The essential requirements of the design of an earth dam are safe and stable structure at a minimum construction and maintenance cost. The essential design criteria are

(i) Safe against overtopping during design flood by providing adequate\spillway and outlet capacity,

(ii) Spillway be of sufficient capacity to pass peak flood by providing adequate spillway and outlet capacity to pass peak flood,

(iii) Safe against overtopping by wave action providing adequate free board,

(iv) Side slopes stable during construction and under all conditions of reservoir operation. Upstream slope is safe against rapid drawdown conditions while the downstream slope is safe against sloughing,

(v) Side slopes upstream and downstream are flat enough so that the shear stress-induced in the foundation is within the shear strength of the material comprising the foundation with a suitable factor of safety,

(vi) Upstream slope is protected against erosion by wave action while the crest and downstream slope is protected against erosion due to wind and rain. Horizontal berms at suitable intervals in upstream and downstream faces may be provided for this purpose,

(vii) Downstream slope is safe during steady seepage under full reservoir condition,

(viii) Portion of the dam downstream of the impervious core is properly drained by the provision of suitable drainage System,

(ix) Seepage flow through the dam and foundation is so controlled that there is no danger of fine particles getting washed out from the downstream by the efflux of seepage. Moreover, quantity of seepage loss is restricted to the minimum,

(x) There is no possibility of free flow of water from upstream through either the dam or the foundation,

(xi) Adequate impervious core to act as water barrier. Top of impervious core is maintained higher than the maximum reservoir level,

(xii) Seepage line, i.e., phreatic line is well within the downstream face so that no sloughing of the slope takes place,

(xiii) Seepage flow through the embankment, foundation and abutments is controlled so that no internal erosion takes place. The amount of ater lost through seepage must be controlled so that it does not interfere with planned project functions,

(xiv) Transition filters are incorporated to satisfy the two essential conditions of filters, viz., to prevent choking of the filter by soil and have minimum of head loss in the filter,

(xv) Dam as a whole is earthquake resistant. The seismic conditions of the region investigated with reference to geological map of vicinity. India has been divided into five sesmic zones. Horizontal seismic coefficient for static in different zones, is taken as under:

Vertical Seismic coefficient, where applicable, is taken as half horizontal seismic coefficient as indicated above.

Seepage In Earth Dams

In earth dam, the entire dam, except for the core, forms part of the drainage system. Filters, made of select materials, allow the water to pass through and in turn the transition zones do the same thing with respect to the filers. Finally, the shells are draining elements and the filtered water is channelled through them downstream shell. Those from the abutments are channelled to the river bed in order to avoid surface erosion.

Seepage through the earth dams is unavoidable. The reservoir water seeps through the body of the dame or through its contact with foundation and abutments. The adverse effects of seepage in earth dame are

1. Loss of reservoir storage capacity,
2. Sloughing of downstream toe due to saturation and progressive development of local shear failures,
3. Weakening of soil strength as finer soil particles are moved through coarse particles resulting in internal erosion or piping failure,
4. Foundation blow out due to excessive uplift, and
5. Reduction in effective strength of soil as seepage caused pore pressures.

For the location of zones of seepage through reservoir foundation or dam embankment, American groundwater consultants of New Mexico have developed a geophysical method known as ZETA-SP. The method is based on the principle that water moving through porous earthen materials creates an electrical field, the magnitude of which is proportional, among other things, to the velocity of the water through earthen materials of the dam embankment or foundation of a reservoir.

In this method, two electrodes are placed side by side in the water at one edge of the reservoir so that they are at zero potentials. To begin measurements, while one electrode is kept in that position, the other is moved to the remote shore and its potential relative to the stationary electrode is measured at that point and at regular intervals as it is drawn across the bottom of the reservoir towards the stationary electrode. The process is repeated from a number of different points around the shore so that a potential grid of the reservoir bottom is derived. Zones of maximum potential are indicative of seepage.

Seepage Control Measures

The purpose of seepage control is to

(i) Contribute to the stability of the dams and their foundations,

(ii) Effect reduction of the loss of water,

(iii) Avert failure of the dam structure by piping, and

(iv) Excessive pore pressure in the dam.

A Suitably designed internal drainage system is essentially required to meet these requirements.

1. Control of Seepage Through Embankment

• Impervious zones: It consists of a watertight core(coefficient of permeability over $10^{-6}$ cm/s) protected on both sides by filter, draining and transition zones. Fig 17.3 shows the most common section of an earth dam. The core depicted is vertical and symmetrical. However, in practice, it is asymmetrical and slanted but this arrangement does not modify the manner in which the drainage elements next to it function. Impervious earth core checks the seepage through the embankment and also enables the portion of dam downstream of the core to act as a drainage zone.

1. Core : The core provides improbable barrier within the dam body. It may be located either centrally or inclined upstream.

Thin core

The core material normally has less shear strength than the rest of the embankment and may further develop appreciable pore pressures. The location of core depends mainly on the availability of materials, topography of site, foundation condition, diversion considerations etc. Thickness is governed by

(i) Availability of impervious and filter materials,

(ii) Resistance to piping,

(iii) Permissible seepage through dam.

Minimum thickness is 3m. Top-level of core is 1 m above, the maximum water level. From stability consideration thinner core is better. A thin core with filter zone and pervious shells develops negligible pore pressure. A core is considered thin if its width is less than 15 to 20 per cent of the water head. Theoretical mainimum volume for a thin core dam is obtained when the core is so located that the upstream and downstream slopes have approximately the same factor of safety. Thin core may be moderately slanting or central. A central core provides higher pressures at the contact between the core and the foundation. Therby reducing the possibily of leakage and piping. Core with thickness less than 10 per cent of the head is not used except where a large leakage through the core would not lead to failure of the dam.

The Advantages of thin core are

(i) Thin core with pervious shells has less unit cost os placement,

(ii) Smaller embankment volume,

(iii) Expeditious construction as lesser time is required for constructing thin impervious zone,

(iv) Lesser requirement of compaction equipment, and

(v) Develops negligible pore pressures.

The disadvantages of thin core are that contact area between the core of the embankment and rock is less and there is likelihood of leakage through thin core. However, this can be taken care of to some extent by increasing the width of the core at junction with the rock to give wider contact area.

Thick Core:

A core with a width of 30 to 50 per cent of the water head is considered as thick core. It is usually central and has a steeper upstream slope and flatter downstream slope. It is suitable for any type of soil and dam height. Thick core provided wider and better contact with the foundation and offers more resistance to piping especially that which may develop due to differential settlement cracks. Since the piping resistance of compacted embankment is largely a result of soil properties, the minimum allowable core thickness depends on the plasticity and gradation of the core.

An Ideal core, according to Cedergern, is that which is moderately slanting, sufficiently wide to eliminate very high gradients and yet confining seepage stresses to within the upstream part of the section.
Having discussed thin and thick cores, it is pertinent to mention merits of vertical and sloping cores.

Vertical Core:

A vertical core dam has a steeper upstream slope and a flatter downstream sope. Advantages of vertical core are

(i) Higher pressure exist on the contact between the core and the foundation thereby reducing the possibility of leakage, and piping,

(ii) Slightly greater thickness of the core is obtained for a given quantity of impervious soil,

(iii) Central core permits carrying out of additional grouting from the crest of the dam after the is completed and without lowering the resevoir.

Sloping Core:

A sloping core dam has steeper upstream and downstream slopes. Advantages of sloping core are

(i) Placement of main downstream portion of the embankment first and core placed later,

(ii) Foundation grouting can be carried out while the embankment is being placed,

(iii) Filter layers on either side of the core can be made thinner and placed more conveniently than vertical core dam,

(iv) Reduces the pore pressure in the downstream part of the dam and thereby increases its safety, and

(v) Allows use of relatively large volume of random material on the downstream.