06 Walberg Calibration of Seepage and Stability Models for analysis...
Transcript of 06 Walberg Calibration of Seepage and Stability Models for analysis...
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Calibration of Seepage and Stability
Models for analysis of Dams and
Levees
Francke C Walberg
Consulting Geotechnical Engineer,
AECOM
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Focus
• Use of models for;
– Evaluation of existing dams and levees
– Rehabilitation
• Role of model calibration for improved
characterization
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Models
Some historical perspective
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Flow Nets: Harry Cedergren
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Levees:
Blanket
Theory
Analysis
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Force Equilibrium Slope Stability
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Analysis Models for Dams and Levees
• Seepage: typically FE or FD
• Slope Stability:
– usually Limit Equilibrium, satisfy both force and moment
– Deformation analysis (Seismic)
• Often used computer codes
– UTEXAS
– SEEP/W, SLOPE/W (GeoStudio)
– SLIDE (Rocscience)
– FLAC (ITASCA)
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Model Calibration
• Verification vs calibration
• Verification: can model get correct answer, i.e. are the mechanics correct? Ex. Development of UTEXAS
• Calibration for project, site specific:
– Get correct result for historic performance event
– enhance the model inputs so that the model will predict dam/levee performance for a give future load condition
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Model Inputs
• Geometry
• Geology/Stratigraphy
• Boundary conditions
• Water level
• Material Properties
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Geotechnical Data
• Survey both land and bathymetry
• Site Recon
• Geomorphology studies
• Subsurface investigations
• Borings, drilling and sampling
• Insitu testing SPT, CPT, geophysics, pump tests
• Instrumentation
• Laboratory testing
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Other Very Important Data
• Historic Performance Data associated with
previous loading
– Often overlooked
– Usually think of instrumentation data records
• But also observations of distress and level of
distress
• Often a full scale field test
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Historical
Performance
• Best indicator of future performance is past performance
• Value of historical performance, construction history and methodology
vs
geotechnical investigations for characterization of embankments and foundations
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Historic Performance Data
• Back analysis of historical performance
• Can be used to – Check reasonableness of model results
• Assumptions
• Input parameters
– Calibrate model: adjust to match performance
• Provides confidence in model results
• Allows model to better predict future performance
• Provides improved parameters for rehab design
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Seepage models
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SEEPAGE-Historic Performance
• Instrumentation
– Dams-piezometers
– Levees not likely
• Observations of distress
• Level of seepage
• Clear vs turbid
• Pin boils – larger boils
• Heave of ground surface
• Spongy to mattress ground consistency
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Seepage Level of Distress
Exit Gradient, i Level of Distress
i < 0.5 Light to Medium Seepage
0.5 < i < 0.8 Heavy Seepage, Boils Flowing Clear
i > 0.8 Boils Carrying Material (Action Required)
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Seepage: Hydraulic Conductivity
Parameters
• Classification based
• Laboratory tests
• Equations from grain size (Chapuis,2003/2004)
• Field pump tests
But don’t forget other field based:
• Values based on historic performance of levees
– Turnbull and Mansur (1959)
– USACE, Kansas City District (flooding early1950’s)
• Correlations (D10 vs k) (Turnbull and Mansur)
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Field Data from Historic Floods
• For thin blankets (<15 ft.)
– Laboratory and Presumptive k too low
– Historic performance data from Mississippi and
Missouri River floods demonstrate
• Macro structure has huge influence
• Riverside lower than landside
• Key parameter: khf/kvbl
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Eppley Airfield (Omaha)
Typical Section 1974 Levee
Documentation:
•Record drawings
•But no design document
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LM 7.43 Cross Section
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Seepage Models: Important
considerations for Levees• Methodology
– Performance/historic distress
– Past investigations, design and construction history
– Geomorphology
– Target subsurface investigations
• Boundary conditions– Extend water side boundary to river
– Seepage entrance conditions (landside blanket)
• Hydraulic conductivity– Consider macro structure
– Consider kf/kbl
• Calibrate model to match performance history
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Levees: Plan and Profiles
Selecting Analysis Sections
• Performance History
• Geomorphology
• HEM profiles
• CPT/boring logs
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Slope Stability
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Safety of Existing Dams
• Precedence of
– Thorough investigations of site conditions and
construction records
• Over
– Stability analysis
Ralph Peck (1988)
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Gardiner Dam
• Construction 1959-
1967
• Main embankment:
– 2500 meters long
– 64 meters high
• Fdtn: pre-sheared
bentonitic Bearpaw
Shale
Gardiner Dam
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Gardiner Dam
• Over 2 m of movement 150 m downstream but
almost none at toe
• Flattened slopes several times
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Gardiner River Section
• Movements occurred near end of construction
• Additional movements with reservoir filling
• Continued movements with each reservoir filling
• Rates are declining
• Limit equilibrium methods not definitive
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The problem with stability analyses for
existing dams
• If F<1 it must erroneous
• If F>1 it merely indicates the obvious
• If progressive movements are occurring it is irrelevant
– Because F is obviously close to one
– Monitor movement with each successive pool cycle
• Real proof: Decreasing increments for comparable reservoir fillings
Ralph Peck (1998)
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Leavenworth State Fishing Lake
• Dam designed and constructed early 1930’s
• 1994 buttressed downstream slope to improve stability
• After 10 years of adequate performance slumps in upper slope occurred
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Past Performance
vs Geotechnical
Investigations
• Past Performance Downstream slope – Performed adequately for 70 years
– Had not been well maintained
– Surficial slump
• Stability calculations– Based on assumed strengths and internal water level
– Led to low safety factors approaching 1.0
• With actual piezometric levels and more realistic strengths factor of safety probably acceptable
•
890
900
910
920
930
940
950
960
970
980
-200 -150 -100 -50 0 50 100 150 200
1.5H:1V
2H: 1V
Puddle Trench
2H: 1V
3H: 1V
Height: 60 Feet
Length: 1200 feet
Unzoned earthfill
Drains
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Leavenworth State Fishing Lake
• 1991 evaluation found concern for slope stability
• 1994 added additional embankment
900
910
920
930
940
950
960
970
980
-60 -10 40 90 140 190
FS = 1.5
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Slump On Downstream Slope
(2005)
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Previous Stability Analysis
Model
900
910
920
930
940
950
960
970
980
-60 -10 40 90 140 190
• Previous Stability Analysis– Doesn’t explain failure
– Critical shear surface not consistent with observations
– 1991 assumed water level and strength don’t support observations
• Model not calibrated
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Back Analysis• Shear strength = average “fully
softened” and residual = 20o
940
945
950
955
960
965
970
975
-50 -30 -10 10 30 50 70 90
FS = 1.0WL = 2'
below
surface
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Calibrated model
• Results Of Investigation
– Back analysis of slide provides rational explanation for cause of slumps
• progressive failure
• triggered by rainfall
– Consequences
• Slumps are shallow, associated with rainfall infiltration
• Not a threat to uncontrolled release of pool
– Piezometric levels near base of embankment, global stability not an issue
• Use calibrated model for remediation
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Role of Slope Stability Analysis
Existing Dams
• Peck, 1988
– Cannot overemphasize importance of past performance
– Engineers too quick to use stability calculations based on unsupported and unverifiable assumptions
– More difficult to do careful investigations followed by application of judgment based on instrumentation data and observations of past performance
• Use calibrated model to get correct parameters for remedial design
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Tuttle Creek Dam Seismic
Rehabilitation
Case History using calibrated soil constitutive model
for deformation analysis (both existing and
remediated conditions)
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Distance (ft) (x 1000)
-1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Ele
vatio
n (
ft)
(x
10
00)
0 .900
0 .925
0 .950
0 .975
1 .000
1 .025
1 .050
1 .075
1 .100
1 .125
1 .150
Deformation Analysis (TARA-FL)
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Fine Grained Blanket: Undisturbed
Sample Testing
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Undisturbed Sample Testing
Fine Grained Blanket Materials
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Deformation Analysis: FLAC
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FLAC
Deformation
Analysis
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Reasonable Result?
Figure 10.Comparison of crest settlements after
Swaisgood (2003) and FLAC results.
0.001
0.01
0.1
1
10
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Cre
st S
ett
lem
en
t (%
)
Peak Ground Acceleration at base of dam (g)
Tuttle Creek Dam
Concrete Faced Rockfill Dam
Earth Core Rockfill Dam
Hydraulic Fill
Earthfill
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Key Points
• Importance of construction and performance
history
• Use to calibrate analysis models
• Improved input parameters
• Greater confidence in results
• Model better able to predict future
performance