Contents preface (VII) introduction 1—37

(x) Relief Wells
Relief wells (Fig. 15.21) are provided when pervious strata of embankment foundation are too deep to be penetrated by rolled-earth cutoffs or toe drains. Relief wells can penetrate most pervious water-bearing strata and relieve uplift pressures effectively. Spacing between the relief wells should be small enough to lower the water pressure to the desired safe level. Relief wells must penetrate through the complete depth of the pervious foundation, if possible. A relief well should preferably have an interior perforated pipe (i.e., the well screen) with a minimum diameter of 15 cm or larger, if heavy foundation seepage is anticipated. The annular space surrounding the well screen is backfilled with graded filter. However, near the surface, the annular space is backfilled with impervious soil or concrete so that upward flow of water outside the relief well pipe may not occur.

90 mm		Embankment
15 mm 60 mm	15 mm
7.5 mm	7.5 mm
	82.5 mm
20 mm	30 mm
20 mm	15 mm

Variable		Impervious
		compacted backfill
	15	Top of pervious stratum
Sand backfill	mm	300 mm
Gravel pack		600 mm
		Wood screen

Gravel pack

Bottom plug

85 mm

Bedding thickness as required
15 mm

Fig. 15.21 Gravael-packed relief well screen (4)

EMBANKMENT DAMS

513

Example 15.3 Using Bennett’s solution for upstream impervious blanket, find the discharge through the foundation of an earth dam having a central core (Fig. 15.22) for the following data:

Water depth over upstream blanket

= 30.0 m

Crest width

= 8.0 m

Freeboard

= 2.0 m

Upstream and downstream slopes of the core

= 1 : 1

Thickness of the impervious blanket

= 2.0 m

Length of the blanket (connected to the core)

= 80.0 m

Depth of pervious foundation

= 30.0 m

Coefficient of permeability of

(i)

foundation soil

= 2 × 10^–4 m/s

(ii)

core and blanket soil

= 2 × 10^–6 m/s

2

8

1

H =

30

1

CORE

Zb = 2

x = 80

xd = 76

Z_f = 30

All dimensions in metres
Fig. 15.22 Sketch for Example 15.3
Solution: Using Eq. (15.22)

	Kb	=	2 × 10^−^6
a =	K f Z f Zb		2 × 10 −4 × 30 × 2 ^= 0.0129

∴ Effective length of blanket,

1

x_r = _a tan h(ax)

1

= _0.0129 tan h(0.0129 × 80) = 60.0 m

From Eq. (15.28),	q		= K Z	H	= 2 × 10^−^4	× 30 ×	30
		60	+ 76
		f		^f f xr + xd

= 1.324 × 10^–3 m³/s/m
15.3.4. Filters

In a homogeneous embankment, the individual soil particles subjected to seepage forces cannot move because they are held in place by the neighbouring soil particles. But, at the boundaries

514 IRRIGATION AND WATER RESOURCES ENGINEERING
between soils of different sizes, the finer soil particles may be washed into the void spaces of the coarser material. To prevent such migration of soil particles, it should always be ensured that the relative gradation of adjacent soil zones meet established filter criteria. If the difference in size of soils of the fine and coarse zones of an embankment is too large to satisfy the filter criteria, zones of intermediate gradation, known as filters must always be provided in between the fine zone and coarse zone.
There are two main conflicting requirements of a filter layer:
(i) the filter layer must be more pervious than the protected soil so that the filter acts as a drain, and
(ii) the size of the particles used in the filter layer must be small enough to prevent migration of the particles of the protected soil into the voids of the filter layer.
Further, the filter layer should be sufficiently thick to provide good distribution of all particle sizes throughout the filter (11). The following rules are widely used for the design of filter layer (4, 11):

(i)		D15 of the filter	≥ 5
D	of the protected soil
	15
(ii)		D15 of the filter	≤ 5
D	of the protected soil
	85
(iii)		D50 of the filter	< 25
D	of the protected soil
	50

(iv) The gradation curve of the filter should have approximately the same shape as the gradation curve of the protected soil.
(v) Where the protected soil contains a large percentage of gravels, the filter should be designed on the basis of the gradation curve of the portion of the material which is finer than the particles passing one-inch sieve.
(vi) Filters should not contain more than about 5% of fines passing no. 200 sieve, and the fines should be cohesionless.
Here, D₁₅, D₅₀, and D₈₅ represent the particle sizes which are, respectively, coarser than the finest 15, 50, and 85 per cent of the soil, by weight. These filter criteria are based on studies with non-cohesive soils and take into consideration only the grain size of the protected soil. As such, these rules may be conservative, particularly for clays which can resist piping action because of cohesion.
Theoretically, the required thickness of a horizontal filter is very small—about 15 cm for sand and 30 cm for gravel. But, from practical considerations, a minimum thickness of 1.0 m is desirable (11). For vertical or inclined filters, the minimum width of filters is about 2 to 3 m for convenience in construction.