The project is an $83 million, 23-foot-diameter (7 m), 328-foot-long (100 m) concrete-lined tunnel and three-story underground transfer facilities shaft for transporting inmates from the Downtown San Diego Central Jail to the new Superior Courthouse. This modified horseshoe-shaped pedestrian connecting tunnel is approximately 60 feet (18.3 m) deep.
Delve Underground was the Engineer of Record for the design of the three-story underground transfer facilities shaft at the existing County Jail and a tunnel connecting the shaft with the County Courthouse. Delve Underground also provided engineering services during construction.
The Courthouse Commons Tunnel crosses an active fault, and the tunnel lining is designed to accommodate an 18-inch (460 mm) offset.
The tunnel was constructed in alluvial deposits using the sequential excavation method (SEM), requiring that the groundwater table be drawn down below the tunnel invert during construction.
The tunnel crosses a city block with less than 6 feet (1.8 m) of clearance below building foundations. Pipe canopies are used for presupport under the mid-block buildings, and pipe spiles are used for the remaining tunnel under Union Street. The tunnel and shaft are fully tanked with a PVC membrane waterproofing system.
American Council of Engineering Companies California, 2022