Text C                                                                  Types of dams

Dams are classified by the material used to construct them. Dams built of concrete, stone, or other masonry are called masonry dams. Dams built of earth or rocks are called embankment dams. Engineers generally choose to build embankment dams in areas where large amounts of earth or rocks are available.

Today nearly all masonry dams are built of large blocks of concrete. There are three main kinds of masonry dams: gravity, arch, and buttress.

Gravity dams depend entirely on their own weight to resist the tremendous force of the oncoming water. They are the strongest and most massive dams built today. A gravity dam is built on a solid rock foundation. The dam transfers the force of the water downward to the foundation below. Gravity dams can hold back enormous amounts of water. However, they are costly to build because they require so much concrete.

Arch dams curve outward toward the flow of water. They are usually built in narrow canyons. As the water pushes against the dam, the arch transfers the water's force outward to the canyon wall. An arch dam requires much less concrete than a gravity dam of the same length.

Buttress dams depend for support on a series of vertical supports called buttresses. The buttresses run along the dam's upstream face—that is, the side facing away from the water's flow. The upstream face of a buttress dam usually slopes outward at about a 45-degree angle. The sloping face and the buttresses serve to transfer the force of the water downward to the dam's foundation. Buttress dams, like gravity dams, are usually built in wide valleys where long dams are needed.

Embankment dams are constructed of materials dug out of the ground, including rocks, gravel, sand, silt, and clay. They are also known as fill dams. An earth-fill dam is an embankment dam in which compacted earth materials make up more than half the dam. Earth-fill dams are constructed by hauling the earth materials into place and compacting them layer upon layer with heavy rollers. The materials are graded by density, and the finest, such as clay, are placed in the center to form a waterproof core. In some cases, concrete cores are used. The coarser materials are placed outside the core and covered with a layer of rock called riprap. The riprap serves as an outer protection against water action, wind, rain, and ice. In addition, thinned-out cement, called grout, is pumped into the foundation to fill cracks. This process makes the foundation watertight.

Where rocks are available, it may prove most economical to build a rock-fill dam. Most dams of this type are constructed of coarse, heavy rock and boulders. Many of them have a covering of concrete, steel, clay, or asphalt on the upstream side. This covering makes the dam watertight. Combinations of rock and earth result in a type of dam called an earth-and-rock-fill dam.

There are other types of dams. Timber dams are built where lumber is available and the dam is relatively small. The timber is weighted down with rock. Planking or other watertight material forms the facing. Metal dams have watertight facings and supports of steel.

Dams with movable gates are built where it is necessary to let large quantities of water, ice, or driftwood pass by the dam. A roller dam has a large roller located horizontally between piers. It can be raised and lowered to allow ice and other materials to pass through the dam without much loss of reservoir water level.

Decide whether the following sentences are true or false according to the text.

a) There are four main kinds of embankment dams.

b) Gravity dams are built in narrow canyons.

c) Buttress dams are usually built in wide valleys.

d) There exist metal dams.

e) Dams are not built with movable gates.

f) Dams are classified by the material used.

Choose the answers to the questions.

1) What are three main kinds of masonry dams? Complete the diagram.

a) buttress dams                                        dams

b) earth-fill dams

c) arch dams                      masonry dams        __________

d) gravity dams

e) rock-fill dams    gravity ____ _____ _______     _______

f) roller dams

2) What are the characteristics of every masonry dam?

(Complete the table, matching the dam with the characteristics given after the table)

a) depend on their own weight          

b) depend for support on buttresses

c) built in wide valleys           

d) built on solid rock foundation

e) a sloping upstream face

f) transfer the water’s force outward to the canyon walls

g) transfer the water’s force downward to the dam’s foundation

h) is curved

i) built in narrow canyons

j) the most massive dam

k) costly to build

Choose the best variant.

1) The finest material in earth-fill dam is placed…

2) Embankment dams are known as…

3) Rip rap serves as an outer protection against…

a) water action, wind, rain, ice           

b) fill cracks

c) leakage

4) An earth-fill dam is an embankment dam in which compacted earth materials make up…

Answer the following questions.

a) When is it most economical to build a rock-fill dam?

b) What are the rock-fill dams constructed of?

c) What covering do they have on the upstream face?

d) What makes the rock-fill dam watertight?

Finish the following sentences according to the text.

1) Timber dams are built where…

2) The timber is weighted down with…

3) Planking or other impervious material forms ….

4) Dams with movable gates are built where …

5) A roller can be raised and lowered to allow …

What parts of the text can you define? Do they correspond to the paragraphs? Give the titles to each part.

1._________________            3._________________

2._________________            … _________________

Match the following words.

Choose the contextual meaning of the words.

1) support

2) like

3) make up

4) forms

5) facing

Make sure you know the nouns formed from the following verbs.

Fill in the correct prepositions, then choose any five items and make sentences.

1) to be built ___ blocks of concrete; 2) different kinds ___ dams; 3) to transfer the force ___ the foundation; 4) to build ___ canyons; 5) ___ a 45-degree angle; 6) to be known ___ fill dams; 7) to make ___ more than half the dam; 8) to compact ___ rollers; 9) to serve ___ protection ___ water action; 10) ___ addition.

Make the following sentences shorter.

1) Embankment dams are constructed of materials dug out of the ground which include rocks, gravel, sand, silt, and clay.

2) Earth-fill dams are constructed by means of hauling the earth materials into place and compacting them layer upon layer with the help of heavy rollers.

3) A large roller which is located horizontally between piers can be raised and lowered to allow ice and other materials to pass through the dam without much loss of reservoir water level.