Read the text to get the gist of it. Answer the following questions.

a) Can earth dams be considered fully impervious?

b) How should the slopes of earth dams be protected from erosion?

Text D Earth-fill Dams

Earth-fill (or embankment) dams are usually used across broad rivers to retain water. The profile of an earth-fill dam is a broad-based triangle.

For the construction of earth dams natural materials with a minimum degree of processing are utilized. These dams may be built with primitive equipment under conditions, where any other construction material would be impracticable. It is not surprising that the earliest known dams were built of earth.

Earth dams are readily adapted to earth foundations. It should not be assumed that the construction of earth dams is a simple operation. Numerous failures of poorly designed earth embankments make it evident that earth dams require as much engineering skill in construction as any other type of dam.

The simple embankment is essentially built from a homogeneous material, although a blanket of relatively impervious material may be placed on the upstream face. There are several types of embankments: zoned embankments, diaphragm-type dams, rock-fill dams.

Hydraulic-fill dams are constructed by using water for transporting the material to its final position in the dam. The material is discharged from pipes along the outside edges of the fill and the coarse material is deposited soon after discharge while the fines are carried into the central pool. The result is a zoned embankment with a relatively impermeable core. Hydraulic fill is best suited for placing well-graded materials containing a considerable amount of coarse sand and gravel. An adequate water supply is necessary as well. Because of the slow drainage of the water from the core, considerable settlement is to be expected over a long period.

Semihydraulic-fill dams are constructed by dumping the material from trucks into its approximate position in the dam. Like the hydraulic-fill method this procedure requires careful control to assure a satisfactory construction of the embankment.

Rolled-fill dams are constructed by placing selected materials in thin layers and compacting them with a heavy roller. Some compaction may also be obtained by proper routing of trucks and other construction equipment. Usually, however, special equipment is used for compacting the fill. Both sheepsfoot rollers and heavy pneumatic-tired rollers are used singly or in combination.

The required height of an earth dam is the distance from the foundation to the water surface in the reservoir when the spillway is discharging at design capacity plus a freeboard allowance for wind-tide as well as for wave and frost action. Recent studies of earth-dam failures indicate that 40 % resulted from overtopping of the dam because of insufficient freeboard or inadequate spillway capacity. Frost in the upper portion of a dam may cause heaving and cracking of the soil that, in turn, may cause dangerous seepage. An additional freeboard allowance up to a maximum of about 5 ft should be provided for dams in areas subject to low temperatures. Parapet walls 2 or 3 ft high as an additional safety factor and as an element of freeboard are sometimes provided on the upstream side of the crest of an earth dam exceeding 30 ft in height.

No earth dam can be considered fully impervious. Some seepage through the dam and its foundation must be expected. Seepage through earth dams may be reduced by the use of a very broad base, by the placing of an impervious blanket on the upstream face, by use of a clay core, or by a diaphragm of timber, steel or concrete. A grout curtain formed by forcing cement grout down through closely spaced drill holes is also effective means of checking leakage through fractured rock. In any case drains are normally provided near the toe of the dam to permit the free escape of seepage water.

The upstream slope of an earth dam should be protected against wave action by a cover of rip-rap or concrete. The rock should be sound and not subjected to rapid weathering and should be placed over a filter layer of graded gravel at least 12 in. thick. Concrete slabs are often employed for facing the upstream slope of earth dams but must be very carefully constructed. The usual failure results from the washing of embankment material through the joints between the slabs. It is therefore desirable that a filter layer be provided under the slab. Upstream slope protection should extend from above the upper limit of wave action to a berm.

The downstream slope of an earth dam may be subjected to erosion resulted from rainfall. On dams having a rock shell this is no problem, but earth slopes should be planted to grass as soon as possible after completion of the dam. Since the erosive action of water increases as the slope length increases, berms should be placed at about 50 ft intervals to intercept rainwater and discharge it safety.