Seismic Slope Stability (CruzLaconsayTamayo)

7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)

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By:

Shiela Laconsay

Glenn TamayoAndre Cruz


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Geologic, hydrologic, topographic, seismic and materialproperties must be obtained

Accuracy of the analysis is as good as the accuracy of the info


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Earthquakes expose slopes to dynamic loads that can reducethe soil shear strength and cause instability


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Are there materials in the slope that will lose

significant strength during cyclic loading?(e.g.,soil liquefaction)

Will the structure undergo significantdeformation that may jeopardize satisfactoryperformance?


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1. Analysis of Inertial Stability Pseudostatic - FS Newmark Sliding Block Analysis - displacement Makdisi-Seed displacement

Stress-Deformation Analysis

2. Analysis of Weakening Instability

-usually associated with liquefaction Flow Failure Analysis Deformation Flow

FS = resisting moment (constant)

static overturning moment+ Earthquake Force

FS = resisting moment (decreasing)

static overturning moment


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Earthquake loading isrepresented by a seismic

horizontal force, Fh

Fh=kW

Seismic coefficient k=a/g

Location of Fh is C.O.G of theslice (Terzhagi)

Recent dynamic analysis showsthat acceleration are amplifiedfrom bottom to top of dams


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Terzaghi (1950)

Severe -0.1

Violent -0.25

Catastrophic -0.5

Seed (1979)

K=0.15, FS =>1.15

Hynes-Griffin & Franklin (1984)

Use k=0.5*PGA, FS=> 1.0, 80% residual strengths


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FS = resisting momentstatic overturning moment

FS = resisting momentstatic + pseudo static


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Used to screen for potential seismic stability

problems

Especially for soils that are not expected to

lose a significant amount of there strengthwhen subjected to seismic loading


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kh = ( aref /g) * (a/aref)= 0.2 * 0.5= 0.1

k=0.15

Earth Dams

Landfills


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An earth structure that satisfies the FS in thescreening analysis criteria may still displacemore than 1m.

This does not mean it is safe for all levels ofperformance


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Pseudostatic method of analysis provides anindex of stability (FoS) but no information ondeformations associated with slope failures

Pseudostatic FoS varies throught anearthquake

Newmark considers behavior of slope whenFos is less than 1.0 i.e. the potential failuremass is not in equilibrium


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The yield coefficient, ky, is the horizontalpseudostatic coefficient that will produce anFoS of 1.0.


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Accuracy of a sliding block analysis dependson the accuracy of the input motion

Since the potential failure mass is assumed tobe rigid, the ground motion at the level of thefailure surface should be considered

In-phase for slopes with very stiff soilsand/orsubjected to low-frequency motion

Out-of-phase for slopes with softer slopes

and/or slopes subjected to higher frequencymotion.


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Simple yet rational approach based on anevaluation of the dynamic response of theembankment

Assumes that failure occurs on a well-definedslip surface

Assumes that material behaves elastically atstress levels below failure but perfectly plasticbehavior above yield


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1. Determine yield acceleration, ky

2. Determine earthquake-induced accelerations

3. If induced acceleration is greater than ky,

movements are assumed to occur along thefailure plane. Magnitude of displacement is

evaluated by double integration procedure


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Yield Accelerationaverage acceleration producing a horizontal inertia

force on a potential sliding mass so as to producea factor of safety of unity and thus cause it toexperience permanent displacements.

Inelasticbehavior


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Very little permanentdeformation

Substantial permanentstrain

Yield Acceleration


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Where:

F(t) = force force

acting along the

boundary of the

sliding mass

Kav = average

acceleration acting

on the sliding mass

at that instant in time

Time History of Earthquake- Induced Average Acceleration


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yh

Variation of Maximum AccelerationRatio with the Depth of Sliding Mass

Kmax = maximum average accelerationmax = maximum crest acceleration


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Factors Affecting Permanent Deformation dueto Earthquake Loading:1. Amplitude of induced average

accelerations, amplifying characteristics ofthe embankment and location of the

sliding mass within the embankment2. Frequency content of the averageacceleration time history

3. Duration of significant shaking


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Summary for several earthquakes and

dams and embankmentsAverage values


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135 ft high Chabot Dam during the 1906magnitude 8 San Francisco Earthquake

max = 0.57g

To = 0.99 sec

y/h =1.0

Ky= 0.14 0.14


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Kmax /max= 0.35

Kmax = 0.35*0.57g = 0.2g

0.35


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U/kmaxg To = 0.013 seconds

Thus, U = 0.013*0.2*32.2*0.99

= 0.08 ft = 2.4 cm


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Simple yet rational approach

Approximate and involves simplifying assumptions,

leading to conservative results Design curves need to be continually updated and

refined as analytical results for embankments are

obtained Significant improvement over the pseudo-static

approach but needs to be used with caution andgood judgment with regards to applicability

Where soil conditions cannot be determined with asignificant degree of accuracy, a more rigorousdynamic method would be more satisfactory

Documents

Seismic Slope Stability (CruzLaconsayTamayo)