ABSTRACT earth structure materials. However, there is still need

ABSTRACT

It has been noted that there is a
need for improvement in earth dams with reference to the preventive measures
against seismic loading. A particular need for improvements can be summarized
in the following two fields: the techniques for calculation of the seismic
response and the properties of soil. Furthermore, a special attention should be
paid to a better comprehension of the impacts of the earthquake acceleration on
the response of the dam as long as the effects of amplification are involved.
Throughout this paper you will find a parametric study that is conducted with
regards to the effects of cohesion of soil and soil friction angle over the
yield accelerations values. Reference to the earthquake acceleration, you will
find that a safety factor has been used as a function of acceleration of
earthquake. Additionally, a thorough analysis is conducted with reference to
the outcomes on crest response spectra of earthquake acceleration input,
reduction of induced deformations of Gmaxsoil strain shear modulus
and velocity shear wave. The conclusions of these analyses are a positive
contribution to the realm of the dam engineering with regards to geotechnical
earthquake.

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Keywords: Dam, earthquake, modulus of
rigidity

 

INTRODUCTION

The
dynamic behavior of soil structures has grabbed a lot of attention of various
geotechnical engineers and researchers, especially with regards to the study of
earth or embankment dams.(Seed et al. 1973,

Marcusson, 1981; Sarma,Wieland, M.
2006)In the past
many researches and analysis have been conducted in the realm of earth dams. As
a result of these, a major improvement has been noted in having an improved
assessment of seismic risk, and furthermore it was also achieved to have an
experimental determination of the properties of earth structure materials.
However, there is still need for further improvement; therefore in this paper
you will find some more elaborated methods for the assessment of dynamic
behavior of embankment dams.

 

Since the beginning, seismic/dynamic
analysis methods have had a great impact in the assessments of dam safety under
earthquake loading. One of the most important incentives in the development of
dam’s seismic analysis was the San Fernando earthquake that took place in CA,
USA on 1971. This earthquake took a major attention for researchers and
geotechnical engineers since it resulted in severe damages for the lower dam of
San Fernando. Till lately, the method used to construct dams that can resist
earthquakes was through utilizing a pseudo-static approach. Hence, relevant for
embankment dams the calculations for the stability of conventional slope were
done by taking into consideration the static inertia forces that are equivalent
to sliding masses. But, it is important to emphasize that this method often has
resulted inappropriate due to the fact that it is unable to quantify the dam
earthquake effects. Going forward, there were a considerate number of like
events where various dams were severely affected by earthquakes. Taking
incentives from these occurrences, very crucial improvements and developments
were done with regards to the comprehension of soil’s cyclic behavior and the
dam’s reaction to seismic loadings.

One of the most utilized methods
for modeling of the materials reaction to cyclic loading is the one that is
based on same linear characteristic soil values. When analyzing earth dams,
generally a two dimensional finite elements model can be sufficient. One could take
into account solicitations as parallel to the main axis of the structure of
earth. Calculations of finite elements are done by dividing the relative
structure of earth in various elements that are connected by nodes. When nodes
are moved from their initial position, they result in stresses and deformations
of the elements. Throughout this study you will find procedures for the seismic
stability analysis. Furthermore, the dam earthquake effects are presented based
on a sensitivity study. As a result, it is noted that earth dam seismic
resistance is based upon characteristics of mechanical properties of the soil
as well as seismic loads.

METHODOLOGY & CASE STUDY

In order to emphasize the
importance of geotechnical properties on the behavior of an embankment dam
under earthquake loading, San Fernando’s lower dam will be presented. Our aim
is to share our results with the exhaustive reassessments of the stability of
the lower San Fernando dam. We must recall that the San Fernando earthquake that
hit California on 9th of February 1971, had a great effect on the
dam, which suffered many damages on the upper side. The earthquake caused a big
shift on the upstream face of the dam, carrying its peak and 9.2 meters of
material from its downstream slope. The recordings showed that the ground
movements near the dam had a maximum acceleration of 0.55 – 0.6g.

To achieve this study, the lower
San Fernando dam is first analyzed on static conditions. The soil is modeled as
a linear-elastic material with the following characteristics: unit weight (gkN/m3),
Young’s modulus (E kPa), Poisson’s ratio (u), cohesion (c kPa) and friction
angle (f degree). Mohr-Coulomb’s criterion is used. This
model will be used set up initial conditions for deformations and stresses.
Then geotechnical parameters are modified to see the variation of the static
safety factor.

Secondly, a horizontal seismic
coefficient is applied to the sliding slope so that different aspects are
studied. The instability of the slope of the dam is caused by the horizontal
acceleration. Soil cohesion will be modified as well to examine the effect on
the critical yield acceleration.

Afterwards, in the third part the
dynamic analysis is conducted using the fine elements method. This is the most
used model by geotechnical earthquake engineers to reproduce the transmission
of the waves in soil layers.

The earthquake accelerogram used
is defined as a movement imposed on the bedrock. This is the El Centro
earthquake on May 18, 1940 at the Imperial Valley in the United States. The
magnitude was 7.1 and the duration 10s.

In the dynamic analysis,
different parameters are carried out to determine their effect the seismic
behavior of the dam. These are the covered themes: variation of the safety
factor based on the max. acceleration; variation of the safety factor according
to the max. deformation of the dam; evolution of the amplification factor based
on the maximum acceleration the rock; variation of the spectral acceleration as
a function of the shear wave velocity; variation of the spectral acceleration
with maximum acceleration on rock; ratio of response spectra as a function of
shear wave velocity, maximum acceleration on rock and plasticity of soils.

The seismic analysis of the dam
remains remains far required for designing and sizing of future dams. The lower
dam of San Bernardo became a famous case in the history of seismic geotechnical
engineering. Many research projects were subject of the dam.

ANALYIS AND DISCUSSION

The height of the dam is 44m. The
upstream slope is 1H: 2.5V and the downstream slope is 1H: 4.5V.

  Static
analysis

The static limit equilibrium
analysis approach scored a factor of safety 2.389. Fig 2 shows the most
critical failure surface under static loading. Keeping all data constant and
changing the cohesion from 30 kPa to 120 kPa increases the safety factor from
1.45 to 2.55, while increasing the friction angle from 10 deg to 40 deg, the
safety factor increases from 2.0 to 3.5.

Pseudo Static analysis

the critical failure surface
gives a lower factor of safety with increasing acceleration factor k. However
these factors of safety remain greater than 1 till reaching an acceleration
factor k 0.412 with a factor of safety equal to 1. This acceleration is called
the yield acceleration. Fig. 3 shows the variation of yield acceleration with
cohesion.

 

Fig. 1.Cross
section of the analyzed dam.                                        Fig. 2.Most
critical surface under static loading conditions.

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