Dams: Classification, Choice and Selection of Site

After reading this article you will learn about:- 1. Classification of Dams 2. Choice of the Type of Dam 3. Selection of Site.

Classification of Dams:

Dams can be classified as per their functions, material of construction, and structural design:

(i) Classification based on function:

(a) Storage (or impounding) dam is constructed across a river to create a reservoir for storing water during the period when the flow iii the river is in excess. This stored water is then used for irrigation, power generation and water supply when the demand exceeds the flow in the river.

(b) Detention dam is primarily constructed to detain all or part of the flood waters of a river and released later in such a way that the region downstream of the dam is safe against flood damages. A dam can also serve both the functions of detention as well as storage.

(c) Diversion dam is constructed for the purpose of diverting water into a canal. Such a dam is usually called diversion weir.

(d) Coffer dam is a temporary dam constructed to keep water away from a specific area to keep it dry for some specific purpose, such as construction of the main dam. The coffer dam too acts like diversion dam.

(e) Debris dam is constructed across a river to trap sediments of the river.

(ii) Classification based on material of construction:

(a) Earth and rock fill dams are constructed of loose earth (ranging from fine material to big rock pieces) without any binding material. An earth dam having its major portion built of rocks is called rock fill dam. Such dams are also known as non-rigid dams.

(b) Masonry dams are rigid dams constructed of either stone or brick masonry.

(c) Concrete dams too are rigid dams constructed of concrete.

(iii) Classification based on structural design:

(a) Earth and rock fill dams are non-rigid dams constructed of loose earth material. Shear strength of the soil used in the construction of these dams opposes the forces exerted on the dam by water.

The difference between an earth and rock fill dam is that in case of rock fill dam major portion of the dam is primarily made of rock pieces, whereas in case of the earth dam it is the soil which is used for the construction of embankment.

Conditions favouring selection of earth and rock fill dams are as follows:

(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.

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

Earth dams are further divided into the following types:

(i) Homogeneous earth dam

(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. 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. 11.1(a)] constructed of material more pervious than the embankment soil. 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 categorized as homogeneous (sometimes ‘modified homogeneous’) dam [Fig. 11.1(a)).

The most common type of an earth dam usually adopted is the zoned earth dam as it leads to an economic and more stable design of the dam. In a zoned earth dam [Fig. 11.1(b)], there is a central impervious core which is flanked by zones of more pervious material. The pervious zones, also known as shells, enclose, support and protect the impervious core.

The upstream shell provides stability against rapid drawdowns of reservoir, while the downstream shell acts as a drain to control the line of seepage and provides stability to the dam during construction and operation of the dam. The central core provides imperviousness to the embankment and reduces the seepage.

The impervious core can be placed either as vertical core or upstream sloping core each of which has some advantages over the other. A vertical core results in higher pressure on the contact between the core and the foundation which, in turn, reduces the possibility of leakage along the contact. Besides, for a given quantity of impervious material, vertical core will have greater thickness.

The main advantage of an upstream sloping core is that the main downstream shell can be constructed first, and the core placed later—an advantageous feature in areas having short spell of dry weather suitable for building a core of fine-grained soil. Also, foundation grouting can be carried out while the downstream embankment is being constructed.

A rock-fill dam (Fig. 11.2) is made of large-sized rock pieces to provide stability and an impervious membrane to provide water-tightness. Materials used for the membrane are: earth, concrete, steel, asphalt and wood. The impervious membrane can be placed either on the upstream face of the dam or as a core inside the embankment.

The major causes of failure of an earth dam are overtopping, piping, and earth slides in a portion of the embankment and its foundation (due to insufficient shear strength). Out of these three, overtopping is the most com­mon cause of complete and catastrophic failure of an embankment dam.

The design of an embankment dam must meet the following safety requirements:

(i) There is no danger of overtopping. For this purpose, spillway of adequate capacity and sufficient freeboard must be provided.

(ii) The seepage line is well within the downstream face so that horizontal piping may not occur.

(iii) The upstream and the downstream slopes are flat enough to be stable with the materials used for the construction of embankment for all conditions during construction, operation and sudden drawdown.

(iv) The shear stress induced in the foundation is less than the shear strength of the foundation material. For this purpose, the embank­ment slopes should be sufficiently flat.

(v) The upstream and the downstream faces are properly protected against wave action and the action of rain water, respectively.

(vi) There should not be any possibility of free passage of water through the embankment.

(vii) Foundation seepage should not result in piping at the downstream toe of the dam.

(viii) The top of the dam must be high enough to allow for the settlement of the dam and its foundation.

(ix) The foundations, abutments and the embankment must be stable for all conditions of operation (steady seepage and sudden drawdown) and construction.

(b) Gravity dam (Fig. 11.3) is a masonry or concrete dam which resists the forces exerted upon it due to all applied loads, and its own weight. The Bhakra dam in Punjab is a gravity dam. The downstream face of a gravity dam usually has a uniform slope which, if extended, would intersect the vertical upstream face at or near the maximum water level in the reservoir.

The upper portion of the dam is made thick enough to accommodate the roadway or other required access as well as to resist the shock of floating objects. The upstream face of a gravity dam is usually kept vertical so that most of its weight is concentrated near the upstream face to resist effectively the tensile stresses due to reservoir water loading.

The thickness of the dam provides resistance to sliding and may, therefore, dictate the slope of the downstream face which is usually in the range of 0.7 to 0.8 (H):1(V). The thickness in the lower part of the dam may also be increased by an upstream batter.

(c) Arch dam (Fig. 11.4) is a curved masonry or concrete dam having its convex face upstream. Such a dam resists forces, exerted upon it due to water pressure, by arch action. The Idduki dam in Kerala is an arch dam.

(d) Buttress dam (Fig. 11.5) has a sloping deck which retains water and is supported by a series of butteresses which are usually in the form of triangular reinforced concrete or masonry walls.

Choice of the Type of Dam:

The following are the important factors which affect the choice of the type of dam:

(i) Topography

(ii) Geology and foundation conditions

(iii) Material available

(iv) Size and location of spillway

The topography of the site dictates the first choice of the type of dam. A Concreter dam would be the obvious choice for a narrow stream flowing between high and rocky abutments (i.e., deep gorges). Broad valleys in plains would suggest an embankment dam with a separate spillway.

Geological and hydro-geological characteristics of the strata, which are to carry the weight of the dam, determine the foundation conditions. Any type of dam can be constructed on solid rock foundations. Well-compacted gravel foundations are suitable for concrete gravity dams of small height, earth-fill and rock-fill dams.

However, effective water cutoffs are required to check the foundation seepage. Silt or fine sand foundations can be used to support concrete dams of small height and earth-fill dams. Problems of settlement, piping and the foundation seepage are associated with this type of founda­tion. Non-uniform foundations containing different types of strata will usual­ly require special treatment before any type of dam is constructed on such foundation.

If the construction materials which are to be used in large quantity for the construction of dam are available in sufficient quantity within a reasonable distance from the site, the cost of the dam will be considerably reduced due to saving on transportation of the construction materials.

If suitable soils for the construction of an earth-fill dam are locally available in nearby borrow pits, choice of an earth-fill dam would be the most economical. The availability of sand and gravel (for concrete) near the dam site would reduce the cost of a concrete dam.

Spillway is a major part of any dam and its size and type and the natural restrictions in its location will affect the selection of the type of dam. Spillway requirements are decided by the runoff and stream-flow characteristics.

As such, spillway on dams across streams of large flood potential can become the dominant part of the dam and put the selection of the type of dam to a seconder position. For large spillways, it may be desirable to combine the spillway and dam into one structure.

This is possible only in concrete dams. Earth and rockfill dams are based on more conservative design assumptions and, hence, spillway is generally not constructed as part of the embankment. On the other hand, excavated material from separate spillway can be advantageously used for the construction of an embankment dam.

Selection of Site for a Dam:

Site conditions and choice of type of dam are closely interrelated. Further, in case of embankment dams, site conditions considerably influence the design of the dam.

Nevertheless, there are some guidelines which must be followed while selecting a suitable site for any type of dam:

(i) Availability of suitable foundation:

While an earth dam can be designed for and constructed on any type of foundation, concrete dams require relatively stronger foundation. Foundations can be improved to some extent by taking suitable steps for foundation treatment.

(ii) Considerable economy will be achieved if the dam is sited at the narrowest section in the river valley subject to fulfillment of other requirements.

(iii) Spillway is an important component of a dam. In case of a concrete dam, it may be accommodated in the main dam section. But, in case of earth dams, it has to be away from the main dam and, hence, suitable site should be available for the spillway too.

(iv) From economic considerations, it would be advantageous if the bulk of the construction material is available close to the dam site. In case of earth dams, the design is so made as to use locally available material as much as possible.

(v) The area upstream of the dam must be suitable for the requirements of a reservoir.

(vi) The environmental considerations, including displacement of local people, require that there is minimum damage to the local environ­ment, and minimum hardship to the local people. At times, these become governing factors in selecting the site of a dam.

(vii) The selected site should be such that it results in overall economy in construction, maintenance and operation of the water resource project.