Contents preface (VII) introduction 1—37

Fig. 15.13 Correction at the discharge point of base parabola
Example 15.2 Calculate the seepage through an earth dam [Fig. 15.14 (a)] resting on an impervious foundation. The relevant data are as follows:

Height of the dam	= 60.0 m
Upstream slope	= 2.75 : 1 (H : V)
Downstream slope	= 2.50 : 1 (H : V)
Freeboard	= 2.5 m
Crest width	= 8.0 m
Length of drainage blanket	= 120.0 m

Coefficient of permeability of the embankment material in x-direction = 4 × 10^–7 m/s

in y-direction = 1 × 10^–7 m/s

504			IRRIGATION AND WATER RESOURCES ENGINEERING
	8		4
	2.5		E
		2.5	23.72
	2.75
		1
1		60
			120	46.16	60

(a) Original section	(b) Transformed section
	All dimensions in metres

Fig. 15.14 Original and transformed sections for Example 15.2
Solution: Since it is a case of anisotropic permeability, the original section needs to be transformed through the transformation
x_t = x K _y /K_x = x 1 × 10^{− 7} /4 × 10⁻⁷ = x/2 With respect to the transformed section [Fig. 15.14 (b)]

		d = 46.16 m and h = 57.5 m
and			= Kx K y = 4 × 10^− ^7 × 1 × 10^−^7 = 2	× 10^–7 m/s
K
From Eq. (15.16)
∴		q =			[ d ^2 + h^2 – d]
	K
			= 2 × 10^–7 [ 46.16 ^2 + 57.5 ^2 − 46.16 ]

• 
  55.2 × 10^–7 m³/s/m
  

2. 
   Methods of Controlling Seepage through Embankment and its Foundation
   

Seepage through an embankment dam and its foundation causes not only the loss of water but, if uncontrolled, also results in piping in the downstream portion of the embankment and foundation. Besides, the stability of slopes is also affected by seepage forces. Piping is the progressive erosion of embankment material due to leaks which develop through the embankment or foundation. Leaks in an embankment are usually caused because of poor construction resulting in insufficiently-compacted or pervious-layered embankment, inferior compaction adjacent to concrete outlet pipes or other structures, and weak bond between the embankment and the foundation or abutments. Piping is caused due to the erosive forces of seepage water tending to move soil particles along with the seepage water. When the forces resisting erosion (viz., cohesion, the interlocking stresses, the weight of soil particles as well as the action of the downstream filters, if present) are less than the erosive forces of the seepage water, the soil particles are washed away and piping begins. Earth slides (or sloughing) are related to piping. Sloughing begins when a small amount of material at the downstream toe has eroded and caused a small slump (or slide) leaving behind steeper slope which eventually gets saturated due to seepage water and slumps again. This may continue and cause complete failure of the embankment.

In order to check these effects of seepage, some measures must be taken to control the seepage in embankment dams so that the seepage line is well within the downstream face and

EMBANKMENT DAMS

505

the seepage water is suitably collected and disposed of. The methods of seepage control for embankment dams can be grouped into two broad categories:

(i) Methods which prevent or reduce the seepage, such as complete vertical barriers (viz., rolled-earth cutoffs, steel sheet piles, and concrete walls), grout curtains, up-stream impervious blanket, and thin sloping membrane.
(ii) Methods which control the seepage water that has entered, such as embankment zoning, horizontal blanket drains, chimney drain, partially penetrating toe drains, and relief wells.
Usually, a combination of these methods is used.
(i) Rolled-Earth Cutoff
Rolled-earth cutoff [Fig. 15.15 ( a and b)] provides an effective barrier for controlling seepage through pervious foundation of moderate thickness (up to about 25 m) over an impermeable bedrock formation. As can be seen from Fig. 15.15 (c), a cutoff penetrating up to 80% of the pervious depth would reduce seepage quantity only by about 50%. Therefore, to be effective, cutoffs must penetrate the full depth of the pervious foundation as shown in Fig. 15.15 (b). Such cutoffs are also advantageous in providing full-scale exploration trenches exposing all soil strata, permitting treatment of the exposed bed rock when necessary and increasing the stability of the dam because of effective replacement of the large mass of foundation soil with stronger material. The major difficulty in the construction of a rolled-earth cutoff is the dewatering of the excavated trench and keeping it dewatered until its backfilling.
At times, the foundation is such that the average permeability of the foundation soil decreases with depth below the surface or there is a single continuous impervious layer (over other pervious layers) to which the cutoff can be connected. In such circumstances a partial vertical cutoff [Fig. 15.15 (a)] may be useful. Such partial cutoffs extending to a depth of 2 to 3 m should be specified for sites which, otherwise, may not need a seepage barrier. This provision would serve the purpose of continuous excavation through the upper layers of soil so as to better understand the subsoil conditions which, at times, may suggest the need for further excavation or provision of other suitable measures.