Construction challenges for soft ground tunneling

For more than 60 years, raw sewage has flowed down the hills of Tijuana, Mexico into the Tijuana River, crossing the U.S.-Mexican border. The sewage contaminates the low-lying river valley in San Diego and then flows into the Pacific Ocean, creating a health hazard for beaches at Border Field State Park, Silver Strand Beach in Coronado, and the most severely affected, Imperial Beach. To alleviate these problems, the South Bay Ocean Outfall (SBOO) will convey effluent down a 11 m diameter by 58 m deep drop shaft, through a 3.35 m diameter by 5.8 km long tunnel, and up a 46 m high by 2.7 m diameter riser shaft connecting to a 1.5 km seafloor pipeline and 1.2 km of diffusers on the ocean floor.