My Favorite Dams – Hoover Dam
Hoover Dam - Photo courtesy Bureau of Reclamation
David W. Harris, PhD,
PE, F.SEI, F.ASCE
I remember standing at the base of Hoover Dam the mist
rising from the Colorado River. You look
up, but, can’t just raise your head, you have to arch your back and lean
backwards to see to the top of this massive structure, 726 feet in height. The concrete arch curves across the canyon in
a smooth graceful form. It is hard to
appreciate that Lake Mead, the largest reservoir in the United States by volume
(when it is full) (Wikipedia) is sitting calmly behind the dam, waiting
patiently to pass through the dam and meander downstream on its way through
Mexico and into the Gulf of California.
It’s not a small job, there are 3,250,000 cubic yards of concrete
sitting there in the valley.
To my left is the powerhouse sending electricity to Las
Vegas about an hour’s drive away. The
bright lights fire up from the turbines spinning using the water’s headed
downstream. The turbines on the Nevada
side and the Arizona side combine for an
installed capacity of 2080 MW generating 4.2 trillion watt hours of electricity
annually.
There are many sources for information on Hoover (e.g. see
links below). Built from 1931 to 1936,
at the peak of construction 5,251 workers were employed at the dam, the 1931
cost was budgeted for $49 million, Lake Mead covers 247 square miles.
I’m a concrete guy, here’s some concrete facts. Concrete generates heat and then
contracts as it cures, the potential for contraction and cracks
caused by the tension of contracting needed careful consideration. Hoover and most large concrete dams are built
in blocks. For Hoover, some of the
blocks were as large as 50 square feet and 5 feet high with steel pipes to
provide flowing cool river water. Ice
cold water was supplied from a refrigeration plant for the final cooling. After curing, pipes were filled with grout,
and grout filled any spaces between columns. The Hoover concrete mix is
referred to as mass concrete, having large rocks in the mix to help build up the
volume and reduce the need for cement.
All concrete mix designs have aggregate, sand, cement and water, but
formerly referred to mass concrete (concrete with very large diameter
aggregate) creates specialized material properties and requires specialized
considerations in review and analysis of the aged structures.
Hoover Dam is considered as the 8th Wonder of the
World (Koch, 2008). It is interesting to
read facts about the dam, it is inspiring to be there, and it was an honor to
be on the Bureau of Reclamation (USBR) engineering staff to study this iconic
structure. Here’s part of the inside
story (pun intended).
To evaluate a dam it is important
to know the current properties of the dam.
I was the Chief of Materials Engineering and Research Laboratory for
USBR. Concrete from dams is obtained
using drilling rigs and obtaining long cylinders of concrete core. It is boxed and shipped to the lab for
testing. Boxes are laid out on exam
tables in the order of depth. The core is shiny when misted with a spray of
water, rocks the size of footballs are found in the core. You look for joints to test joint strengths
in shear. Some joints are intact and may
require careful investigation with a magnifying glass to even be found. Other planes can be found across the diameter
of the core that can be breakoff to obtain the core, can be spins occurring
during the drilling, or can be open or less strong joints. Intact core becomes the samples for
compression testing. I’m a stress strain
guy, I evaluate the data to help the investigation of the dam‘s condition with
its aged properties, and loading conditions that may be greater than those
expected years ago when the dam was designed.
Strengths, the stress at failure; and failure strains, the normalized
deflection at failure, are found using the laboratory tests. These will be used to compare with computer
analysis of the dam with current conditions, and with extreme conditions such
as an earthquake.
After 911 I helped setup the
security procedures and evaluations for Reclamation’s more than 350 dams. For Hoover I wish I could tell you more, but
I can tell you the dam stands tall and strong after more than 75 years. A credit to Civil Engineers even today of
what can be accomplished.
As an ASCE Fellow of the Structural
Engineering Institute I am proud to have worked on this magnificent structure
and to represent the Civil Engineering profession. For younger students, come join us in an
interesting and rewarding profession. We
need your creativity to solve the problems of today for a better tomorrow.
References:
Wikipedia, Hoover Dam, Obtained
from the Internet https://en.wikipedia.org/wiki/Hoover_Dam,
September 7, 2018
Comments
Post a Comment