screen analysis procedure for seismic slope stability 2014 file

8/12/2019 Screen Analysis Procedure for Seismic Slope Stability 2014 File

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Screen Analysis Procedure for

From:

J.P. Stewart, T.F. Blake, and R.A. Hollingsworth, Earthquake Spectra, Vol. 19, No.

3, pp. 697-212, 2003

Two important components of seismic

slope stability analysis:

,stability hazard.

Displacement Analysis when the slope fails thescreening, or when large displacement would becritical

Note: It is very important that thestrength evaluation procedures

follow the recommendations of Blake(2002)


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Methods to establish k, historical

compilation

k is the seismic coeficient giving a SF = 1 in the pseudo static analysis. Oftenreferred as ky

Makdisi & Seed (1978) related Newmarks displacement withM andky/kmax

LA County (1978): ky= 0.15 FS 1.10 regardless of seismicity

Seed (1979) (for earth dams) : ky= 0.1 forM = 6.5= 0.15 forM = 8.25

Slo e is safe if SF >1.15Strength Loss < 15 %, and acceptable displacement as high as 0.9 m

(displacement was calibrated with Newmarks sliding block

method).Notes: It is unreasonable to utilize k = amax or 0.65 amax, as the maximum

acceleration only occurred once for a very short period, and soil has the ability todeform to absorb the shaking.

Hynes-Griffin (1984), for earth dams, based on: Embankment shaking evaluated using linear elastic shear beam models after

Sarma (1979).

(1977) method.

Results were statistically correlated, i.e. amplifications (kmax/MHAr), depth ofsliding surface, Newmarks displacement, and ky/kmax.

The developed pseudo static procedure uses 95th %ile amplification valuefor deep sliding, with upper bound ky/kmaxvalue that produces a 1.0 mdis lacement.

The result is k = 0.5 MHArand screen is passed when SF 1.0, providedthat 80% of shear strength is used for non degrading materials. Method isnot recommended for areas subject to large earthquakes, embankments onliquefiable soils and embankments with small displacement tolerances


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Important Issues for Screening

Procedures

Level of tolerable displacements

Earthquake magnitude used for displacement analysis

Level of conservatism in interpretation of the statistical

distributions

Comparisons Limiting displacements: Seed (1979) & Hynes-Griffin

(1984), 100 cm for earth dams; Bray (1998)15-30 cm for

,

cm.

Earthquake Magnitude: SeedM = 8.25, Hynes-GriffinM =

3.8 to 7.7 (most around 6.6); Bray used duration ~M =

7-8, closer to 8.

Seed used upper bound displacement for given k /kmax,

Hynes-Griffin used 95% amplification level & upper bounddisplacements for given ky/kmax, Bray used near upper

bound amplification and 84% percentile displacements.


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Arguments

Need a screen procedure for residential and commercialdevelopments (mostly embankment dams and landfills).

residential and commercial developments.

The level of earthquake magnitude will be different for

residential and commercial developments.

The amplification and distributions from the said

embankments are at a different risk level than that of the

ground motions, i.e. ground motion ~ hazard analysis ~

return period. Slope displacement on that ground motionis extreme, rare, and having a longer return period.

Developments of a screening procedure

Purpose:

To obtain a filter where a site has no or low landslide

potential during earthquakes.

Outline of method:

Obtain a k value to be used in pseudo=static analysis

If FS 1 slo e asses the screen no more anal sis

If FS < 1, slope fails the screen, need displacement analysis.


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MHEA vs MHAr

Actually must useMHEA of the sliding body

k = feq*x MHEA/g

In practice

k = feqx MHAr/g

feq*

= feqx (MHAr/MHEA)

Brays (1998) results:

-r

propagation analysis

Normalize it toMHArx NRF(non linear response factor)

Not applicable to deep soft clays (Site Class E), for which

a site specific analysis is required.

Com ile data for different T in ut motion mean eriod

and Ts (small strain/elastic mean period of sliding mass ~Ts = 4H/vs)


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Data was for urban Ca,M = 6.5 to 7.5,r< 10 km,associated Tm,

For H > 20 m, and vs = 300 m/sec (soft rock & compacted fill) Ts/Tm >

and vs = 300 m/sec. or less

Not conservative forshallow surface slides (useAshford & Sitar 2002)

. r

For Ts/Tm < 0.5 averageMHEA/(MHArx NRF) ~1

k = feq* xMHEA/g = feq

* xMHArx NRF= feq xMHAr

We can use

k = feq xMHAr

if feq includes effectsof NRF

Formulation offeq

For the sameMHAr, larger earthquake causes more

dama e due to it lon duration and lon er T .

Bray & Rahtje (1998) run 309 Newmarks displacement

analysis, M = 6.25 to 8, 19 Input Rock Motions, and

ky/kmax= 0.2, 0.4, 0.6 and 0.8.

Correlate u (displacement) to amplitude of shaking (kmax= MHEA/g), significant duration of shaking D5-95, and

ky/kmax


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Influence of Mand r(and therefore

MHAr), and NRF, tofeq

From k = feq* xMHEA/g, when k = ky, feq

* represents the

reduction factor toMHEA (= kmax) to achieve a SF = 1 at* , eq y max.

Since we can simplify, for most casesMHEA/(MHArx NRF)

~ 1.0; feq = feq* x NRF= ky/ kmaxx NRF

Establish threshold u = 5 and 15 cm, should be

understood as indices, of conditions where very small or

moderate to small displacements are likely.

In the last equation, kmax=MHArx NRF/g, and D5-95 can be

estimated from available attenuation functions.

Thus feq can be calculated.


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Example


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Example

Medianfeqvalues


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Design Ground Motions

Must use a PSHA, usually 475 return period

Use deaggregation analysis for a specific site, to obtain

mode MagnitudeM and mode distance r

A severe deterministic scenario would have a higher

return period most of the times.


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Limitations

Many would not be familiar to the procedure Not applicable for soils with significant strain softening

Must be used with shear strengths as recommended by

Blake (2002)

Not applicable for slopes over soft clays.

Typical to California tectonic regions

Not applicable when 5 cm displacement is an

nappropr ate t res o .

Not applicable for surficial slopes and rock falls

screen analysis procedure for seismic slope stability 2014 file

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