Extreme Engineering: A Bridge Built On Quicksand

A Bridge Built On Quicksand Anchored on either side with 130-foot-deep pylons, the bridge’s main arch supports the full weight of the roadway below. Nick Kaloterakis

Name: Sheikh Rashid bin Saeed Crossing
Where: Dubai, United Arab Emirates
Cost: $817 million
Estimated Completion: 2012
The Challenge: Construct the world's tallest arch bridge on a bed of sand

The winning entry in a design competition held by Dubai's Roads and Transportation Authority, the so-called Sixth Crossing will be a mile-long, 12-lane highway with the larger of its two arches reaching twice the height of the current record-holding Lupu Bridge in Shanghai, China. Designed by FxFowle Architects and engineered by Parsons Corporation, the bridge will support more than 2,000 vehicles per hour and a commuter train. Between the two arches, a man-made island will hold an amphitheater and a ferry terminal.

An arch bridge gets its strength from its shape. The weight of the roadway and everything on it is distributed outward along the curve of the arch and down to concrete walls, or abutments, on either side. Like bookends, the abutments keep the arch from flattening out.

But anchoring them to Dubai's soft and unstable terrain poses one of the project's biggest engineering hurdles. "We typically build arches at locations where you have very good, solid rock, because the full weight of the arch is pushing against the ground," says Parsons engineer Ken Serzan. "But there is no rock here, just sand and then sandstone." So instead of installing the standard arch abutment, engineers will drill 200 holes -- six and a half feet wide and 130 feet deep, half vertical and half inclined -- and fill them with steel- reinforced concrete. Spaced about 20 feet apart and topped off with a concrete cap, the enormous foundations can absorb and dissipate the full force of the bridge. Once the Sixth Crossing is completed in 2012, it will be more than simply a traffic fix. It will be a destination in its own right.