scoggins dam geotechnical analysis and risk analysis

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Scoggins DamGeotechnical Analysis and

Risk Analysis

July 18, 2012

Overview and General Topics

• Facility Description• Types of Reclamation Dam Safety Studies• 2004 Comprehensive Facility Review• 2008 Issue Evaluation• 2010 Issue Evaluation• Ongoing Corrective Action Study

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Detailed Analysis Topics

• Geotechnical Analyses– Field Investigations– Embankment Analyses

• Risk Analysis in Reclamation• Potential Failure Modes• Estimation of Failure Consequences• Estimation of Annual Probabilities of Failure• Summary of Risks• Conclusions• SOD Recommendation• Corrective Action Study

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Facility Description

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Scoggins Dam• Scoggins Dam is an earthfill embankment located on Scoggins

Creek about 25 miles west of Portland, Oregon

• Dam construction was completed in 1975

• Reservoir (Henry Hagg Lake) has a capacity of 53,323 acre-ft at the top of joint use capacity, elev. 303.5 ft

• Structures at this facility include:– Embankment dam– Gated spillway – Tunnel outlet works

Location

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Embankment Dam

• Dam has length of 2,700 ft

• Maximum structural height of 151 ft (Crest El. 313)

• Zoned embankment

• Due to presence of soft foundation soils, dam was designed with two foundation (cutoff) trenches

Dam Cross Section

Appurtenant Structures

• Outlet works consists of a tunnel through the left abutment, with a capacity of 400 ft3/s

• Spillway is a gated structure located on left abutment, with a capacity of ~14,000 ft3/s

Plan View

Dam Safety Studies

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Types of Reclamation Dam Safety Studies

• Comprehensive Facility Review (CFR)– Every 6 years– “Screening” level

• Issue Evaluation Study (IE)– Detailed– Range in scope

• Corrective Action Study (CAS)

• Modification Final Design

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Risk Analysis UsedThroughout These Studies

• Potential Failure Mode (PFM) analysis conducted at all phases

• CFR risks are estimated by simple means, and typically use “best estimates” with limited uncertainty analysis– Include all loading conditions (static, hydrologic, seismic)

• For all higher level studies, risk analysis is accomplished by a facilitated team– Likely to focus only on specific loading conditions

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Scoggins Dam - 2004 CFR

• Concluded that static and hydrologic risks do not exceed Reclamation Public Protection Guidelines (PPG) thresholds and thus provide decreasing justification for any additional actions– This finding verified in recent 2010 CFR

• Concluded that latest earthquake loadings were higher than used in previous engineering analyses, and that seismic risks may exceed guideline values, justifying additional actions to better define risks

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Scoggins Dam - 2004 CFR

• Resulted in one new Safety of Dams (SOD) recommendation

• 2004-SOD-A: After the study to update the potential seismic hazards has been finalized, evaluate the need to perform additional investigations and dynamic analyses

• This led to IE studies

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Scoggins Dam – 2008 IE

• Updated the PSHA, but did not develop site-specific ground motions

• No new explorations or investigations

• Simplified engineering analyses, including:– Screening-level spillway analyses with earth pressures– Foundation “triggering” analyses– Post-EQ stability analyses– Newmark analysis using ground motions from other sites

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Scoggins Dam – 2008 IE

• Conducted team risk analysis

• Concluded that estimated risks from dam overtopping or internal erosion due to seismic loading justified further risk reduction actions

• Concluded that estimated risks from spillway wall failure or separation at embankment-structure interface due to seismic loading justified further risk reduction actions

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March 2008 Decision

• Although risks appear to justify corrective actions, need additional study since findings were based on “preliminary” studies– Essentially, conclusion that a well-built dam could fail under

subduction zone earthquake, with no assumed strength loss, is a critical conclusion, and needs verification

• To withstand scrutiny, perform a more detailed Issue Evaluation study to fully verify risks– Gather additional embankment/foundation data– Update seismic loading– Perform state-of-practice engineering analyses

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Scoggins Dam – 2008 IE

• 2008-SOD-A: Develop and perform a field exploration program for Scoggins Dam that will obtain information that will better define the earthquake loading and the site’s response to large earthquakes.

• 2008-SOD-B: Perform detailed stability and numerical dynamic analyses using the information obtained in the field exploration program, updated (if necessary) seismic hazard analyses and ground motions.

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Scoggins Dam – 2008 IE

• 2008-SOD-C: Perform an issue evaluation risk analysis for Scoggins Dam using the information obtained from the field exploration program and dynamic analyses.

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Scoggins Dam – 2008 IE

• 2008-SOD-D: Have all work done for this issue evaluation reviewed by a Consultant Review Board (CRB).

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Scoggins Dam – 2010 IE

• This latest round of IE studies resulted from the preceding SOD recommendations, and included the following activities:– Updated PSHA and development of ground motions– Extensive field program and geologic/testing reports– Evaluation of in situ and laboratory testing of embankment

and foundation soils– Detailed engineering analyses of embankment and

spillway (strength loss triggering, post-EQ stability, Newmark deformations, FLAC deformations, LS-DYNA spillway analysis

– Facilitated, team risk analysis

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Scoggins Dam – 2010 IE• Concluded that estimated mean seismic risks from dam

overtopping or internal erosion brought about by earthquake-induced slope failures and cracking justify further risk reduction measures

• Concluded that estimated mean seismic risks from failure of spillway wall, which could lead to an erosional failure of the embankment, justify further risk reduction measures

• Concluded that seismic risks from spillway pier failure did not justify additional action

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Scoggins Dam – 2010 IE

• 2010 IE led to one new SOD recommendation• 2010-SOD-A: Initiate a Corrective Action

Alternatives Study to evaluate potential alternatives to mitigate the high risks of seismic failure modes of the embankment and spillway at Scoggins Dam

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Scoggins Dam – Ongoing CAS

• Initiated, based on findings of 2010 IE

• Conducted concurrently with finalization of IE studies, and convening of a Consultant Review Board

• In progress, with ongoing analyses and design work

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Geotechnical Analyses

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Field Investigations

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Field Investigations Program• CPT testing along downstream toe and beneath shell

– Screening exploration to better locate additional borings– Determine peak/remolded undrained strength of clayey

overburden– Strength loss potential of clays/silts

• SPT testing in basal sand/gravel unit– Liquefaction potential of sands/gravels

• Vane shear testing in overburden– Peak and remolded undrained strengths of clayey overburden

• Four shear wave velocity crossholes– Also have seismic cone downhole data– Plus a line of surface shear wave data at toe

• Undisturbed sampling holes– Used shear wave holes for undisturbed sampling

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Fall 2008 Explorations

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Observations from Field Investigations

• Strengths of foundation soils are generally higher than as measured in pre-construction explorations

• Foundation soils to the right of Scoggins Creek still appear to be of generally lower strength and also contain more sandy, silty materials

• Foundation soil strengths increase under the dam footprint• There was relative agreement between strengths

measured by the CPT and by vane shear tests, although the CPT values were typically lower

• Clay sensitivity, or ratio of peak undrained to remolded strength, typically ranges from 2 to 3

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Laboratory Testing• Contracted with then-URS lab in New Jersey• Both embankment and foundation samples were tested,

but focus was on strength of foundation clays• Tests included:

– 1-D consolidation – U-U triaxial shear (peak undrained strength of clays)– C-U triaxial shear– DSS (both peak and remolded undrained clay strengths)– Lab vane shear (peak/remolded clay strength)– Cyclic triaxial– Cyclic DSS

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Observations from Undisturbed Sampling and Lab Testing

• Foundation clays are lightly overconsolidated, with OCR typically around 2 or 3

• Most foundation soils are plastic, with an average PI value of 22

• Laboratory testing of undisturbed foundation soil samples confirmed the presence of low strength soils (similar to what was determined by CPT and vane shear testing)

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Embankment Analyses

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Embankment Analyses

• Strength loss triggering in foundation– CPT, SPT, Vs, and vane shear test data– Looked at liquefaction in coarse-grained soils, strength loss in

clayey overburden

• Limit equilibrium post-EQ stability• “Squashed dam” analyses• Newmark analyses

– Used both DYNDSP and QUAKE/W

• FLAC analyses

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Liquefaction Triggering in Coarse-Grained Foundation Soils

• Focused on those foundation soils that had a plasticity index (PI) of less than 7

• Generally limited to the basal sand/gravel (Qalb) and the sandy soils to the right of Scoggins Creek

• Potential for liquefaction of these types of soils was evaluated using Standard Penetration Test (SPT) blow counts and by shear wave velocities

• Evaluated in accordance with state-of-the-practice procedures (Seed simplified method for SPT, Andrus and Stokoe for shear wave velocity)

• Looked at both earthquakes – local and subduction zone earthquake, as well as several different return periods, ranging from 500 years to 50,000 years

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Liquefaction Triggering in Coarse-Grained Foundation Soils

• SPT blow count analysis and the shear wave analysis yielded similar findings

• Both indicated that the silty and sandy soils at the toe and beneath the downstream slope of the embankment immediately right of Scoggins Creek were potentially liquefiable

• Also, liquefaction could be triggered for the 500-year EQ• Weak zone of low plasticity soils was in the vicinity of Station

7+00 and was generally from elevation 170 to 185• Liquefaction potential was limited in all other areas of the

foundation. Basal sand/gravel appeared relatively dense based on both the SPT and shear wave tests, and there were no other continuous areas of coarse-grained soils.

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Strength Loss Triggering in Fine-Grained Foundation Soils

• The potential for strength loss or cyclic failure of the fine-grained soils which comprise the majority of the foundation overburden was evaluated by 3 methods -Boulanger and Idriss, Seed et al, and Bray and Sancio

• The Boulanger and Idriss approach will indicate a potential for the soils to lose strength past the peak undrained strength, but not necessarily all the way to remolded strength

• Appears that the Seed et al and Bray and Sancio methods may assess the potential that the soils will go to remolded strengths

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Strength Loss Triggering in Fine-Grained Foundation Soils

• Boulanger and Idriss– Utilized vane shear tests and CPT results to measure resistance to

cyclic loading– Widespread cyclic failure across the entire valley– Strength loss would occur to some degree even during a 500-year

earthquake– Most widespread during earthquakes with return periods of 5,000

years or more

• Seed et al – Moisture content and Atterberg limits– 22 to 30 percent of all samples may be liquefiable

• Bray and Sancio– About 20 percent of the samples would be potentially liquefiable

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Assignment of Foundation Strengths

• Considered strength results from field vane shear testing, CPT, and laboratory tests on undisturbed samples – focused on clay strengths

• Used test data to estimate both peak undrained and remolded undrained strengths for clayey soils

• Estimated strengths in terms of reasonable low, best estimate, and reasonable high values

• Used different strengths for left and right sides of Scoggins Creek, as well as for different areas under the embankment - e.g. beneath crest, under downstream (and upstream) slope, and at downstream toe

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Assignment of Foundation Strengths

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Peak Undrained Strengths Remolded Undrained StrengthsLow

Estimate Su (psi)

Best Estimate Su (psi)

High Estimate Su (psi)

Low Estimate Sur (psi)

Best Estimate Sur (psi)

High Estimate Sur (psi)

Right Side of Valley (Approximate Dam Stations 6+00 to 10+00)Beneath Crest

12 19 25 5 7 10

Beneath D/S Shell

8 12 15 3.5 5 8

D/S Toe 4 5 10 1.5 2.5 5Beneath U/S Shell

6 8.5 12.5 2.5 3.75 6.5

Center and Left Side of Valley (Approximate Dam Stations 11+00 to 22+00)Beneath Crest

14 19 25 5 7 10

Beneath D/S Shell

9 12 16 3 5 8

Beneath U/S Shell

9 12 16 3 5 8

Post-Earthquake Stability Analyses

• SLOPE/W was used to assess post-earthquake stability• Stability evaluated at three different embankment cross

sections (stations 9+00, 15+00, and 21+00)• Modeled several different assumptions of strength loss• Different foundation strengths were used under various

portions of the embankment• Looked at deep-seated failure surfaces that would take

out the crest (ignored shallow failure surfaces that may have lower factors of safety but would be less likely to lead to dam failure)

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Post-Earthquake Stability Analyses – Typical Failure Surface

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Post-Earthquake Stability Analyses –Results

• Analyses consistently indicated that upstream failure surfaces resulted in higher factors of safety; thus, downstream failures pose greater risk of failure

• Embankment is not stable if earthquake loading leads to remolded/residual strengths (sur) in either the fine-grained or coarse-grained foundation soils

• Greatest chance for instability appears to be in that portion of the embankment located to the right of Scoggins Creek

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“Squashed Dam” Analysis• A “squashed dam” analysis refers to a progressive

analysis of the failed embankment using a limit equilibrium procedure

• SLOPE/W was utilized to iteratively determine the stability of the dam by applying a pseudo-seismic load on the dam and deforming the dam along the resulting failure surfaces

• After each failure, the dam geometry was changed to represent the estimated deformed dam, and the stability reanalyzed

• This analysis suggested that progressive sliding under seismic load could ultimately lead to the embankment deforming to almost half its original height

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“Squashed Dam” Results

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Newmark Deformation Analyses• Newmark displacements were calculated using two methods

– using QUAKE/W and using DYNDSP• Different time histories were used for the two types of

earthquake (local and subduction zone)• Two different embankment cross sections were analyzed –

Station 9+00 and Station 21+00• Different foundation overburden strengths were modeled.

Since Newmark analyses essentially require that the initial factor of safety must be above 1.0, not all strength assumptions (particularly the lower values) could be modeled

• Critical failure surfaces were selected from the post-earthquake stability analyses. The failure surfaces were deep-seated, and located in the downstream slope of the dam.

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Newmark Deformation Analyses - Results• Largest predicted Newmark deformations resulted from the

lowest strength assumptions, which correspond to the lower safety factors and lower yield accelerations (Sta. 9+00)

• Predicted deformations are much larger due to the subduction zone earthquake than due to the local earthquake

• Predicted Newmark deformations were similar whether calculated by DYNDSP or by QUAKE/W

• During the 50,000-yr subduction zone earthquake, very significant crest loss (on the order of 40 feet) is predicted even for a drained strength scenario - this is due to the long duration of severe shaking that results in frequent exceedance of the yield acceleration and thus makes large embankment deformations likely

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Newmark Analyses - Results

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Predicted Newmark Vertical Deformations at Station 9+00 Local Earthquake with USBR time histories

DYNDSP values shown first; QUAKE/W values follow in parentheses

Loading DrainedStrength

Best su Low su High sur

500-yr EQ 0.01 ft (n/c) 0.2 ft (0.2 ft) 0.8 ft (1.3 ft) 1.0 ft (1.6 ft)

1,000-yr EQ 0.2 ft (n/c) 0.9 ft (1.1 ft) 2.6 ft (3.5 ft) 3.0 ft (4.2 ft)

5,000-yr EQ 1.4 ft (n/c) 3.4 ft (3.6 ft) 8.0 ft (10 ft) 8.7 ft (12 ft)

10,000-yr EQ

1.9 ft (n/c) 4.6 ft (4.5 ft) 11 ft (13 ft) 12 ft (15 ft)

50,000-yr EQ

4.4 ft (n/c) 11 ft (9.4 ft) n/c (22 ft) n/c (27 ft)

Newmark Analyses - Results

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Predicted Newmark Vertical Deformations at Station 9+00Subduction Zone Earthquake with USBR time histories

DYNDSP values shown first; QUAKE/W values follow in parentheses

Loading DrainedStrength

Best su Low su High sur

1,000-yr EQ 0.3 ft (n/c) 5.0 ft (4.2 ft) 24 ft 28 ft

5,000-yr EQ 6.0 ft (n/c) 32 ft (28 ft) n/c (n/c) n/c (n/c)

10,000-yr EQ

14 ft (n/c) 52 ft (48 ft) n/c (n/c) n/c (n/c)

50,000-yr EQ

42 ft (n/c) 122 ft (70 ft) n/c (n/c) n/c (n/c)

QUAKE/W Deformations

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Scoggins - Sta 9 - E70 - h2

Deform

ation (ft)

Time (sec)

0

10

20

30

40

50

60

0 100 200 300

Scoggins - Sta 9 - E70 - h2

Deform

ation (ft)

Time (sec)

10

15

20

25

30

35

40

50 60 70 80 90 100

Scoggins - Sta 9 - E70 - h2

Deform

ation (ft)

Time (sec)

24

25

26

27

70 70.5 71 71.5 72

FLAC Analyses• FLAC (theoretically) has the advantage of being able to

estimate potential deformations using even the lowest assumed strength scenarios, while the Newmark analyses discussed above were limited to higher assumed strength scenarios

• As with the Newmark analyses, deformations were modeled at two stations – Station 9+00 and Station 21+00

• In addition, both the local and the subduction earthquakes were evaluated, as well as a number of different strength scenarios for the foundation overburden

• FLAC model deformation results were driven by the value of the reduced strength assigned to the foundation soils, as well as the severity and duration of dynamic loading

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FLAC AnalysesTypical deformed mesh

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FLAC (Version 6.00)

LEGEND

13-Nov-09 15:28 step 2079824Dynamic Time 3.6000E+01 7.552E+01 <x< 1.470E+03 -4.313E+02 <y< 9.634E+02

Exaggerated Grid DistortionMagnification = 1.000E+00Max Disp = 7.035E+01Exaggerated Boundary Disp.

Magnification = 0.000E+00Max Disp = 7.005E+01Max. shear strain increment 0.00E+00 1.00E+00 2.00E+00 3.00E+00 4.00E+00 5.00E+00 6.00E+00 7.00E+00

Contour interval= 1.00E+00Extrap. by averaging

-3.000

-1.000

1.000

3.000

5.000

7.000

9.000

(*10^2)

0.200 0.400 0.600 0.800 1.000 1.200 1.400(*10^3)

JOB TITLE : 50k-bbl, Low Sur, Sta 9+00

FLAC Analyses - Results

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Predicted Vertical Deformations (from FLAC) at Station 9+00 Local Earthquake (USBR time histories)

Loading DrainedStrength

Best su High sur Low sur

Gravity only not calculated not calculated not calculated 8 ft

1,000-yr EQ 1.5 ft 1.7 ft 3 ft 33 ft

5,000-yr EQ 4 ft 5 ft 7 ft 34 ft

10,000-yr EQ

5 ft 6 ft 9 ft 34 ft

50,000-yr EQ

9 ft 11 ft 15 ft 36 ft

FLAC Analyses - Results

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Predicted Vertical Deformations (from FLAC) at Station 9+00Subduction Zone Earthquake (USBR time histories)

Loading DrainedStrength

Best su High sur Low sur

Gravity only not calculated not calculated not calculated 8 ft

1,000-yr EQ 5 ft 6 ft 13 ft 43 ft5,000-yr EQ 15 ft 21 ft 31 ft not calculated

10,000-yr EQ

21 ft 30 ft 40 ft not calculated

50,000-yr EQ

34 ft 46 ft not calculated not calculated

Risk Analysisin

Reclamation

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Risk Analysis Overview• Reclamation uses quantitative risk analysis to aid in making

risk-informed dam safety decisions

• For Issue Evaluation (and higher level) studies, risk analyses involve a team of “experts” led by a facilitator

• Steps include PFMA, creation of event trees, discussion of factors influencing nodal probabilities, consensus assignment of probabilities and distributions, Monte Carlo analysis, team discussion of risk results, and ultimately portrayal of risk

• Risk numbers a key part of decision, but not the sole factor56

Measures of “Risk”• Reclamation’s Public Protection Guidelines define two

measures of acceptable performance for our dams

• The Annual Probability of Failure (APF) is the probability that the dam will fail in a given year, and is expressed as (Prob. of Loading) x (Prob. of Structural Response)

• The Annualized Life Loss (ALL) combines the probability of failure and the consequences. It is expressed by the equation (Prob. of Loading) x (Prob. Of Structural Response) x (Consequences)

• For Reclamation dam safety studies, “consequences” refer solely to loss of life

Potential Failure Modes

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Embankment Seismic Failure Modes

• Risk team brainstormed potential seismic failure modes for embankment; 12 mechanisms were identified

• Most were judged to pose low risk, or at least risks substantially below that posed by more critical failure modes

• Four failure modes were judged to pose potentially significant risks, and each of these was carried into the risk analysis and evaluated

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Brainstormed Failure Modes• Overtopping due to foundation liquefaction• Overtopping due to foundation strength loss in clays• Overtopping from Newmark displacements (no strength loss)• Internal erosion from cracking from Newmark displacements• Internal erosion from cracking from foundation liquefaction• Internal erosion from cracking from clay strength loss• Internal erosion from embankment/spillway separation• Internal erosion from cracking from left abutment landslide• Internal erosion from cracking from foundation fault offset• Overtopping from seiche wave – reservoir landslide• Overtopping from seiche wave – fault offset in reservoir• Internal erosion from differential settlement cracking

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Most Plausible/Critical Embankment PFMs

• PFM A - Dam overtopping (deformation > freeboard) due to slope failures caused by significant strength loss in foundation soils

• PFM B - Dam overtopping (deformation > freeboard) due to Newmark-type displacements (without significant strength loss in foundation soils)

• PFM C - Internal erosion resulting from cracking due to partial slope failures (and associated extensive cracking) caused by significant strength loss in foundation soils

• PFM D - Internal erosion due to cracking caused by Newmark-type displacements (without significant strength loss in foundation soils)

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Potential Failure Mode A

• Large earthquake causes strength loss in foundation soils, either due to liquefaction in coarse-grained soils or cyclic failure in fine-grained soils

• Strength loss leads to deep seated failure surface• After initial slide, progressive sliding possible due to long

duration of subduction zone earthquake• Slope failures result in a remnant of remaining

embankment that is lower than the reservoir level• Reservoir flows over the top of the remnant, resulting in a

fairly rapid breach by erosion

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Potential Failure Mode B

• Large and prolonged earthquake shaking leads to a Newmark-type slope failure in the embankment (due to accelerations repeatedly exceeding the yield acceleration)

• Progressive sliding (due to long duration of subduction zone earthquake) occurs along a deep-seated failure plane

• Slope failures result in a remnant of remaining embankment that is lower than the reservoir level

• Reservoir flows over the top of the remnant, resulting in a fairly rapid breach by erosion

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Potential Failure Mode C

• Large earthquake causes strength loss in foundation soils, either due to liquefaction in coarse-grained soils or cyclic failure in fine-grained soils

• Strength loss leads to deep seated failure surface, but not one that leads to overtopping

• Shearing and associated extensive cracking resulting from the slope failure create continuous seepage paths in dam

• Seepage begins to erode embankment materials• If no self-healing or intervention, dam breaches by gross

enlargement of seepage/erosion path or from progressive sloughing of downstream slope

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Potential Failure Mode D

• Large and prolonged earthquake shaking leads to a Newmark-type slope failure in the embankment (due to accelerations repeatedly exceeding the yield acceleration)

• Progressive sliding (due to long duration of subduction zone earthquake) occurs along a deep-seated failure plane, resulting in extensive shearing and cracking but no overtopping

• Seepage through the continuous shearing/cracking begins to erode embankment materials

• If no self-healing or intervention, dam breaches by gross enlargement of seepage/erosion path or from progressive sloughing of downstream slope

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Estimation of Failure Consequences

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Consequences

• Failure of Scoggins Dam would be expected to cause life-threatening flooding and significant property damage along Scoggins Creek and the Tualatin River

• Expected peak breach flow is about 675,000 ft3/s

• Inundation area includes a lumber mill and portions of several towns

• Quantitative approach used

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Estimation of Annual Probabilities

of Failure

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Risk Analysis Process

• Team effort, with multi-disciplined teams & facilitators• Evaluated embankment and spillway seismic failure

modes separately• Developed event trees to model failure modes• Estimated probabilities for each node/branch of tree

– Based on newly gathered data and latest analysis methods– Involved thorough team discussions– Used judgment (degree-of-belief estimates)

• Performed Monte Carlo analysis (10,000 iterations) to multiply nodal probabilities and estimate mean annual failure probabilities

• Reviewed estimates for reasonableness

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Embankment Risk Analysis

• Decided to combine the four potential failure mechanisms into two basic failure modes

• First: Dam overtopping resulting from seismic-induced deformations that exceed available freeboard – with or without significant strength loss in the foundation– includes Newmark-type deformations as well as flow slides

• Second: Internal erosion resulting from cracking in the embankment due to slope failures or Newmark-type displacement– with or without significant strength loss in the foundation

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Dam Overtopping Resulting from Large Deformations

• Probability of failure was estimated by using an event tree that included: type of ground motion model, probability of the earthquake loading, probability of widespread foundation strength loss, and probability that deformations would exceed freeboard

• Two event trees were considered: one for a subduction zone earthquake, and one for a local earthquake. The most severe loading condition, or the one that generated the highest risk, was used to represent the risks of this failure mode

Main Trunk of Event

Tree

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Branches for each Loading

Increment

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Ground Motion Model Weights• Several methods were used to de-aggregate the ground

motions – these included the “USBR method,” the CMS method, and a hybrid approach termed USBR-1

• Seismotectonic group suggested a weighting of 33% to each model (all potentially viable)

• Our screening analyses indicated little differences between USBR and USBR-1 approaches, but did show noticeable differences between USBR and CMS models (CMS ground motions were smaller and resulted in somewhat smaller deformations)

• Team decided to weight the models as 60% for the USBR and 40% for the CMS

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EQ Loading Increments• Increments were chosen to bracket the 5 return periods

developed in the ground motion study• No failures assumed for EQ smaller than 300-yr event

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Basic Return Period

Loading Increment Probability of Load

Approximate Ground Motion

Range < 300-yr 99.667 % < .23g

500-yr 300- to 800-yr 0.208 % .23 to .42g

1,000-yr 800- to 3,000-yr 0.092 % .42 to .76g

5,000-yr 3,000- to 8,000-yr 0.021 % .76 to 1.05g

10,000-yr 8,000- to 25,000-yr 0.008 % 1.05 to 1.42g

50,000-yr > 25,000-yr 0.004 % > 1.42g

EQ Loading Example

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Strength Scenarios• Key question is how the EQ loading will affect the strength

of the foundation soils• Our analyses assumed a number of different strength

scenarios including drained strengths, low/best/high peak undrained strengths, and low/best/high residual/remolded strengths

• For efficiency in event tree, 3 strength scenarios were developed– SS1 (lowest) – reasonable low to best estimate remolded

undrained– SS2 (intermediate) – reasonable high remolded undrained to

reasonable low peak undrained– SS3 (highest) – best estimate to reasonable high peak undrained

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Strength Scenarios (continued)• Since analyses did not show dramatic differences between

the USBR and CMS ground motions, strength scenarios were assumed to be the same for each

• Large differences in the intensity of subduction versus local earthquakes justified different strength scenarios

• A key factor influencing the assignment of probability estimates to the strength scenarios is that the deformation analyses indicated appreciable deformations during the larger subduction zone earthquakes, even with drained strengths. The potentially large straining of the soils suggested to the team that remolded strengths would be expected under those conditions.

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Team Estimate

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Probability of Strength Loss ScenariosLoad Increment Strength Scenario Probability during

Subduction EQProbability during

Local EQ

300-yr to 800-yrLow (SS1) .01 .01

Intermediate (SS2) .09 .09High (SS3) .90 .90

800-yr to 3,000-yrLow .32 .05

Intermediate .37 .35High .31 .60

3,000-yr to 8,000-yrLow .65 .15

Intermediate .27 .45High .08 .40

8,000-yr to 25,000-yrLow .78 .40

Intermediate .19 .50High .03 .10

> 25,000-yrLow .94 .60

Intermediate .06 .35High 0 .05

Predicted Deformations• For each type of EQ, each ground motion model (for the

subduction zone EQ), each loading increment, and each strength scenario, the team estimated the reasonable lower bound, best estimate, and reasonable upper bound of deformations that might be expected

• The estimates were based primarily upon the deformation analyses, which included both FLAC and two different Newmark approaches

• The estimated deformations were developed into probability functions

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Predicted Deformations (CSZ EQ)

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Expected Deformation (feet of Vertical Crest Loss) - Cascadia Subduction Zone Earthquake using USBR approach

Load Increment Type of Estimate SS1 SS2 SS3

300-yr to 800-yrUpper Bound 60* 5* 4* Best Estimate 33* 3* 2*Lower Bound 10* 1* 1*

800-yr to 3,000-yrUpper Bound 70 35 12 Best Estimate 40 20 5Lower Bound 10 5 2

3,000-yr to 8,000-yrUpper Bound 75 50 35 Best Estimate 45 30 20Lower Bound 20 10 5

8,000-yr to 25,000-yrUpper Bound 75 60 50Best Estimate 45 35 25Lower Bound 20 15 7

> 25,000-yrUpper Bound 75 65 60Best Estimate 45 40 30Lower Bound 20 15 12

Predicted Deformations (Local EQ)

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Expected Deformation (feet of Vertical Crest Loss) - Local Earthquake (identical for USBR and CMS approaches)

Load Increment Type of Estimate SS1 SS2 SS3

300-yr to 800-yrUpper Bound 60 5 4 Best Estimate 33 3 2 Lower Bound 10 1 1

800-yr to 3,000-yrUpper Bound 60 8 6 Best Estimate 33 4 3 Lower Bound 10 2 1

3,000-yr to 8,000-yrUpper Bound 65 15 8 Best Estimate 34 8 4Lower Bound 15 4 2

8,000-yr to 25,000-yrUpper Bound 65 20 10Best Estimate 34 10 5Lower Bound 15 5 2

> 25,000-yrUpper Bound 65 30 15Best Estimate 34 15 8Lower Bound 15 8 3

Probability of Dam Failure• During the Monte Carlo simulation, the probability of deformation

was sampled 10,000 times, as was the probability of the reservoir elevation (taken from the reservoir exceedance curve)

• The result of each sampling was a value of “residual freeboard,” which is the difference between the amount of deformation and the amount of pre-existing freeboard

• The risk team then developed a fragility curve that estimated the likelihood of dam failure for given amounts of residual freeboard

• Factors considered in developing the curve included the erodibility of the embankment, the severity of damage and expected configuration of the remnant, the filter compatibility of embankment zones, and whether materials could sustain a crack

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Probability of Dam Failure• Two fragility curves were developed, to differentiate

between very large and smaller amounts of deformation

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Fragility Curves

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

-10 -5 0 5 10 15 20 25

Residual Freeboard

P(f) Def < 20'

Def > 20'

Overtopping versus Internal Erosion Failures

• The fragility curves just shown account for either a sudden (overtopping-type) failure, or a failure resulting from internal erosion through the damaged embankment

• A sudden (overtopping-type) failure was assumed to result whenever the residual freeboard was less than 0.1 feet.

• When residual freeboard is greater than 0.1 feet, an internal erosion failure due to EQ-induced cracking was considered possible

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Resulting Embankment Risks

• Dam overtopping failure mode– Annual probability of failure estimated at 6x10-4

• Internal erosion failure mode– Annual probability of failure estimated at 1x10-4

• Subduction zone earthquake controlled the risks

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Key Factors Influencing Risk Numbers

• Very long duration and high peak accelerations associated with the Cascadia Subduction Zone earthquake

• Presence of silts and clays beneath dam that will likely experience some strength loss

• Large deformations predicted by different analysis techniques, even without strength loss

• Reservoir operations that result in a minimum freeboard of about 10 feet, and less than 20 feet approximately 50 percent of the time

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Sensitivity Studies

• Key observation is that most of the risk comes from the smaller earthquakes – the 1,000- and 5,000-year events

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Sensitivity Studies (continued)

• A number of sensitivity studies were conducted in which key variables were adjusted to determine the effect on the summary probability estimates

• Different scenarios included considering only USBR or only CMS ground motion models, looking solely at subduction or at local earthquakes, and evaluating risks for different strength assumptions, including only high strengths throughout

• For all these variations, the summary resulting annual probability of failure was always above 1x10-4

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Sensitivity Studies (continued)• Some of the key sensitivity models and results

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Results from Select Sensitivity Runs

Sensitivity Condition Annual Probability of Failure

Baseline Risk (no changes to team estimates) 7.5x10-4

Ground motions with only USBR method 7.6x10-4

Ground motions with only CMS method 7.4x10-4

Only local earthquake (no subduction zone) 2.2x10-4

Assuming peak undrained strengths during 1,000- and 5,000-year loading increments

4.0x10-4

Note: Results are the total values, or the sum of both the overtopping and internal erosion failure modes

Summary of Risks

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Summary of Mean Embankment Seismic Risks

FAILURE MODE

ANNUAL PROBABILITYOF FAILURE

Overtopping due to excessive deformations 6x10-4

Internal erosion due to EQ-induced cracking 1x10-4

All Risks Portrayed on

f-N Plot

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Risk Analysis Conclusions

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Risk Analysis Conclusions

• The estimated mean seismic risks from dam overtopping or internal erosion brought about by deformations resulting from earthquake shaking exceed Reclamation guidelines and provide justification to take risk reduction measures.

• The uncertainty with regard to these estimated risks does

not suggest a need for additional studies.

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Key Considerations

• The seismic hazard at Scoggins Dam is among the most severe earthquake loadings within Reclamation’s inventory of dams. The principal seismic source of concern is the Cascadia Subduction Zone, which has the potential for very large earthquakes with very long durations of strong shaking, and with relatively frequent return periods.

• Foundation soils within the footprint of Scoggins Dam are comprised largely of low density and low strength silts and clays, which have the potential to lose strength during earthquake shaking.

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Key Considerations• However, it is not necessarily the foundation strength

loss that poses the greatest concern. Current, state-of-the-practice analyses indicate that even without any strength loss, large embankment deformations are predicted. This is most likely due to Newmark-type displacements resulting from amplification of the bedrock accelerations within the embankment and resulting frequent exceedance of the yield acceleration.

New SOD Recommendation• 2010-SOD-A Initiate a Corrective Action Alternatives

Study to evaluate potential alternatives to mitigate the high risks of seismic failure modes of the embankment and spillway at Scoggins Dam.

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Corrective Action Study

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Input Parameters• Loadings revised based on CRB input and recent events

• Material properties revised based on CRB input and additional data analysis

• PFMs remain the same

• Several alternatives considered

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Modification Alternatives• Add downstream berm and shear key composed of

sandstone or rhyolite/basalt rockfill• Add downstream berm composed of rockfill with a soil

cement shear key• Secant piles near centerline and middle of downstream

slope for additional strength• New dam with concrete core and rockfill shells• New dam with concrete core and sandstone shells• Crest realignment with concrete core and rockfill shells• Crest realignment with concrete core and sandstone shells

Preferred Alternative

• Add downstream berm and shear key composed of sandstone or rockfill

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Downstream Berm• Significant modification to existing dam, with associated

cost and schedule

• Deformation is still significant, but much smaller than for existing dam

• Risks are significantly reduced, and meet Reclamation guidelines

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Summary• Loadings associated with Cascadia Subduction Zone

present a risk to Scoggins Dam• This is true even without strength loss in the foundation• Risk is driven by relatively short return period loads, not

“extreme” events• Risk mitigation alternatives are large and complex

• No easy solution

Questions ?

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