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Corps of EngineersBUILDING STRONG®
Geotechnical Aspects of Dam SafetyWilliam Empson, PE, PMP Senior Levee Safety Program Risk ManagerU.S. Army Corps of EngineersRisk Management Center [email protected]
Dam Safety WorkshopBrasília, Brazil20-24 May 2013
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Geotechnical Aspects of Dam SafetyTopics
Concrete Dams►To be presented by Structural Instructor
Earth and Rock Fill Dams►Failure modes►Seepage►Filters►Stability
Emergency Spillways►Erosion
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Geotechnical Aspects of Concrete DamsFailure Modes
Foundation Leakage, Piping 11 Overtopping 9 Deterioration 6 Flow Erosion 3 Gate Failure 3 Sliding 2 Deformation 2 Faulty Construction 2
*Lessons From Dam Incidents, ASCE/USCOLD 1975
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Geotechnical Aspects of Concrete Dams-Foundation Piping
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Geotechnical Aspects of Concrete Dams-Uplift Pressure
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Geotechnical Aspects of Concrete Dams-Flow Erosion
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Geotechnical Aspects of Concrete Dams-Sliding
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Geotechnical Aspects of Concrete Dams-Foundation Improvements
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Geotechnical Aspects of Concrete Dams-Arch Dam Abutments
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Geotechnical Aspects of Dam SafetyTypes of Embankment Dams
Earth FillHydraulic FillHomogenous Rolled FillZoned Rolled Fill
Rock fillDiaphragm Rock FillCentral Core Rock Fill
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Geotechnical Aspects of Dam SafetyTypes of Embankment Dams
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Geotechnical Aspects of Earth Dams- Hydraulic Fill Dam
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Geotechnical Aspects of Earth DamsFailure Modes
Cause Failures Incidents Total
Embankment Piping 23 14 37Foundation Piping 11 43 54Overtopping 18 7 25Flow Erosion 14 17 31Sliding 5 28 33Deformation 3 29 32Slope Protection Damage 0 13 13Deterioration 2 3 5Gate Failure 1 3 4Earthquake Instability 0 3 3Faulty Construction 0 3 3
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Geotechnical Aspects of Earth DamsFailure Modes (Cont.)
Piping► Along outlet conduits► Through cracks across the impervious core► Inadequately compacted core material at contact
with uneven surfaces► In zones susceptible to erosion within the
foundation Overtopping
► Inadequate spillway capacity► Large, rapid landslides in the reservoir► Too little freeboard
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Geotechnical Aspects of Earth DamsFailure Modes (Cont.)
Slope Failure► Design deficiencies► Neglected remedial actions
Instability► Excessive deformations► Excessive stresses► Excessive loss of materials due to erosion
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Geotechnical Aspects of Earth DamsFailure Modes (Cont.)
Earthquake conditions► Excessive deformation► Excessive pore pressure buildup► Sudden densification of loose, saturated, non-
cohesive soils that causes rapid build-up of pore fluid pressures
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Geotechnical Aspects of Earth DamsTechnical Requirements
Dam and foundation must be sufficiently watertight and have adequate seepage control for safe operation
Must have “sufficient spillway and outlet capacity” as well as “adequate freeboard” to prevent over topping by the reservoir
Must be stable under all loading conditions
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Geotechnical Aspects of Earth Dams Seepage
Seepage through the foundation or abutments causing piping or solutioning of rock
Seepage through embankments, along conduits, or along abutment contacts causing piping or internal erosion
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Geotechnical Aspects of Earth Dams Through Seepage
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Geotechnical Aspects of Earth Dams Milford Dam, KS
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Geotechnical Aspects of Earth Dams Foundation Seepage
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Geotechnical Aspects of Earth Dams- Hodges Village Dam - Seepage
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Geotechnical Aspects of Earth DamsPiping Into Voids
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Geotechnical Aspects of Earth DamsSink Hole, Clearwater Dam, MO
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Geotechnical Aspects of Earth DamsInternal Drains
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Geotechnical Aspects of Earth Dams Blanket Drain Exit
Embankment
Foundation
Blanket Drain
Gravel swale
Proper configuration – facilitates free drainage
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Geotechnical Aspects of Earth DamsBlocked Drain Exit
Embankment
Foundation
Blanket Drain
Swale
Improper configuration – blocks drainage
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Geotechnical Aspects of Earth Dams-
Uplift in Rock and Seepage
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Geotechnical Aspects of Earth Dams Seepage Reduction Measures
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Geotechnical Aspects of Earth DamsToe Drains and Relief Wells
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Geotechnical Aspects of Earth DamsEmergency Repairs
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Geotechnical Aspects of Earth Dams Emergency Repair for Boils
i = h / l
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Geotechnical Aspects of Earth Dams Conduits
Seepage collars – designers thought they would stop seepage
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Geotechnical Aspects of Earth DamsFilter Design
Facilitates the controlled flow of water and prevents movement of soil particles►Collection and control►Adequate carrying capacity►Prevents migration of fines
Criteria►Permeability►Stability
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Geotechnical Aspects of Earth DamsSlope Stability
Type slopes►Embankment slopes►Cut slopes►Reservoir rim slopes
Failure modes►Shallow Slide►Deep Slide►Wedge (Block) Slide
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Geotechnical Aspects of Earth Dams Shallow Slide
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Geotechnical Aspects of Earth DamsShallow Slide
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Geotechnical Aspects of Earth DamsDeep Slide
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Geotechnical Aspects of Earth Dams Waco Dam, TX
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Geotechnical Aspects of Earth DamsAbutment Slide, Libby Dam, MT
Reservoir Rim Slides
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Geotechnical Aspects of Earth DamSpillway Erosion
Painted Rock Dam, AZ
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Earthquakes & Dams
• 162 COE dams in high seismic areas (2 and above) subject to damage
• Most built in 1940’s and 1950’s with no seismic design
• Seismic design for liquefaction came into practice in the late 1970’s early 1980’s
43210
Seismic ZonesLocation of Embankment Dams
Low hazard to life & property High hazard to life & property
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Earthquake Engineering
Near failure of Lower San Fernando DamSan Fernando Earthquake - 1971
Seismic dam safetybecomes a priority
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Earthquake SizeIntensity Scale Damage based
Modified Mercalli I-XII
Magnitude Scales (Instrumental) Energy basedRichter M 1-9Local MLSurface Wave MsMoment Mw
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Comparison of earthquake energy release to the seismic energy yield of quantities of the explosive TNT
Magnitude Energy Yield (approximate) -1.5 6 ounces Breaking a rock on a lab table 1.0 30 pounds Large Blast at a Construction Site 1.5 320 pounds 2.0 1 ton Large Quarry or Mine Blast 2.5 4.6 tons 3.0 29 tons 3.5 73 tons 4.0 1,000 tons Small Nuclear Weapon 4.5 5,100 tons Average Tornado (total energy) 5.0 32,000 tons 5.5 80,000 tons Little Skull Mtn., NV Quake, 1992 6.0 1 million tons Double Spring Flat, NV Quake, 1994 6.5 5 million tons Northridge, CA Quake, 1994 7.0 32 million tons Hyogo-Ken Nanbu, Japan Quake, 1995; Largest Thermonuclear Weapon 7.5 160 million tons Landers, CA Quake, 1992 8.0 1 billion tons San Francisco, CA Quake, 1906
8.5 5 billion tons Chilean Quake, 196010.0 1 trillion tons (San-Andreas type fault circling Earth)12.0 160 trillion tons (Fault Earth in half through center)
160 trillion tons of dynamite is a frightening yield of energy. Consider, however, that the Earth receives that amount in sunlight every day.
Richter TNT for Seismic Example
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New Madrid Earthquakes, 1811-1812 (Isoseismals)
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Earthquake Effects
Transient loading or shaking Changes material properties Settlement Liquefaction Permanent ground displacement Dynamic response
►Each thing has it own shaking response
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Buildings
Bridges
Problem: Earthquake Induced Liquefaction Causes Failures
Slide in Lower San Fernando Dam - 1971 Dams
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Earthquake Effects
Liquefaction►Sand boils►Settlement►Slope failures
Alluvial valleys often involve liquefiable materials
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Earthquake Effects
Liquefaction►Sand boils►Settlement►Slope failures
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Seismic Failure Mechanism
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
Elev
ation (ft) (x 1000)
0.900
0.925
0.950
0.975
1.000
1.025
1.050
1.075
1.100
1.125
1.150
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Earthquake Effects Permanent Ground
Displacement
>15 ft of thrust faulting created this waterfall and destroyed the bridge (Chi Chi Earthquake, Taiwan, 1999)
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Seismic Considerations in Dam Design
Freeboard design pools, analysis -> design geometry Crack stoppers filters, transition zones, drains, material properties Seepage & pore relief well, weep holes pressure control Foundation stability siting, in situ: replacement, improvement Embankment stability deformation and dynamic material properties
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Possible Earthquake Induced Modes of Failure
Disruption of dam/levee by fault movement in foundation Loss of freeboard due to settlement or differential tectonic
ground movements Slope failures induced by ground motions Sliding of dam/levee on weak foundation materials Piping failure through cracks induced by ground
movements Overtopping of dam/levee due to seiches in waterway Overtopping of dam/levee due to slides or rockfalls into
waterway
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Taiwan earthquake
Dams Damaged by Earthquakes
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Dams Failed by Earthquakes Sheffield Dam, CA
► Santa Barbara Eqk 1925, M=6.3 @ 7 mi distance
► Slide failure induced by liquefaction Izu Tailings Dams, Japan
► Earthquakes in 1978, M=7 and 5.7 ► Slide failures induced by liquefaction
World Total: 3 Dams
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Earthquake Performance of Dams
Well built dams usually survive strong earthquake loading
- Kirazdere Dam 100 m height dam 10 km from epicenter, M=7.4 Izmut Turkey Eqk 1999
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Vulnerability Assessment(Phased approach, to be detailed in upcoming new EM
1110-2-6001)
Seismic vulnerability of levees and dams are similar and are evaluated as such
► Liquefaction triggering analysis
► Seismic slope stability analysis
► Post-earthquake stability analysis
► Deformation analysis, if warranted
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Inspection After Earthquake (paraphrased from USSD Guidelines for
Inspection of Dams After Earthquakes, 2003)
If an earthquake is felt at or near the dam (levee), or has been reported to occur, with:► M ≥ 4.0 w/in 25 miles, ► M ≥ 5.0 w/in 50 miles, ► M ≥ 6.0 w/in 75 miles, ► M ≥ 7.0 w/in 125 miles, or ► M ≥ 8.0 w/in 200 miles, …immediate inspection is
indicated.
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Thank You !