Contents preface (VII) introduction 1—37

Fig. 15.18 Definition sketch for Bennett’s analysis

510 IRRIGATION AND WATER RESOURCES ENGINEERING
Having determined the effective length of the blanket using a suitable equation [Eq. (15.23) for infinite length and Eq. (15.26) for finite length], one can determine the reduction in the foundation seepage as follows:

as
If x_d is the base width of the impervious core of the dam, the foundation seepage is given

^qf1 = ^Kf	H	Zf	(15.27)
	^xd

When an impervious blanket, whose effective length is x_r, is available, the foundation seepage is given as

qf2 = K f	H	^Zf	(15.28)
x r + xd

Here, H is the total head. Also, the loss of head up to the end of the blanket will be equal
to

^xr	H
xr + xd

The optimum length of an upstream impervious blanket x_o is given (10) by the expression ax_o = 2 . Length of the blanket beyond x_o will increase its effective length x_r only marginally.




(viii) Zoning
Most often embankment dams and particularly earth dams have an internal impervious core surrounded on both sides by outer sections called shell. The core provides water tightness to the embankment. The core can either be a thin sloping core, [Fig. 15.2 (a)], or a somewhat thicker sloping core, [Fig. 15.2 (b)] or even a thick central core [Fig. 15.2 (c)]. Impervious core dissipates hydrostatic energy rapidly but increases the chances of piping due to the presence of large hydraulic gradients. Therefore, at the boundaries of the core and shells, filters must always be used. Whenever sufficient quantity of highly impermeable soils are readily available, a thick central core type embankment [Fig. 15.2 (c)] should be constructed. A thick core provides wide zone for energy dissipation, relatively low hydraulic gradients, and longer contacts with foundations. Dams having extremely narrow cores induce large hydraulic gradients within the embankment.
(ix) Downstream Drains
Dams with heights of more than about 6 m should always be provided with some type of downstream drain which is constructed using material many times more pervious than the embankment soil. Such drains reduce the pore water pressures in the downstream shell and, thus, increase the stability of the downstream slope against sliding. These downstream drains could be in the form of toe drains, [Fig. 15.1 (a)], horizontal blanket drains [Fig. 15.1 (b)], chimney drains [Fig. 15.1 (c )], strip drains [Fig. 15.19] or partially penetrating toe drains [Fig. 15.20] depending upon the height of the dam, the cost and availability of previous material, and the permeability of the foundation.
Horizontal Blanket Drains: These drains [Fig. 15.1 (b)] are widely used for dams of low to moderate heights and are highly effective only on relatively uniform foundations and in non-stratified embankments. Such drains collect seepage from the embankment as well as the downstream portion of the foundation and accelerate the consolidation of the foundation. Such drains are, however, not suitable for stratified embankments. These drains usually extend from the downstream toe to the upstream ranging in length from 25 to 100 per cent of the projected length of the downstream slope. The thickness of such drains should always be more

EMBANKMENT DAMS

511

than 1 m and should be sufficient to be able to carry the maximum anticipated seepage with a conservative factor of safety. The drain would consist of relatively coarse particles and should, therefore, be surrounded by suitable filter layers to prevent the migration of finer material of embankment or foundation. At sites where pervious material is available in small amount, the general effect of horizontal blanket can still be obtained by constructing strip drain (Fig. 15.19).

Continuous strip drain

Pervious strip drains discharging to downstream toe
Fig. 15.19 Strip drain


^Temporary Original slope

stable

slope
Back-filled soil
Perforated

pipe Filter

Gravel
Fig. 15.20 Partially penetrating toe drain
Toe Drains: If sufficient quantity of boulder or quarried rock is available, a toe drain (or rock-toe) [Fig. 15.1 (a)] of height about 0.25 to 0.35 times the height of the embankment will be very effective in controlling seepage from stratified embankments. The rock-toe also protects the lower part of the downstream slope from tail-water erosion. Generally, the inner slope of rock-toe is provided at 1 : 1.
Chimney Drains: Chimney drains [Fig. 15.1 (c)] combine the advantages of horizontal blanket drains as well as toe drains. These drains can completely intercept embankment seepage irrespective of the extent of embankment stratification. Besides, these drains keep the seepage line well below the downstream face. For effective functioning, these drains and their outlets must have sufficient permeability to discharge seepage water without building up excessive head.