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 commissioned the Hoover Dam Bypass Project. The centerpiece of the project is a concrete arch, steel deck bridge – with 1,500 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 United States.

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, downhole imaging and orientation, and Goodman Jack tests.

Goodman Jacks, from Durham Geo Slope Indicator, are used for in-situ investigations of the deformability of rock masses. The Goodman Jack is coupled to the drill rod and inserted into the borehole, along with its hydraulic lines and signal cable. When the jack is in position, a hand pump is used to activate the pistons within the jack. The pistons push a curved bearing plate against the borehole wall, producing a uniform, uni-directional stress field. The applied pressure is measured with a pressure gauge, and the deformation of the rock is measured by two linear variable-differential transformers. After the test, the bearing plates are retracted and the jack is withdrawn from the borehole.

The extreme exposure of working on 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 that had been under consideration.

The Hoover Dam Bypass Project also includes construction of 3 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 this year.

Thanks to Scott Tunison and Erik Mikkelsen for their contributions to this article.