U.S.A.
U.S.A. facts & history in brief
U.S.A. Map
Hoover Dam in the Black Canyon of the Colorado River,
in the Arizona State, in the United States is one of
the highest concrete dams in the world.
The Colorado River originates in
the snow-capped mountains of north
central Colorado and winds southwest
for more than 1,400 miles before
reaching the Gulf of California.
The river and its tributaries, the Green, the Gunnison,
the San Juan, the Virgin, the Little Colorado, and the
Gila rivers - are called the "Colorado River Basin".
These rivers drain 242,000 square miles in the United States,
or one-twelfth of the country's continental land area, and
2,000 square miles in Mexico. Seven western states and Mexico
have interests in the Colorado River Basin.
The Colorado River Compact apportioned water to two basins,
Upper (Arizona, Colorado, New Mexico, Utah, and Wyoming) and
Lower Basin States (California, Arizona, and Nevada), each the
use of 7,500,000 acre-feet of water per year from the Colorado
River system in perpetuity.
The United States and Mexico signed a treaty in 1944, guaranteeing
Colorado River water annually.
The dam is part of the Boulder Canyon Project, consisting of a
dam, a hydroelectric power plant, and a reservoir (Lake Mead).
The Dam controls floods of the Colorado River and supplies
domestic and irrigation water and electric power for a large
area of the Pacific Southwest, including Las Vegas and Los Angeles.
The 221 metres high and 379 metres long Dam was built during the
'Great Depression' years, and was completed in 1936.
It was a giant project and is still 20th highest dam in the world
at, 226 metres.
The Dam is in Black Canyon on the Colorado River, about 30 miles
southeast of Las Vegas, Nevada.
The Dam weighs more than 6,600,000 tons and it is a concrete
arch-gravity type, in which the water load is carried by both
gravity action and horizontal arch action.
The maximum water pressure at the base of the dam is about
10000 kg per sq metres.
Over three million cubic metres of
concrete was used in the dam, using more than 5 million barrels
of cement.
To dissipate the chemical heat caused
by the setting cement in the dam
more than 936 kilometres of 2.5
cm steel pipe was embedded in the
concrete and circulating ice water
through it from a refrigeration
plant, that could produce 1,000
tons of ice in 24 hours.
The dam was built in blocks or vertical columns
varying in sizes.
Adjacent columns were locked together
by vertical keys on the radial joints and horizontal keys on
the circumferential joints.
Concrete placement in any one
block was limited to 5 feet in 72 hours.
After the concrete
was cooled, a cement and water mixture (Grout) was forced
into the spaces between the columns by the contraction of
the cooled concrete to form a monolithic structure.
The first partial surveys of the over 2250 km Colorado River,
the third longest river in the United States, were made by
Major John Wesley Powell in the late 1860s and the
early 1870s.
In 1920,as the federal Water Power Act was
enacted, more than 1200 km of the river was still
unsurveyed and uncharted.
In 1921, four 18-foot, special,
wooden, kayak-like boats, the Grand, the Glen, the Boulder
and the Marble were designed and constructed to withstand
the wear and tear of the turbulent stretches of the river.
They were made of oak, spruce and cedar wood, and
featured watertight compartments in the bow and stern
to keep them afloat if they overturned.
In September
1921, Geological Survey and Edison Company engineers
retraced much of Major Powell's original route down the
Green and Colorado Rivers, to develop a topographic map
of the area that would be flooded if a dam was
constructed at Glen Canyon, just above the Grand Canyon
encountering several rapids in the uncharted waters of
the upper Colorado River.
The survey took more than
two years due to seasonal low waters and flooding.
After the survey work and actual
siting and planning of the reservoir
and dam access highways, Boulder
City for the construction staff,
a railway line from Las Vegas to
Boulder City and to the dam site
and a nearly 360 km long power transmission
line from San Bernardino, California,
to the dam site to supply energy
for the construction had to be constructed.
The dam's
foundation and abutments are "andesite breccia",
rock of volcanic origin.
The rock is hard
and very durable.
All loose rock had to be
removed from the canyon's wall, before construction
could begin by the High-Scalers, whose job was to climb
down the canyon walls on ropes and working with
jackhammers (20 kg) and dynamite they had to strip
away the loose rock.
The broken rocks sometimes had
to be levered free using crowbars.
They were all agile
men, unafraid to swing out over
empty space on slender ropes.
It was hard and dangerous work, perhaps the most
physically demanding work on the entire project.
Live air hoses, electrical lines, bundles of drill
steel festooned the cliffs.
The scalers laden with
tools and water bags had to carefully pick their way
through the resulting maze.
The danger from falling
rocks and dropped tools was extreme.
The most common cause of death
during the building of the dam
was being hit by falling objects.
The men began making improvised hard
hats for themselves by coating
cloth hats with coal tar.
These "hard-boiled hats"
were extremely effective.
Before construction could
begin on the dam, to isolate the construction site,
and protect it from flooding, two cofferdams were
constructed.
Construction of the upper cofferdam
began before the river was diverted.
The Colorado River was diverted around
the dam site through four
15 metres diameter tunnels, two on each side of the
river drilled through the canyon walls.
The tunnel,
with a total length of about 5 km, were excavated to
18 metres and lined with 1 metre thick concrete.
The tunnels could carry over 6 million litres of water
per second.
The Nevada and Arizona States border
runs and marked in the middle of the Colorado River
and the Dam.
The first diversion tunnels were built on
the Arizona side first.
There was also a rock
barrier built just over 100 metres down river from
the lower cofferdam.
A good supply of aggregate
had to be found and a concrete batching plant had to
built before construction could begin in earnest.
Another concrete batching plant had to built too at
a later stage.
The dam was built in five years.
The contractors were allowed 7 years, but concrete
placement in the dam was completed in 1935, and all
features were completed by March 1936.
There were 96 industrial fatalities during the
construction of the dam including deaths from
drowning, blasting, falling rocks or slides,
falls from the canyon walls, struck by heavy
equipment, truck accidents, but not including
deaths from heat, pneumonia, heart trouble, etc.
A Bureau of Reclamation surveyor, J.G. Tierney
fell from a barge in the Colorado River and drowned
in 1922.
Thirteen years later his son J.G.
Tierney, fell to his death, from one of the
intake towers, in 1935.
The dam needed a
reservoir for flood control and ensuring an adequate
supply of water all year around.
The reservoir now called Lake Mead
is at 375 metres above sea
level and about 5 metres above the spillways
storing about 36,702,300,000 cubic metres of water.
It is about 185 kilometres long and 180 metres
deep and one of the world's largest
artificial lakes.
They are four reinforced-concrete
structures located above the
dam, two on each side of the canyon supplying
the of water for the powerplant turbines, through
10 metres diameter penstocks (pipes) installed in
concrete-lined tunnels.
As nearly everything like tunnels
the spillways run on each side of
the border of the two states and
are concrete-lined open channels
about over 200 metres long, 50 metres
wide, and 60 metres deep on each
canyon wall.
The U-shaped powerplant structure
is at the base of the dam.
High speed lifts ascend the nearly
50 storeys into the dam, without
reaching its base.
The concrete base
is 200 metres thick.
There are 17 main turbines
in Hoover Powerplant.
The original turbines were
all replaced between 1986 and 1993.
This includes the two station
(Arizona and Nevada) service
units, which are rated at 2,400 kilowatts each.
The dam was originally known as
the Boulder Dam.
It was renamed Hoover Dam to honour President Hoover.
Many thanks to the Bureau of Reclamation's pamphlets.
Hui Chin and I visited Hoover Dam and Lake Mead
on the way to the Grand Canyon.
It is something Gigantic and many films
were made to impress people.
I, as a very mechanically
minded person, who likes anything that ticks or moves or
do things in general, was impressed by it and I tried
to learn about it as much as I could.
You can click on these photos for an enlargement.
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Hoover Dam |
You can click on these photos for an enlargement.
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