Tunneling a Bridge

Linking an addition to the Georgia World Congress Center in Atlanta required a pedestrian tunnel through a railroad embankment 100 ft wide and 40 ft high, with no disruption of train service during construction. Nannis & Associates, Atlanta consulting engineers, designed the tunnel to be built in two stages and from the top down. In the first phase, half the embankment was excavated to the level where a steel bridge structure and tunnel ceiling could be built. Sheet piling driven between two of the three tracks, tie rods and continuous walters prevented damage to the existing structure. Abutment walls for the bridge are reinforced concrete caissons. After completion of the second phase in the same manner, the tunnel was excavated from below.