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Why Hoover Dam was Constructed?
Colorado, worlds one of the most
powerful and unpredictable rivers, would break its banks in every spring and
flood the area. The Government instructed the Bureau of Reclamation to come
up with a solution and they decided to build world's largest dam. The
site chosen for the megastructure Hoover Dam was Black
Canyon. It is an 800 ft high deep gorge through which the river flowed.
The spot, Black canyon is in the middle of the desert, so there was no
infrastructure, no labors, no transportation and the weather too was harsh.
Frank Crow, was the Chief Engineer of
Hoover Dam and was assigned the job to get it completed in the span from 1931
- 1935. The construction of Hoover took 7 years at a cost of $ 125 million.
Nowadays this amount is about 788 million pounds. If the dam was not
completed in the given time it would have cost the contractors $ 3000 / day
in financial penalties.
Hoover Dam Statistics - Hoover
Facts
- 726.4 feet high (221 m)
- 1,244 feet wide (379 m)
- 660 feet (203 m) thick at the base
- 45 feet (13 m) thick at the top
- $165 million dollars to build
- 4.5 years to build
- 4.4 million yards of concrete used for
construction
- March 1931 building began
- September 30, 1935 President Franklin D.
Roosevelt dedicated the completed dam
Powerhouse
- 17 generators
- 4+ billion kilowatt hours produced each year
- 10 acres of floor space
Power used by:
- 56% California
- 25% Nevada
- 19% Arizona
Lake Mead
- 6.5 years to fill (A slow filling process
was required to lessen the pressure change on the dam and to help
prevent small earthquakes due to land settlement.)
- 589 feet (181 m) at the deepest point
- 247 square miles in size
- 110 miles (176 km) long
- Named after Dr. Elwood Mead, Commissioner of
the Bureau of Reclamation (1924 - 1936)
- Largest man made reservoir in the United
States
Hoover Construction - Stage 1
Hoover Dam Tunnels
In April 1931 blasting for
construction of plain dry area, upon which dam would be built, began. To
divert the Colorado river 4 tunnels were to be excavated on each side of the
Canyon, measuring 4000 ft long and the diameter of the tunnel was 56 ft,
these were acting as diversion channels. Two tunnels would be constructed on
the Nevada side, and another two were to be constructed on the Arizona side.
2 small cofferdams were built to force water into the tunnels. In May 1931
the drilling continued. The digging, blasting, and debris removal continued
for 13 months, with men working 3 shifts 24 hours a day, 7 days a week.
Holidays were observed only at Christmas, 4rth July and Labor Day.
At Hoover's Construction site, the
workers had to face harsh conditions but were paid only 40% extra. No proper
ventilation was provides, work was extremely physically demanding. Men had to
swing 100's of feet down the canyon walls to remove dangerous loose rocks,
using jacks and dynamites. Due to lack of safety measure men required nerves
of steel. The most common cause of death was, being hit by falling rocks.
Because no roads led into the canyon,
men (as well as equipment) arrived at the work site by boat. Workers used 500
pneumatic drills, hoses, and compressors to make holes in the canyon rock
where explosives could be placed. Once holes were drilled, workers used
dynamite to blast into the rock and break it into smaller pieces that could
be hauled away by dump trucks. A ton (0.9 metric tons) of dynamite was
required for every 14 feet (4.3 meters) of tunnel that workers dug into the
canyon wall. Special team then visited the inside of the tunnels to ensure it
would remain same for workers to work inside it. The tunnels were then lined
with concrete and By sliding sticks of dynamite into holes bored into the
canyon wall, workers were able to blast and excavate large diversion tunnels.
These tunnels, each about the size of a 4-lane highway, were lined with 3
feet of concrete, allowing river water to be transported away from the
construction site at a rate of 1.5 million gallons per second.
Till November, 14, 1932 four 4
tunnels were completed and the water was allowed to flow through it. Hoover's
Construction was in full swing.
Coffer Dams
Workers made the cofferdams by using
100 trucks to dump dirt, rock, and debris into the water at a rate of one
truckload every 15 seconds. This amazing pace of dredging and dumping went on
for five months. The largest flow ever recorded at Black Canyon 200,000 cubic
feet per second, was used by the Engineers to design the coffer dams.
Stage 2 of construction
In this stage building the dam itself
was the task. The work was too huge, there were many problems in design which
needed to be solved.
Design of the Hoover Dam
Hoover is an arch gravity dam,
incorporating two principles.
According to the first principle, the
weight of the dam forces it into the ground due to its weight, thus helping
it to remain stable.
In another principle, the arch shape
of the dam deflects the force of the water into the canyon walls through the
compression of dam's concrete walls, using the compressive strength of
concrete (concrete is very strong in compression). Major problem was the
pouring of 3.4 million cubic meters of concrete. Plants were installed at the
construction site to produce concrete locally. But the dam was too big to be
made into a single concrete mount. If the concrete in the dam was poured in
only one go, the concrete would not have settled even today. It is because
when ingredients of concrete - cement, sand, coarse aggregate combine in the
presence of water, they start a chemical reaction, resulting in the
generation of internal heat, thus slowing down the curing process. The large
the pour, the larger the cure. If heat is not dispersed, cracks would form,
weakening the structure.
Hoover's Heat of hydration
To counteract the problem of heat
generation, Hoover dam was built in series of inter locking blocks. This idea
was conceived by a previous dam called Lower Crystal Spring dams. But Hoover
was even 20 times massive than gigantic Lower Crystals Spring Dam. Each block
was 5 ft high and was inter locked with the neighboring one and water was
forced between them. To accelerate the setting of concrete, cool water pipes
were passed through each block. Concrete mix was cooled and cured faster.
To speed up pouring of concrete in
the mega structure, an elaborate overhead network of cables and pulleys was
designed, carrying vast buckets of concrete. Labors stayed on the site to
spread, place and compact the poured in concrete. Due to this new method, a
record breaking volume - 8000 cubic meters of concrete was poured in a single
day.
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