Presentation on theme: "Geotechnical Aspects of Dam Safety"— Presentation transcript:
1Geotechnical Aspects of Dam Safety William Empson, PE, PMPSenior Levee Safety Program Risk ManagerU.S. Army Corps of EngineersRisk Management CenterDam Safety WorkshopBrasília, Brazil20-24 May 2013
2Geotechnical Aspects of Dam Safety Topics Concrete DamsTo be presented by Structural InstructorEarth and Rock Fill DamsFailure modesSeepageFiltersStabilityEmergency SpillwaysErosionGiving general overview of key aspects.Concrete Dams-TypeCauses of FailureFailure MechanicsFoundation Improvements Embankment Dams-Type, Causes of FailureSeepageSlope StabilitySpillway Erosion
3Geotechnical Aspects of Concrete Dams Failure Modes Foundation Leakage, Piping 11OvertoppingDeteriorationFlow ErosionGate FailureSlidingDeformationFaulty Construction 2*Lessons From Dam Incidents, ASCE/USCOLD 1975Foundation issues-piping, sliding, liquefaction, subsidence, settlementCharacteristics of consolidated foundations-hardness and strength, discontinuities (openings and fractures), weathering, and solutioning.Static failure can be divided into two broad categories: sliding and overturning. Primary failure mode.Seismic events:Can open foundation and abutment joints that can reduce foundation strength.Can increase infiltration of water along bedding planes and lead to erosion of shear zone material by piping.Induces liquefaction, which transforms loose cohesionless foundation soil into a fluid mass. Such liquefaction lowers bearing capacity, and induces settlement and or overturning.In Arch Dams, seismic failure occurs in the abutments.
4Geotechnical Aspects of Concrete Dams-Foundation Piping Infiltration of water along bedding planes can lead to erosion of joint filler or shear zone material by piping.The process can cause strength reduction as well as lead to sliding, settlement or overturning due to undermining of the dam.
5Geotechnical Aspects of Concrete Dams-Uplift Pressure Without proper means to relieve water seepage induced by the reservoir, it can literally caused the dam to float.During an earthquake excessive movements can open up faults and cracks in the foundation, which result in increased seepage and a corresponding rapid increase in uplift pressure.
6Geotechnical Aspects of Concrete Dams-Flow Erosion Flow erosion undermines the structure and can causes sliding and overturning.
7Geotechnical Aspects of Concrete Dams-Sliding Hydrostatic loads push the dam downstream, inducing shear failure along the concrete-rock contact or within the rock foundation.Sliding failures often result from deficiencies in the foundation.Low foundation strength.Bedding planes and joints containing weak materials.Seams of pervious material, if seepage through them is not controlled to prevent detrimental uplift.Infiltration of water along bedding planes due to open cracks and fissures can cause reduced foundation shear strength.Faults and shear zones.
8Geotechnical Aspects of Concrete Dams-Foundation Improvements Add grout curtains to reduce uplift pressures.Add foundation drains to reduce uplift pressures.Add grout blankets (consolidation grout) to improve bearing and resistance to overturning. It also seals joints and reduces seepage.Add rock anchors to reduce overturning and sliding.Add a buttress to the toe to resist sliding.Add toe reinforcement to prevent flow erosion.Add relief wells
9Geotechnical Aspects of Concrete Dams-Arch Dam Abutments Concrete arch dams are usually high in relation to their length, and their safety depends on the competence of the abutments.Failure is frequently induced by displacement of the foundation or abutment or by the erosion of the foundation or abutment when the dam is overtopped.Sliding along the foundation or high uplift pressures can induce bending, tension, and then cracking on the downstream face.An earthquake can induce tensile stresses in the rock causing cracking.An initial break usually develops into a complete failure of a dam.St. Francis Dam, NPR specials, 75th anniversary of failure.
10Geotechnical Aspects of Dam Safety Types of Embankment Dams Earth FillHydraulic FillHomogenous Rolled FillZoned Rolled FillRock fillDiaphragm Rock FillCentral Core Rock FillEarth Fill-Contains 50% or more fines to gravel size material.Rock Fill-Contains 50% or more cobble size or larger materialEmbankments dams can be built on either rock or soil foundations, because the rock foundation and abutments required to support the loads of a concrete dam are not necessary.Embankment dams are a preferred choice for sites with wide valleys and difficult foundation conditions because of their flexibility. However, soil is a difficult engineering material because of its three-phase nature, diverse composition, and our incomplete understanding of its behavior. Soil behavior under load is nonlinear, time dependent, and strain softening.Embankment dams are built from materials excavated at or near the dam site, usually with only minimal processing. This type of construction is considerably less costly than construction involving production of mass concrete.Principal disadvantage of an embankment dam is that it will be damaged or even destroyed by water erosion if insufficient height or spillway capacity allows overtopping.
11Geotechnical Aspects of Dam Safety Types of Embankment Dams Four kinds.
12Geotechnical Aspects of Earth Dams- Hydraulic Fill Dam Hydraulic fill dams were constructed by using water for transporting embankment material to its final position in the dam.In this method of construction, the material is discharged from pipes along the outside edges of the fill. Coarser material is deposited soon after discharge, while the fines are carried into the central portion of the fill. The result is a zoned embankment with a relatively impermeable core.Several problems are inherent to this type construction. Because of the slow drainage of water from the core, considerable settlement over a long period of time is expected. Also, this type construction leaves a relatively loose soil structure that is subject to liquefaction.Hydraulic Fill construction was economical prior to the advent of large earth moving and compaction equipment.The advent of this equipment made practical the construction of modern rolled-fill embankments.
13Geotechnical Aspects of Earth Dams Failure Modes Cause Failures Incidents TotalEmbankment PipingFoundation PipingOvertoppingFlow ErosionSlidingDeformationSlope Protection DamageDeteriorationGate FailureEarthquake InstabilityFaulty ConstructionHistorical Data of Embankment Dam Failures and Accidents to 1979 for Dams of Heights to Feet or Greater.Source-Development of Dam Engineering in the US by E. B. Kollgaard and W. L. Chadwick, 1988.
14Geotechnical Aspects of Earth Dams Failure Modes (Cont.) PipingAlong outlet conduitsThrough cracks across the impervious coreInadequately compacted core material at contact with uneven surfacesIn zones susceptible to erosion within the foundationOvertoppingInadequate spillway capacityLarge, rapid landslides in the reservoirToo little freeboardA Dam as a sloping ground surface wants to move downward and outward under the influence of gravity, i.e. sag.Often rapid failures are preceded by a period of slow movements. Therefore, it pays to review the instrument data periodically to recognize a stability problem in the early stages of its development.Failures due to subsurface erosion are catastrophic. They occur with little warning, usually at full reservoir, and occasionally many years after the reservoir is first put into operation.
15Geotechnical Aspects of Earth Dams Failure Modes (Cont.) Slope FailureDesign deficienciesNeglected remedial actionsInstabilityExcessive deformationsExcessive stressesExcessive loss of materials due to erosionDesign deficiencies-upstream slope too steep for sudden drawdown; crest width too narrow, inadequate seepage control and collect; poorly compacted soils.Neglected remedial actions-drainage features, vegetation, animal borrows.Earthquakes can cause sloughing (flow like movements) and loss of freeboard which increases chance for overtopping.Landslides along the reservoir rim can cause waves which may overtop the dam. Vaiont Dam, Italy.Transverse cracking allows water to flow through can lead to dam failure by erosion.Poor compaction along conduits or open joints in conduits allows flow that can induce erosion.
16Geotechnical Aspects of Earth Dams Failure Modes (Cont.) Earthquake conditionsExcessive deformationExcessive pore pressure buildupSudden densification of loose, saturated, non-cohesive soils that causes rapid build-up of pore fluid pressuresMentioned EQ aspect discussed by Joe Koester.
17Geotechnical Aspects of Earth Dams Technical Requirements Dam and foundation must be sufficiently watertight and have adequate seepage control for safe operationMust have “sufficient spillway and outlet capacity” as well as “adequate freeboard” to prevent over topping by the reservoirMust be stable under all loading conditionsThe structural safety of an embankment dam is dependent primarily on:the absence of excessive deformation and pore fluid pressure build-up under all conditions of environment and operations,the ability to safely pass flood flows,and the control of seepage to present migration of materials and thus preclude adverse effects on stability.
18Geotechnical Aspects of Earth Dams Seepage Seepage through the foundation or abutments causing piping or solutioning of rockSeepage through embankments, along conduits, or along abutment contacts causing piping or internal erosionWater stored behind a dam always seeks to escape or flow along a path of least resistance.There are four basic seepage problems, all which can lead to failure. (Intergranular seepage-flow thru pores.)Piping-Intergranular seepage that results in erosion (low or non-plastic soils.Piping occurs when water moving through pores of the soil exerts a tractive force on the soil particles through which it is flowing sufficient to remove them at the exit point.The removal of soil moves progressively upstream, forming the characteristic pipe from which the phenomenon derives its name.Internal erosion-Flow thru cracks that result in erosion (plastic soils)Solutioning of soluble rockInternal pressure and/or saturationMost common causes:Poor constructionInadequate compactionFailure to provide or meet filter criteriaLack of adequate treatment at the contact between the dam and foundation abutments.2
19Geotechnical Aspects of Earth Dams Through Seepage Example of phreatic surface.
20Geotechnical Aspects of Earth Dams Milford Dam, KS Picture stretched in horizontal direction.
21Geotechnical Aspects of Earth Dams Foundation Seepage Schematic of boil mechanism. Prominent in low/non-plastic soils. Common in levees along Mississippi and American Rivers.
22Geotechnical Aspects of Earth Dams- Hodges Village Dam - Seepage
23Geotechnical Aspects of Earth Dams Piping Into Voids The type of seepage illustrated above is difficult to detect since nothing is visible until the embankment starts to collapse, or until a vortex appears in the reservoir.A vortex is the rotational movement that will appear as the water rapidly enters the foundation, same action as pulling the plug in the bathtub.If you happen to be in a boat, get out of there.
24Geotechnical Aspects of Earth Dams Sink Hole, Clearwater Dam, MO
27Geotechnical Aspects of Earth Dams Internal Drains Example of internal chimney and blanket drain.
28Geotechnical Aspects of Earth Dams Blanket Drain Exit EmbankmentGravel swaleBlanket DrainEmphasize the need to keep open and functioning.FoundationProper configuration – facilitates free drainage
29Geotechnical Aspects of Earth Dams Blocked Drain Exit EmbankmentSwaleBlanket DrainExample of neglected maintenance or remedial repairs.FoundationImproper configuration – blocks drainage
30Geotechnical Aspects of Earth Dams- Uplift in Rock and Seepage Curwensville Dam, PA
31Geotechnical Aspects of Earth Dams Seepage Reduction Measures Go down list o measures to control leakage.Add toe trenches and relief wells-see next slide.
32Geotechnical Aspects of Earth Dams Toe Drains and Relief Wells
33Geotechnical Aspects of Earth Dams Emergency Repairs Example of how to treat seepage exiting an embankment. Mention heroic measures.Fabric for short term solution only. Clog.
34Geotechnical Aspects of Earth Dams Emergency Repair for Boils i = h / lExample of emergency treatment for boil.Sand bag to contain sediment and increase downstream pool elevation. Reduces head driving the boil.2
35Geotechnical Aspects of Earth Dams Conduits Prior to 1970, seepage collars were used on several projects. Experienced problems with inadequate compaction and or inadequate drainageGuidance to eliminate seepage collars and add appropriate filters and drainageSeepage collars – designers thought they would stop seepage2
36Geotechnical Aspects of Earth Dams Filter Design Facilitates the controlled flow of water and prevents movement of soil particlesCollection and controlAdequate carrying capacityPrevents migration of finesCriteriaPermeabilityStabilityFilters retain protected material, allow relatively free movement of water, and have sufficient discharge capacity.2
37Geotechnical Aspects of Earth Dams Slope Stability Type slopesEmbankment slopesCut slopesReservoir rim slopesFailure modesShallow SlideDeep SlideWedge (Block) SlideReview type slopesFailure modes in following slides.2
38Geotechnical Aspects of Earth Dams Shallow Slide Within the slope.
39Geotechnical Aspects of Earth Dams Shallow Slide Example of shallow slide.2
40Geotechnical Aspects of Earth Dams Deep Slide Extends into the foundation.
41Geotechnical Aspects of Earth Dams Waco Dam, TX Example of deep slide. Looks the same, but note displacement in downstream area.
42Geotechnical Aspects of Earth Dams Abutment Slide, Libby Dam, MT Reservoir Rim SlidesExample of abutment/reservoir rim slide.
43Geotechnical Aspects of Earth Dam Spillway Erosion Painted Rock Dam, AZ Example of spillway erosion, February 1993.Recent Texas floods, erosion block downstream area causing floodwaters to back-up.
45Earthquakes & Dams162 COE dams in high seismic areas (2 and above) subject to damageMost built in 1940’s and 1950’s with no seismic designSeismic design for liquefaction came into practice in the late 1970’s early 1980’s4321Location of Embankment DamsSeismic ZonesLow hazard to life & propertyHigh hazard to life & property
46Earthquake Engineering Seismic dam safetybecomes a priorityNear failure of Lower San Fernando DamSan Fernando Earthquake
47Earthquake Size Modified Mercalli I-XII Richter M 1-9 Local ML Intensity Scale Damage basedModified Mercalli I-XIIMagnitude Scales (Instrumental) Energy basedRichter M 1-9Local MLSurface Wave MsMoment Mw
48Comparison of earthquake energy release to the seismic energy yield of quantities of the explosive TNTRichter TNT for Seismic ExampleMagnitude Energy Yield (approximate)ounces Breaking a rock on a lab tablepounds Large Blast at a Construction Sitepoundston Large Quarry or Mine Blasttonstonstons,000 tons Small Nuclear Weapon,100 tons Average Tornado (total energy),000 tons,000 tons Little Skull Mtn., NV Quake, 1992million tons Double Spring Flat, NV Quake, 1994million tons Northridge, CA Quake, 1994million tons Hyogo-Ken Nanbu, Japan Quake, 1995; Largest Thermonuclear Weaponmillion tons Landers, CA Quake, 1992billion tons San Francisco, CA Quake, 1906billion tons Chilean Quake, 1960trillion tons (San-Andreas type fault circling Earth)trillion tons (Fault Earth in half through center)160 trillion tons of dynamite is a frightening yield of energy. Consider, however, that the Earthreceives that amount in sunlight every day.
55Earthquake Effects Permanent Ground Displacement >15 ft of thrust faulting created this waterfall and destroyed the bridge (Chi Chi Earthquake, Taiwan, 1999)
56Seismic Considerations in Dam Design Freeboard design pools, analysis -> design geometryCrack stoppers filters, transition zones, drains, material propertiesSeepage & pore relief well, weep holespressure controlFoundation stability siting, in situ: replacement, improvementEmbankment stability deformation and dynamic material properties
57Possible Earthquake Induced Modes of Failure Disruption of dam/levee by fault movement in foundationLoss of freeboard due to settlement or differential tectonic ground movementsSlope failures induced by ground motionsSliding of dam/levee on weak foundation materialsPiping failure through cracks induced by ground movementsOvertopping of dam/levee due to seiches in waterwayOvertopping of dam/levee due to slides or rockfalls into waterway
59Dams Failed by Earthquakes Sheffield Dam, CASanta Barbara Eqk 1925, 7 mi distanceSlide failure induced by liquefactionIzu Tailings Dams, JapanEarthquakes in 1978, M=7 and 5.7Slide failures induced by liquefactionWorld Total: 3 Dams
60Earthquake Performance of Dams Well built dams usually survive strong earthquake loading- Kirazdere Dam100 m height dam10 km from epicenter, M=7.4Izmut Turkey Eqk 1999
61Vulnerability 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 suchLiquefaction triggering analysisSeismic slope stability analysisPost-earthquake stability analysisDeformation analysis, if warranted
62Inspection 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, orM ≥ 8.0 w/in 200 miles, …immediate inspection is indicated.