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determinedenchiladaUrban and Civil

Nov 25, 2013 (3 years and 4 months ago)

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Embankment Dams

History

Historically, dams have been embankment dams. Farmers would pile up soil to collect a pool of water to
water their crops. One of the earliest recorded dams was an embankment dam built in Egypt
constructed around 1800 B.C. It was 70 feet high, and 300 feet
thick at the base
[citation]
. The middle of
the dam was composed of clay, and cut stone was on both sides of this. The reservoir it cr
eated was
known as Lake Moeris.

Until 1900, either embankment dams or dams built of cut stone fitted with hydraulic cement

were
commonly built. Then in the early part of the twentieth century, the United States Department of
Reclamation took giant steps in the design of dams and the material used to build them; specifically,
they started using concrete and reinforced concrete

instead of cut stone. T
he Hoover and the Grand
Coulee D
ams were built during this time period. These new breeds of dams, although impressive,
required enormous amounts of concrete and a huge labor force. These two factors drove up the cost
significantly.

In the middle of the twentieth century, embankment dams became a more attractive option due to the
availability of better earth
-
moving equipment, allowing dam builders to use fewer people to move very
large amounts of material. Because of this and because
embankment dams use more local materials,
agencies such as the Department of Reclamation took a fresh look at embankment dams.

Overview

From outer space, the pyramids of Egypt, the Great Wall of China, and dams are the only visible traces of
man’s handiwor
k on planet Earth. Water and power are key elements in any modern society. Dams
provide both. Embankment dams, when properly designed and built, hold back the same amount of
water for less cost than other types of dams.

Types of Dams

Dams are built in a va
riety of shapes and sizes. One of the most famous, the Hoover Dam, is a gravity
arch dam. Gravity arch dams are a good choice when there are solid rock abutments on either side with
which to anchor the dam. Gravity arch dams are also preferable when a dam
must be taller than it is
wide. Another famous dam is the Grand Coulee Dam, which is a concrete gravity buttress dam. This type
of dam is a good choice when the dam must be very high. However, a disadvantage to both gravity arch
dams and concrete buttress
gravity dams are that millions of yards of concrete are needed to construct
the dams.

Embankment dams may be less glamorous than gravity arch dams or concrete gravity buttress dam, but
they get the job done. An embankment dam can hold back just as much wat
er as a gravity arch dam or a
concrete buttress gravity dam. Embankment dams, the most common type of dams in the world, can be
any size and are built with local materials, such as rock, clay, gravel, and sand. Concrete is only used in
strategic spots such

as the spillways, locks, gatehouses and fish ladders. Embankment dams can be built
when there is no solid bedrock to anchor the dam to, and some settlement of the dam during
construction is allowable, unlike a massive concrete dam.

Aswan Dam

The Aswan Hig
h Dam, a classic example of an embankment dam, is located in Egypt. There have been
many dams in the Aswan area, but in 1952, the Egyptian president Nassar pledged to control his
country’s annual flood and to provide electric power and irrigation. The plan
s for the dam were drawn
up by the British, but due to Cold War politics, the British withdrew and the Russians lent technical
engineering personnel and paid for the dam at an estimated cost of $1 billion. The Aswan Dam created
the fourth largest reservoir

in the world. This dam is 360 feet high, 12,562 feet long, and 57,940,000
cubic yards of material was used in construction
[
c
itation]
.

The construction of the Aswan Dam provided many challenges to the dam builders. Perhaps the biggest
challenge was that
under the river bed, bedrock was over 600 feet down, and the material between the
river bed and the bedrock was all layers of sand. The engineers were worried that if they placed the dam
on top of the sand, the water would run under it. They came up with t
he answer of an alluvial grout
curtain. They drilled parallel rows of drill holes every 1.5 meters down to the bedrock, and then filled
these holes with pressurized grout. Above this grout curtain they layered packed sand, gravel, dirt, and
rubble. This so
lution prevented seepage under the dam. The impermeable layer in the dam, above the
grout curtain, is a clay core. Six large tunnels were laid before the building of the dam was started. These
tunnels transfer water from the front to the rear of the dam to

12 generators that provide 2.1 million
kilowatts of electric power.

Dams on the Missouri River

The lakes behind the Hoover and Glen Canyon Dams, Lake Meade and Lake Powell, are the first and
second largest reservoirs by volume in the United States, but th
e third, fourth, and fifth in volume are
the reservoirs behind the six embankment dams on the Missouri River. In the mid
-
twentieth century,
the Department of Reclamation started work on six major dams on the Missouri River. They decided to
build embankment

dams because there were no classic canyon walls on the Great Plains where a
concrete dam could be anchored.

First Dam on the Missouri: Fort Peck

The first dam built on the Missouri was the Fort Peck
Dam
. This dam was unique because the engineers
used the
hydraulic fill method instead of the rolled
-
fill method, which is the usual method for building
embankment dams. First, they built the power generation plant and the spillways, and then they filled
the river in and piled rock and various other fill materia
l into the river while it was still moving, much like
a kid would dam up his neighborhood stream. The problem with the hydraulic fill method is that an
impermeable layer is very hard to install, if it’s even possible at all. While the river is being filled

in, the
fill is very prone to liquefaction. This problem was demonstrated when part of the Fort Peck
D
am
slipped 19 feet downstream during construction. To make up for this problem of liquefaction and no
impermeable layer, huge amounts of material are add
ed to hold the dam down and to keep it from
sliding.

Dams That Followed
the
Fort Peck

Dam

The other dams on the Missouri River

the Garrison, the Gavins Point, the Fort Randall, the Oahe, and
the Big Bend
Dam
s

were all built using the rolled
-
fill method. Wi
th the rolled
-
fill method, the river is
first diverted with coffer dams, and then alternating layers of soil and rock are placed and compacted at
every level. In the rolled
-
fill method, the moisture content of the fill is very high as the fill is applied i
n
layers so that it approximates what its moisture content will be upon completion of the dam. Because
the river has been diverted, it is much easie
r to add the impermeable layer.

Largest Embankment Dam: Itaipu Dam

The Itaipu Dam is the most recent and mod
ern embankment dam in the world. It is also the largest
power plant in the world. Located on the Parana River between Brazil and Paraguay, the Itaipu is an
embankment dam constructed mostly of compacted rock. On the upstream face of the dam, a one
-
meter th
ick concrete sheet covers the rock. This thin layer of flexible concrete acts as the impermeable
layer for the dam. The final cost of the Itaipu was roughly estimated at 20 million dollars. The builders
estimated that if the dam had been made out of all co
ncrete, such as a gravity buttress dam, it would
have doubled the cost.

Construction

Large embankment dams are constructed of a variety of materials in alternating layers. Because the
materials used are soil and rocks from the surrounding area, and there i
s no need to bring other
materials, such as portland cement and reinforcing steel, to the dam site from great distances, the
transportation costs are greatly reduced.

Slope

With embankment dams, water seeps into the dam and under it and tries to lift the d
am up. An
embankment dam holds back the water purely by its own weight, so it must be heavy enough that it
does not slide sideways as the water pushes against it. Therefore, embankment dams are sloped so that
they can use gravity to help stop the force of
the water as the water pushes against it. This is much like
the shore at the ocean’s edge.

Other forces also need to be taken into account when designing an embankment dam, such as ice and
wave load on the upstream side, the effects of an earthquake, and t
he weight that might be on the top
of the dam such as bridges, roads, and vehicles.

Most embankment dams are sloped at least at a 3 to 1 ratio or less on both the upstream and
downstream sides of the dam to enable them to resist all the forces working agai
nst them. On many
dams, this ratio is increased to as much as 40 to 1 at the upstream toe, or base, of the dam.

Permeability

Dams need an impermeable layer. In embankment dams, there are two locations for the impermeable
layer. The first is in the middle
of the dam. This type of impermeable layer is usually clay or bitumen. The
second location is on the upstream face of the dam. This is usually accomplished by using a sheet of
reinforced concrete with riveted metal. The advantage of putting the impermeable

layer here is that it
also can resist the wave action of the water.

Problems

The problems with embankment dams, even designed correctly, are erosion and overtopping. The
upstream side of an embankment dam is usually faced with rocks to prevent wave action

from eroding
the structure. The downstream side must be protected from rain that lands on the top of the dam and
runs down. Usually, either gravel or vegetation are used to control erosion.

The biggest danger to an embankment dam is when it becomes overto
pped. Overtopping is when the
level of the water rises higher than the top of the dam. In 1889, the Jonestown Dam in Jonestown,
Pennsylvania, was overtopped during a big storm when its spillway became clogged with debris. The
dam was overtopped and failed,

resulting in a massive wave 37 feet high sweeping down and killing
2,200 people in the town a few miles below the dam
[
c
itation
]
. To avoid the overtopping problem,
spillways need to be very carefully designed. One way to do this is to build a spillway cap
acity of double
that of the highest expected flow into the spillways. It is also very important that a spillway be designed
to dissipate the energy of the falling water so that a scour hole that could erode under the dam does not
occur. Many times this pro
blem is solved by adding a ski jump shape at the bottom of the spillway to try
to get as much air as possible into the water as it falls.

Throughout the centuries, mankind has harnessed water for irrigation, flood control, power, and
navigation. The twenti
eth century saw many massive dams constructed around the world, but when
looking for the lowest cost, an embankment dam must be considered as the most economical dam type
where conditions permit.