Contents preface (VII) introduction 1—37

Fig. 15.1 Typical sections of homogeneous earth dam
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. 15.2), 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 its construction

		Water surface	Water surface
	Thin core		Filters	Rock
Rock
			(Shell)	Filters
	Shell				Rock or gravel
						Rock or gravel
		(a) Thin sloping core dam	(b) Thick sloping core dam
					Core
				Filter	Filter
				Rock or	Shell
			Shell	Rock or
					gravel
				gravel

(c) Central core dam

Fig. 15.2 Typical sections of zoned earth dam

490 IRRIGATION AND WATER RESOURCES ENGINEERING
and operation. The central core provides imperviousness to the embankment and reduces the seepage. The maximum width of the impervious core will be governed by stability and seepage criteria and also by the availability of the material. An earth dam with a sufficiently thick impervious core of strong material with pervious outer shells can have relatively steeper embankment slopes limited only by the foundation and embankment characteristics. However, a thin core dam is usually more economical and more easily constructed because of lesser amount of fine-grained soil to be handled. Core widths of 30 to 50% of the water head are usually adequate for any type of soil and any dam height while core widths of 15 to 20% of water head are thin and considered satisfactory, if adequately designed and constructed filter layers are provided (4). Core widths of less than 10% of water head should not be used as far as possible.

The impervious core can be placed either as a vertical core or as an 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 foundation which, in turn, reduces the possibility of leakage along the contact. Besides, for a given quantity of impervious material, the vertical core will have greater thickness. The main advantage of upstream sloping core is that the main downstream shell can be constructed first and the core placed later – an advantageous feature in areas which have short periods of dry weather suitable for building a core of fine-grained soil. Besides, foundation grouting can be carried out while the downstream embankment is being constructed.

A rock-fill dam (Fig. 15.3) is an embankment which uses large-sized rock pieces to provide stability and an impervious membrane to provide watertightness (5). 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 upstream face of the dam is, however, more suitable for placing the impervious membrane due to the following reasons (4):

Impervious

membrane

Rock
Fig. 15.3 Typical section of a rock-fill dam

(i) The upstream impervious membrane, with a suitable drain behind it, prevents seep-age from entering the embankment. This reduces pore pressures and prevents the embankment mass from being submerged. Both these effects result in greater stabil-ity of the embankment.
(ii) The upstream impervious membrane is accessible for inspection and repair.
(iii) The upstream impervious membrane also serves a secondary function of wave pro-tection.
(iv) The upstream impervious membrane can be built after completion of the embank-ment. This would permit initial settlement of the embankment without affecting the membrane adversely.

EMBANKMENT DAMS

491