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!