Tunneling Under Pressure

by Stephen J. Navin, Manager; Underground Construction, Parsons Engineering Science, Inc., La Jolla, CA,
Jon Y. Kaneshiro, Senior Supervising Engineer; Parsons Engineering Science, Inc., La Jolla, CA,
Larry J. Stout, Civil Engineer; International Boundary and Water Commission,
Gregory E. Korbin, Consultant; Orinda, CA,

Serial Information: Civil Engineering—ASCE, 1996, Vol. 66, Issue 2, Pg. 64-67

Document Type: Feature article


Work will begin this month on the South Bay Ocean Outfall (SBOO) project, a $140 million undertaking to build 3.6 mi of tunneling through the saturated ground of the Pacific Ocean. When completed in 1998, the SBOO, combined with the new South Bay International Wastewater Treatment Plant (SBIWTP) and the existing South Bay Land Outfall (SBLO), will stop the 60 years of sewage discharge into the Tijuana River Valley and the South Bay beaches of San Diego. The finished project will deliver raw sewage originating in Tijuana, Mexico to the SBIWTP, the design of which proceeds in parallel with the design of the SBOO. From there the treated effluent will pass through a distribution structure and an energy-dissipation structure before entering the existing SBLO, a 144 in. diameter, 12,300 ft pipeline. An anti-instrusion structure will be constructed at the end of the SBLO and a drop shaft will convey effluent from the SBLO to an elevation of about 165 ft below sea level. At that point, a tunnel will be constructed with an internal diameter of 121 in. and an overall length of about 19,000 ft. The tunnel will cross under land (onshore) for 5,000 ft and will terminate about 14,000 ft offshore. There it will connect to a 108 in. diameter, 147 ft long riser, which will convey effluent vertically to a 120 in. pipeline on the seabed. This 5,000 ft long sea floor pipeline will convey the effluent to a wye, where it will be discharged at 25 ft canters along two 2,000 ft diffuser legs on the seabed. Because of the unique geological conditions of the site, the tunnel will have to carry effluent under internal differential operating heads of 16-39 psi, and it will be constructed under the ocean with potential for high external ground-water pressure up to 100 psi, averaging 86 psi. Consequently, the liner design criteria must consider the internal and high external heads and soil cover, while the tunnel boring machine (TBM) must be designed to withstand the high external head.

Subject Headings: Effluents | Bays | Sea floor | Tunneling | Wastewater treatment plants | Existing buildings | Sewage | Pipelines | Pacific Ocean | California | United States | Mexico

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