Seismic Stability Evaluation of Lookout Point … of uniform shear stress loading (typically at 65% of max). Then it can be compared to cyclic simple shear laboratory test results,

US Army Corps of Engineers

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Seismic Stability Evaluation of Lookout

Point Embankment Dam

Kenji Yamasaki, P.E.

Cornforth Consultants

David Scofield, P.E., C.E.G.

U.S. Army Corps of Engineers, Portland District

February 10, 2015

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PORTLAND DISTRICT 2

Outline (Part I)

Purpose of Seismic Evaluation

Project Overview

Geologic Section

Embankment Dam Design

Previous Seismic Screening Analysis

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PORTLAND DISTRICT 3

Purpose of Seismic Evaluation USACE conducts dam safety evaluations of all

projects at a five year schedule.

USACE has adopted a “Risk Based”

methodology to quantify the risk and make risk

based decisions.

To accomplish this, there is a need for more

detailed analyses to conduct risk assessment.

Lookout Point Dam is one of the first two dams

in Portland District undergoing this process.

This presentation covers the seismic analyses

used to develop parameters that went into the

risk process

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PORTLAND DISTRICT 4

Project Overview Location:

Lookout Point Dam, Oregon

Middle Fork Willamette River

(R.M. 7.5), 22 Miles upstream

of Eugene, OR

It is one of 13 Corps projects in

the Willamette Basin

The

Willamette

Basin

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PORTLAND DISTRICT 5

Project Overview

Concrete Gravity Section

Spillway/Regulating Outlets

Embankment

Powerhouse

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PORTLAND DISTRICT 6

Concrete Dam

Embankment

Powerhouse

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PORTLAND DISTRICT 7

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PORTLAND DISTRICT 8

Embankment Dam Design

Bedrock (Lava Flows, Tuffs with Clay Seams)

Summer Conservation (95%)

50% Pool

Minimum Flood Pool

Random Gravel shells are coarse alluvial gravels. Average 10

inch diameter.

Downstream Filter Zone is 8 Feet Thick. Processed Recent

Alluvium. Cohesionless.

Alluvial Gravel Foundation averages about 10-inch diameter.

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PORTLAND DISTRICT 9

Geologic Section

Overburden removed from the

foot print of the core trench.

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Embankment Dam Design Embankment is well constructed and documented

Foundations are either rock or overburden with low

liquefaction susceptibility

Filter Zone is thinner than would like for seismic design

There are seismically favorable design features

► Test fill was constructed using actual construction equipment.

Extensive laboratory testing.

► Impervious core and downstream filter zone wrap around the

concrete gravity dam

► Random gravel zone was designed with finer materials against the

filters and become coarser away to provide a secondary filter zone.

However, permanent construction record samples do not

substantiate this and it cannot be counted on.

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10% Normal High Pool El. 929.3

45% Spillway Crest Pool El. 891.2

50% Pool El. 886.5

Previous Seismic

Screening Analysis

Stability analysis used is the Hynes-Griffin and Franklin

pseudo-static method developed by the Corps of Engineers.

Conclusion:

The embankment may crack with displacements that are

approaching the level of concern.

More sophisticated analyses are required.

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Kenji Yamasaki with Cornforth Consultants will

present the new analyses conducted for the

Corps of Engineers to support our Dam Safety /

Risk Assessment Program

Outline (Part II)

Analysis Model

Seismic Hazard and Input Motions

Results

Cyclic Softening of Core

Analysis Model

Downstream shell is dry.

Estimated undrained shear

strength of compacted

clay core

Clay core material was

compacted at about 2% wet

of optimum in 12-in lifts

with an 85,000-pound rubber-

tired roller.

Overconsolidated

Normally consolidated

Static Factors of Safety - Upstream

FS ≈ 2.0

Seismic Hazard and Input Motions

AMEC Geomatrix and

Quest Structures, 2009

Uniform Hazard Response Spectra (UHRS) and Cascadia Deterministic Spectra

2,500-Year UHRS

matches Cascadia

M8.7 to M9.1 84th

Deterministic Spectra

(Max. Credible

Earthquake = MCE)

Seismic hazard is

dominated by Cascadia

subduction zone

interface earthquakes

M8 - M9+

0.7 g

at

0.2 s 0.3 g

PGA 0.4 g

at

1.0 s

Interface Earthquake Time Histories

• Scaled to match 1,000-Year, 2,500-Year, and 10,000-

Year UHRS (three levels of severity)

• Valparaiso, 1985, M8, U.F.S.M.D.I.C., 70°

• Michoacan, 1985, M8, Caleta de Campo, 180°

• Geomatrix (1993) M9.0 Synthetic

• PGA Values:

1,000-year: 0.14 g - 0.16 g

2,500-year: 0.24 g - 0.27 g

10,000-year: 0.45 g - 0.51 g

60 s 2,500-Year Level

Acceleration Time Histories

Valparaiso 1985 M8

Michoacan 1985 M8

0.25 g

Wait for completion of

NGA-Sub Project by

PEER (UC Berkeley and others).

Will include data from

2010 MW8.8 Chile and 2011 MW9.0 Japan

subduction earthquakes.

SEEP/W, QUAKE/W, SLOPE/W

Finite Element Mesh

Time histories applied along the base (rigid boundary)

QUAKE/W Response Analysis for Michoacan 2,500-Year Motion

Peak Horizontal Accelerations – Amplification from Base to Crest

Base Peak Acceleration = 0.24 g

Crest Peak Acceleration = 0.57 g

Base to crest

amplification of

acceleration is

about 1.5 to 3,

and is reasonable

for an embankment

dam.

Embankment

Base to Crest Amplification

Cyclic Softening Potential of Clay Core

Evaluate τxy at Points in Core (HP: History Points) τxy

Dynamic shear stress on

horizontal plane caused

by earthquake shaking

Represent irregular shear stress time history by equivalent

cycles of uniform shear stress loading (typically at 65% of max).

Represent irregular shear stress time history by equivalent

cycles of uniform shear stress loading (typically at 65% of max).

Then it can be compared to cyclic simple shear laboratory test results,

which are based on cyclic uniform stress loadings.

Idriss and Boulanger (2008): data from Andersen et al. 1988, Azzouz et al. 1989,

Hyodo et al. 1994, Lefebvre and Pfendler 1996, Woodward-Clyde 1992,

Zergoun and Vaid 1994.

(to cause 3% cyclic strain)

1.3





0.9





0.7

“Cyclic softening” likely

in upper part of core

under 2,500-year and 10,000-year

Cascadia M9 subduction earthquakes.

“Cyclic softening” not likely

in lower part of core.

dN = 10 – 20 in. (2,500-y)

Estimate Displacements Caused by Subduction Earthquakes

NEWMARK DISPLACEMENTS, dN (U/S SIDE)

dN = 70 – 110 in. (10,000-y)

Shallow Failure Surface

Strength of upper core

reduced to 30% of su

ky ≈ 0.21

dN = 0.1 – 2 in. (2,500-y)



dN = 10 – 20 in. (10,000-y)

Intermediate Failure Surface

ky ≈ 0.20

dN = 0 (2,500-y)



dN = < 3 in. (10,000-y)

Deep Failure Surface

ky ≈ 0.22

Conclusions

• “Cyclic softening” of core may occur under the 2,500-year level of Cascadia subduction earthquake shaking. However, the strength loss potential is probably low: core compacted wet of optimum with rubber-tired rollers.

• Seismic displacement under the 2,500-year level subduction earthquake shaking is not likely to offset the downstream filter.

Thank you for your kind attention.

Documents

Seismic Stability Evaluation of Lookout Point … of uniform shear stress loading (typically at 65% of max). Then it can be compared to cyclic simple shear laboratory test results,