Pushing Back

Cameron Run flows into the Potomac River in Alexandria, Virginia, just south of Washington, D.C. This unremarkable urban stream primarily carries runoff and serves as a flood control channel. In 1975 the city built seven parallel 20 ft (6 m) diameter tunnels to convey the stream through an earthen rail embankment, but after 20 years sections of the corrugated steel liner plate had deformed as much as 1.25 ft (0.4 m) on each side of the crown in six of the seven tunnels.