Chapter 3 – Design of Embankment Dam

CHAPTER 3: DESIGN PRINCIPLES OF EMBANKMENT DAMS

3.1 INTRODUCTION

The generic term" Embankment" is used to designate a large variety of granular materials
(natural, processed) which can be used for construction of Dams, such material s include: clayey
and sandy soil, sans gravel and rock. In principle any granular material may be used if provides
adequate characteristics of durability (chemical, mechanical) and mechanical resistance. Many of
the largest and highest dams in the world are embankments. There is a continuous strain towards
new developments in the knowledge of material properties, construction techniques and Enova
tike uses. Recent example: reinforced earth, hydraulic fills, soil cement etc.

This chapter briefly discusses the embankment dam design principles and construction methods
which will finally be followed by the discussion on seepage, stability and settlement ad the key
factors in design. It concludes with a brief section dealing with rock fill embankments.
Therefore, students are need to recapitulate basic elements of soil mechanics, and geology in
section dealing with the nature and classifications of engineering soils & with their
characteristics.

Some of the soil parameters are

 Soil identification and classification

 Liquid limit (from plastic to liquid), plastic limit (soil is too dry to exhibit plasticity),
plasticity index= WL-WP
 Phreatic surface pore water pressure is zero in the soil
 Stability equilibrium between forces & moments and the mobilized soil strength.
 Deformation
 Share strength of soil the maximum resistance to shearing stress. Has two components
 Apparent cohesion, C, electrical forces binding clay-size particles together
 Angle of shearing resistance, , developed by inter particle frictional resistance and particle
inter looking
 =C+ tan ,

 Normally consolidated- if the insitu effective stresses are the greatest to which the clay has
historically subjected
 Over consolidated previous effective stress levels have been relieved.
 Consolidation gradual expansion of water under applied load
 Compaction densification by expulsion of air from the soil void space.
dh
V  K Q  Ki As gross area subjecte toflow
dl
 Permeability  (balk , in site)
d (dry density ) 
(1  w( water content ))

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 Chapter 3 – Design of Embankment Dam

3.2 TYPES OF EMBANKMENT DAMS

Generally embankment dams have two components
1) An impervious water retaining element or core of very low permeability soil, e.g. soft clay
2) Supporting shoulders of sheet earth fill (or of rock fill), to provide structural stability
Common classification of embankment dams is based on materials used:
A) Earth fill dams: using a variety of clayey to sandy soils
1) Homogenous dams: basically only one material is used. It is preferable where only one type
of material is economically or locally available. Such a section is used for small and less
important dams, high section required to make it safe against piping, stability etc. They
required internal drainage system (filter) to control seepage. The internal drainage keeps the
phreatic line will within the body of the dam.
2) Zoned dam: different materials used in different zones of the dam. Most common for large
embankment dams. Usually provided with a central impervious core, covered by a
comparatively pervious transition zone which is finally surrounded by a much more pervious
outer zone. The central core cheeks seepage, the transition zone prevents piping through
cracks which may develop in a core. The outer zone gives stability to the central impervious
fill and distributes the load. There are three types of zoned dams
i) Thick core: (core base width is
now (20-40%) of the height of the embankment
ii) Inclined core: advantageous in moderating the list of core cracking as result of load
transfer b/n compressible core and steffer embankment.
iii) Slender core
3) Reinforced earth: a special class is represented by earth dams with reinforcement (by steel
base or geo-synthetics)
B) Rock fill dams: using mainly rock fill materials such as quarry rock of different grain size or
course gravel found in river bass. A rock fill is comparatively pervious and a sealing element
has to be provided. Classification of rock fill dams is based on this sealing element:
a) Upstream face sealing
ii. Concrete face rock fill dam
iii. Asphaltic face rock fill dam
iv. special type of sealing (metallic)
b) "Core" sealing
 Concrete, Asphatic- concrete, bentonite diaphragms
 Clay core
c) Mixed type
In practice mixed type solution using earth rock fill materials are common. the receive the name
of earth rock fill dam use of d/t materials in zones.
C) Hydraulic fills dams: using sand, small fraction of silt or clay. Constructed by dredging and
pumping operation
Advantage of embankments
 Compatibility with soft foundation
 Deformability
 Use of natural construction materials often inexpensive

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 Chapter 3 – Design of Embankment Dam

Main problems
 Stability
 Permeability
 Deformations settlement
3.3 DESIGN PRINCIPLE OF EMBANKMENT DAMS

The design criteria for earth fill dams requires

1) Embankment and foundation stability sliding of slope zones etc) under all reasonably
postulated conditions; including flood and earthquake.
2) Control of seepage quantities and pressure in the embankment and its foundation.
3) Safeguards against overtopping; and
4) Control of surface erosion (usually downstream slope)
5) Excessive settlement deformations; in particular different settlements inducing cracks.

3.4 LOADS AND LOAD CASES (STABILITY)

Loads commonly considered are

A) Dead load: weight of the dam body
B) Water pressure: include
 Hydrostatic external pressures
 internal seepage pressures
 hydrodynamic pressures due to waves and earth quakes
C) Other external pressures; earth, silt; ice and wind
D) Internal forces due to earthquakes

It is common practice to consider a combination of such loads representing possible conditions at

the end of construction. Normal operation condition, exceptional operation condition.

Condition Min. factor of safety

a) End of construction (U/s & D/s slopes 1.25
With earth quake loading >1.00
b) Steady seepage, (full reservoir d/s slope) >1.50
With earth quake loading 1.25
c) Rapid draw down (u/s slope) 1.25
With earth quake loading >1.00

3.5 DESIGN FEATURES

Thus there are two aspects of the design of an earth dam, viz.
i) To determine the X-section of the dam and
ii) To analyses the stability of the proposed x-section.
3.5.1 Free board

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 Chapter 3 – Design of Embankment Dam

Earth fill dams particularly need free board as a margin of safety to accommodate unusual
events. Because of their relative high erodiability, they should be safeguarded against even
temporary overtopping during floods or earth quakes. There is a general agreement that free
board should be adequate to pass the probable maximum flood or to serve an extreme earthquake
without overtopping. The provision is also necessary for long term settlement of the dam. it is
therefore customary to construct the crest of the dam to a longitudinal camber to accommodate
the varying settlement.

The overall minimum free board from spillway sill to dam crest should be at last 1.5m on the
smallest reservoir embankment.
3.5.2 Crest details

The top width of an earth fill dam may be governed by requirements for passage of vehicles,
both during construction and in service.
The Japanse code gives the following dimension

Height (m) Crest width (m) Or the following dimensions can also be used
30 8
H/5 + 3 ; for very low dams
50 10
70 11 0.55 H  0.2 H For dams lower than 30m
100 13 1.65(H+1.5)1/3 For dams higher than 30m
200 18
Where: - H is the height of the dam.

3.5.3 Zoning
The careful and correct zoning of the available material is an important aspect of embankment
design.

Core: The seepage barrier in a zoned earth dam usually consists of a core of compacted
impervious soil located centrally or sloping u/s. it is dimensions will deep on the availability &
properties of materials in and near the site, and the need to avoid high seepage gradients. A
commonly used rule specifies that the base width of the core should be at the least 25% of the
maximum difference between reservoir and tail water elevations.

A normal hydraulic gradient through the core of the order of 1-5 to 2.5 is satisfactory. It is
preferable that the core be approximately central and it can core down stream filter placing if that
face is kept vertical.

Outer zones: The shells of an earth fill dam may be constructed from a wide range of materials
at or near the site. Preferred properties include adequate shell strength and drainage capability to
ensure economical slopes. The upstream shell generally should be free drainage for stability
during draw down and for resistance to liquefaction during earth quake.

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 Chapter 3 – Design of Embankment Dam

Drains: safe interception and conveyance of seepage are necessary to ensure that vital
embankment or foundation materials are not washed away, or that internal water pressures do not
jeopardize stability, placement of more pervious materials in the outer zones is a fundamental
approach for seepage control.

The pervious material should be selected carefully. Aggregate drains must function as filters to
retain soil or rock particles and as conduits to convey water safely to discharge points.
To meet the filter requirement, a graded filter a course aggregate protected by fine aggregates is
effective.

3.5.4 Slope protection

Earth fill dams must be protected from erosion. Wherever wave action can be expected, the
upstream slope will need to be armoured effectively. The downstream face may require less
extensive treatment to provide resistance to runoff & general weathering.

Upstream slope: The extent of treatment required at the u/s face will depend on the operational
regime, the size and the shape of the reservoir, he climate & the typical wind patterns. The
primary protection is conventionally extended from the so to about 1.5 to 3 meters below the
active storage range.

Riprap is the most common armor for upstream face of earth dams specifications typically
require well graded riprap. Riprap should be placed up on bedding material that is compatible
with it so that it will remain in place while retaining the soils in the main body of the
embankment. the required total bedding layer thickness is b/n 14cm & 60cm.Two layers of
bedding may be necessary to satisfy filter criteria where acceptable riprap is not available at a
reasonable cost, soil cement or shell compacted concrete can be considered as an alternative.

Down stream slope: The protection needed on d/s face will be governed by the extent of
exposure to runoff, wind, general weathering, and tail water washing. The potential for erosive
damage can be reduced by using the proper thickness of free draining surfacing (blanket of
gravel or broken rock) or grass cover.

If part of the d/s slope face is subject to submergence by law water, a filter protected riprap layer
or rock fill toe zone may be placed to control erosion. The table below shows slopes
recommended by Terzaghi.
Type of Material u/s slope (V:H) D/s slope (V:H)
Homogeneous, well graded material 1:2.5 1:
Homogeneous, coarse silt 1:3 1:2.5
Homogeneous clay or silt clay
i) < 15m height 1:2.5 1:2
ii) > 15m height 1:3 1:2.5
Sand or sandy gravel with silty core 1:3 1:2.5
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 Chapter 3 – Design of Embankment Dam

Sand or sandy gravel with R.C. core 1:2.5 1:2

3.5.5 Foundation seepage control

In designing to control foundation seepage, a reasonable balance must be attained between the
drainage and provisions for reducing flows. The conditions peculiar to each site will dictate the
optimum combination of protective measures.
 Grouting
 Impervious Blankets
 Cutoff
 Toe Drains
 Relief Wells

3.5.6 Spillway location

The spillway and discharge channel should be kept clear f the embankment. Spillway are
therefore generally built on natural ground with the channel by passing the flank of he dam and
discharging to a stilling basin care of the d/s toe. The alternative is to use a drop shaft-type
spillway located within the reservoir and discharging via and out let tunnel or culvert.

3.5.7 Seepage Analysis

Filter design: is a critical item in the design of earth fill dams. Seepage occurs at various places
in an earth fill dam and its foundation: from the core of the dam to the d/s shell; from the u/s
portion of a dam of homogeneous section to the chimney drain, from the u/s face of the dam to
riprap slope protection etc.

The phereatic line: if this line of saturation is allowed to intersect the d/s slope above the toe, a
serious sloughing will always occur unless prevented by construction of toe drains or filters or
rock fill toe.

3.6 SLOPE STABILITY ANALYSIS

Failure of an embankment dam can result from instability of either the upstream or down stream
slopes. The failure surface may lie with in the embankment or may pass through the embankment
and the foundation soil. The critical stages in u/s slope are at the end of construction & during
rapid draw down. The critical stages for the d/s slope are at the end of construction & during
steady seepage when the reservoir is full. Stability of an embankment is determined by its
resistance to shearing stresses that may result from external loads (such as reservoir pressure &
earth quake) and internal or body force.

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 Chapter 3 – Design of Embankment Dam

There are several methods for analyzing the stability of embankment dams: Of these, the limit
equilibrium methods are the ones most commonly used. In these methods, a number of failure
surfaces are analyzed to determine their factors of safety. The minimum of these values is taken
as the factor of safety for the slope under consideration. The failure surface corresponding to the
minimum factor of safety, F is termed the ‘‘critical failure surface’’. Commonly recommended
methods of stability analysis are:

(i) The method of slices (have further two types)

1. ** The standard method of slices (Swedish method of stability analysis): which
assumes that the forces acting on the sides of a slice do not affect the maximum shear
strength which can develop on the bottom of the slice and forces due to pore pressures
and, the intergranular forces on one side of a given slice are assumed to be equal (in
magnitude) and opposite (in direction). By this method of analysis, one obtains a
conservative value of the factor of safety (i.e safer, due to neglect of the inter granular
forces and pore pressures acting on the sides of slices)

2. ** Alternatively the factor of safety for the chosen slip surface is computed using
Taylor’s ‘‘Modified Swedish Method. This method is graphical and it considers inter
granular force and the pore pressure. It should be used for final stability analysis in all
major embankment dams.

(ii) The wedge method:

In such circumstances, an accurate stability analysis can be carried out by dividing the
trial sliding mass into two or three blocks of soil and examining the equilibrium of each
block.

Other important points:

 Width of slices is selected in such a way that the width of the arch should be approximately
equal to the arc length of the bottom of that specific slice.
 6 to 12 slices are usually sufficient to do stability analysis,
 When there is no pore pressure given and when it is found difficult to estimate, you can take
the submerged unit weight (for the normal component) and the saturated unit weight (for the
tangential component) to estimate the factor of safety.
 To get a critical failure surface (both for upstream and downstream), one can use the fellinius
graphical method of drawing.
 An earth dam should be treated as a flexible structure for determining dynamic pressure due
to earthquake. However, a simple method to account for earthquake forces in the design of
structures is based on seismic coefficients.
 The overall equation for factor of safety considering both seismic and pore pressure is as
follows.
 cL   N  U   h * T  * tan 
Fs 
 T   h N 
Where
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 Chapter 3 – Design of Embankment Dam

- c , corresponds to unit cohesion

- ∆L is length of the slice
- N is normal component of the weight (Wcosά).
- U pore water pressure
- T tangential component of the weight (Wsinά)
- Φ angle of internal friction
- αh horizontal seismic coefficient

 Steps of embankment dam stability analysis, if only the dam section view and dimensions,
and soil characteristics are provided:
Draw the pheratic line (put the entire seepage analysis result, the flow net)
Find critical failure surface which fits with your selected u/s or d/ slope.(use Fellinius
graphical method etc)
Divide the critical section in to slices
Measure their corresponding angle with the vertical
Estimate the areas, percentage wetted, percentage of slices lying in different soil
types, identify soil characteristics etc
Identify load combinations and do the analysis as per the condition of combinations.
Repeat most of the steps again and again if the dam is found not safe.
After all these steps, one will have such kind of section.

Design Example: Stability analysis:

Determine the factor of safety for the slip surface shown in Fig. below for sudden drawdown condition
with the following properties of the embankment material.
Saturated weight = 21.0 kN/m3
Submerged weight = 11.0 kN/m3
Cohesion = 24.5 kN/m2
Angle of internal friction, Φ’= 35°
Angle ά, arc length and area of different slices are given in the first four columns of table.

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 Chapter 3 – Design of Embankment Dam

Fig. Slip surface for the above Example

Solution
Since pore pressures are not known, the driving force (T-component) and the resisting force (N-
component) are calculated using saturated and submerged weights, respectively, and are shown in Table.

3.7 CAUSES OF FAILURE OF EARTH DAMS

Earth dam failures are caused by improper design frequently based on insufficient investigation
and Lack of control and maintenance. The various causes may be grouped in to the following
three broad categories:-
i. Hydraulic failure
ii. Seepage failure
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 Chapter 3 – Design of Embankment Dam

iii. Structural failure

Hydraulic failure: - Caused by surface erosion of the dam by water. They include
washout from overtopping, wave erosion of upstream face, scour from the discharge
of spillways & erosion of the d/s slope by rain.
Seepage failure: - un controlled or concentrated seepage through the dam body or
through the foundation may lead to piping and sloughing and subsequent failure of
the dam.
Structural failure: - Consists of foundation slide and or embankment slide.
The following critical conditions must be analysed:
1) end of construction (both slopes)
2) steady state, reservoir full (d/s slope critical)
3) rapid draw down (u/s slope critical)
4) seismic loading to 1, 2, and 3 if appropriate

3.8 ROCKFILL DAMS

3.8.1 Introduction

Rock fill in its various forms dumped, compacted in layers hand-placed cobbles and masonry,
equipment-placed masonry, and wet masonry- has been known since ancient times as a useful,
reliable, and durable construction material. Rock fill dams have been successfully constructed to
great heights worldwide since 1900s.

The disadvantage of rock fill lie in the difficulties in controlling the gradation of crushed rock,
e.g. from excavations of tunnels, and in the construction and post construction settlements, which
are relatively high. This can result in interface problems where rock fill shoulders are adjacent to
a compressible clay core. Other disadvantages are:
Shear strength- cohesion less materials are used
Permeability – rock materials are highly permeable
On the other hand, the basic and very important function of rock fill is to provide structural
support for whatever types of impervious zone in the dam.

3.8.2 Design considerations

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 Chapter 3 – Design of Embankment Dam

 Rock fill sources

 Physical properties of source rock
 Zoning
 Foundation requirement
 Lift thickness and characteristics
 Compaction
 Slope stability
 Drainage
 Embankment section
 Settlement
Homework: Stability analysis:
Given conditions:
 Material properties of both the embankment and the foundation soil

γ emb = 21KN/m3 γ foun = 20KN/m3

Cemb = 85KN/m3 Cfound = 77KN/m3

emb = 5 0
found = 20
 Embankment slope of (3:1)
 10 slices with their corresponding width, height share both in the embankment and in
foundation ,base inclination  (the magnitudes are written in the computation table)
 Use standard slice method (swedish method)
 Analysis is about to be performed at the end of construction case.
The shear strength of both soil are characterized by total friction angle greater than zero. For this specific
question all water pressure are zero, because it is assumed that there is no external water.
Show the detail procedure and calculation. Refer to the table below for the major characteristics and
measurements of the dam body.

Sliding Hight Slope of slip

surface Slice Width (Havg) surface (α)
in No (B),m m degree
EMBANKME

1 17.4 4 48
NT

2 13.4 11 43

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 Chapter 3 – Design of Embankment Dam

3 11 16.5 37

4 11.3 19.8 31

5 8.5 21 24
19.5
6 7.9 1.5 18
15.9
FOUNDATION

7 8.8 4 11
12.2
8 6.7 5.5 4
7.9
9 6.7 5.2 -5
3
10 5.2 2.4 -15

Soil characteristic:

Properties of the embankment Properties of the foundation

Y emban= 21 KN/m3 Y foun= 20 KN/m3
Cemban= 85 KN/m3 Cfound= 77 KN/m3
Y emban= 5 degree Y found= 2 degree

3.9 ROLLER COMPACTED CONCRETE DAMS (RCC DAMS)

In the 1970’s different soils and concrete materials developed a new method of dam
construction. The development of the roller compacted concrete dam started both in the United
States, England and other countries almost at the same time. But the technical approaches were
different in each of the countries and the development went in various directions.

The interest in this new dam construction concept was driven by the idea of building dam faster
than by conventional methods with less expensive materials. The two main considerations.
Economy and speed of construction, were demonstrated through construction of the early dams
around 1980 , and cleared the way for this new method of construction which has been proved to
be one of the most striking development in Hydropower the last 20 yrs .The result is that the
RCC dam today are competing with both rock- fill dams and conventional gravity or arch
concrete dam .At the end of 1996 RCC dam have been completed or are under construction in
approximately 23 countries around the world.

The concept of the RCC construction method is to combine the advantage of the rapid placing
techniques of the rock- fill dams and the use of material properties (strength and durability) of
concrete.
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 Chapter 3 – Design of Embankment Dam

This lead to a relatively simple definition of the technique;

A conventional concrete gravity or arch dam, using the construction method of a rock-fill dam.

The RCC is a “Zero-slump “concrete which easily can be vibratory rollers in its unhardened
state. The properties of hardened RCC are similar to those of the traditionally placed RCC.
In addition to cement, the lion’s share of the cementation content has been a pozzolan; mostly
flash from a coal fired plant nearby. The use of pozzolan is the aspect which makes this
construction method interesting possible. The pozzolan has effect of lower the speed of hydration
of the concrete. Which further prevent the temperatures developing beyond acceptable limits in
the dam during the construction period. These allows a higher speed of construction. At the same
time cheap pozzolan replace the expensive cement and thereby represent lower cost of the
materials.
The concrete gravity dam shares with the embankment the attributes of a simple concept and
adaptability, but mass concrete construction method remain essentially as they were in the1930’s
the volume instability of mass concrete due to thermal effects imposes sever limitations on the
size and rate of concrete pour, causing delay and disruption through the need to provide
contraction joints and similar design features. Progressive reductions in cement content and
partial replacement of cement with PFA have served only to contain the problem mass concert
construction remains a semi continuous and labor-intensive operation of low overall productivity
and efficiency.
In some circumstances the technical merits of the gravity dam and the embankment may be
evenly balanced, selection resting on estimated construction cost. Economic advantage will
almost invariably favor the embankment, particularly if constructed in compacted rock fill. In
some instance, however, factors such as locating spillway of sufficient capacity etc.. may
indicate the concrete gravity dam as being a preferable design solution, provided that the cost
differential lies within acceptable limits. Despite incentive to develop a cheaper concrete gravity
dam
The problem of optimizing concrete dam construction can be approached is several ways in the
absence of progress toward an ideal cement and a dimensionally stable concrete the most
promising lines of approach may be classified as follows;
1. A reappraisal of design criteria, particularly with regard to accepting modest tensile
stresses

2. The development of improved mass concretes through the use admixtures to enhance
tensile strength and to modify stress –strain response, and /or the use of modified cements
with reduced thermal activity;

3. The development of rapid continuous construction techniques based on the use of special
concretes.

The concept of construction using RCC, first developed in the 1970’s, is based primarily on
approach 3 several variants of RCC have now been developed and offer the prospect of
significantly fastest and cheaper construction for large gravity dams.

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 Chapter 3 – Design of Embankment Dam

3.10 Best features of RCC

 Rapid and continuous construction
 Economical and using local materials
 Integrated spillway
 Strength and durability
 Energy dissipaters
 Admixtures and pozolanas – delivering strength
 Lower speed of hydration there by prevent the temperature not to rise beyond the
limit
 Much vertical water load on the u/s slope batter.

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