Read Seismic Deformation Analysis of Earth and Rockfill Dams text version

Reliability of simplified methods for evaluation of earthquake-induced displacement in earth and rockfill dams

Mojtaba E. Kan and Hossein A. Taiebat

School of Civil and Environmental Engineering University of New South Wales, NSW

Presentation Outline

· Introduction

Methods of estimating deformations of dams Simplified semi-empirical methods

· Is the simplified method conservative? · Effects of tuning ratio on reliability of the simplified methods · Range of reliability of the simplified methods · Conclusions

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Introduction

Earth and Rockfill Dams c Subjected to deformation and settlement during and after earthquake loading

Settlement c Loss of Freeboard Loss of Freeboard c Breach and failure Field of interest in research and practice

Australia: Far from tectonic plate margins Intra plate stress build up Meckering (1968, M6.9), Newcastle (1989, M5.6) Re-evaluation of seismic resistance of all dams

Major deformation patterns in earth dams (After Ambraseys, 1958, taken from Yan, 1991)

http://geology.about.com/library

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Methods of estimating deformations of dams

How to deal with earthquake loading in design?

Pseudo static analysis: Based on limit equilibrium Application of a horizontal acceleration on a wedge Simple but inaccurate Unable to take cyclic nature of loading into account Unable to predict settlement Not recommended for final design Numerical simulations: Complex and expensive Performed for marginal cases if necessary

Jinto, 2009

Jinto, 2009 Empirical and semi-empirical methods: Empirical equations based on performance of existing dams Newmark (1965) and Makdisi and Seed (1978) semi-empirical methods Semi-empirical methods, recommended as screening tool

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Methods of estimating deformations of dams

Empirical methods

Based on the record of performance in the existing dams 1 Jansen (1990): 0.9

= . ( 0.8 ) ( - )

Swaisgood (2003): 69 dams 0.7

0.6 (%) = (. +. -.) 0.5 Pells and Fell (2002 and 2003): 305 dams 0.4 Singh and Roy (2009): 152 dams 0.3 0.2

Settlement (%)

0.1

0 0 0.1 0.2 0.3 0.4 0.5 0.6 Peak Ground Acceleration (g) 0.7 0.8

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Simplified Semi-empirical Methods

Newmark (1965)

Assumption:

Sliding mass on an inclined surface Slide occurs if earthquake acc. > yield acc. Earthquake acceleration = Ground acceleration Yield acceleration : FOS = 1 Displacement: Double integration of acceleration

Concepts of Newmark Approach (Newmark, 1965)

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Simplified Semi-empirical Methods

Makdisi and Seed (1978)

Modified and improved the Newmark approach based on:

Deformability of dam Variation of acceleration in dam height 9 cases of 30 to 60 m dams in FEM Equivalent linear material behaviour

Variation of seismic coefficient with depth of the base of the potential sliding

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Variation of normalized permanent displacement with ratio of proportional yield acceleration

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Is the simplified method conservative?

Current Approach in Practice

Makdisi and Seed (1978) method, still widely used Believed as conservative approach Recommended as screening tool (ANCOLD, 1998)

Results of Recent Studies

Rathje and Bray (1999 and 2000)

Coupled model (dynamic response + movement of sliding block) Generalized distribution mass Mainly for deformation of landfills 19 (1999) and 24 (2000) earthquake ground motions Main focus on medium plasticity clay

Wartman et al. (2003)

Rigid block (upper diagram) and soil column (lower diagram) test setup and instrumentation plan (Wartman et al., 2003)

Behaviour of a rigid block and a flexible soil column Inclined surface and shaking table

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Is the simplified method conservative?

Summary of previous theoretical studies on conservatism of simplified methods Reference (Authors) Rigid Block Analysis (e.g. Newmark, 1965) Non-conservative for 0.2< To/Tm<2 Decoupled Analysis (e.g. Makdisi and Seed, 1978) Conservative for To/Tm<2 and ky/kmax<0.6 Non-conservative for To/Tm>4

Rathje and Bray (1999)

Rathje and Bray (2000) Significantly Non-conservative or Conservative

Conservative for To/Tm<1 Might be Non-conservative for To/Tm>1 Potentially Non-conservative for Large To/Tm, and ky/kmax>0.4 Primarily Non-conservative for Large To/Tm, , Low ky and Intense Ground Motion

Wartman et al. (2003)

Non-conservative for 0.2<tuning ratio<1.3

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Critical Threshold: To/Tm>1

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Is the simplified method conservative?

Some Recent applications of MSA, leading to non-conservative results:

Ghanooni and Roosta (2002)

115 m high dam, BCCRD PGA, 0.54g FDM (1.2 m), Newmark Method (0.55 m)

Ghahreman Nejad et al. (2010 and 2011)

75 m and 84 m high dams (BCCRD and Tailing Dam) PGA, 0.8g, M7.5 FDM: (2~4) × (Newmark and Makdisi & Seed Methods)

Feizi-Khankandi et al. (2009)

110 m high dam, BCCRD (Garmrood Dam) PGA, 0.54g, M7.5 FDM (1.5 m), Newmark Method (0.8 m)

Sengupta (2010)

Tehri Dam, 260.5 m high Unconservative results for M7: FEM: 2 × (Makdisi & Seed Method) Extremely conservative for M8: FEM: 1.1 m, Makdisi & Seed: 7.5 m

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Effect of tuning ratio on reliability of the simplified methods

A case study: A dam with non-conservative results in simplified method

84 m

10

Cross section of the 84 m high asphaltic core rock fill dam (Ghahreman Nejad et al., 2010)

0.6

Acceleration (m/sec2)

8

6 4 2 0 -2 -4 -6 -8 0 5 10 15 20

Fourier Amplitude

0.5 0.4 0.3 0.2 0.1 0 0 0.5 1 1.5 2 2.5 3

Tm=0.39 sec

Time (Sec)

Period (Sec)

Acceleration time history of Loma Prieta Earthquake (E-W, Gilroy #1, scaled to 0.8g)

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Fourier amplitude transform of the earthquake record

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Effect of tuning ratio on reliability of the simplified methods

A case study: A dam with non-conservative results in simplified method

1.0

0.9

0.8

Results of shear beam analysis using four stress-strain dependencies

Seed et al. (1986) Shibuya et al. (1990) Ishibashi et al. (1993) Rollins et al. (1998)

0.7

G/Gmax

0.6

Stress- Strain Dependency

0.5

0.4

To (Sec) To/Tm Seed et al. (1986)

0.74 1.90

0.41 1.05

0.39 1.01

0.60 1.54

0.3

0.2

Shibuya et al. (1990) Ishibashi & Zhang (1993) Rollins (1998)

0.1

0.0 1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

Shear Strain

Variation of the stiffness vs. shear strain for coarse grained materials

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Range of reliability of the simplified methods

Critical threshold: To/Tm >1.0 Range of variation for Tm (Rathje et al., 1998): For a dam within 100 km far from a causative fault:

0.45 to 0.80 sec in active plate regions (e.g. : Western US) 0.21 to 0.45 sec in stable continental regions (e.g. : Eastern US and Australia) Range of variation for T0:

Using Shear beam theory:

Variation of shear modulus along the height of a dam (Gazetas, 1987) Maximum shear modulus of granular material based on Seed and Idriss (1970) Estimation of T0 in terms of dam height and material stiffness

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Range of reliability of the simplified methods

200 180 160 140 Dam Height (m) 120

T = 0.45 Sec

K2max = 50 K2max = 60 K2max = 70 K2max = 80

100

80 60 40 20 0 0 0.2 0.4 0.6 0.8 To (Sec) 1

H = 20 m

T = 0.21 Sec

H = 50 m

K2max = 90 K2max = 100 K2max = 110

1.2

1.4

Variation of To vs. H for earthfill dams (K2max from 50 to 110)

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Range of reliability of the simplified methods

200 180 160 140 Dam Height (m) 120 100 80 60 40 20 0 0 0.2 0.4 To (Sec) Variation of To vs. H for earth/rockfill dams (K2max from 120 to 180)

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T = 0.45 Sec

K2max = 120 H = 90 m K2max = 130 K2max = 140 K2max = 150 K2max = 160 H = 35 m K2max = 170 K2max = 180

T = 0.21 Sec

0.6

0.8

1

Range of reliability of the simplified methods

200 180 160 140 200 180 160

T = 0.45 Sec

Dam Height (m)

140

T = 0.45 Sec

K2max = 50 K2max = 60 K2max = 70 K2max = 80

120 100 80 60 40 20 0 0

T = 0.21 Sec

Dam Height (m)

120 100 80 60 40 20

H = 35 m K2max = 170 K2max = 180 H = 90 m K2max = 120 K2max = 130 K2max = 140 K2max = 150 K2max = 160

T = 0.21 Sec

H = 50 m K2max = 90 K2max = 100

H = 20 m K2max = 110

0.5

1

1.5

0 0 0.2 0.4 0.6 0.8 1

To (Sec)

To (Sec)

Variation of To vs. H for earthfill dams (K2max from 50 to 110)

Variation of To vs. H for earth/rockfill dams (K2max from 120 to 180)

Region Earthfill Dam Rockfill Dam

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Active seismic >50 m >90 m

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Stable continental >20 m >35m

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Conclusion

Simplified Semi-Empirical Methods:

Practical and widely used Based on :

Limited number of studies Records of the western US earthquakes Dams with lower than 60 m hight

Assumed to be conservative c Recommended as screening tool (ANCOLD, 1998)

Recent Studies:

Effect of frequency content (e.g. tuning ratio) on reliability of results Non-conservative results in some cases of recent numerical simulations

Are they conservative for Australian earthquakes?

Present Study:

Frequency content and tuning ratio is important in results Potentially non-conservative for earth and rockfill dams higher than:

50m to 90m in active seismic regions (e.g. west US) 20m to 35m in stable seismic regions (e.g. Australia)

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Thank you

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Seismic Deformation Analysis of Earth and Rockfill Dams

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