter housing a 1067 mm (42 inch) gravity line, a 762 mm (30 inch) diameter forcemain, and a 1067 mm (42 inch) diameter forcemain for approximately 2530 m (8300 feet). During final design several of the following critical design issues along the alignment will need to be addressed 1) the location of the shafts along the highly traveled roadways, 2) potential settlement effects on a pile supported portion of Interstate-10, 3) potential settlement effects on a elevated water tank supported on a shallow foundation, and 4) under a man made lake.