Constraining Models of Extended Gravity using Gravity Probe B and LARES experiments
Modelling Issues in the Seismic Analysis of Concrete Gravity Dams
Transcript of Modelling Issues in the Seismic Analysis of Concrete Gravity Dams
DAM ENGINEERING Vol XXIV Issue 2 1
Modelling Issues in the Seismic Analysis ofConcrete Gravity Dams
Arnab Banerjee(1), D K Paul(1) & R N Dubey(1)
ABSTRACT
Modelling issues in the seismic analysis of dams are challenging. This paper addresses the
various modelling issues of a concrete gravity dam, with the help of various field problems.
The issues regarding the modelling of these types of dams, e.g. infinite reservoir with truncated
boundaries, and semi-infinite soil medium with finite domain, are discussed. The issues
related to the input motion of earthquake excitation are also considered, a comparative
study of different modelling issues has been made, and the appropriate modelling of a
dam-reservoir-foundation system, along with suitable simulation of seismic input, has
been suggested for the evaluation of a dam’s response.
Keywords: Dam-foundation-reservoir interaction; concrete gravity dams; compressible fluid
element; incompressible fluid; absorbing boundary; seismic analysis.
1. INTRODUCTION
Concrete gravity dams are considered to be one of the most important civil engineering
structures in the world. A gravity dam stores a large amount of water in its reservoir,
the pressure of which is resisted by the dam. Earthquakes, however, can cause catastrophic
failure of these dams and, hence, safety analysis is of utmost importance. Many
researchers have investigated the dynamic behaviour of gravity dams using different
modelling techniques.
Modelling of a dam-reservoir-foundation system has the following major issues: depending
upon the accuracy of results, modelling of the concrete gravity dam is chosen. The dam can
either be analyzed by assuming the dam is fixed at the base[19], or by considering its foundation
flexibility[9]. Simplified analysis procedures for the earthquake-resistant design of concrete
gravity dams is also used to estimate the stability of the dam section[7,8]. CADAM software
can effectively be used to measure a dam’s response, and its stability against sliding and
overturning[14]. Use of the finite element method is another effective way to analyze the
dam-foundation-reservoir water system together.
(1)Department of Earthquake Engineering, Indian Institute of Technology Roorkee, Roorkee, India.
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Reservoirs have considerable effect on the earthquake response of a gravity dam. There
are three ways to consider reservoir effect in the seismic response analysis of the dam system,
i.e. Westergaard, Lagrangian and Eularian approaches.
The Westergaard approach considers a virtual lumped mass on the wet surface, which
simulates the hydrodynamic effect on the upstream face of the dam[22,25,1]. In the Eulerian
approach[3,6,16,26,4,18,2,12] the degree-of-freedom in structure and fluid are displacement and
pressure, respectively, and a special compatibility equation is required to establish the
compatibility between reservoir water and the dam-foundation system. In the Lagrangian
approach displacement is the variable for both the impounded reservoir water and the dam-
foundation system, and no compatibility equation is required[23,10].
The generation of an infinite medium is a big modelling issue in any finite element analysis.
To solve a field problem a finite boundary is required to be specified. Generally, the boundary
of an infinite field can be formulated using four approaches, i.e. the elementary boundary,
the local boundary, the consistent boundary, and the infinite element approach[24,27,20,21].
Rollers assigned at the truncated edge of the soil or rock are known as an elementary
boundary. In a local boundary viscous dampers are used at the truncated face of the soil
layer. The foundation system is represented as Kelvin elements in a consistent boundary.
Infinite element has both near- and far-field nodes. Far-field nodes map the infinite extent
of soil. The interpolation function of this element is different compared to the continuum
plane strain or plane stress elements[5].
The application of earthquake input motions has great importance in seismic analysis.
Issues such as rock outcrop motion, free field motion, base motion at foundation level, and
motion at the soil boundaries, need to be understood and implemented correctly. The free
field earthquake motion is generally specified at the rock outcrop and, as a rule, a site-specific
response spectrum is also given for rock outcrop. Deconvolution is performed by calculating
the complex impedance of the rock for a particular depth[13].
In this paper the comparison of responses between Westergaard’s approach and the
acoustic element approach are presented. The scope of this paper is extended to allow
comparison of the seismic responses for elementary and local boundaries assigned at the
truncated edge of the semi-infinite soil. The site-specific response spectra, and corresponding
accelerogram, is given at the rock outcrop. The outcrop accelerogram is deconvoluted at the
base of the soil boundary, and this deconvoluted input motion is applied at the base of the
considered soil profile for analysis.
2. DAM DATA
A concrete gravity dam with a height of 235m, a base width of 260m, a downstream slope
of 1:0.9, and an upstream slope of 1:0.625, is chosen as a numerical example. The thickness
of every monolith is 15m, and each one is separated by a construction joint. The dam section
is shown in Figure 1.
ARNAB BANERJEE, D K PAUL & R N DUBEY
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The peak ground acceleration of the site is taken as 0.2g. The geotechnical data resulting
from the analysis is tabulated in Table 1.
There are two philite strips of 10m width present at 60m and 160m from the upstream
face of the dam. The two strips are parallel to each other, totalling 220m along the horizontal.
The property of the philite strips is the same as the fragmented rock layer. The property of
concrete is given in Table 2.
MODELLING ISSUES IN THE SEISMIC ANALYSIS OF CONCRETE GRAVITY DAMS
10
10
10
225
260
50
80
Figure 1. Dam section
Table 1. Geotechnical data
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Bulk modulus and density of water are taken as 2.07GPa and 1000kg/m3, respectively.
The damping values of each material is tabulated in Table 3.
3. FINITE ELEMENT ANALYSIS OF THEDAM-RESERVOIR-FOUNDATION SYSTEM
The dam’s body and foundation are modelled using the Lagrangian approach. Two-dimensional
plane stress elements are used to model the dam’s monoliths.
The foundation soil is infinitely extended for both in-plane and out-of-plane directions.
Two-dimensional plane strain elements are suitable to model the foundation soil or rock.
The appropriate near-field portion of soil/rock is considered for modelling the foundation.
The waves reflected back from the truncated boundary. To generate the non-reflecting
boundary, P-wave and S-wave dampers are used at the nodes of the boundary. The
transmissibility coefficient (TC) and reflection coefficient (RC) for a wave falling at the
interface of the two mediums is calculated by:
(1)
(2)
To ensure the radiating boundary condition, TC = 1 and RC = 0 should be satisfied. The
coefficient of these dampers are such that impedance (Z) given by dampers should be equal
ARNAB BANERJEE, D K PAUL & R N DUBEY
Table 2. Property of concrete
Table 3. Damping value
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to the impedance of the foundation soil/rock. Impedance of a material is the product of
density (ρ) and shear wave velocity (Vs).
(3)
The coefficient of P-wave dampers (Cp) and S-wave dampers (Cs) are calculated as:
(4)
(5)
The impounded reservoir effect can be modelled taking water as:
1. Incompressible
In the case of incompressible water the effect of the reservoir is considered as Westergaard’s
added mass on the upstream face of the dam. The virtual masses of water are tied or lumped
at the nodes of the upstream wet face of the dam according to IS:1893-1984. Using pressure
distribution at the upstream face, according to Equations 6 and 7, virtual masses are calculated
due to hydrodynamic effect:
(6)
(7)
2. Compressible
If the impounded reservoir water is considered compressible then the reservoir can be modelled
as acoustic fluid elements. In an acoustic fluid or Eularian system the fluid field is of constant
volume. The acoustic element has a degree-of-freedom in terms of pressure at the nodes of the
element. When fluid is flowing through a constant volume field then pressure is induced at the
inner boundary of the field. For example, pressure is induced at the inner surface of a pipe
(which is constant in volume) when water flows through it. In acoustic analysis the mesh
remains stationary (Eularian), but the acoustic media is assumed to flow through the mesh.
The acoustic element needs the bulk modulus and density of fluid for analysis.
The equilibrium equation for the acoustic medium is given by:
(8)
(9)
MODELLING ISSUES IN THE SEISMIC ANALYSIS OF CONCRETE GRAVITY DAMS
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where p is the dynamic pressure, x is the spatial position of the fluid particle, uf and uf are
the velocity and acceleration of the fluid particle, ρf is the density of fluid, γ is the volumetric
drag coefficient, and Kf is the bulk modulus of the fluid.
Element type AC2D8, an 8-node quadratic 2D acoustic quadrilateral, is used to model the
impounded reservoir water.
The link element is used to model the fluid-structure interface, which can convert pressure
into displacement. The conceptual representation of the link element is shown in Figure 2,
and the connecting matrix is given in Equation 10.
(10)
A free surface is assigned at the top surface of the impounded reservoir water, where
pressure is zero.
The non-reflecting (absorbing) boundary is generated at the truncated edge of the infinite
reservoir. The equation of the absorbing boundary is as follows:
(11)
Assigning impedance to the truncated boundary infinite domain is generated in the
case of the acoustic medium. Using Equation 12, a non-reflecting boundary is created in
ABAQUS:
ARNAB BANERJEE, D K PAUL & R N DUBEY
v v
v
vu
u
u
u
uuuv v
7
6 65 5p
p p
p p p
p p
8 8v4 4
1 2 3 1 2
7
3
Lagrangesolid
element
Acousticßuid
element
Link element
Figure 2. Fluid link element
. ..
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(12)
where is the wave number, ρ is the complex density, f is the geometric factor for
the curvilinear coordinate system used at the boundary, and β is the spreading loss.
Combining Equation 8 with Equation 12, the non-reflecting boundary condition is generated
in terms of impedance:
(13)
by Fourier inversion:
(14)
For γ = 0, Equation 14 converts to:
(15)
Comparing Equation 11 with Equation 15 it can be concluded that for a non-reflecting
boundary the geometric factor for the curvilinear coordinate (f) = 1 and, correspondingly,
the spreading loss (β) = 0.
The four different types of dam models, having different modelling aspects, are
shown in Figure 3. For Model 1 the reservoir water is considered incompressible and
the elementary boundary assigned at the truncated soil boundary. In Model 2 the reservoir
water is considered incompressible and the viscous boundary assigned at the truncated
soil boundary. For Model 3 the reservoir water is considered compressible and the elementary
boundary assigned at the truncated soil boundary, and in Model 4 the reservoir water
is considered compressible and the viscous boundary assigned at the truncated soil
boundary.
MODELLING ISSUES IN THE SEISMIC ANALYSIS OF CONCRETE GRAVITY DAMS
~
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4. GROUND MOTION DECONVOLUTION
A response spectrum compatible synthetic accelerogram of the site at the rock outcrop is
specified, but this accelerogram cannot be put directly onto the ground’s surface as an input
motion. As the soil strata consists of soil, sand and rock at different depths this input motion
is applied at the base of the foundation soil/rock, i.e. at a depth of 250m considered here.
Therefore, the ground motion must be deconvoluted in order to obtain the input response at
the base of the soil/rock.
The following flowchart shows the various steps of deconvolution processes:
ARNAB BANERJEE, D K PAUL & R N DUBEY
Added mass
(a)
Added mass
(b)
(c) (d)
(a) Model 1 (b) Model 2
(c) Model 3 (d) Model 4
Figure 3. Different dam models
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The transfer function depends on the complex impedance ratio, depth, shear wave velocity,
and damping of the soil.
To consider the soil-structure interaction it is convenient to apply the motion at the base
of the foundation soil. The necessity of the deconvolution is explained in Figure 4. In this
process the input motion at the base of the foundation soil/rock is determined based on the
specified rock outcrop motion. This generally gives a motion having less peak amplitude.
This input motion is applied at the base of the dam-foundation-reservoir system to obtain
the dam response.
The accelerogram at the rock outcrop is given in Figure 5, having a PGA of 0.2g. The
deconvoluted time history at the base of the foundation soil/rock at the depth of 250m is
given in Figure 6. The peak value of the motion is reduced to 0.12g.
MODELLING ISSUES IN THE SEISMIC ANALYSIS OF CONCRETE GRAVITY DAMS
Surface
Base Deconvolution
Rockoutcrop
Figure 4. Deconvolution of motion
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After deconvolution the corresponding new response spectrum at the base of the soil
foundation is shown, along with the response spectra at rock outcrop (see Figure 7).
These same input motion/response spectra at the base of the soil foundation are taken as
the input to the dam-foundation-reservoir system.
ARNAB BANERJEE, D K PAUL & R N DUBEY
0.2
0.1
0
-0.1
-0.2
5 10 15 20 25 30 35Time (sec)
Accelerogram at 250m depth
Acc
eler
atio
n(g
)
Figure 6. Motion at 250m depth
0.2
0.1
0
-0.1
-0.2
5 10 15 20 25 30 35Time (sec)
Accelerogram at Rock outcrop
Acc
eler
atio
n(g
)
Figure 5. Motion at rock outcrop
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5. RESULTS AND DISCUSSIONS
5.1. FREQUENCY ANALYSIS
Free vibration analysis of the dam is performed for the incompressible water (Westergaard’s
lumped mass) model, as well as the compressible water (acoustic water) model. The Lanczos
solver is used to determine frequency. Natural time periods are tabulated in Table 4 for the
dam section.
The natural time period of the two models gives almost similar values, and indicates that
the Westergaard idealization of a reservoir is acceptable. However, due to the consideration
of the compressibility of impounded reservoir water, the dam-foundation-reservoir system
becomes slightly flexible. Mode shapes of the dam sections for the two cases, considering
both compressible and incompressible water, are shown in Figure 8.
MODELLING ISSUES IN THE SEISMIC ANALYSIS OF CONCRETE GRAVITY DAMS
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Pse
udo
spec
tral
acce
lera
tion
(Sa/
g)
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5Time period (sec)
Rock outcrop motion
Deconvoluted at base
Figure 7. Response spectra
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ARNAB BANERJEE, D K PAUL & R N DUBEY
Table 4. Natural time period of dam section
+3.273e-07+3.000e-07+2.727e-07+2.455e-07+2.182e-07+1.909e-07+1.636e-07+1.364e-07+1.091e-07+8.182e-08+5.455e-08+2.727e-08+0.000e+00
U, magnitude (a)
+1.039e+00+9.525e-01+8.659e-01+7.793e-01+6.927e-01+6.062e-01+5.196e-01+4.330e-01+3.464e-01+2.598e-01+1.732e-01+8.659e-02+0.000e+00
U, magnitude (b)
First mode shape in dam (T = 1.06sec) - water compressible
First mode shape in dam (T = 1.03sec) - water incompressible
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The pressure variations of the impounded water in the reservoir in different modes are
shown in Figure 9.
The first mode shape shows the parabolic distribution of pressure, which matches the
parabolic pressure distribution as reported in previous literature.
MODELLING ISSUES IN THE SEISMIC ANALYSIS OF CONCRETE GRAVITY DAMS
+1.440e-07+1.320e-07+1.200e-07+1.080e-07+9.601e-08+8.401e-08+7.201e-08+6.000e-08+4.800e-08+3.600e-08+2.400e-08+1.200e-08+0.000e+00
U, magnitude (c)
+1.206e+00+1.106e+00+1.005e+01+9.047e-01+8.042e-01+7.037e-01+6.031e-01+5.026e-01+4.021e-01+3.016e-01+2.010e-01+1.005e-01+0.000e+00
U, magnitude (d)
Figure 8. Mode shapes in dam body
Second mode ahape in dam (T = 0.75 sec) - water compressible
Second mode shape in dam (T = 0.76sec) - water incompressible
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5.2 RESPONSE SPECTRUM ANALYSIS
Response spectrum analysis is not possible when reservoir water is considered compressible,
and modelled using acoustic elements. In such cases time history analysis is carried out. A
response spectrum determined at the base of the foundation at a depth of 250m is applied.
Response spectrum analysis is performed for the incompressible impounded water model.
The principal stresses are shown in Figure 10.
The maximum tension obtained at the heel is found to exceed the maximum allowable
tensile strength of concrete. Therefore cracks may initiate at the heel point.
ARNAB BANERJEE, D K PAUL & R N DUBEY
+1.000e+00+9.167e-01+8.333e-01+7.500e-01+6.667e-01+5.833e-01+5.000e-01+4.167e-01+3.333e-01+2.500e-01+1.667e-01+8.333e-02+0.000e+00
(a)
+1.000e+00+9.059e-01+8.118e-01+7.177e-01+6.235e-01+5.294e-01+4.353e-01+3.412e-01+2.471e-01+1.530e-01+5.885e-02-3.526e-02-1.294e-01
(b)
First mode shape of the reservoir
Second mode shape of the reservoir
Figure 9. Mode shapes in the reservoir
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MODELLING ISSUES IN THE SEISMIC ANALYSIS OF CONCRETE GRAVITY DAMS
+2.826e+07+2.591e+07+2.355e+07+2.120e+07+1.884e+07+1.649e+07+1.413e+07+1.178e+07+9.424e+06+7.069e+06+4.714e+06+2.359e+06+4.283e+03
S, Max. In-plane principal(Avg: 75%)
(a)
+2.448e+05-1.981e+05-6.410e+05-1.084e+06-1.527e+06-1.970e+06-2.413e+06-2.855e+06-3.298e+06-3.741e+06-4.184e+06-4.627e+06-5.070e+06
S, Min. In-plane principal(Avg: 75%)
(b)
In plane maximum principal stress
In plane minimum principal stress
Figure 10. Stress contours static + earthquake
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5.3 TIME HISTORY ANALYSIS
Prior to application of the acceleration shown in Figure 6, baseline correction is applied
to the acceleration time history. The accelerogram is applied at the bottom of the rock
foundation. The free surface of the impounded reservoir water is modelled by assigning
p = 0 at the top surface. Non-reflecting boundary is assigned at the truncated edge of
the reservoir. The principal stresses at the heel for the different models are plotted in
Figure 11.
ARNAB BANERJEE, D K PAUL & R N DUBEY
18
14
10
6
2
-20 10
Time (sec)3020 40
Str
ess
(Mp
a)
(a)
0 10Time (sec)
3020 40
Str
ess
(Mp
a)
18
14
10
6
2
-2
(b)
Principal stress at heel Model 1
(max 12.90MPa)
Principal stress at heel Model 2
(max 10.10MPa)
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Application of the absorbing boundary does not allow the incident waves to reflect back but,
in the case of the elementary boundary, reflection of waves are not arrested. Models 1 and 3, with
the elementary boundary, show higher responses. While the stress response in the cases of the
absorbing boundary (Models 2 and 4) reduces significantly beyond 20sec, the stress response in
the cases of the elementary boundary (Models 1 and 3) remains high. This phenomenon is due to
the reflection of waves from the truncated boundary of the elementary boundary model.
In Models 1 and 2 water mass is considered, but the bulk modulus of water flexibility is
neglected. Surface waves cannot be generated in a lumped mass model, but Models 3 and 4
can generate surface waves. The energy loss due to the generation of these waves, and the
outgoing waves at the truncated boundary, attribute to a lesser response when water is
considered compressible.
MODELLING ISSUES IN THE SEISMIC ANALYSIS OF CONCRETE GRAVITY DAMS
0 10Time (sec)
3020 40
Str
ess
(Mp
a)
18
14
10
6
2
-2
(c)
0 10Time (sec)
3020 40
Str
ess
(Mp
a)
18
14
10
6
2
-2
(d)
Principal stress at heel Model 3
(max 15.50MPa)
Figure 11. Principal stresses at heel
Principal stress at heel Model 4
(max 9.08MPa)
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The maximum stresses at the upstream locations are reduced by 10-20% due to the fluid
compressibility, but at the toe point the stress is not so much affected due to the compressibility
of the impounded water.
A plot of dam crest accelerations for each model are shown in Figure 12.
ARNAB BANERJEE, D K PAUL & R N DUBEY
25
20
15
10
5
0
-5
-10
-15
-20
-25
Acc
eler
atio
n(m
/s2)
Time (Sec)0 10 20 30 40
(a)
25
20
15
10
5
0
-5
-10
-15
-20
-25
Acc
eler
atio
n(m
/s2)
Time (Sec)0 10 20 30 40
(b)
Crest acceleration Model 1
(max 2.29g)
Crest acceleration Model 2
(max 1.69g)
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The maximum acceleration at different locations along the dam-foundation-reservoir
system due to both the compressible and incompressible water models is tabulated in
Table 5.
MODELLING ISSUES IN THE SEISMIC ANALYSIS OF CONCRETE GRAVITY DAMS
25
20
15
10
5
0
-5
-10
-15
-20
-25
Acc
eler
atio
n(m
/s2)
Time (Sec)0 10 20 30 40
(c)
25
20
15
10
5
0
-5
-10
-15
-20
-25
Acc
eler
atio
n(m
/s2)
Time (Sec)0 10 20 30 40
(d)
Crest acceleration Model 3
(max 2.23g)
Crest acceleration Model 4
(max 1.27g)
Figure 12. Crest acceleration for different cases
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ARNAB BANERJEE, D K PAUL & R N DUBEY
The acceleration at the heel is modified by acceleration of the rock outcrop, due to the presence
of the structure and different soil/rock strata. Acceleration at the heel is almost 30% more than
the acceleration of the rock outcrop. Acceleration of the crest, heel and upstream slope change
location, and are reduced by 18-24% due to the influence of compressible water. Westergaard’s
virtual lumped mass model produces results a little on the conservative side.
6. CONCLUSIONS
Some modelling issues like modelling of a dam using 2D plane stress element, soil/rock
domain by 2D plane strain elements, modelling of reservoir assuming water as a compressible
(acoustic elements) and incompressible (virtual lumped mass) fluid, generation of a dam-
reservoir-foundation interface, and modelling of infinite fluid and soil using the finite element
approach, are discussed. The estimation of earthquake motion at the base of the dam’s
foundation is also included.
The specified motion at the rock outcrop is deconvoluted at the base of the dam’s
foundation for evaluating the response of the system, and the deconvoluted motion is
applied at the base. Maximum acceleration of the deconvoluted motion is 50-60% of
the outcrop motion, depending upon the depth of deconvolution. The non-reflecting
boundary is used to absorb outgoing waves at the truncated rock boundary. In the case
of a reflecting boundary the waves reflect back to the system, which results in a higher
response. After 20sec, when earthquake motion is reduced, the elementary boundary still
gives a significant response.
The impounded reservoir water is either considered compressible (modelled using acoustic
elements), or incompressible (by assigning lumped mass at the wet surface of the dam). Where
water is considered compressible the response spectrum method is not applicable, and only
time history analysis needs to be performed. Natural frequency analysis shows comparable
results when the water is considered compressible (acoustic element model) and incompressible
(Westergaard’s model). The fundamental frequency of the dam system in the two cases is found
to be almost identical, suggesting that Westergaard’s model is a good idealization.
Table 5. Maximum acceleration of compressible and incompressible water models
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The distribution of pressure in water shows an almost parabolic variation in the first
mode of the reservoir, in the case of a changing upstream slope. Due to the interaction
with fluid, the acceleration and stresses at the upstream face of the dam are found to be
18-24%, and 10-15% less than the lumped mass approach, respectively, as wave propagation
is not considered in the incompressible water model, whereas in the acoustic water model
surface waves are generated and not reflected back from the boundary. The energy loss
can be considered in terms of damping at the upstream face. The assignment of a higher
damping value (7% instead of 5%) for the full dam section in the case of an incompressible
model gives a comparable response to the compressible water model.
The most suitable model of a dam-foundation-reservoir system is obtained when acoustic
elements are used for the impounded water. Truncated reservoir and soil boundaries are
replaced with absorbing boundaries, and appropriate input motion is considered at the base
of the foundation rock.
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Vol 59, Issue 5, pp877-890 (1996).
[4] Dungar, R, ‘An Efficient Method of Fluid-structure Coupling in the Dynamic Analysis of
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