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Capitol Annex Building State Street Utility Tunnel


The State Street Utility Tunnel project connects the Old Ada County Courthouse (renamed to the Capitol Annex Building when purchased by the State of Idaho) to the State of Idaho’s utility tunnel system. The Capitol Annex is just two blocks east of the State Capitol and is used for legislative offices. The State’s utility tunnel system connects all of the state government buildings and has a parallel pedestrian tunnel.

This project consisted of constructing a 200-foot long tunnel from the north side of State Street to the Capitol Annex Building to the south. The new tunnel is 7-foot tall by 7-foot wide and connects to the basement of this historic building. Project challenges included crossing State Street, a collector road with high peak hour traffic volumes, poor soil conditions, multiple conflicting utility crossings, deep excavations, and a tight construction window.

Keller Associates, as a sub-consultant to the mechanical engineer, designed the tunnel, excavations, utility crossings, construction staging, traffic control, and the roadway reconstruction. In addition, Keller coordinated and gained approvals from utility companies, City of Boise, and the Ada County Highway District.