mohanad.alfach (4)

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Seismic Response and Design of Dams
Damascus University MSc in Disaster Risk
Management

• Dr. Mohanad Al affach
• 2016

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TABLE OF CONTENTS PART. 1 Introduction and
Basic Concepts about Dams PART. 2 Requirements
for Seismic Design of Dams PART. 3 Seismic
Response of Earth and Concrete Gravity Dams PART.
4 Simplified Methods for Seismic Design of Dams
and Numerical Applications PART. 5
Instrumentation, Inspection and Monitoring
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1.1- INTRODUCTION A dam can be defined as a
structure built across a stream, river or estuary
to retain water. Dams are made from a variety of
materials such as rock, steel and wood as
presented in the next sections herein. For
centuries, dams have provided mankind with such
essential benefits as water supply, flood
control, reaction, hydropower, and irrigation.
They are integral part of society's
infrastructure. In the last decade, several
major dam failures have increased public
awareness of the potential hazards caused by
dams. Because of several disasters in recent
years, the safety of dams has received increasing
attention throughout the world. Governments at
all levels have come to recognize and, in many
cases, to accept their responsibilities in this
area.
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As example, the Dam Safety and Security Act of
2003 indicates that out of 78,000 dams in the
United States, 10,000 have a high-hazard
potential, meaning that their failure could
result in loss of life or severe property damage.
In today's technical world, dam failures are
rated as one of the major "low probability, high
loss" events. In this concern, it is important
to mention that underestimating the dam seismic
response could put downstream populations at an
unacceptable or unknown level of risk. Finding
the elements that contribute to stability is
vital, and key parameters of the analysis and
their level of uncertainty must be identified, as
well as the sensitivity of the results to various
parameters. There must be good communication
between all interested parties, including the
materials engineers, seismologists, structural
engineers, geotechnical engineers, geologists,
and management so that the use of input from each
group and the effect on results is understood.
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In addition, management must have confidence in
results on which decisions are based. At the
conclusion of an analysis, the analysts and
decision-makers must know how reliable the
analysis really is. Accordingly, the provisions
presented in this course are applicable to small
to intermediate size earth and rock- fill dams as
classified in some international codes. Table.1.
shows the definition of small intermediate and
large dams as per IS 11223-1985. While many of
the provisions of presented herein will be
applicable to large earth dams, the design
requirements for such dams are, in general, more
strict than those included herein
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Table.1 Size Classification of Dams (IS
1123-1985) (Size classification is greater of
that indicated by either of the two criteria
given in this Table)
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1.2- STRUCTURE OF DAMS AND BASIC
DEFINITIONS Figure.1. shows the structure of dam
along with its main components as follows
Fig.1. Structure of Dam
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Basic Definitions 1. Axis of the dam is the
line of the upstream edge of the top (or crown)
of the dam. The axis of the dam in plan is also
called the base line of the dam. The axis of the
dam in plan is usually straight. 2. Length of
the dam is the distance from one abutment to the
other, measured along the axis of the dam at the
level of the top of the dam. 3. Structural
height of the dam is the difference in
elevations of the top of the dam and the lowest
point in the excavated foundation. It, however,
does not include the depth of special geological
features of foundations such as narrow fault
zones below the foundation. In general, the
height of the dam means its structural
height. 4. Toe and Heel The toe of the dam is
the downstream edge of the base, and the heel is
the upstream edge of the base.
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5. Maximum base width of the dam is the maximum
horizontal distance between the heel and the toe
of the maximum section of the dam in the middle
of the valley. 6. Hydraulic height of the dam
is equal to the difference in elevations of the
highest controlled water surface on the upstream
of the dam and the lowest point in the river
bed. 7. Galleries small rooms like structure
left within the dam for checking operations. 8.
Spillways It is an arrangement near the top to
release the excess water of the reservoir to
downstream side. 9. Abutment Sides of the
valley on which the structure of the dam rest.
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• 1.3- TYPES OF DAMS, SALIENT FEATURES AND GENERAL
• CONSIDERATIONS
• This part describes briefly the type of dams,
  salient features and adaptability of the
  different types of dams to outline the important
  characteristics which influence their choice for
  a particular site.
• 1.3.1- Classification of Dams
• Classification based on function
• ? Storage Dam
• ? Detention Dam
• ? Diversion Dam
• ? Coffer Dam
• Debris Dam

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• Classification based on hydraulic design
• ? Overflow Dam/Overfall Dam
• Non-Overflow Dam
• Classification of dams based on material of
  construction
• ? Concrete Dams
• Earthen Dams
• The usual types of dams based on structural
  behaviour can be summarized as follows
• Concrete Gravity Dams
• (2) Roller-Compacted Concrete(RCC) Dams
• (3) Concrete Arch Dams
• (4) Concrete Buttress Dams
• (5) Embankment Dams
• (6) Rock-fill Dams (7) Other
  Dams

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1.3.1.1- Concrete Gravity Dams Basically,
gravity dams are solid concrete structures that
maintain their stability against design loads
from the geometric shape and the mass and
strength of the concrete. Generally, A
cross-section (or slice) through a gravity dam
will usually look roughly triangular. Gravity
dams are suited to sites with either wide or
narrow valleys, but they do need to be built on
sound rock. These hold the water back because of
their own weight as shown in Figure.2. Moreover,
they are particularly appropriate across valleys
with very steep side slopes where earth dams
might slip and are usually cheaper than earth
dams if suitable soils are not available for
their construction. It is useful to mention that
when good foundations are available, gravity dams
can be built up to any height. It is the most
lasting type of dams, and requires little
maintenance.
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Fig.2. Gravity Dams
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The choice of type best suited to a particular
location or use is a matter on which experienced
engineers differ considerably it is quite often
purely a matter of judgment and experience.
However, an intelligent study of existing
conditions and requirements will assist
materially in the choice. Safety, of course, is
the first consideration. It is impossible to
build with safety some types of dams if certain
foundations and other characteristics of the site
exist. Consideration of these factors will often
decrease considerably the number of possible
types from which to choose. The principal cost
of structure, as affected by the availability and
price of construction materials and other
characteristics of the site, is perhaps of next
importance.
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The choice of type is often limited by the funds
available. It will sometimes be found that the
difference in cost between an expensive,
permanent dam and inexpensive structure of
adequate safety but of short life and high
maintenance charges, in general, the most
permanent dam will be found to be the most
economical, and it is usually adopted for
ordinary sites, unless the structure is for
temporary use, or if sufficient funds are not
available. In a few words, there is no type of
dam more permanent than one of solid concrete,
nor does any other type require less for
maintenance. It is adaptable to all localities,
but its height is limited by the strength of
foundations, an earth-fill dam has frequently
been found to be more economical, particularly
when a dam of great height is required, because
the earth-fill dam does not have to rest on a
rock foundation.
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An earth dam will almost always cost considerably
less than any form of gravity concrete dam, if
materials for the former are found con-venient to
the site. Therefore, if conditions admit of an
embankment, that type of dam is usually to be
preferred.
1.3.1.2- Roller-Compacted Concrete (RCC)
Dams Roller-compacted concrete gravity dams
involve a new construction technique designed to
reduce material and labour costs. A fairly dry
concrete mix is put into place and compacted
using similar methods to those employed in the
construction of embankment dams as shown in
Figure.3. It uses low cement contents so result
in cost reduction as well as minimizing the
problem of hydration heat control.
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Fig.3. Roller-Compacted Concrete (RCC) Dams
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1.3.1.3- Concrete Arch Dams Arch dams are made
from concrete. They are curved in the shape of an
arch, with the top of the arch pointing back into
the water. An arch is a strong shape for
resisting the pushing force of the water behind
the dam. Arch dams are usually constructed in
narrow, steep sided valleys. They need good rock
for their foundations, and for the sides of the
valleys, to resist the forces on the dam as shown
in Figure.4. Unlike concrete gravity dams, whose
load bearing capability is achieved simply by
their dead load, the thin concrete shell of arch
dams transmits vertical and horizontal loads into
the foundation and abutments.
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Fig.4. Concrete Arch Dams
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Excellent homogeneous rock conditions as well as
appropriate ground treatment measures prior to
concrete placement are therefore imperative to
achieve a sound interface between bedrock and dam
concrete. This type is adaptable when the length
is small in proportion to the height, and when
the sides of the valley are composed of good rock
which can resist the end thrust. Under
favourable conditions, it contains less material
than other concrete types, and being equally
permanent, it is usually adopted where conditions
permit. Unfortunately, few sites are suitable
for this type of dam. The weight of arched dams
is not counted on to assist materially in the
resistance of external loads. For this reason,
uplift on the base is not an important design
factor.
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1.3.1.4- Concrete Buttress Dams Buttress dams
are made from concrete or masonry. They have a
watertight upstream side supported by triangular
shaped walls, called buttresses. The buttresses
are spaced at intervals on the downstream side.
They resist the force of the reservoir water
trying to push the dam over. The buttress dam was
developed from the idea of the gravity dam,
except that it uses a lot less concrete due to
the clear spaces between the buttresses. Like
gravity dams, they are suited to both narrow and
wide valleys, and they must be constructed on
sound rock as shown in Figure.5.
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Fig.5. Concrete Buttress Dams
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1.3.1.5- Embankment Dams Embankment dams are
massive dams made of earth or rock. They rely on
their weight to resist the flow of water as shown
in Figure. 6. Embankment-type dams have been
classified in a number of different ways,
classification generally recognize 1. The major
material containing the embankment, either earth
or rock 2. The method by which the material were
placed in the embankment 3. The geometric
configuration or internal zoning of the
cross-section. Embankment dams are constructed
of natural materials obtained from borrows and
quarries and from waste materials obtained from
mining and milling operations. The two primary
types are the earth-fill dam, an embankment dam
in which more than one-half of the total volumes
is formed by compacted or washed fine-grained
material, and the rock-fill dam in which more
than one-half of the total volume is formed
by compacted or damped previous natural or
quarried stone.
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Fig.6. Embankment Dam
A homogeneous rolled-earth dam is entirely
constructed of one type of material but may
contain an impermeable concrete or clay core or
upstream face, or sometimes with a hydraulic fill
to produce a impermeable core and a drain layer
to collect seep water as shown in Figure.7.
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A zoned-earth dam has different parts or zones of
different material, typically a locally
sufficient shell with an impermeable clay core.
Modern zoned-earth embankments employ filter and
drain zones to collect and remove seep water and
preserve the integrity of the downstream shell
zone. Rolled-earth dams may also employ an
impermeable facing or core in the manner of a
rock-fill dam. Furthermore, when plenty of
material are convenient to the site, earthen dams
can usually be built for considerably less cost
than any form of concrete gravity dam. The use of
this type, however, is often limited by necessity
of providing a more suitable spillway for the
passage of floods. It is not safe to allow water
to spill directly over the embankment, even if it
is well paved, unless the volume of the flood per
linear meter of crest is small.
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Fig.7. Earth-fill Dams
1.3.1.6- Rock-fill Dams These types of dams are
made of rocks and gravel and constructed so that
water cannot leak from the upper stream side and
through the middle of the structure. It is best
suited in the area where rocks are around as
shown in Fig. 8.
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Fig.8. (a) Rock-fill Dam. (b)
Differences in Confining Pressure in a Rock-fill
Dam Body
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• 1.3.1.7- Other Types of Dams
• Timber Dams
• A timber dam is the ideal temporary type
  although when well designed, constructed, and
  maintained, it may last 50 years or more.
  Maintenance charges, however, are very high,
  compared with those for other types.
• Steel Dams
• Only a small number of steel dams have been built
  all over the world. This type is claimed to be
  more economical than any type of concrete gravity
  dam.
• However, steel dams already built require
  anchoring to the foundation, a provision which is
  possible but not considered good practice for
  concrete gravity dams.

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Moreover, Various other types of dams have been
designed and built. These include noticeably
shaped masonry dams, the many forms of movable
dams, and others. These, however, may be
considered as structures of unique character,
suitable for special conditions not admitting of
comparison in the general sense.
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1.4- FAILURE MECHANISMS Major contributors to
earth dam failure are overtopping, piping, and
structural failure. Earthquake loading may
trigger any one of these failure modes or their
combinations. 1.4.1- Overtopping Overtopping
is defined as uncontrolled flow of water over the
crest of the dam or embankment. Non-overflow
(other than spillway) portions of a dam are not
usually designed for erosional effect of flowing
water, overtopping may lead to failure of the dam
due to excessive erosion or saturation of the
downstream slope. Adequate spillway capacity
should be provided to prevent such damages.
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1.4.2- Piping Due to the permeable nature of
earth dams, the dam body acts as pathways for
water seepage. If such seepage is uncontrolled
in terms of volume and velocity, and material
used in constructing the dam body are not
carefully selected, particles of soil with which
the dam body is constructed may be taken into
suspension by seepage water and carried away. In
order to prevent such an occurrence, there are
some recommendations that should be taken into
consideration as follows (a) the seepage
gradient is kept well below the critical
gradient, (b) the particle size distribution of
the filter material used in constructing the dam
body is carefully chosen to meet the filter
criteria, and
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(c) hydraulic stability of dam core and
potentially distribution of fine-grained soils
within foundation is demonstrated with pin-hole
tests.

• Additionally, there should be appropriate seepage
  control measures at the contact between dam and
  foundation such as
• (a) drainage blanket for soil foundation,
• (b) removal of rock mass affected by excessive
  cracking or jointing at dam-foundation interface
  before dam construction, and
• (c) ensuring absence of slopes steeper than 10
  vertical to 1 horizontal at dam-foundation
  interface.

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• 1.4.3- Structural Failure
• Structural failure includes failure of upstream
  or downstream slopes of the dam, as well as
  cracking, deformation and settlement of the dam
  body that may lead to overtopping or a piping
  failure.
• Moreover, it is important to mention that the
  possible damaging effects of earthquakes on earth
  dams and embankments include
• Slope failure because of inertial loading and/or
  softening of materials strength or liquefaction.
• Fault displacement under the foundation.
• ?Crest settlement of dam caused by settlement or
  by earthquake
• generated water waves in the reservoir.

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• Permanent deformation of foundation soils or dam
  body.
• Sliding failure of an embankment composed of weak
  or liquefiable soils.
• ?Piping and erosion.

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PART. 2 Requirements for Seismic Design of
Dams 2.1- TYPES OF DAM FOUNDATIONS (PROVISIONS
AND FEATURES FOR DESIGN) Embankment dams can be
constructed on foundations that would be
unsuitable for concrete dams. The foundation
requirements for earth-fill dams are less strict
than those for rock fill dams (Engineering
Foundation,1974). Foundations for embankment
dams must provide stable support under all
conditions of saturation and loading without
under-going excessive deformation or settlement.
The foundation must also provide sufficient
resistance to leakage where excessive loss of
water would be uneconomic. Foundations are
extremely variable in their geologic,
topographical, strength, and water retaining
characteristics.
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Each is unique and is an integral part of a dam.
During design and Construction, the foundation
characteristics can be modified and improved by
such treatments as excavation, shaping, curtain
and consolidation grouting, blanketing,
densification, installation of sheet piling etc.
These various forms of treatment are mainly for
the purpose of (1) strengthening, (2) safely
controlling seepage and leakage, and (3)
limiting the influence of the foundation on
embankment deformations. However, for an
existing dam one can only evaluate the
effectiveness of the treatment from the
construction record and observable
performance. Based on strength and resistance to
seepage and leakage, foundations can be typified
as (1) Rock, (2) sand and gravel, and (3) silt
and clay or combination.
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Earth-fill dams have been improved to all three
of these types of foundations. Type2 and 3 have
generally been determined unsuitable for faced
rock-fill dams. Type3 has been determined
unsuitable for impermeable core rock-fill dams
without extensive foundation excavation and
treatment. The foundation types have been
treated in a variety of ways depending on the
designers adaptability and objectives and the
type and configuration of the dam. The
foundations of many existing dams will not have
received any special treatment and present safety
concerns. Where treatment was afforded, it
varies under the different zones of the dam,
depending on the intended functions of the zones
and the foundation type.
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Foundations have been treated for strengthening
by (1) excavating weak materials and
formations (2) consolidation grouting (3)
pre-wetting collapsible soils (4) installing
vertical drain to accelerate consolidation and
accompanying strength gain during embankment
placement and (5) to limited extent, vibratory
densification. Foundations containing saturated,
fine, cohesion-less sand of low density are
suspect, especially in regions of higher
seismicity, because of tendency of the sand to
collapse and liquefy during long-duration ground
shaking from earthquakes. Many foundations of
this type have probably received no treatment for
such a condition. Dams built of cohesive soils
on stable foundations have been found to perform
quite well during earthquakes, and they pose a
much smaller hazard than do dams constructed of
or founded on loose, liquefiable, cohesion-less
soils (Seed et al. 1977).
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Even though they may remain stable during an
earthquake, cohesive soil embankments may suffer
permanent deformations as a consequence of
earthquake shaking, which may take the form of
bulging of the slopes, physical movement of the
dam, and possibly settlement of the top. 2.2-
DEFENSIVE DESIGN MEASURES Defensive design
features should be incorporated in the foundation
and embankment design of new dams regardless of
the method of seismic analysis. These features
according to (USACE 2004) include 1. Additional
dam height to accommodate the loss of crest
elevation due to deformation, slumping, and fault
displacement. 2. Crest details that will
minimize erosion in the event of overtopping.
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3. Wider transition and filter sections as a
defense against cracking. 4. Use of rounded or
sub-rounded gravel and sand as filter
material. 5. Adequate permeability of the filter
layers. 6. Near vertical chimney drain in the
center portion of the embankment. 7. Zoning of
the embankment to minimize saturation of
materials. 8. Wide impervious cores of plastic
(non-brittle) cohesive fine-grained soils to
accommodate deformation. 9. Well-graded core and
uniformly graded filter materials to ensure
self-healing in the event cracking should
occur. 10. Stabilization of reservoir rim slopes
to provide safety against large slides into the
reservoir. 11. Ground improvement or removal and
replacement of foundation material to mitigate
liquefaction potential. 12. Stabilization of
slope adjacent to operating facilities to prevent
blockage from slide associated with the
earthquake. 13. Flaring embankment sections at
the abutment contacts. 14. Installation of
suitable features to prevent piping through
earthquake generated seepage cracks.
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• 2.3- SEISMIC DESIGN APPROACH FOR DAMS AND
  EMBANKMENKS
• It is convenient to mention that understanding
  potential dam failure modes is an essential step
  in appreciating dam response to earthquakes and
  validating current analysis methods.
• For the design of intermediate dams or
  embankments whose failure entails unacceptable
  level of risk, a two-stage seismic design
  approach is usually adopted so that
• the dam or embankment remains operational
  following an earthquake that has a reasonable
  probability of occurrence during the service life
  of the facility with distress of a minor nature,
  and
• (2) the dam or embankment does not collapse
  following an earthquake that has a small
  probability or occurrence during the life of the
  facility.

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(1) usually corresponds to a probability of
exceedance of 50 over the operational life of
the facility and that for condition (2) is often
specified as that having a probability of
exceedance of 10 over the operational life of
the facility. Operational life of a dam or an
embankment is usually 50 to 100 years. However, a
longer operational life should be assumed in
situations such as (1) a water-retaining dam
that is not appropriately decommissioned at the
end of operational life or where there is an
unacceptable downstream risk in the event of a
dam break, and (2) a tailings dam retaining
radioactive or other wastes that may pollute
groundwater or threaten downstream public health
and safety in the event of dam break.
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2.4- SEISMIC EVALUATION AND INVESTIGATION OF DAMS
AND EMBANKMENTS It is important to mention that
the seismic evaluation and design of earthen dams
should involve the participation of geologists,
seismologists, and geotechnical engineers. The
entire effort can be grouped into four main
areas field investigations, site
characterization, analysis, and evaluation. The
investigations and site characterization should
be thoroughly evaluated to establish the nature,
extent, and in-situ geotechnical properties of
the materials in foundation, embankment, or dam
being investigated. In this concern, an
assessment of site geologic and geotechnical
conditions is one of the important aspects of the
dam safety evaluation. Evaluation of safety of
new and existing dams requires, among other
things, that its foundation has been adequately
examined, explored, and investigated.
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The investigation should include the
following Seismological Investigations Studi
es should be made of the past occurrence of
earthquakes in the general region of the site,
and on this basis estimates are made of the
probability of future earthquakes. In order for
this approach to be valid, a sufficiently long
seismic history must be available.
Geotechnical Investigations Investigations are
made of geological formations, soil deposits and
rock in and around the construction site for
assessing their behaviour during earthquake
shaking, and how they might affect the ability of
a structure to resist earthquake including
evaluation of liquefaction potential, if
appropriate.
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The investigation should focus on topics
including 1. Topographic conditions. 2.
Description of geology. 3. Composition and
structure of foundation soils, soils from borrow
area and bedrock. 4. Principal engineering
properties of the rocks and soils including grain
characteristics, plasticity, compaction
characteristics, shear strength, dispersivity and
hydraulic properties. 5. Geotechnical
investigation would typically include drilling
and sampling, in-situ testing (piezocone
penetration test, Standard Penetration Test or
Field Vane Shear Test, seismic velocity
profiling) as appropriate. Small and
intermediate dams or embankments whose failure
entails negligible risk may be designed for
earthquake ground accelerations as specified in
the later parts herein.
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However, site-specific seismic assessment should
be performed for all projects located in active
fault zones. 2.5- LOCATION OF DAMS AND DESIGN
MEASURES Earth dams and embankments should be
ideally located away from any potentially active
fault or an area underlain by liquefiable or
sensitive soils or abutments prone to static or
seismic instability. Earthquake damage to
embankment can be due to actual ground rupture
beneath the embankment and/or seismic shaking.
Failure of a dam due to ground rupture is
possible only when the dam is built over an
active fault zone or across reactivated or newly
activated landslide zone. The location of the
dam over the fault zone should be reviewed at the
time of site selection and appropriate measures
should be taken in the design and construction of
a dam over a fault. By far the more common
problem in a dam design is to ensure that the dam
will be stable under predicted levels of seismic
shaking.
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• Also, If necessary, an earth dam or an embankment
  may be constructed over a fault or at a site
  triggered by potentially liquefiable or sensitive
  soils only if
• the dam is designed for the displacements and
  other dynamic effects of an earthquake that is
  likely to occur, and
• (b) potential failure is unlikely to lead to any
  loss of life and the risk
• associated with such a selection of site is
  acceptable.
• 2.6- FREEBOARD
• The freeboard of all embankment dams should be
  based on most extreme conditions expected for
  which the dam is designed.
• The maximum reservoir elevation is determined for
  the design flood, wind speed, fetch and expected
  wave run-up conditions.
• In general, overtopping of the dam is not
  acceptable.

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• Sufficient freeboard should be provided to avoid
  the possibility of overtopping by
• earthquake-generated water waves,
• settlement and permanent deformation of crest due
  to liquefaction which may cause densification or
  loss of stiffness of the materials or fault
  rupture.
• In addition, it may be prudent to use riprap or
  other crest details that will resist erosion by a
  succession of overtopping waves.
• It is recommended to provide a freeboard of at
  least 2 to 3 of dam height, but not less than
  2m if there is a potential for occurrence of
  landslides near the dam abutment within the
  slopes of the reservoir margins or 1m if there is
  a negligible landslide potential near the dam
  abutments.

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2.7- LIQUEFACTION INFLUENCE Since soil
liquefaction has a major influence on the seismic
performance earth dams and embankments, this
section will highlight the significance of
considering liquefaction influence on
dams. Prior to the 1971 near-breach of Lower San
Fernando Dam, caused by liquefaction of the
embankments hydraulic fill section and sliding of
much of the upstream slope, seismic analysis of
embankment dams focused largely on the seismic
coefficient method, essentially just checking
the embankments yield acceleration against some
acceleration value imposed by building code or
local practice. The 1971 San Fernando Valley
earthquake made it obvious that more was needed.
Since then, the main focus of embankment-dam
seismic analyses has been assessment of
liquefaction potential of loose materials in the
foundation or embankment, and analysis of
post-earthquake stability.
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Liquefaction potential is most often assessed
using empirical correlations with in situ tests
such as the standard penetration test
or shear-wave velocity. Post-earthquake
strengths are estimated using laboratory tests
and/or additional empirical correlations with in
situ tests. There are rather severe difficulties
with either of these approaches. Laboratory
stress-strain tests can only deal with small
samples and cannot replicate the effects of
material heterogeneity or of certain large-scale
phenomena such as migration of excess pore-water
pressure from loose, liquefied material into
denser material, or formation of a water film at
the base of a low-permeability layer overlying a
layer of loose sand. In situ can be calibrated
against field behaviour, but the number of
earthquake case histories is limited, and there
is only so much information that can be gained
from back Predecisional draft - Internal agency
review not complete 11 analyzing from known
behaviour.
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For example, if a slope did not fail, that
information provides only a lower bound on the
strength of the soil. For larger embankments
potentially subjected to severe earthquake
loadings, deformation analysis is gaining
prominence in practice. It is recognized that
even a dam with an adequate post-earthquake
factor of safety can undergo large dynamic
deformations during the earthquake. Simple
Newmark-type sliding block analysis (or a chart
solution based on it) has been used since the
1960s, but advances in computing power have
allowed finite-element and finite-difference
codes such as FLAC and PLAXIS to be developed and
used. During an earthquake, the pore pressure
within saturated soil often increases if the
deposit is loose, sensitive or young and the
earthquake is of moderate to large magnitude and
intensity.
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The increase could be so large that the effective
stress may approach zero. As a result, frictional
soils may loose a substantial fraction of shear
strength leaving the soil to behave like a
viscous liquid. Such a phenomenon is referred to
as liquefaction. Although many deposits never
attain the state of zero effective stress, they
may deform substantially during earthquakes
leading to the development of liquefaction-like
failures. Liquefaction is therefore often
functionally assumed to be the state in which a
double amplitude shear strain of 5 develops.
This definition will be adhered to in this
commentary. The pore pressure generated in the
soil due to cyclic stresses depends mainly on the
state of packing (i.e., whether the soil is loose
or dense), geologic age of the deposit for
non-cohesive soils, and on plasticity and
sensitivity for cohesive soils.
57
Saturated cohesive soils usually do not attain
the state of zero effective stress during
earthquakes. Nevertheless, many such deposits
deform appreciably during earthquakes. As a
result liquefaction-like features develop within
such deposits. Among cohesive deposits, sensitive
soils of moderate to low plasticity are
especially vulnerable to liquefaction. Among
cohesion-less soils, dense sands or
well-compacted deposits that are of early to mid
Pleistocene age (or older) are not susceptible to
liquefaction. Holocene to late Pleistocene,
loose, saturated sands with relative densities of
up to 30-50 are more susceptible
to liquefaction. In other words, dam and
embankment failure under seismic loading may be
caused by liquefaction of embankment and/or
foundation materials. Dam and embankment
constructed using loose, un-compacted material or
those founded on liquefiable foundations are
prone to catastrophic flow failure.
58

• If an unacceptably large deformation of an earth
  dam or embankment is estimated because of
  liquefaction, considerations may be given for
  liquefaction potential mitigation measures such
  as
• In situ ground improvement (e.g., deep dynamic
  compaction, vibro-compaction, vibroreplacement,
  jet grouting, deep soil mixing, blast
  densification, sand and gravel piles)
• 2. Removal and replacement of liquefaction
  susceptible soil
• 3. Surcharging, dewatering and reinforcement.

59
2.7.1- Foundation Liquefaction of saturated
sandy soils in a dam foundation may be caused due
to pore water pressure build-up during earthquake
shaking. Liquefaction leads to loss of strength
and increase in settlement. Appropriate methods,
such as the one described in this document,
should be used to evaluate the liquefaction
potential. If the material is found to be
liquefaction susceptible, the deformation of the
embankment should be estimated also for the
design ground motion as presented in the next
sections herein.
60
2.7.2- Compaction The material of all new
embankments and dams should be compacted to a
density that will cause them to dilate rather
than liquefy during earthquake shaking. It is
recommended that the compacted density of
material should exceed 95 of Standard Proctor
Maximum Dry Density (SPMDD) for rail or road
embankments and 98 SPMDD for dam
embankments. Cohesive materials used within the
dam or embankment body should be placed with
moisture content 2 to 4 higher than the optimum
moisture content. For cohesion-less soils, a
relative density of 80 may be used as an
alternative indicator of the minimum compaction
requirement.
61
PART. 3 Seismic Response of Earth and Concrete
Gravity Dams In earth dams, seismic forces or
shaking can induce destabilizing deformation or
outright failure if not made earthquake
resistant. A permanent simplified procedure can
be adopted to estimate permanent horizontal
displacements of the dams using finite element
method that account for non-linear material
behavior and strength reduction due to
liquefaction or strain softening. It has been
shown ((Hatami, 2001) that the seismic
performance of earth dams has been related to the
nature and state of compaction of the fill
material. Concrete dams structural safety and
stability are jeopardized due to the hydrodynamic
load of the reservoir that is subjected to ground
motion.
62

• 3.1- FORCES ACTING ON DAMS
• Weight of the dam
• 2. Water pressure
• 3. Uplift pressure
• 4. Earth and Silt pressure
• 5. Wind pressure
• 6. Ice pressure
• 7. Thermal loads
• 8. Wave pressure
  9. Earthquake forces

63

• The above-mentioned forces fall into two
  categories as
• Forces, such as weight of the dam and water
  pressure, which are directly calculable from the
  unit weights of the materials and properties of
  fluid pressures and
• Forces, such as uplift, earthquake loads, silt
  pressure and ice
• pressure, which can only be assumed on the
  basis of assumption
• of varying degree of reliability.
• As for the second category of the forces, a
  special care has to be taken and confidence
  placed on available data, experience, and
  judgment.
• Moreover, It is convenient to compute all the
  forces per unit length of the dam.

64

• Many of the forces that must be considered in the
  design of the dam structure are of such a nature
  that an exact determination cannot be made.
• The intensity, direction, and location of these
  forces must be estimated by the designer after
  consideration of all available facts and, to a
  certain extent, must be based on judgment and
  experience.
• Weight of the dam
• Weight of the dam is considered as the main
  stabilizing force in a gravity dam. In this
  concern we may remark the following points
• Dead load (weight of concrete or masonry or
  both)
• (weight of such accessories as piers, gates
  and bridges).
• ? Weight of the dam per unit length is equal to
  the product of the area of cross-section of the
  dam and the specific weight (or unit weight) of
  the material.

65

• Unit weight of concrete (24 kN/m3) and masonry
  (23 kN/m3) varies considerably depending upon the
  various materials that go to make them.
• For convenience, the cross-section of the dam is
  divided into simple geometrical shapes, such as
  rectangles and triangles, for the computation of
  weights.
• The areas and centroids of these shapes can be
  easily determined.
• Thus the weight components W1, W2, W3 etc. (as
  shown in Fig.9) can be found along with their
  lines of action.
• The total weight W of the dam acts at the C.G. of
  its section.

66
Fig. 9. Calculation of dam weight
67
2. Water Pressure Conversely, water pressure on
the upstream face is the main destabilizing (or
overturning) force acting on a gravity dam.
Tail water pressure helps in the
stability. Although the weight of water varies
slightly with temp., the variation is usually
ignored. Unit Mass of water is taken as 1000
kg/m3 and specific weight 10 kN/m3 instead of
9.81 kN/m3. The water pressure always acts
normal to the face of dam. It is convenient to
determine the components of the forces in the
horizontal and vertical directions instead of the
total force on the inclined surface directly.
68
The water pressure intensity P (kN/m2) varies
linearly with the depth of the water measured
below the free surface y (m) When the upstream
face of the dam is vertical, the water pressure
diagram is triangular in shape with a pressure
intensity of ?wh at the base, where h is the
depth of water. The total water pressure per
unit length is horizontal and is given by PH1/2
(?wh)2 The water pressure acts horizontally at a
height of h/3 above the base of the dam.
69
Fig. 10. Water Pressure (Reservoir and Tail Water
Loads)
70
When the upstream face ABC (as shown in Fig.10)
is either inclined or partly vertical and partly
inclined, the force due to water pressure can be
calculated in terms of the horizontal component
PH and the vertical component PV. The
horizontal component is given as earlier and acts
horizontal at a height of (h/3) above the base.
The vertical component PV of water pressure per
unit length is equal to the weight of the water
in the prism ABCD per unit length. For
convenience, the weight of water is found in two
parts PV1 and PV2 by dividing the trapezium ABCD
into a rectangle BCDE and a triangle ABE. Thus
the vertical component PV PV1 PV2 weight of
water in BCDE weight of water in ABE. The
lines of action of PV1 and PV2 will pass through
the respective centroids of the rectangle and
triangle.
71
3. Uplift Pressure Water has a tendency to seep
through the pores and gaps of the material in the
body of the dam and foundation material, and
through the joints between the body of the dam
and its foundation at the base. The seeping water
exerts pressure. The uplift pressure is defined
as the upward pressure of water as it flows or
seeps through the body of dam or its
foundation. A portion of the weight of the dam
will be supported on the upward pressure of
water hence net foundation reaction due to
vertical force will reduce. The area over which
the uplift pressure acts has been a question of
investigation from the early part of this
century.
72

• One school of thought recommends that a value
  one-third to two-thirds of the area should be
  considered as effective over which the uplift
  acts. While, the second school of thought,
  recommend that the effective area may be taken
  approximately equal to the total area.
• According to the code of Indian Standards (IS
  6512-1984), we can summarize the following
  points
• There are two constituent elements in uplift
  pressure the area factor or the percentage of
  area on which uplift acts and the intensity
  factor or the ratio which the actual intensity of
  uplift pressure bears to the intensity gradient
  extending from head water to tail water at
  various points.
• The total area should be considered as effective
  to account for uplift.
• ?The pressure gradient shall then be extending
  linearly to heads
• corresponding to reservoir level and
  tail-water level.

73
Fig.11. Uplift Pressure
74

• In case of drain holes the uplift pressure at
  the line of drains exceeds the tail-water
  pressure by one-third the differential between
  the reservoir and tail-water heads. The pressure
  gradient shall then be extended linearly to heads
  corresponding to reservoir level and tail-water
  level.
• ?In case of a crack The uplift is assumed to be
  the reservoir pressure
• from the upstream face to the end of the
  crack and from there to
• vary linearly to the tail-water or drain
  pressure. appropriate tail-
• water pressure at the downstream face.
• ?Uplift pressures are not affected by
  earthquakes.
• 4. Earth and Silt Pressure
• ?Gravity dams are subjected to earth pressures on
  the downstream and upstream faces where the
  foundation trench is to be backfilled.

75
Except in the abutment sections in specific
cases, earth pressures have usually a minor
effect on the stability of the structure and may
be ignored.
Fig.12. Earth and Silt Pressure
76

• IS code recommends that
• Horizontal silt and water pressure is assumed to
  be equivalent to that of a fluid with a mass of
  1360 kg/m3, and
• (b) Vertical silt and water pressure is
  determined as if silt and water
• together have a density of 1925 kg/m3.
• 5. Ice pressure
• Ice expands and contracts with changes in
  temperature. In a reservoir completely frozen
  over, a drop in the air temperature or in the
  level of the reservoir water may cause the
  opening up of cracks which subsequently fill with
  water and freezed solid. When the next rise in
• temperature occurs, the ice expands and, if
  restrained, it exerts pressure on the dam.
• It is convenient to mention that good analytical
  procedures exist for computing ice pressures, but
  the accuracy of results is dependent upon certain
  physical data which have not been adequately
  determined.

77
Also, ice pressure may be provided for at the
rate of 250 kPa applied to the face of dam over
the anticipated area of contact of ice with the
face of dam. The problem of ice pressure in the
design of dam is not encountered in India except,
perhaps, in a few localities. 6. Wind
Pressure Wind pressure does exist but is not
often a significant factor in the design of a
dam. Therefore, wind loads may be ignored. 7.
Thermal Loads The cyclic variation of air
temperature and the solar radiation on the
downstream side and the reservoir temperature on
the upstream side affect the stresses in the dam.
Even the deflection of the dam is maximum in the
morning and it goes on reducing to a minimum
value in the evening. Measures for temperature
control of concrete in solid gravity dams are
adopted during construction. Therefore, thermal
are not significant in gravity dams and may be
ignored.
78
8. Wave Pressure The upper portions of dams are
subject to the impact of waves. Wave pressure
against massive dams of appreciable height is
usually of little consequence. The force and
dimensions of waves depend mainly on the extent
and configuration of the water surface, the
velocity of wind and the depth of reservoir
water. The height of wave is generally more
important in the determination of the free board
requirements of dams to prevent overtopping by
wave splash. An empirical method has been
recommended by (T. Saville) for computation of
wave height hw (m), which takes into account the
effect of the shape of reservoir and wind
velocity over water surface rather than on land
by applying necessary correction.
79
 
80
 
Fig.13. Wave Pressure on Gravity Dams
81

• 9. Earthquake Forces
• It is useful to take into consideration the
  following points
• An earthquake sets random vibrations (waves) in
  the earth's
• crust, which can be resolved in any three
  mutually perpendicular
• directions. This motion causes the structure
  to vibrate.
• ?The waves transfer accelerations to the
  foundations under the dam
• and causes its movement.
• ?Acceleration introduces an inertia force in the
  body of dam and sets
• up stresses initially in lower layers and
  gradually in the whole
• body of the dam.
• ?The vibration intensity of ground expected at
  any location depends
• upon the magnitude of earthquake, the depth
  of focus, distance
• from the epicenter and the strata on which
  the structure stands.

82

• The response of the structure to the ground
  vibration is a function of the nature of
  foundation soil materials, form, size and mode
  of construction of the structure and the
  duration and the intensity of ground motion.
• Earthquake causes impulsive ground motion which
  is complex and irregular in character, changing
  in period and amplitude each lasting for small
  duration.
• Earthquake is not likely to occur simultaneously
  with wind or maximum flood or maximum sea waves.
• The value of elastic modulus of materials,
  wherever required, may be taken as for static
  analysis unless a more definite value is
  available for use in such condition.
• Whenever earthquake forces are considered along
  with other normal design forces, the permissible
  stresses in materials, in the elastic method of
  design, may be increased by one-third.
• The earthquake force experienced by a structure
  depends on its own dynamic characteristics in
  addition to those of the ground motion.

83

• Response spectrum method takes into account these
  characteristics and is recommended for use in
  case where it is desired to take such effects
  into account.
• IS1893-1984 code specifies design criteria under
  earthquake condition, as per IS Code, for dams up
  to 100 m height, the seismic coefficient method
  shall be used for the design of the dams while
  for dams over 100 m height the response spectrum
  method shall be used.

84
Basic seismic coefficients (a0) and seismic zone
factors (F0) for different zones shall be taken
as given in Table.2. The design seismic forces
shall be computed on the basis of importance of
the structure I (Table.3) and its soil-foundation
system ß (Table.4). In Seismic Coefficient
Method the design value of horizontal
seismic coefficient (ah) shall be computed
as ah ßIa0
85
Table.2. Basic seismic coefficients (a0) and
seismic zone factors (F0) for different zones
86
Table.3. importance factor of structure I
87
Table.4. Soil-foundation system factor ß
88
 
Where H height of the dam in (m), B base
width of the dam in (m), ?m unit weight of the
material of dam in (N/m3), g acceleration due to
gravity in (m/s2), and Em modulus of elasticity
of the material in (N/m2).
89
Fig.14. Average acceleration coefficient Vs.
Natural period of vibration
90
Where a number of modes are to be considered for
seismic analysis ah shall be worked out
corresponding to the various mode periods and
damping and then design forces shall be computed.
If actual response spectra is available then the
same may be used directly instead of the above
equation.
3.2- EFFECT OF HORIZONTAL ACCELERTATION Effect
of Horizontal Acceleration causes two forces (1)
Inertia force in the body of the dam, and (2)
Hydrodynamic pressure of water. 3.2.1- Inertia
force in the body of the dam The inertia force
acts in a direction opposite to the acceleration
imparted by, earthquake forces and is equal to
the product of the mass of the dam and the
acceleration.
91
For dams up to 100 m height the horizontal
seismic coefficient shall be taken as 1.5 times
seismic coefficient ah at the top of the dam
reducing linearly to zero at the base. This
inertia force shall be assumed to act from
upstream to downstream or downstream to upstream
to get the worst combination for design.
Inertia force causes an overturning moment about
the horizontal section adding to that caused by
hydrodynamic force. For dams over 100 m height
the response spectrum method shall be used. The
base shear, VB and base moment MB may be obtained
by the following formulae VB 0.6Wah
MB 0.9Wh
ah Where W total weight of the masonry or
concrete in the dam in (N), and h' height of the
center of gravity of the dam above the base in
(m).
92
For any horizontal section at a depth y below top
of the dam shear force, Vy, and bending moment
My, may be obtained as follows
Vy Cv'VB My
Cm'MB
Fig.15. Coefficients C'v and C'm Vs. y/H
93
3.2.2- Hydrodynamic pressure of water To
include the earthquake-related hydrodynamic
effects in stability analysis of water retaining
dams, the hydrodynamic pressure, pey, at depth y
below reservoir water level may be estimated from
the relationship developed by (Zangar 1952) which
assumes that the dam is relatively rigid and
water is incompressible as follows Pey C (ah
/ g)? w h where C is hydrodynamic pressure
coefficient which varies with shapes of upstream
face and depth of water obtained from Fig.16. ?w
is the unit weight of water, and h is the height
of water surface above the base of the dam (depth
of reservoir) in (m).
94
Fig.16. Hydrodynamic Pressure Coefficient
(Zangar, 1952)
95
Limited experimental evidence that are available
in the literature (Memos et al. 2001) indicates
that these assumptions are reasonable for earth
dams as well. While hydrodynamic effects could
be significant for near vertical dam-water
interface, for earth dams and embankments its
influence is usually limited. The moment of
pressure about the joint up to which the pressure
is taken is given by Mey0.299 pey y2 where
Pey hydrodynamic shear in (N/m) at any depth y,
and Mey moment in (N.m/m) due to hydrodynamic
force at any depth y.
96
3.3- EFFECT OF THE HORIZONTAL ACCELERATION ON THE
VERTICAL COMPONENT OF RESERVIOR AND TAIL WATER
LOAD Since the hydrodynamic pressure (or
suction) acts normal to the face of the dam,
there shall be a vertical component of this force
if the face of the dam against which it is acting
is sloping, the magnitude at any horizontal
section being pev (pey2 - pey1) tan ? where
peV increase (or decrease) in vertical
component of load due to hydrodynamic force, Pey2
total horizontal component of hydro-dynamic
force at elevation of the section being
considered, Pey1 total horizontal component of
hydrodynamic force at the elevation at which the
slope of dam face commences, and ? angle
between the face of dam and the vertical.
97
Effect of Vertical Acceleration The effect of
vertical earthquake acceleration is to change the
unit weight of water and concrete or masonry.
Acceleration upwards increases the weight and
acceleration downwards decreases the weight. Due
to vertical acceleration a vertical inertia force
F aVW is exerted on the dam, in the direction
opposite to that of the acceleration. When the
acceleration is vertically upwards, the inertia
force F aVW acts vertically downwards, thus,
increasing momentarily the downward weights.
When the acceleration is vertically downwards
the inertia force F aVW acts upwards and
decreases momentarily the downward weight. For
methods of design (seismic coefficient up to 100
m and response spectrum over 100 m) Vertical
seismic coefficient (aV) shall be taken as 0.75
times the value of ah (of the respective method)
at the top of the dam reducing linearly to zero
at the base.
98

• PART. 4 Simplified Methods for Seismic Design of
  Dams and Numerical Applications
• 4.1- SEISMIC SLOPE STABILITY ASSESSMENT
• Seismic slope stability is influenced by the
  following two factors
• Cyclic stresses induced by earthquake shaking,
  and
• The cyclic stress-stain behavior of the materials
  within the
• body of the dam or embankment and that of
  foundation soils.
• Potential instability of an earth dam or an
  embankment during an earthquake may be due to the
  inertial effects or due to cyclic softening of
  soils.

99

• Techniques ranging from very approximate to very
  elaborate are available for seismic stability
  analysis of dam and embankment.
• These methods include
• Equivalent-static Stability Analysis
• Sliding Block Method
• Dynamic Analysis (Simplified or Rigorous)
• Seismic slope stability analysis often begins in
  a staged approach, which usually involves
  starting with a simpler analysis
  (equivalent-static) and progressing to more
  rigorous analyses (the sliding-block method and
  the simplified dynamic analysis) if appropriate.
• An earth dam or an embankment is usually
  considered safe if it is found safe by
  equivalent-static or the sliding-block method. In
  such cases a more sophisticated analysis is not
  usually undertaken.

100
On the other hand, if the sliding-block analysis
indicates a potential for instability, then
either a simplified dynamic analysis or a
rigorous dynamic analysis could be undertaken to
assess stability of the earth dam or embankment.
The designer may also skip the
equivalent-static or the sliding-block method and
proceed directly to simplified dynamic analysis
provided that high quality material-and
site-specific input parameters are available for
undertaking the dynamic analysis. It should be
noted that adoption of a more intricate
analytical procedure in the design should also
require detailed and appropriate characterization
of pre-failure undrained deformation behavior of
the soils within the embankment and foundation as
well as a suite of earthquake time histories
which the earth structure may reasonably be
expected to encounter during its design life.
101
4.2- EQUIVALENT-STATIC METHOD FOR SLOPE STABILITY
ANALYSIS For many years the standard method of
evaluating the safety of embankment dams against
sliding during earthquakes has been the
equivalent-static method of analysis.
Equivalent-static method of analysis involves the
computation of the minimum limit equilibrium
factor of safety by including in the analysis
static horizontal and vertical forces that
represent the inertial effects of earthquake
shaking. These equivalent-static forces are
usually expressed as a product of horizontal or
vertical seismic coefficients and the weight of
the potential sliding mass. The horizontal
equivalent-static force decreases the factor of
safety by reducing the resisting force and
increasing the driving force. The vertical
equivalent-static force typically has less
influence on the factor of safety. As a result,
it is often ignored.
102
Although the equivalent-static approach to
stability analysis is simple and straight forward
producing an index of stability (factor of
safety) which engineers are used to appreciating,
it suffers from many limitations as it can not
really simulate the complex dynamic effects
of earthquake shaking through a constant
unidirectional equivalent-static acceleration.
These limitations are well recognized (Terzaghi
1950, Seed 1966 and Marcuson 1981). Of particular
importance is the fact that in case of soils that
build up large pore water pressures or have a
degradation in strength of more than say 15 due
to the earthquake shaking the analysis can be
unreliable. As shown by Seed (1979) a number of
dams such as the Upper and Lower San Fernando
Dams, Sheffield Dam have in fact failed due to
earthquakes although the calculated factors of
safety were well above 1.0.
103
In equivalent-static analysis, the dynamic
(random) earthquake shaking is replaced by a
single constant unidirectional equivalent-static
acceleration. Slope stability analysis is
similar to that for static conditions except for
the application of horizontal and vertical
inertia forces over every portion of the
potentially unstable soil mass. This approach is
based on seismic coefficients. Upon
multiplication of the weight of the potential
sliding mass with these coefficients an estimate
of earthquake-related inertial forces are
obtained. These forces are considered in
addition to other (conventional) static forces in
seismic slope stability assessment. Usually the
horizontal seismic coefficient used in this
method is equal to the free-field peak ground
acceleration corresponding to design level of
earthquake shaking.
104
A limit equilibrium factor of safety of 1.0 is
usually considered acceptable in the
equivalent-static seismic slope stability
assessment. In the last couple of dec