Site Effects

7/28/2019 Site Effects

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NATIONAL TECHNICAL UNIVERSITY OF ATHENSLABORATORY OF EARTHQUAKE ENGINEERING

Site effects on ground motion

Ioannis N. Psycharis


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NATIONAL TECHNICAL UNIVERSITY OF ATHENS Ioannis PsycharisLABORATORY FOR EARTHQUAKE ENGINEERING Site effects on the ground motion

Site effects

Local ground response

Influence of the soil response on the seismic motion at theground surface.

Usually it is considered through the nonlinear one-dimensional

response of a soil column.

Basin effects

Influence of two- or three-dimensional sedimentary basin

structures on ground motions, including wave reflections and

surface wave generation at basin edges.

Effect of surface topography

Ridges

Canyons

Slopes


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Basic definitions

Free surface motion

= the motion at the surface of a soil deposit

Bedrock motion

= the motion at the base of a soil deposit

Rock outcropping motion

= the motion at a location where bedrock is exposed at the

ground surface

Free surface Rock outcropping

Bedrock


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Local ground response


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Effect of soil on response spectra

Average normalized response spectra for 107 earthquake records

grouped in four soil categories (Seed et al. 1976)


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Ground motion amplification

Average spectral amplification vs. Vs-30recorded during the

1989 Loma Prieta earthquake (Borcherdt & Glassmoyer, 1994)

Amplification of SA Amplification of SV

(average for T=0.1-0.5 s) (average for T=0.4-2.0 s)


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Soil classification (EC8)

Typically based on the average shear wave velocity at the top 30

m of the soil profile (e.g. EC8)

hi = thickness of layer (m)

vi = elastic shear wave velocity

= no. of layers at the top 30 m of soil deposit

Ground

type DescriptionParameters

S,30 (m/s) SPT cu (kPa)A Rock or other rock-like geological formation > 800 _ _B Deposits of very dense sand, gravel, or very

stiff clay360 - 800 > 50 > 250

C Deep deposits of dense or medium-dense sand,gravel or stiff clay

180 - 360 15 - 50 70 - 250

D Deposits of loose-to-medium cohesionless soilor of predominantly soft-to-firm cohesive soil < 180 < 15 < 70

EA soil profile consisting of a surface alluviumlayer with vs values of type C or D andthickness varying between about 5 - 20 m,underlain by stiffer material with vs > 800 m/s

N,1i i

i30,S

v

h

30v


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EC8 Elastic response spectrum

, T(sec)

Se / ag

2.5S

S

0 C TD

2.5STC/T

2.5STCTD/T2

Groundtype (sec) C (sec) D (sec) S 0.15 0.40 2.50 1.00 0.15 0.50 2.50 1.20C 0.20 0.60 2.50 1.15D 0.20 0.80 2.50 1.35E 0.15 0.50 2.50 1.40


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EC8 Elastic response spectrum

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 1 2 3 4 5

Se/

ag

Period, (sec)

Type A

Type B

Type C

Type D

Type E


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Soil response for weak earthquakes

Small shear strains (< 10-5) elastic response

The elastic shear modulus, Gmax=Vs2, is used for thecalculation of the response

Profiles ofVs can be obtained from in situ measurements

(downhole, crosshole, geophysical techniques)


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Elastic response (homogeneous soil)

Shear cantilever behavior

Equation of motion

where

Eigenperiods

Eigenmodes

Participation factors

gs xz

uV

t

u

2

22

2

2

GVs

max

...,,i,V)i(

H

2112

4

si

H

z)i()z(

2

12sini

12

4i

)i(

2

max1

2

H

G

2

max2

2

3

H

G

2

max3

2

5

H

G

Bedrockg

x

H

z

u(z,t)Gmax, , Vs


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Elastic soil response

Surface amplification (Kanai 1962)

T = period of seismic waves

Tsoil = predominant period of soil

2

soilsoil

22

soil

301

1

1

1)(

T

T

T

.

T

T

TA

Bedrockg

x

soil , Vsoil , Tsoil

rock Vrock(period T)

rockrock

soilsoil

V

V

Soil


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Soil response for strong earthquakes

Large shear strains inelastic response

The secant shear modulus and the hysteretic damping areused for the calculation of the response through an iteration

procedure.

Relations ofG/Gmax and damping as functions of the shear

strain are given in the literature for common soil types.


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Nonlinear soil response

Definition ofsecant shear modulus and hysteretic damping

ED ES = Gs 2

S

D

4 E

E


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Nonlinear soil response

G/GmaxSand: Seed & Idriss Average

Clay: Seed and Sun, 1989

Damping (%)

Sand: Seed & Idriss Average

Clay: Idriss, 1990


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One-dimensional analysis

Considers effects of soil response on one-dimensional (nearly

vertical) wave propagation

Assumptions:

All soil layers are horizontal

SH-waves that propagate vertically from the bedrock

Cannot model: Slopping

Irregular ground surfaces

Basin effects

Embedded structuresIn such cases 2-D and 3-D analyses are required


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Equivalent linear analysis

Soil layers Equivalent MDOF lamped-mass model

(From Park & Hashash, 2004)


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Equivalent linear analysis

Iterative procedure

Make an initial estimation of shearmodulus and damping

Calculate the strain time-histories for

each layer for the given seismic motion

at the bedrock

Obtain the maximum strain values for

each layer and calculate the

corresponding effective shear strain

(~65% of peak strain)

Use the G/Gmax and curvesto obtain

better estimation of shear modulus and

damping

Repeat until convergence is reached

Limitation: Constant shear modulus anddamping is used during each iteration for thewhole time-history (overestimates stiffness)


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Verification

Comparison of recorded ground surface accelerations and

predictions by SHAKE

E-W component

(Borja et al. 1999)

N-S component


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Verification

Comparison of acceleration response spectrum for Treasure

Island strong motion (Loma Prieta, 1989 earthquake)

Recorded motion

Calculated

motion usingnearby rockrecordings ascontrol motion

(Idriss 1993)


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Basin effects


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Basin effect

Flat soil layer case

The seismic waves may resonate in the layer but cannot become

trapped

Basin case

The seismic waves become trapped within the basin if incidence

angles larger than the critical are developed

(Stewart et al. 2001)


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Basin effect

(Stewart et al. 2001,

Graves 1993)


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Basin effect

Amplification factors, defined relative to the prediction for

the site class, vs. basin depth (Field et al. 2000)


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Effect of surface topography


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Effect of ridges

Crest amplification maximizes

at wave lengths

corresponding to the ridge

half-width

Maximum spectral

amplification is about 1.6 for

this case

(Stewart et al. 2001,

Geli et al. 1988)


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Effect of canyons

Maximum amplification

near the canyon edge

at wave lengths similar

or smaller to the

canyon dimension

Maximum amplification

is about 1.4 for this

case

(Trifunac 1973)


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Effect of slopes

Maximum crest

amplification is about 1.2

(Stewart & Sholtis 1999)

Spectral amplification at crest ofa 21 m tall, 3:1 (h:v) slope


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NATIONAL TECHNICAL UNIVERSITY OF ATHENS Ioannis PsycharisLABORATORY FOR EARTHQUAKE ENGINEERING Sit ff t th d ti

EC8 Topographic amplification factors

EN-1998-5, Appendix A

Isolated cliffs and slopesA value ST > 1,2 should be used for sites near the top edge

Ridges with crest width significantly less than the base width

A value ST > 1,4 should be used near the top of the slopes for

average slope angles greater than 30 and a value ST > 1,2

should be used for smaller slope angles

Presence of a loose surface layer

In the presence of a loose surface layer, the smallest ST value

given above should be increased by at least 20%

Spatial variation of amplification factorThe value of ST may be assumed to decrease as a linear

function of the height above the base of the cliff or ridge, and

to be unity at the base.

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