Urban tunneling through variable geologic conditions―A case study of the Pipeline 3 Relocation Project at State Routes 125/94, San Diego, California

摘要

The project consisted of relocating a segment of the San Diego County Water Authority's 66-inch diameter Pipeline 3 to facilitate construction at the State Route 125/94 Interchange by Caltrans. The project had approximately 1,170 feet of tunneled pipeline, 480 feet of cut-and-cover pipeline, three vertical shafts, and two tie-ins to the existing Pipeline 3 at each end of the project. Variable geologic conditions consisted of fractured metavolcanic rock, soft sedimentary rock and unconsolidated sediments, and required several shaft and tunnel excavation and support methods. Project challenges included variable geologic conditions, saturated ground conditions, an urban setting with third party impacts, and an aggressive construction schedule. The greatest technical challenge was tunneling beneath the existing Pipeline 3 with only six feet of cover, with pipeline settlement limited to 1/8-inch.