21st Century Dam Design - Advances and Adaptions

8/6/2019 21st Century Dam Design - Advances and Adaptions

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21st Century Dam Design

Advances and Adaptations

31st Annual USSD Conference

San Diego, California, April 11-15, 2011


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On the CoverArtist's rendition of San Vicente Dam after completion of the dam raise project to increase local storage and provide

a more flexible conveyance system for use during emergencies such as earthquakes that could curtail the regions

imported water supplies. The existing 220-foot-high dam, owned by the City of San Diego, will be raised by 117

feet to increase reservoir storage capacity by 152,000 acre-feet. The project will be the tallest dam raise in the

United States and tallest roller compacted concrete dam raise in the world.

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advertising or promotional purposes and may not be construed as an endorsement of any product or

from by the United States Society on Dams. USSD accepts no responsibility for the statements made

or the opinions expressed in this publication.

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Enhancing practices to meet current and future challenges on dams; and

Representing the United States as an active member of the International Commission onLarge Dams (ICOLD).


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Myonga Dam Stability Evaluation 131

MYPONGA DAM STABILITY EVALUATION: MODELING STRESS

RELAXATION FOR ARCH DAMS USING LINEAR FINITE ELEMENT

ANALYSIS

Scott L. Jones, P.E.1

Guy S. Lund, P.E.

2

Bill Moler, P.E.3

Derek Moore, P.E.4

ABSTRACT

Myponga Dam, a concrete arch dam owned and operated by the South Australia WaterCorporation (SA Water), is located on the Myponga River, approximately 55 km (34

miles) south of Adelaide, South Australia. As part of a previous safety inspection, a

cursory pseudo-static study of the extreme (seismic) loading condition performed in 2003

indicated the potential for overstressing of the concrete and a more detailed dynamicanalysis of the dam was recommended. To address these recommendations, SA Water

contracted with URS to perform a linear finite element analysis as part of a Stage 1

Safety Review of Myponga Dam. The linear finite element analysis was used to performa stability evaluation of Myponga Dam for the usual (full supply level), unusual (inflow

design flood), and extreme (maximum design earthquake) loading conditions. The initial

results from the preliminary evaluation indicated the potential for overstressing,especially in the colder winter months. Based on the initial results and the understanding

that the joints in the foundation would not carry significant tension, adjustments were

made to the material properties in the linear model to better simulate conditions andunderstand the response of the dam. The updated results from the modified model

indicated that the stresses in the dam were less than the allowable strength of the

concrete; therefore, no further analysis was deemed necessary, allowing SA Water to

move forward with the Stage 2 Safety Review without further expense on seismicity orstability evaluations.

INTRODUCTION

Myponga Dam is a concrete arch dam on the Myponga River, approximately 4 km west

of Myponga township and 55 km south of Adelaide. The original supply was unfilteredwater, but since the completion of a filtration plant at the dam site in 1993, the Myponga

Reservoir has provided a filtered water supply to southern Adelaide and as far south as

Victor Harbor and Goolwa, Australia. The Myponga River and its tributaries rise in a

low range of hills, which extend from 10 km east to 17 km north east of Myponga

township. The catchment area is 124 square km.

1 Civil/Structural Engineer, URS, 8181 E. Tufts Ave., Denver, CO 80237; [email protected] Principal Civil/Structural Engineer, URS, 8181 E. Tufts Ave., Denver, CO 80237;

[email protected] Principal Engineering Geologist, URS Australia Pty Ltd, Level 4, 70 Light Square, Adelaide, SA 5000,

Australia; [email protected] Principal Dams and Geotechnical Engineer, SA Water, 250 Victoria Square, Adelaide, SA 5000,

Australia, [email protected]


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132 21st Century Dam Design Advances and Adaptations

Myponga Dam was completed in 1962 to a height of 47.6 m and impounding a storage of26,800 Mega Liters (ML). The dam consists of central arch with thrust blocks at the

abutments. A gated spillway was constructed on the left abutment adjacent to the left

thrust block. A public roadway traverses the crest of the dam and a bridge over thespillway. A view of the dam is presented in Figure 1.

The central arch of the dam has a crest width of 4.57 m at El. 213.52. The central archhas a length of approximately 166 m along the upstream edge of crest. A 1.48-m-high

parapet wall was constructed at the upstream edge of the crest to El. 215.00 in 2009. The

upstream face of the dam subtends an angle of 104 degrees with a radius of 91.44 m. Thecurvature of the downstream face of the dam is defined by a line of centers.

A comprehensive inspection of the dam was performed in 2003 to inspect the condition

of the dam, review the design methods, and review operations and maintenance

procedures for the dam [1.]. A finding in the review of previous design documentation

was that the dam had not been designed for seismic loading. To address this, a cursorypseuso-static study was performed and described in the inspection report. The cursory

study indicated the potential for overstressing during the Maximum Design Earthquake

(MDE). Based on these results, recommendations were made to perform a site-specificseismic evaluation and a more detailed seismic stability evaluation that includes the

temperature response of the dam.

Figure 1. Photograph of Myponga Dam.

In 2009 to 2010, South Australia Water (SA Water) managed a Stage 1 Safety Review of

Myponga Dam to inspect and assess key elements of Myponga Dam in order to identify

Stage 2 investigations and designs of remedial work that might be necessary. As part ofthe Stage 1 Safety Review, URS performed an updated stability evaluation of Myponga


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Dam using a three-dimensional linear finite element model of the dam. As per therecommendation of the 2003 inspection, the finite element analysis evaluated site specific

ground motions [2.] and included the effects of temperature. The finite element analysis

also included an evaluation of normal operating conditions and the updated flood [3.].

This paper describes the approach used to demonstrate the stability of the dam usinglinear finite element analysis methods. The analysis was performed in two stages. First,the analysis was run using previously developed material properties for the dam and

foundation, assuming continuity at all joints. The results from this analysis were

evaluated to determine if the model predicted tensile stresses at known joints in the model(e.g., along the dam/foundation interface, contraction joints, etc.). In the second stage of

the analysis, the material properties were modified locally to eliminate tensile stresses

from these areas and simulate the redistribution of the stresses that would naturally occurin the dam. The following sections describe the geometry, material assumptions, loading

conditions, and results of the finite element analyses.

MODELING PARAMETERS

Finite Element Model

The finite element computer program ANSYS [4.]was used to perform analyses of

Myponga Dam to determine the maximum stresses and deformations in the dam due to

the assumed loading conditions. The ANSYS finite element program is a fully three-

dimensional finite element method of analysis modeling tool.

The ANSYS model that was used to evaluate the stability of Myponga Dam consists of31,401 elements and 34,787 nodes, resulting in 103,461 degrees of freedom. The model

includes a significant portion of the foundation in addition to the concrete dam. The

foundation extends approximately one dam height into the abutment and upstream anddownstream from the extreme edges of the dam. Myponga Dam is simulated in the finite

element model using 8-noded brick elements and 4-noded contact elements. Note thatthe two foundation types (slate and limestone) were modeled. The bonded contact

elements were included along the dam/foundation interface. The ANSYS model that was

used to evaluate the stability of Myponga Dam is shown in Figure 2. Fixity within the

foundation was modeled using displacement boundary conditions, which were placed onthe exterior edges of the foundation portion of the finite element model.


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Figure 2. Finite Element Model of Myponga Dam.

Material Properties

Myponga Dam was constructed using two different concrete mixes, one for the core of

the dam (Class D) and one for the faces of the dam (Class C). The structural material

properties of the concrete assumed in the model were selected based on a review ofdesign documentation for Myponga Dam [5.] and on typical values for concrete [6.]. The

material properties of the concrete are summarized in Table 1.

Table 1. Concrete Material Properties

Values

Properties Class D Concrete Class C Concrete

Compressive Strength 28.6 MPa 32.2 MPa

Tensile StrengthStaticDynamic

2.84.2

MPaMPa

3.14.6

MPaMPa

Modulus of ElasticityStaticDynamic

23,00034,500

MPaMPa

Unit Weight 23.5 kN/m3

Poissons Ratio 0.20

Coefficient of Thermal Expansion 9.0 x 10-6 / C

Thermal Conductivity 13,600 J/hr/m/C

Specific Heat 950 J/kg/C

BRIGHTON

LIMESTONE

PURPLE

SLATE

DAM MAXIMUM

SECTION


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Table 2. Foundation Material Properties

ValuesProperties

Limestone Slate

Unconfined Compressive Strength 60 MPa 43 MPa

Deformation Modulus 24,800 MPa 13,000 MPa

Poissons Ratio 0.3 0.3

Assumed Shear Strength ParametersFriction AngleCohesion

550

DegreesMPa

450

DegreesMPa

The structural material properties for the foundation were selected based on rock core test

data [7.] and comparison with data from tests on similar rock types [8.]. The dam

foundation consists primarily of slate and limestone Adelaidean bedrock of Precambrian

age. The left abutment is primarily limestone (Brighton Limestone) and the rightabutment is primarily slate (Purple Slate). The maximum section of the dam is also

founded on slate. The material properties of the foundation are summarized in Table 2.

Loads

The behavior of the dam was analyzed for static, flood, and dynamic loads. The static

loads include gravity, normal reservoir elevation, seasonal temperature variations, andnormal tailwater elevation. The flood loads include assumed probable maximum flood

(PMF) reservoir and tailwater elevations. The dynamic loads include the design ground

motion time histories and the hydrodynamic added mass. Sediment against the dam was

determined to be negligible. The individual loads are summarized in Table 3 and shownon Figure 3.

Table 3. Individual Loads

Load Description

Gravity Dead weight of dam.

Reservoir Hydrostatic water pressure applied to the upstream face of the dam.

Temperature Load caused by variation in reservoir and air temperature.

Tailwater Hydrostatic water pressure applied to the downstream face of the dam.

Uplift Hydrostatic pressure applied to the dam/foundation interface.Hydrodynamic Added Mass Added mass to represent the reservoir/dam interaction during the earthquake.

MCE Ground accelerations due to the Maximum Design Earthquake (MDE).


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Uplift

TailwaterReservoir

Gravity

Upstream-Downstream

Ground Acceleration

Vertical

Ground Acceleration

Mass Elements on Upstream Face of Dam

Based on Westergaard to Represent the

Reservoir/Dam Interaction during the

Earthquake

TW

HW

Temperatures Applied as a Body Load

(Induce a Strain relative to the Stress-Free

Temperature of the Concrete)

Cross-Canyon

Ground Acceleration

Figure 3. Summary of Assumed Loads for Myponga Dam Stability Evaluation.

The studies evaluated the usual loading conditions (i.e. normal operations), unusual

loading condition (PMF event), and extreme loading conditions (MDE event). The fiveload combinations and the corresponding individual loads are summarized below.

Usual Load Combinations:Winter Gravity, FSL reservoir El. 211.69, winter temperature, and uplift.

Spring/Fall Gravity, FSL reservoir El. 211.69, spring/fall temperature, and uplift.

Summer Gravity, FSL reservoir El. 211.69, summer temperature, and uplift.

Unusual Load Combination:PMF Gravity, PMF reservoir El. 216.87, summer temperature, PMF

tailwater El. 173.1, and uplift.

Extreme Load Combination:MDE Gravity, FSL reservoir El. 211.69, spring/fall temperature, uplift,

hydrodynamic added mass, MDE acceleration time histories.

EVALUATION CRITERIA

The structural adequacy of Myponga Dam was evaluated for the following failure modes,

in accordance with ANCOLD criteria:

- Overstressing. The computed stresses from the structural analysis were comparedwith the allowable strength of the concrete to determine the potential for cracking or

crushing of the material. The required factor of safety was taken as 4.0, 2.7, and 1.3


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for the usual, unusual, and extreme load combinations, respectively (allowablestresses presented in results tables below).

- Sliding Stability. The computed sliding stability factor of safety for thedam/foundation interface was compared to the required factor of safety as described

in the ANCOLD guidelines. The required factor of safety was taken as 1.5, 1.3, and

1.3 for the usual, unusual, and post-earthquake load combinations, respectively.

Note that overturning (moment equilibrium) is not considered to be a viable failure mode

for an arch dam because of the redundancy of load paths in this type of dam. The

following paragraphs describe in detail the criteria that were used to evaluate the

structural adequacy of Myponga Dam.

ANALYSIS RESULTS

The results of the analyses are presented in this section for the assumed usual winter andextreme load combinations. These two load combinations were found to be the critical

load combinations on the dam. The results are used to evaluate the behavior of the damwith regard to the typical failure modes, which include overstressing and sliding stability

along the dam/foundation interface.

The results from the finite element analysis were \transformed into local cylindricalcoordinate systems so that the stresses in the model of the dam could be more easily

evaluated. The local cylindrical coordinate systems are oriented such that the X-axis is

normal to the face (i.e. radial), Y-axis is horizontal and tangent to the curvature of theface (i.e. arch), and Z-axis is vertical (i.e. cantilever). The local stress results are

evaluated using the horizontal Y- and vertical Z-components, defined as arch andcantilever stresses, respectively. The stress results from the analysis are presented in

the form of color contour plots with the contour units in Pascals (Pa). Negative andpositive values correspond to compressive and tensile stress, respectively.

Usual Load Combination Winter

The computed arch and cantilever stresses in the dam were compared to the allowable

compressive and tensile stresses (8.0 MPa and 0.8 MPa, respectively, based on thestrength values reported in Table 1 and the required factor of safety of 4.0). The arch and

cantilever stress plots are shown on Figures 4 and 5. The plots show that the computed

compressive stresses are less than the allowable compressive stress. These plots also

show that significant areas of the upstream and downstream face are in tension, and themaximum computed tensile stresses are greater than the allowable tensile strength of the

concrete. The estimated tensile stresses from the model indicate the potential forcracking of the concrete; however, due to structural characteristics of the dam andinterpretation of the plots (as described in the following paragraphs), the stress results do

not indicate that there is a concern regarding the safety of the dam.


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Figure 4. Arch Stresses Computed for Usual Winter Load Combination.

Figure 5. Cantilever Stresses Computed for Usual Winter Load Combination.

The analyses for Myponga Dam were performed using linear analysis assumptions and

techniques. Linear analyses use simplified assumptions to simulate structural and

Stress (Pa)

Stress (Pa)


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material behaviors. The benefit to linear analysis is that the response of concrete dams toloads similar to those used for Myponga Dam is generally well within the limits of linear

behavior, and linear evaluations are more cost effective. However, there are limitations

in some of the assumptions used to perform linear analysis, and these limitations must beunderstood to properly evaluate the behavior of the dam.

The dam is simulated using homogeneous and monolithic assumptions, thus, the finiteelement model behaves linearly. This means that external loads on the model can

generate tensile stresses as well as compressive stresses. However, Myponga Dam

utilized monolithic construction techniques with vertical joints between adjacent concreteblocks. These joints can open and close when subjected to different loads. For example,

the colder temperature loads on the dam will cause contraction of the concrete. The

vertical joints between the monoliths will open to allow the contraction to develop.During warmer summer temperatures, the joints will close when the concrete expands.

The horizontal arch tensile stresses shown in Figure 4 indicate that the load will result inopening of the vertical joints between monoliths. The opening of the joints will

significantly reduce, or eliminate, the horizontal arch tensile stresses in the dam. Also,

the arch stress results shown on the selected sections of the dam indicate that the highmagnitude horizontal tensile stresses are confined to the area near the face. These tensile

stresses develop in the model because the effect of the colder temperatures is limited to

the face; the thermal diffusivity of concrete requires longer periods of time for thermalloads to affect the core of the dam.

The vertical cantilever stresses, shown in Figure 5, indicate that high magnitudes oftensile stress will develop near the base of the upstream face of the dam. These stresses

are primarily due to the reservoir load on the dam, which induces a bending moment.

However, the re-entrant corner between the foundation and the upstream face of the dam

is also influencing the results. The pressures on the upstream face of the model used tosimulate the effect of the hydrostatic reservoir load cause the foundation in the model todevelop horizontal tensile stresses (upstream/downstream direction) between the

boundary conditions on the upstream edge of the foundation and the dam portions of the

model. These tensile stresses are transferred into the dam, which result is an overmagnification of the vertical stresses in the dam model.

The foundation will contain discontinuities, such as joints, bedding planes, and shears,

that prevent the horizontal tensile stresses from developing in the foundation. If the

foundation discontinuities were simulated in the finite element model, then the cantilever

tensile stresses would be reduced, or eliminated.

The results from the analysis demonstrate that the finite element model is behaving asanticipated. The computed stress results indicate that minor cracking may develop in the

dam along the upstream face, but at worst the cracking would be limited to the surface

area of the dam. The linear assumptions have likely resulted in over-estimation of thetensile stresses, and the actual tensile stresses in the dam are significantly less, or

eliminated. Based on the discussion presented above, the material properties were

changed locally (i.e., deformation modulus was reduced for individual foundation


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elements) to prevent the formation of significant tension in areas that are unlikely to resisttension.

The arch and cantilever stress plots for the modified analysis are shown on Figures 6 and

7. The results from the analysis show that the maximum computed compressive stresses

in the modified finite element model are greater than those for the original model(increased by as much as 80 and 160 percent for the arch and cantilever stresses,respectively). The results indicate that the maximum computed compressive stresses are

still less than the allowable compressive strength of the concrete in the dam. This

indicates that the dam will have sufficient compressive strength to support the assumedusual winter load combination.

The results from the analysis show that the maximum computed tensile stresses in the

modified finite element model are less than those for the original model (decreased by as

much as 20 and 65 percent for the arch and cantilever stresses, respectively). This

reduced tensile stress in the model is a direct result of the reduced deformation modulusin the foundation elements. The results indicate that the cantilever tensile stresses are

less than the allowable tensile strength of the concrete in the dam; whereas, they were

greater than the allowable strength according to the original model. The results indicatethat the maximum computed arch tensile stresses are still greater than the allowable

Figure 6. Arch Stresses Computed for Modified Analysis of Usual Winter Load

Combination.

Stress (Pa)


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Figure 7. Cantilever Stresses Computed for Modified Analysis of Usual Winter LoadCombination.

tensile strength of the concrete in the dam. As was previously discussed, the linear

assumptions in the finite element model likely resulted in an over-estimation of the

magnitude of the tensile stresses, and the actual tensile stresses in the dam aresignificantly less, or eliminated. If further modifications to the model were performed to

simulate the behavior of the vertical joints in the dam, then the horizontal arch tensile

stresses would be reduced more than shown. Additional modifications were notconsidered necessary, because the horizontal tensile stresses are isolated to the area nearthe face of the dam.

The sliding stability factors of safety along the base of the dam, predicted using the

original and modified linear finite element analyses, are summarized in Table 4. The

results show that the sliding factor of safety increased from the original model to themodified model. The computed increase is due to the modifications in the foundation

elements, which reduced the effect of the re-entrant corner at the upstream heel of the

model. The reduced foundation modulus allowed the arches in the dam to support more

of the load, and the redistribution increased the thrust on the abutments, which increased

the normal component of the load and the resistance due to friction. Similarly, themodifications to the deformation modulus in the foundation elements reduced the effect

of the vertical tensile stresses at the upstream heel of the dam near the base, whichresulted in an increase in the normal force and an increase in the frictional resistance.

Therefore, the study shows that simple modifications to the model resulted in an increase

in the computed sliding factor of safety.

Stress (Pa)


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Table 4. Summary of Sliding Stability Factors of Safetyfor the Usual Winter Loads.

Analysis

Description Original Modified

Spillway 2.9 3.3Left Abutment 0.9 1.4

Right Abutment 0.8 1.4

Minimum Allowable Factor of Safety 1.5 1.5

Additional modifications could be made to the model to simulate the effect of openedvertical joints; however, the effect would be similar, in that the dam would relax and

redistribute the load to the arches, and the sliding stability would also be increased.

The computed sliding factors of safety along the right and left abutment are slightly lessthan the minimum allowable factor of safety. The lower factors of safety are not a

concern regarding the stability of the dam. Based on the change in behavior between theoriginal and modified model, the sliding factor of safety will likely be greater than that

computed, if the effects of vertical joints were included in the analysis. Based on these

results, the dam is considered to satisfy sliding stability criteria for the assumed usualwinter load.

Extreme Load Combination

The linear dynamic analysis was performed using modal superposition. First, a modalanalysis was performed on the finite element model using the computer program

ANSYS. The dam/reservoir interaction was simulated using mass elements attached to

the upstream face of the dam. The first 10 mode shapes were extracted for use in thetransient modal superposition analysis.

The time history plots (three to twelve seconds after the start of the ground motion) of the

maximum horizontal and vertical stresses at any point on the upstream and downstream

faces of the dam are shown on Figures 8 and 9. The results show that the computed

stresses are less than allowable strength of the concrete for the extreme loading condition.Based on these results, the dam is considered to satisfy overstressing criteria for the

assumed extreme load.


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Figure 8. Arch Stress Envelopes Computed for the Extreme Load Combination.

Figure 9. Cantilever Stress Envelopes Computed for the Extreme Load Combination.

-6.00E+06

-5.00E+06

-4.00E+06

-3.00E+06

-2.00E+06

-1.00E+06

0.00E+00

1.00E+06

2.00E+06

3.00E+06

4.00E+06

3 4 5 6 7 8 9 10 11 12

Time (s)

Stress(Pa)

Upstream FaceDownstream Face

-6.00E+06

-5.00E+06

-4.00E+06

-3.00E+06

-2.00E+06

-1.00E+06

0.00E+00

1.00E+06

2.00E+06

3.00E+06

4.00E+06

3 4 5 6 7 8 9 10 11 12

Time (s)

Stress(Pa)

Upstream FaceDownstream Face

Allowable Tensile Stress

Allowable Tensile Stress


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CONCLUSIONS

An evaluation of the results from the analyses of the Myponga Dam led to the followingconclusions:

A linear static analysis of the dam using the measured material properties of thedam indicates the potential for overstressing (i.e., cracking) and sliding during the

usual loading conditions. As the dam has withstood this loading condition

approximately every year since construction was completed, modifications to the

model were required to more accurately describe the behavior of the dam.

A linear static analysis of the dam using modified material properties in localareas (areas not capable of carrying the predicted tensions) indicates that the

computed stresses in the dam are generally within acceptable limits and that thesliding stability of the dam satisfies safety criteria.

A linear dynamic analysis of the dam indicates that the computed stresses in the

dam are within acceptable limits; therefore, the dam is considered to satisfy safetycriteria for the Maximum Design Earthquake (MDE).

Based on the results of these analyses, no further studies under the Stage 2 Safety Review

were recommended to demonstrate the stability of the dam.

REFERENCES

[1.] URS Australia Pty Ltd (2003).Myponga Dam Comprehensive Inspection FinalReport, Hackney, SA, Australia, May 7.

[2.] URS (2010).Myponga Dam Ground Motion Time History Draft Report,Pasadena, CA, January 14.

[3.] SKM (2010).Myponga Dam Flood Hydrology, Malvern, VIC, Australia,February 26.

[4.] ANSYS, Inc. (2007).ANSYS Mechanical, Canonsburg, PA.

[5.] Hryniewicz, Z. W. and R. B. Stevens (1958). Report of Design of Concrete Mixesfor Myponga Dam, The Engineering and Water Supply Department, South

Australian Government, 22 May.

[6.] Bureau of Reclamation (2006). State-of-Practice for the Nonlinear Analysis ofConcrete Dams at the Bureau of Reclamation, U.S. Department of the Interior,January.

[7.] The Engineering and Water Supply Department (1962). Myponga DamGeological Committee Part I: Meeting Minutes and Part II: Geological Reports,

South Australian Government.


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[8.] Bureau of Reclamation (1974).Rock Mechanics Properties of Typical FoundationRock Types, REC-ERC-74-10, U.S. Department of the Interior, Denver, Colorado.

[9.] ANCOLD (1998). Guidelines for Design of Dams for Earthquake, AustralianNational Committee on Large Dams, August.


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21st Century Dam Design - Advances and Adaptions