Design Considerations Of Earth Dams

Rosul Ahmed & Lujain Haider

An earth dam is a dam built with highly compacted earth. This dam is

classified as a type of embankment dam, being built in the shape of

an embankment or wedge which blocks a waterway. These dams

have been built by various human societies for centuries, and they

continue to be produced in some regions of the world when they

appear to be suitable for the location and intended use.

Introduction

The characteristics of the particular site have a greater

influence on the design of an earth dam than they do other

engineering structure .the and locations of the soils available

for the construction of the embankment may dominate the

entire design ,although in most cases the soils must be

considered in conjunction with conditions such as the nature

of the foundation, the climate of the region , the size and

shape of the valley.

Factor influence design

A thorough exploration of the foundation and abutment, and an evaluation of the quantities and characteristics off all the embankment construction materials available within a reasonable distance of the site.

A study of all the conditions which may influence the design. The selection of possible trail design. An analysis of the safety of the trail design. The modification of the designs in order to meet minimum stability

requirements. The preparation of detailed cost estimation. The final selection of design which seems to offer the best combination of

economy , safety And convenience in construction.

The fundamental in designing an earth dam are:

On of the principle economic advantages of the earth dam is that nature has already fabricated the construction materials and placed them free of charge at the site. In some cases only one type of soil is readily available ,and the principle design problem is to determine the most economical dam that can be made from it.

If it is an impervious soil, the design consist of a homogeneous embankment with only a small amount of pervious material to control internal seepage .

If it is a pervious sand and gravel , a dam with a very thin core may be used where enough impervious is available to make a core.

At the other extreme are sites with a great number of varied soils which can be incorporated in different combinations into the embankment. In such case it in advisable to plan a zoned dam with the finer and more impervious materials placed in an internal core, and the coarser materials utilized to provide strength and drainage in upstream and downstream zones.

where two types of soils occur in horizontal layers in the same borrow pit, they can either excavated selectively and placed in different zones of the dam or excavated together and blended into single material with intermediate properties.

Materials available embankment construction

Removal Of Oversized Cobbles:•The largest cobble used in an embankment for example the core the max. diameter is set at 5 or 6 in and for embankment sections up to diameter of 24 in .

•If the soil for the core of the dam contains only a few percent of oversized cobbles, the purpose of removing them is to improve the compaction and quality of the embankment .but if the soil contained a large quantity of oversized cobbles, two other benefits may be obtained :The embankment can be made more impervious and the oversized cobbles can become an important source of material for the construction of free-draining zones .

The design of a dam is affected by climate, because of the importance of he weather during the construction period.

In regions where construction must be carried on during a rainy season, it is often preferable to design the dam with a sloping upstream core instead of a central core.

In arid or semiarid regions it is much easier than in wet climates to control the construction water content of fine-grained soils.

: CLIMATE

It possible to build an earth dam on almost any kind of foundation. The foundation characteristics may or may not have a large influence on the embankment design.

The embankment that is on a foundation of soil with low shear strength must usually be made broader and with flatter sloped than would otherwise e required .

For pervious soil foundation either a seepage cutoff must be provided or the under seepage must be controlled to avoid the danger of the piping.

In the case of the rock foundations ,special design details are used to seal the core of the dam to the rock.

Character of the foundation

The shape and size of the valley at the dam site may have an important influence on the design of the dam in a number of ways . In board valleys with gently sloping abutments, the embankment section is not usually much influenced by either design or construction problems.

In narrow valleys or in valleys with steep abutments ,special design provisions may be necessary.

In very narrow valleys the area of the construction surface is small ,and large quantities of equipment cannot be used efficiently.

For dams in very narrow valleys with steep walls, it may be necessary to use piping-resistant cores ,special filters, or other design details which will minimize the danger of embankment cracking due to differential settlement.

Shape and size of the valley

1-Homogeneous dams: Homogenous are constructed entirely or almost entirely of a single

embankment material. Homogenous embankments are used most often in dams of low to

moderate height. Homogenous dams are usually composed of impervious or semi-

pervious soils, but many successful embankment have been built of relatively pervious sands and sand-gravel mixture.

Any homogenous dam with a height of more than about 20-25 ft. should have provided with some type of downstream drain constructed of material appreciably more pervious than the embankment soil.

Types Of Earth Dams

The design of the downstream drainage system is governed mainly by the height of the dam, the cost and availability of pervious material and permeability of the foundation.

For low dams , a simple drain of the type can be used .toe drains such as this were installed in some of the oldest homogenous dams in an effort to prevent softening.

Horizontal drain blankets widely used for dams of moderate height.

Higher homogenous have been constructed with inclined and vertical “chimney” drains .although “chemeny” drains were used on some very old dams.

There are several conditions which may make a thin core dam the best design for a given site. Where there is an ample supply of pervious embankment material but only a limited supply of soil suitable for an impervious core, the designer may have no other choice than a thin core dam. Where there are ample supplies of both pervious and impervious soil, a thin core dam be more economical or more easily constructed for a combination of

reasons : 1 -The unit cost of placing pervious material may be less than that of

placing the available impervious materials .2 -The embankment volume can be made smaller in a thin core dam

than in any other .3 -The weather and the construction time available may not permit

the use of a large zone of impervious material .

2-Thin Core Dams:

In this type of dam the core may be sloped upstream or placed in a vertical position near the center of the embankment.

The pervious zones are constructed either of sand or gravel or of rock obtained by quarrying or screening earth rock mixtures.

The problem of determining the min. safe thickness is not amenable to theoretical treatment and no definite rules can be given.

It is governed for practical purposes by the following factors :1- The tolerable seepage loss.2- The min. width which will permit proper construction.3- The type of material available for the core and shells.4- The design of the proposed filter layers.5- precedent on similar projects.

The min. allowable core thickness should always depend to a great extent on the plasticity and gradation of the core material .

Advantages of vertical and slopping cores in earth dams are:

1 -the higher pressure will exist on the contact between the core and the foundation and will provide more protection against the possibility of

leakage along the contact .

2 -The thickness of the vertical core is slightly greater for a given quantity of impervious soil than thickness of slopping cores.

3 -The filter layers between the core and the upstream and the downstream pervious zones can be made thinner with less difficulty in construction than the filter layers for vertical core dams.

Disadvantages of the slopping core is that the location of the area of the contact between the core and the foundation depends on the depth of

foundation excavation.

Typical embankment sections: For the great majority of earth dams, the embankment is

constructed with the same design section (the same zoning and the slopes) over the hole length.

At some sites, however, have been divided into two or more sections and different design used in each.

For dams of short length there is usually little or no reason for changing the design, but for long dams several circumstances may make different sections advisable . For example, different portions of the dam may have different foundation condition or different heights .

Reference: Sherard, J. L., Woodward, R. J., Gizinski, S. F., and Clvenger, W. A. (1967). Earth and earth rock dams, Wiley, New York.