The document discusses several design considerations for earth dams. It addresses factors that influence earth dam design such as site characteristics, available construction materials, foundation conditions, climate, and valley shape and size. It also describes different types of earth dam designs including homogeneous dams, thin core dams, vertical and sloping core dams, and dams with different sections. The key design steps are outlined as thorough exploration of the site, evaluation of materials, selection of trial designs, analysis of stability, and modifications to meet requirements.

1. Design Considerations Of Earth Dams
Rosul Ahmed & Lujain Haider

2. 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.

3. 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.

4.  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.

6. 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.

8. 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 .

10. 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.

11. 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.

13. 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.

14. 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.

15. 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.

17. 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
.

19.  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 .

20. 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 .

21.  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 .

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