COMBINED EFFECT OF GROUND PRESENCE AND HEAVING MOTION ON A RACE CAR WING
Identification of Failure Modes for Dam Safety Monitoring ... · Spillway Failures During Flood •...
Transcript of Identification of Failure Modes for Dam Safety Monitoring ... · Spillway Failures During Flood •...
Identification of Failure Modes for Dam Safety Monitoring and Evaluation (Part 2)
David Rees Gillette, PE, PhD
Causes of Embankment Dam Failures
Flood-Induced PFMs for Embankment Dams
• Overtopping• Internal erosion• Slope instability• Erosion by spillway flow• (Spillway failure)
● Flow over the top of the core
● Reservoir water acting against untested embankment or abutment areas
● Higher pressures and gradients
High Reservoir – Seepage Concerns
Flood Overtopping
Overtopping by Major Flood
Virginia Kendall Dam, Ohio USA, after being overtopped 35 cm deep by thunderstorm flood.
Note concrete core wall, which prevented breach.
Considering only the design crest elevation – small depth of overtopping flow over the length of the dam – no problem
Looking at the actual crest profile – sufficient flow depth at low area to start erosion and breaching process
Concentrated overtopping flow
Delhi Dam, Iowa, USA
Delhi Dam, Iowa, USA• Concrete core wall effective in
controlling seepage until reservoir rose above top of core wall.
• No internal drains.• Spillway capacity reduced because one
of three gates could not open all the way, apparently because of repairs that were not completed – Maintenance does affect dam safety!
Failure by Flood-induced Internal Erosion Leading to Slope Failure
Normal PoolLevel
Erosion of Embankment by Flood Discharge
Horseshoe Dam USA
Flood-Induced PFMs for Concrete Dams• Sliding• Overturning• Overtopping and foundation erosion
Overtopping flow erodes foundation material
Gibson Dam
Overtopping Erosion Protection for the Abutment
Gibson Dam - Modifications
Splitters to Introduce Air Under Overtopping Flow
Gravity Dam Sliding or Overturning
AB C D E
Increased Uplift Pressure
on Base
Normal RWS
Flood Level
½ γW H2
Spillway Failures During Flood
• Heaving of floor slab due to stagnation pressure –"slab jacking"
• Failure due to cavitation damage to concrete• Loss of material from foundation from internal
erosion or undercutting• Overtopping of chute walls due to inadequate
capacity
Uplift Pressures Under Chute Floor Slabs from Stagnation
Potentially high uplift pressure
Big Sandy Dam Spillway Failure due toStagnation Pressure and “Slab Jacking”
Big Sandy Dam SpillwayThe Results
Defense: Good Details for Joints in Floor of Chute
Continuous Reinforcement
Water Stop
Under Drain with Filter
Overlapped Slabs
Concrete CutoffAnchor Bar
CavitationBoiling of water due to localized vacuum from high-velocity flow. Collapsing bubbles cause shock waves that can erode concrete or even steel.
MethodsWhere cavitation can occur
See references –Cavitation in Chutes and Spillways
How bad it can getGlen Canyon Dam1983
Two years earlier
Erosion of Material From Beneath the Spillway
Voids beneath the spillway floor could lead to structural collapse of the spillway floor slabs
Erosion of Material From Beneath the Spillway
Voids beneath the spillway floor could lead to structural collapse of the spillway floor slabs
Damaged flow surface could result in flow erosion failure
FUENTE: HIDROVEN
Chute Overtopping - El Guapo Dam
FUENTE: HIDROVEN
FUENTE: HIDROVEN
FUENTE: HIDROVEN
Monitoring – Flood-Related PFMsPre-Flood Baseline• Photographs (embankment, spillway, etc.)• Aerial topographic survey• Measurement point survey
Flood Response• Visual observation (flow conditions, seepage, etc.)• Photographs/video• Monitor for other failure modes also
Post-Flood• Inspection• Measurement point survey• Aerial survey
Seismic PFMs for Embankment Dams• Liquefaction and instability• Crack erosion• Movement of fault in foundation
Liquefaction and Instability
Sheffield Dam, 1925 Santa Barbara Earthquake
Liquefaction of Foundations or Embankments
Liquefaction of EmbankmentLower San Fernando Dam, California1971 San Fernando Earthquake
Lower San Fernando Dam
La Marquesa Dam, 1985 Central Chile EQ, M 8.0, PHA≈ 0.6 g
Crest settlement 2.3 m (23%). Remnant freeboard < 1 m.
De Alba et al. (1988)
Failure of Fujinuma Dam, 2011 Tohoku Offshore Earthquake, Japan, 0.3-0.4 g
Photos: Geotechnical Extreme Events Reconnaissance Association, 2011
Caused by weak/sensitive silt/clay in foundation?
Flow appears to be on a level surface with some erosion resistance, consistent with clayey fill in field reports. Picture taken 25 minutes after earthquake.
Construction Shutdown and
Material Change?
From Matsumoto, Sasaki, and Ohmachi (2011)
Crack Erosion? Rogers Dam, Fallon Nevada Earthquake 1952
After rebuilding embankment. Note very small freeboard above stoplogs.
Zipingpu Dam, 10 km from Wenchuan Earthquake (7.9)
• 150 m high CFRD• Reservoir low at
time of EQ
www.connect.in.com
Settlement ~1/2 % of height; cracks 20 mm by "several m" at joints in upper part of face
www.internationalrivers.org
www.connect.in.com
Looking up u/s slope
Looking across u/s slope
www.bbc.co.uk
waterpowermagazine.com
But what if Zipingpu Reservoir had been full at the time of the earthquake?
What potential failure modes are there?
What governs their likelihood?
Movement of Foundation Faults
1906 San Francisco Earthquake: four embankment dams on San Andreas Fault, few details available.
Lower Howell – FailedUpper Howell – Didn’t
Why the difference? MAYBE because Lower Howell had a steel conduit through it at the site of the fault rupture. Most of the evidence was washed away, of course.
Langalda Dam, Iceland – Aseismic opening of "tectonic fractures" in foundation – no breach.
Failure by Foundation Fault Movement
Baldwin Hills Dam, California
Underdrain System
Foundation Faulting due to Oil Well Pumping – No Earthquake
Attempted Intervention
Baldwin Hills Dam, CA.Aseismicmovement of fault
Seismic PFMs for Concrete Dams• Sliding on foundation or lift lines• Overturning• Structural failure of arch dam
Sliding along disbonded or weak lift lines in earthquake (or flood)Could also occur under normal operations if there is plugging
of drains in the dam
Keyed Lift Line
Tensile Stresses Induced by Seismic Shaking
Crack
58
Pacoima Dam, CA (earthquake)• 113m high flood
control arch dam• 1971 M 6.6 San
Fernando and 1994 M 6.8 Northridge EQs
• Opening of joint between dam and ltthrust block, cracking of thrust block, left abut rock movements
• Reservoir was low, or dam might have failed.
Pacoima Dam
California
Left abutment thrust block
Pacoima Dam
Pacoima Dam• Opening of joint between dam and lt. thrust
block, cracking of thrust block, lt. abut. rock movements
• Reservoir was low, or dam might have failed
Sefid-Rud Dam, Iran
Concrete buttress dam subjected to coseismic movement of abutments, 1990 Manjil Earthquake
Sefid-Rud Dam
1990 Manjil EQ
Failure of Gateor Walls Under Earthquake Loading
Monitoring – Seismic PFMsPre-Earthquake Baseline• Seepage conditions (flow rate, piezometers)• Water pressures• Survey measurement points• Inclinometers• Visual
Earthquake Response• Visual inspection• Instrument readings (measurement points,
inclimometers, piezometers, flow rate)
Post-Earthquake Monitoring• Potential hidden damage
New seepage, or just relief of excess pore water pressure?
Monitoring During Extreme Loading Conditions (Floods and Earthquakes)Little can be done during event
- the key recognition and action in advance
Learn from lesser magnitude events- focused monitoring effort during the
event
Routine monitoring- baseline of pre-event condition- compare to post-event condition
“Things Happen...”• Gates do not operate or fail.• Stoplogs cannot be removed.• Power for operating the gates is lost.
– Storm outage or electrical line to gates lost.– Auxiliary power eitherDoes not exist.Does not function as planned.
• Access to site is lost.• Procedures break down.• Operational errors.
Operational ErrorsTaum Sauk Pumped Storage Facility, Missouri, USA
• 25 m high concrete-face rockfill dam on hilltop
• No spillway, small freeboard
• Water-level gauges were out of correct position, and pumps did not turn off.
• Failure of dam from overtopping leading to erosion and slope instability.
“You won’t findwhat you aren’t looking for.”
Random, unfocused activity is neither effective nor efficient.
Focused attention on key performance indicators is the best way to assure that a failure mode has not initiated or is not developing.
Potential Failure Mode Descriptions
Initiating Condition Flood or earthquake Unusually high reservoir level Deterioration/aging of structure Operator error or malfunction
Failure Mechanism Piping of core material through the foundation Erosion of the downstream slope and crest Sliding of a block of abutment rock
How Dam Failure Actually Occurs Seepage erosion eventually leads to breach Overtopping leads to breach Abutment slide leads to cracking and failure of
concrete structure
Potential Failure Modes Analysis
1. Assemble a team with the necessary expertise and knowledge:
EngineersGeologistsSeismologistsFlood HydrologistsDam OperatorsPFMAFacilitator
2. Study the dam, geology, and loadings, and develop "brainstorm" list of potential failure modes (PFMs).
Component Events
• What loading makes failure start?
• How and where does it start?
• And then what happens?
• And then what?
3. Decompose each PFM into its component conditions and events.
[0.3 to 0.4 g∩ High Res.]
WidespreadLiq’n?
SlopeInstability?
Crest Mvmt > Freeboard?
Cracks ->ErosionFailure?
NoFailure
NoFailure
OT Failure?
Failure
NoFailure
NoFailure
Failure
YY
Y
Y
Y
N
N
NN
N
Example event tree for seismic failure
Performance Monitoring Program
1. Routine visual monitoring2. Routine instrumented monitoring3. Periodic exam and review by specialists4. Earthquake response5. Flood response
Visual InspectionsExpected Performance
• Not easy to define
• Knowledge and experience are vital
• Understanding the potential failure modes, and how observations relate to them, helps greatly
Failure Mode Identification andPerformance Parameters
Dam safety concerns definedUnified, focused monitoring programReasons for monitoring are clearExpected performance definedCosts low - benefits high
Schedule for Periodic
Monitoring(L-23)
Discussion?
Shi Kang Dam,
Taiwan
The one that did not fail!