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.

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.