ABSTRACT
Ground freezing was used for excavation support and groundwater control of a 13 m diameter, 61 m deep drop shaft for the South Bay Ocean Outfall Tunnel in San Diego, CA. During the shaft excavation process, external piezometer readings indicated a sudden drop in groundwater levels, followed by excessive inflows through the bottom of excavation at 30-m depth. As the shaft began to flood, subsidence of the surface soils adjacent to the shaft started at a rapid rate. After backfilling the shaft, an extensive program of freeze pipe profiling and groundwater pump tests was implemented to locate the breach in the freeze wall. After the extent of the breach was determined, grouting and localized liquid nitrogen freezing was used to reduce groundwater flows and freeze a high permeability zone. The use of investigative methods and remedial actions were able to overcome these adverse conditions resulting in a successful ground freezing project.
