Project Update by John Stolz, PE
McMillen Jacobs Associates, Developer Holland Partner Group, Holland Construction, Architects Carrier Johnson, and Atkinson Construction were selected by the County of San Diego to construct the below-grade Courthouse Commons Tunnel in downtown San Diego. McMillen Jacobs is responsible for the design of a Transfer Facilities Shaft and tunnel that connect the County Central Jail with the new State Courthouse. The tunnel crosses beneath a city block that is part of a larger multiblock development of new high-rise mixed-use residential and commercial projects by Holland Partner Group.
The 23-foot-diameter (7 m) modified horseshoe tunnel is approximately 320 feet long (98 m) and is to be constructed in alluvial soils across an active fault. The Courthouse Commons Tunnel is approximately 60 feet (18 m) deep, and cover above the tunnnel (the mid-block building foundations) is approximately 6 feet (1.8 m). The groundwater table will be drawn down below the tunnel invert while the tunnel is constructed using sequential excavation methods (SEM). Presupport comprises three arrays of canopy tubes between 80 (24 m) and 100 feet (30 m) in length under the mid-block buildings and pipe spiles for the remainder of the tunnel. The low cover requires extensive geotechnical instrumentation to perform a real-time grouting program under sensitive foundations as the tunnel is advanced.
The Transfer Facilities Shaft is a three-story underground structure immediately adjacent to and connected with the existing Central Jail. The shaft is supported by a combination of secant piles and soldier piles and lagging with three bracing levels. Everything about the project is congested: the arrangement of shaft bracing and center strut providing only two 12-foot by 22-foot (3.7 x 6.7 m) openings for access to the tunnel below; the shaft blocking the Central Jail loading dock so that a sliding bridge deck is required to make periodic deliveries; and a “postage stamp–sized” shaft staging area some 200 feet long by 25 feet wide (61 x 7.6 m).
Design was fast-tracked over nine months, with an average time between design milestones of less than two months. Now in construction, the shaft has been excavated to grade and installation of the first array of canopy tubes is underway.
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