Contents preface (VII) introduction 1—37

EXERCISES

1. 
   Discuss the physical, economic, and environmental considerations which influence the planning of a water resource project.
   

2. 
   What are the main factors which affect the selection of a dam?
   

3. 
   Write a note on the planning of reservoirs.
   

REFERENCE
1. Golze, AR (Ed.), Handbook of Dam Engineering, Van Nostrand Reinhold Company, 1977.


15
EMBANKMENT DAMS
15.1. GENERAL
Embankment dams are water impounding structures composed of natural fragmental materials (such as soil and rock) and consist of discrete particles which maintain their individual identities and have spaces between them. These materials derive strength from their position, internal friction, and mutual attraction of their particles. Unlike cemented materials, these fragmental materials form a relatively flexible structure which can deform slightly to conform to the foundation deflection without causing failure.
Embankment dams have been in existence for many centuries. The earliest forms of these dams were made naturally by landslides and rockfalls which cut off streams and formed natural dams. A 300-m natural dam of this type was created by a landslide which occurred in 1840 on the upper reaches of the Indus river (1). This dam, however, burst just after six months of its formation resulting in great loss of life and property in the valley.
Man-made tanks (or reservoirs) constructed in the early days of civilisation are found in the southern part of India and Sri Lanka. These tanks have been constructed by building earthen embankments. One such earthen embankment 17.6 km long, 21.34 m high, and containing about 13 million cubic metres of earth material was completed in 504 BC (2).
Till around 1925, the methods of design of an embankment dam were based on thumb rules and the heights of such dams rarely exceeded 30 m. The recent developments in soil mechanics have, however, made it possible to design an embankment dam with more confidence. This has resulted in much higher embankment dams such as Beas (116 m) and Ramganga (125 m) dams of India, Goschenenalp dam (156 m) in Switzerland, Oroville dam (224 m) in the USA, Mica Greek dam (235 m) in Canada, and Nurek dam (300 m) in the erstwhile USSR.
Conditions favouring the selection of an embankment dam are as follows (3): (i) Significant thickness of soil deposits overlying bedrock,

(ii) Weak or soft bedrock which would not be able to resist high stresses from a concrete dam,

(iii) Abutments of either deep soil deposits or weak rock, (iv) Availability of a suitable location for a spillway, and

(v) Availability of sufficient and suitable soils from required excavation or nearby bor-row areas.

Embankment dams are mainly of two types: (i) Earth-fill or earth dams, and

(ii) Rock-fill or earth-rock dams.

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EMBANKMENT DAMS

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The bulk of the mass in an earth-fill dam consists of soil, while in the rock-fill dam it consists of rock material. The design principles for the two types of embankment dams are similar. Earth dams are further divided into the following types:

(i) Homogeneous earth dam, and (ii) Zoned earth dam.

Homogeneous earth dams are constructed entirely or almost entirely of one type of earth material. A zoned earth dam, however, contains materials of different kinds in different parts of the embankment. A homogeneous earth dam is usually built when only one type of material is economically available and/or the height of the dam is not very large. A homogeneous earth dam of height exceeding about 6 to 8 m should always have some type of drain (Fig. 15.1) constructed of material more pervious than the embankment soil (4). Such drains reduce pore pressures in the downstream portion of the dam and thus increase the stability of the downstream slope. Besides, the drains control the outgoing seepage water in such a manner that it does not carry away embankment soil, i.e., ‘‘piping’’ does not develop. Such a dam is also categorised as homogeneous (sometimes ‘modified homogeneous’) dam (Fig. 15.1). Some of the benefits of a zoned earth dam can be achieved in a homogeneous earth dam by either selective placement of soil or using different construction methods in different parts of the embankment and thus creating zones of different characteristics.

(a) Rock toe or toe drain (b) Horizontal blanket drain (c) Chimney drain