The 435-foot-long (133 m) Capitol Connector Tunnel (CCT) is a new below-grade pedestrian and utility corridor that was constructed to provide a secure, accessible connection between the existing Cordell Hull State Office Building (CHB) and the Tennessee State Capitol Building (TSC). The renovated CHB serves as the new home for Tennessee’s legislature. Since there is over 60 feet (18.3 m) of elevation gain and numerous stairs between 6th Avenue North and the entrance to the TSC, this ADA-compliant corridor was required between the two buildings.
Delve Underground was responsible for engineering and design development, detailed design of the tunnel and shaft excavations, preparation of bid documents, development of a geotechnical and blasting instrumentation program, and construction inspection and administration services. Delve Underground worked with the geotechnical consultant to develop the Geotechnical Baseline Report. We also assessed the impact of construction to adjacent structures, including the impact of blasting on historic buildings in close proximity to the tunnel alignment and to an existing utility tunnel within 5 feet (1.5 m) of the tunnel crown.
Construction took place in a congested urban environment surrounded by high-profile historic structures. Staging and laydown areas were limited in size to the footprint of a demolished building, and excavation of the shafts required access through the existing marble-lined Motlow Tunnel, which necessitated the use of small, limited-access equipment. The majority of the tunnel and shafts were excavated with controlled blasting techniques, with extensive real-time monitoring evaluated after every blast.
Additionally, an innovative construction sequence was required to complete the shaft excavations since two existing elevator shafts were extended down to the new tunnel. Because the legislators needed to be relocated prior to the 2018 legislative session, the project required very aggressive design and construction schedules, with less than 24 months from the start of design until installation of final tunnel lining and structural invert.