平假名翻译-阀块
Comparison of Design and Analysis of
Concrete Gravity Dam
ABSTRACT
Gravity dams are
solid concrete structures that maintain their
stability against design loads
from the
geometric shape, mass and strength of the
purposes of dam construction
may
include navigation, flood damage
reduction,hydroelectric power generation, fish and
wildlife
enhancement,water
quality,water
supply,and
design
and
evaluation
of
concrete
gravity
dam
for
earthquake
loading
must
be
based
on
appropriate
criteria
that
reflect
both
the
desired level of safety and the choice
of the design and evaluation Bangladesh, the
entire
country
is
divided
into
3
seismic
zones,
depending
upon
the
severity
of
the
earthquake
intensity. Thus, the main aim of this
study is to design high concrete gravity dams
based on the
U.S.B.R. recommendations
in seismic zone II of Bangladesh, for varying
horizontal earthquake
intensities from
0.10 g - 0.30 g with 0.05 g increment to take into
account the uncertainty and
severity of
earthquake intensities and constant other design
loads, and to analyze its stability and
stress conditions using analytical 2D
gravity method and finite element method. The
results of the
horizontal
earthquake
intensity
perturbation
suggest
that
the
stabilizing
moments
are
found
to
decrease significantly with
the increment of horizontal earthquake
intensity while dealing with
the
U.S.B.R.
Recommended
initial
dam
section,
indicating
endanger
to
the
dam
stability,
thus
larger
dam
section
is
provided
to
increase
the
stabilizing
moments
and
to
make
it
safe
against
failure.
The
vertical,
principal
and
shear
stresses
obtained
using
ANSYS
5.4
analyses
are
compared with those obtained using 2D
gravity method and found less compares to 2D
gravity
method, except the principal
stresses at the toe of the gravity dam for 0.10 g
- 0.15 g. Although, it
seems apparently
that smaller dam section may be sufficient for
stress analyses using ANSYS 5.4,
it
would not be possible to achieve the required
factors of safety with smaller dam is
observed during stability analyses that
the factor of safety against sliding is satisfied
at last than
other factors of safety,
resulting huge dam section to make it safe against
sliding. Thus, it can be
concluded
that
it
would
not
be
feasible
to
construct
a
concrete
gravity
dam
for
horizontal
earthquake
intensity
greater
than
0.30
g
without
changing
other
loads
and
or
dimension
of
the
dam and keeping
provision for drainage gallery to reduce the
uplift pressure significantly.
Keywords:
Comparison
Concrete
Gravity
Dam
Dam
Failure
Design
Earthquake
Intensity Perturbation
Stability and Stress
uction
Basically, a gravity
concrete dam is defined as a structure,which is
designed in such a way
that its own
weight resists the external forces. It is
primarily the weight of a gravity dam which
prevents it from being overturned when
subjected to the thrust of impounded water [1].
This type
of structure is durable, and
requires very little maintenance. Gravity dams
typically consist of a
non
overflow
section(s)
and
an
overflow
section
or
spillway.
The
two
general
concrete
construction methods for concrete
gravity dams are conventional placed mass concrete
and RCC.
Gravity dams, constructed in
stone masonry, were built even in ancient times,
most often in Egypt,
Greece, and the
Roman Empire [2,3].
However, concrete
gravity dams are preferred these days and mostly
constructed. They can
be constructed
with ease on any dam site, where there exists a
natural foundation strong enough
to
bear
the
enormous
weight
of
the
dam.
Such
a
dam
is
generally
straight
in
plan,
although
sometimes, it may
be slightly curve. The line of the upstream face
of the dam or the line of the
crown
of
the
dam
if
the
upstream
face
in
sloping,
is
taken
as
the
reference
line
for
layout
purposes,
etc.
and
is
known
as
the
“Base
line
of
the
Dam”
or
the
“Axis
of
the
Dam”.
When
suitable conditions are available, such
dams can be constructed up to great heights. The
ratio of
base width to height of high
gravity dams is generally less than 1:1.
A typical cross-section of
a high concrete gravity dam is shown in
Figure . The upstream
face may be kept throughout vertical or
partly slanting for some of its length. A drainage
gallery
is generally provided in order
to relieve the uplift pressure exerted by the
seeping es
applicable
to
dam
construction
may
include
navigation,
flood
damage
reduction,
hydroelectric
power
generation, fish and wildlife enhancement, water
quality, water supply, and recreation.
Many
concrete
gravity
dams
have
been
in
service
for
over
50
years,
and
over
this
period
important
advances
in
the
methodologies
for
evaluation
of
natural
phenomena
hazards
have
caused the design-basis events for
these dams to be revised upwards. Older existing
dams may
fail to meet revised safety
criteria and structural rehabilitation to meet
such criteria may be costly
and
difficult.
The
identified
causes
of
failure,
based
on
a
study
of
over
1600
dams
[4]
are:
Foundation
problems
(40%),
Inadequate
spillway
(23%),
Poor
construction
(12%),
Uneven
settlement (10%), High poor pressure
(5%), Acts of war (3%), Embankment slips (2%),
Defective
ma
terials(2%),
Incorrect
operation
(2%),
and
Earthquakes
(1%).Other
surveys
of
dam
failure
have been cited by
[5], who estimated failure rates from
2
×
10-4to7
×
10-4per damyear based on
these surveys.
In
the
design
of
gravity
concrete,
it
is
essential
to
determine
the
loads
required
in
the
stability
and
stress
analyses.
The
forces
which
may
affect
the
design
are:
1)
Dead
load
or
stabilizing force; 2) Headwater and
tailwater pressures; 3) Uplift; 4) Temperature; 5)
Earth and
silt
pressures;
6)
Ice
pressure;
7)
Earth
quake
forces;
8)
Wind
pressure;
9)
Subatmospheric
pressure; 10)
Wave pressure, and 11) Reaction of foundation.
The seismic safety of such
dams has been a serious concern since damage to
the Koyna Dam
in
India in
1967 which
has been regarded
as
a
watershed event
in
the development
of seismic
analysis
and
design
of
concrete
gravity
dams
all
over
the
world.
It
is
essential
that
those
responsible must
implement policies and proce dures to ensure
seismic safety of dams through
sound
professional practices and state-of-the-art in
related technical areas. Seismic safety of dams
concerns
public
safety
and
therefore
demands
a
higher
degree
of
public
confidence.
The
Estimations and descriptions of various
forces are provided briefly in the following
sections.
2.1. Water
Pressure
Water pressure (P)
is the most major external force acting on gravity
dams. The horizontal
water pressure
exerted by the weight of water stored on the
upstream and downstream sides of the
dam can be estimated from the rule of
hydrostatic pr essure distribution and can be
expressed by
p
?<
/p>
1
?
w
H
2
2
where,
H is the depth of water and
?
w
is the unit
weight of water.
2.2.
Uplift Pressure
Water
seepage through the pores, cracks and fissures of
the foundation materials, and water
seepage through dam body and then to
the bottom through the joints between the body of
the dam
and its foundation at the base
exert an uplift pressure on the base of the dam.
According to the [6],
the
uplift
pressure
intensities
at
the
heel
and
toe
of
the
dam
should
be
taken
equal
to
their
respective
hydrostatic
pressures
and
joined
the
intensity
ordinates
by
a
straight
line.
When
drainage
galleries
are
provided
to
relieve
the
uplift,
the
recommended
uplift
at
the
face
of
the
gallery
is
equal
to
the
hydrostatic
pressure
at
toe
plus
1/3rd
of
the
difference
between
the
hydrostatic pressures at the heel and
the toe, respectively.
2.3.
Earthquake Forces
An
earthquake
produces
waves,
which
are
capable
of
shaking
the
earth
upon
which
the
gravity dams rest, in every possible
direction. The effect of an earthquake is,
therefore, equivalent
to
imparting acceleration to the
foundations of the dams in the direction in which
the wave is
traveling at the moment.
Generally, an earthquake
induces horizontal acceleration (h) and vertical
acc eleration (v).
The
values of these accelerations are generally
expressed as per centage of the acceleration due
to
generally
sufficient
for
high
dams
in
seismic
zones.
In
extremely
seismic
regions
and
in
conservative Designs even a value up to
0.30 g may sometimes be adopted [7].
Earthquake
loadings
should
be
checked
for
horizontal
as
well
as
vertical
earth
quake
accelerations.
While
earthquake
acceleration
might
take
place
in
any
direc
tion,the
analysis
should be performed for the most
unfavorable direction.
The
earthquake loadings used in
the design
of concrete
gravity dams
are
based on design
earthquakes and
sitespecific motions determined
from seismological eva luation. At a
minimum,
a seismological evaluation
should be performed on all pro jects located in
seismic zones 1, 2, and
3 of Bangladesh
[8], depending upon the severity of earthquakes.
The
seismic
coefficient
method
of
analysis
should
be
used
in
determining
the
resultant
location and sliding stability of dams.
In strong seismicity areas, a dynamic seismic
analysis is
required for the internal
stress analysis.
2.3.1.
Effect of Vertical Acceleration
(
ɑ
v)
A
vertical
acceleration
may
either
act
downward
or
upward.
When
it
acts
in
the
upward
direction, then the
foundation of the dam will be lifted upward and
becomes closer to the body of
the dam,
and thus the effective weight of the dam will
increase and hence, the stress developed
will increase.
When the vertical acceleration acts
downward, the foundation shall try to move
downward
away from the dam body; thus,
reducing the effective weight and the stability of
the dam, and
hence is the worst case
for design. The net effective weight of the dam is
given by
w
w
?
w
)
(2)
k
v
g
?
(
1
?
k
v
g
acceleration, such as 0.10
or 0.20, etc.
where,
W
is
the
total
weight
of
the
dam,
kv
is
the
fraction
of
gravity
adopted
for
vertical
In other words,
vertical
acceleration
reduces the unit weight of the dam material and
that
of water to (1
–
kv) times their original
unit weights.
2.3.2.
Effects of Horizontal Acceleration
(
ɑ
h)