Hoover Dam Bridge

Hoover Dam Bypass Project, USA: Each day, more than 14,000 cars and trucks travel the steep and winding approach roads to Hoover Dam.

There, they cross the Colorado River by driving across the dam on a two-lane highway. Built in the early 1930s when there was much less traffic, the approach roads and the road across the dam have now become a major bottleneck and safety hazard for travelers on US Highway 93.

To solve this problem, the Federal Highway Administration has commissioned the Hoover Dam Bypass Project. The centerpiece of the project is a concrete arch, steel deck bridge with 1500 feet of clear span, 840 feet above the river, that will carry four lanes of traffic. It will be the largest concrete arch bridge in the US.

As designs for the bridge took shape, Engineers had questions about the quality of the rock on the Nevada side of the canyon. To find the answers, AMEC Earth & Environmental contracted Crux Subsurface Inc to provide geotechnical drilling services. Crux Subsurface specializes in geotechnical drilling at difficult-to-access sites. The work involved drilling seven 250-foot borings into the near-vertical canyon walls. The borings were used for core samples, Goodman Jack tests, and downhole imaging and orientation.

The extreme exposure of working on the the steep canyon walls required safety anchoring and fencing for fall protection to be set prior to placement of the drill rigs and geotechnical instruments. The heavy equipment was lifted into position by helicopter. Expert piloting was required because, at times, the helicopter had to place the equipment directly under the high power transmission lines that cross the canyon.

Slope Indicator's Alan Jones and consultant Eric Mikkelsen assisted Crux and AMEC personnel with the Goodman Jack tests. The tests, along with other geotechnical data, revealed that rock was of sufficient quality to support a foundation for any of the various bridge designs then under consideration.

The Hoover Dam Bypass Project also includes construction of three miles of approach bridges and roadways in Nevada and Arizona, a tunnel in Nevada, interchanges with the existing Highway 93, and several wildlife underpasses and overpasses. The project is scheduled for completion in 2007.

Thanks to Scott Tunison of Crux Subsurface Inc for providing the information and photographs for this story. Also thanks to Erik Mikkelsen for photographs and background information.