Transcript Slide 1

Scoggins Dam
Geotechnical Analysis and
Risk Analysis
1
July 18, 2012
Overview and General Topics
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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
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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
6
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 Used
Throughout 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
14
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:
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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
17
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
22
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:
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–
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–
–
–
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)
32
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 finegrained 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
Peak Undrained Strengths
Low
Best
High
Estimate Estimate Estimate
Su (psi)
Su (psi)
Su (psi)
Remolded Undrained Strengths
Low
Best
High
Estimate Estimate Estimate
Sur (psi)
Sur (psi)
Sur (psi)
Right Side of Valley (Approximate Dam Stations 6+00 to 10+00)
Beneath
12
19
25
5
7
Crest
Beneath D/S
8
12
15
3.5
5
Shell
D/S Toe
4
5
10
1.5
2.5
Beneath U/S
6
8.5
12.5
2.5
3.75
Shell
Center and Left Side of Valley (Approximate Dam Stations 11+00 to 22+00)
Beneath
14
19
25
5
7
Crest
Beneath D/S
9
12
16
3
5
Shell
Beneath U/S
9
12
16
3
5
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Shell
10
8
5
6.5
10
8
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 finegrained 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
45
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 postearthquake 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
47
Newmark Analyses - Results
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
Drained
Strength
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
50,000-yr
EQ
1.9 ft (n/c)
4.6 ft (4.5 ft)
11 ft (13 ft)
12 ft (15 ft)
4.4 ft (n/c)
11 ft (9.4 ft)
n/c (22 ft)
n/c (27 ft)
48
Newmark Analyses - Results
Predicted Newmark Vertical Deformations at Station 9+00
Subduction Zone Earthquake with USBR time histories
DYNDSP values shown first; QUAKE/W values follow in parentheses
Loading
Drained
Strength
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
50,000-yr
EQ
14 ft (n/c)
52 ft (48 ft)
n/c (n/c)
n/c (n/c)
42 ft (n/c)
122 ft (70 ft)
n/c (n/c)
n/c (n/c)
49
Scoggins - Sta 9 - E70 - h2
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QUAKE/W Deformations
40
30
20
Scoggins - Sta 9 - E70 - h2
10
40
0
100
200
300
35
Time (sec)
30
25
20
Scoggins - Sta 9 - E70 - h2
15
27
10
50
60
70
80
90
100
Time (sec)
D eform ation (ft)
0
D eform ation (ft)
D eform ation (ft)
50
26
25
50
24
70
70.5
71
Time (sec)
71.5
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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 Analyses
Typical deformed mesh
JOB TITLE : 50k-bbl, Low Sur, Sta 9+00
(*10^2)
9.000
FLAC (Version 6.00)
LEGEND
7.000
13-Nov-09 15:28
step 2079824
Dynamic Time 3.6000E+01
7.552E+01 <x< 1.470E+03
-4.313E+02 <y< 9.634E+02
5.000
Exaggerated Grid Distortion
Magnification = 1.000E+00
Max Disp = 7.035E+01
Exaggerated Boundary Disp.
3.000
Magnification = 0.000E+00
Max Disp = 7.005E+01
Max. 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
1.000
-1.000
-3.000
Contour interval= 1.00E+00
Extrap. by averaging
52
0.200
0.400
0.600
0.800
(*10^3)
1.000
1.200
1.400
FLAC Analyses - Results
Predicted Vertical Deformations (from FLAC) at Station 9+00
Local Earthquake (USBR time histories)
Loading
Drained
Strength
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
50,000-yr
EQ
5 ft
6 ft
9 ft
34 ft
9 ft
11 ft
15 ft
36 ft
53
FLAC Analyses - Results
Predicted Vertical Deformations (from FLAC) at Station 9+00
Subduction Zone Earthquake (USBR time histories)
Loading
Drained
Strength
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 ft
5,000-yr EQ
15 ft
21 ft
31 ft
not calculated
10,000-yr
EQ
50,000-yr
EQ
21 ft
30 ft
40 ft
not calculated
34 ft
46 ft
not calculated
not calculated
54
Risk Analysis
in
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 factor
56
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
58
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
59
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
60
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)
61
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
62
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
63
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
64
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
65
Estimation of Failure
Consequences
66
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
67
Estimation of Annual
Probabilities
of Failure
68
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
69
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
70
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
71
Main Trunk
of Event
Tree
72
Branches for
each Loading
Increment
73
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
74
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
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
75
EQ Loading Example
76
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
77
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.
78
Team Estimate
Probability of Strength Loss Scenarios
Load Increment
Strength Scenario Probability during Probability during
Subduction EQ
Local EQ
Low (SS1)
.01
.01
300-yr to 800-yr
Intermediate (SS2)
.09
.09
High (SS3)
.90
.90
Low
.32
.05
800-yr to 3,000-yr
Intermediate
.37
.35
High
.31
.60
Low
.65
.15
3,000-yr to 8,000-yr
Intermediate
.27
.45
High
.08
.40
Low
.78
.40
8,000-yr to 25,000-yr
Intermediate
.19
.50
High
.03
.10
Low
.94
.60
> 25,000-yr
Intermediate 79
.06
.35
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
80
Predicted Deformations (CSZ EQ)
Expected Deformation (feet of Vertical Crest Loss) Cascadia Subduction Zone Earthquake using USBR approach
Load Increment
300-yr to 800-yr
800-yr to 3,000-yr
3,000-yr to 8,000-yr
8,000-yr to 25,000-yr
> 25,000-yr
Type of Estimate
Upper Bound
Best Estimate
Lower Bound
Upper Bound
Best Estimate
Lower Bound
Upper Bound
Best Estimate
Lower Bound
Upper Bound
Best Estimate
Lower Bound
Upper Bound
Best Estimate 81
Lower Bound
SS1
60*
33*
10*
70
40
10
75
45
20
75
45
20
75
45
20
SS2
5*
3*
1*
35
20
5
50
30
10
60
35
15
65
40
15
SS3
4*
2*
1*
12
5
2
35
20
5
50
25
7
60
30
12
Predicted Deformations (Local EQ)
Expected Deformation (feet of Vertical Crest Loss) Local Earthquake (identical for USBR and CMS approaches)
Load Increment
Type of Estimate
SS1
SS2
Upper Bound
60
5
300-yr to 800-yr
Best Estimate
33
3
Lower Bound
10
1
Upper Bound
60
8
800-yr to 3,000-yr
Best Estimate
33
4
Lower Bound
10
2
Upper Bound
65
15
3,000-yr to 8,000-yr
Best Estimate
34
8
Lower Bound
15
4
Upper Bound
65
20
8,000-yr to 25,000-yr
Best Estimate
34
10
Lower Bound
15
5
Upper Bound
65
30
> 25,000-yr
Best Estimate 82
34
15
Lower Bound
15
8
SS3
4
2
1
6
3
1
8
4
2
10
5
2
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
83
Probability of Dam Failure
• Two fragility curves were developed, to differentiate
between very large and smaller amounts of deformation
Fragility Curves
1
0.9
0.8
0.7
P(f)
0.6
Def < 20'
Def > 20'
0.5
0.4
0.3
0.2
0.1
0
-10
-5
0
5
10
84
Residual Freeboard
15
20
25
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
85
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
86
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
87
Sensitivity Studies
• Key observation is that most of the risk comes from the
smaller earthquakes – the 1,000- and 5,000-year events
88
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
89
Sensitivity Studies (continued)
• Some of the key sensitivity models and results
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 1,000and 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
90
Summary of Risks
91
Summary of Mean
Embankment Seismic Risks
FAILURE
MODE
ANNUAL
PROBABILITY
OF FAILURE
Overtopping due to excessive
deformations
6x10-4
Internal erosion due to EQinduced cracking
1x10-4
92
Risk Estimates
Scoggins Dam
1.E-01
1.E-02
1.E-03
Annual Failure Probability, f
All Risks
Portrayed on
f-N Plot
1.E-04
1.E-05
1.E0
1
1.E-06
1.E-01
1.E-07
1.E-02
1.E-08
1.E-03
1.E-04
1.E-09
0.1
1
10
100
1000
10000
100000
Loss of Life, N
Static-IE of Embankment
Static-IE of Foundation
Static-IE Emb into Fnd
Hydrologic-Increased Threat of IE
Seismic-EQ-induced overtopping
Seismic-EQ-induced internal erosion
Seismic-Spillway wall failure
Seismic-Spillway pier failure
Total Static Risk Estimate
Total Hydrologic Risk Estimate
93
Total Seismic Risk Estimate
Total Probability of Failure - All Loadings
Notes:
Static and
hydrologic risks are
from the 2004 CFR.
Summary
hydrologic risk plots
below the bottom
axis of the graph.
Risk Analysis
Conclusions
94
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.
95
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.
96
Key Considerations
• However, it is not necessarily the foundation strength
loss that poses the greatest concern. Current, state-ofthe-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.
97
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.
98
Corrective Action Study
99
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
100
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
101
Preferred Alternative
• Add downstream berm and shear key composed of
sandstone or rockfill
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
103
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
104
Questions ?
105