Application Stories
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. Click any of the
photographs above or below to see a larger image.
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