"I'll Take One of Everything": Utilizing a Menu Approach to Tunnel Design

The new South Platte Interceptor (SPI) project for the Metro Wastewater Reclamation District (the District) in Denver, Colorado, required design of multiple tunneled crossings with pipe diameters ranging from 30 in. to 78 in. and tunnel lengths ranging from 190 ft up to 3,700 ft. The District's goals for the overall project, including the tunnel work, were threefold: (1) Design robust infrastructure that will require minimal maintenance. (2) Promote optimal pricing from qualified contractors. (3) Manage the District's exposure to risk from construction claims and environmental upsets. Using these goals as a charter, the project team consisting of the District staff, seasoned pipeline designers, permitting specialists, and expert tunnel engineers collaborated to prepare design documents for the project. In addition to variations in diameter and length, the geology of each tunneled crossing also varied greatly from dry claystone bedrock to sands and cobbles below the water table. Tunnels were required at major roadways, the South Platte River, irrigation canals, and a golf course. Early on, the project team concluded that tunneling technologies that were technically appropriate for one tunnel definitely were not appropriate for other tunnels. To meet all of the District's goals, the team used a menu approach for tunnel design. With this approach, the design documents describe all of the challenges and operational requirements unique to each tunneled crossing and provide a menu of tunnel technologies from which the contractor may choose to construct the tunnel. This paper will describe the process used to select acceptable tunneling technologies for each crossing and steps taken to have only experienced and qualified contractors bid on the work.