HOW TO EVALUATE TUNNELING AND ASSOCIATED RISKS AS AN OPTION, AND LESSONS LEARNED

The decision to build collector, conveyance, and outfall sewer systems by tunneling or conventional cut and cover is based on a myriad of elements: (1) environmental sensitivity, (2) geological conditions, (3) organizational relationships, (4) societal issues, (5) construction risks, (6) construction costs, and (7) construction schedule. These factors are typically evaluated in a ranking matrix along with costs to choose the best alternative. The general role and philosophy of risk management at the various stages of the design and construction process of tunnels are also discussed.A number of case histories are included to demonstrate how tunneling or conventional options were chosen. Lessons learned from case histories include the San Diego's Tunneled Outfall; the Saint Thomas Wastewater Treatment Plant outfall pipeline; the North Fork Peach Tree CSO; the Los Angeles East Central Interceptor Sewer; and projects with San Diego's Metropolitan Wastewater Department.