Introduction to site effects Influence of the local geology on the - PowerPoint PPT Presentation

Introduction to site effects Influence of the local geology on the ground motion Fabian Bonilla (fabian.bonilla@ifsttar.fr) Université Paris Est - IFSTTAR 1

Presentation Outline ü Introduction and general concepts ü Empirical evidence of site effects ü Linear site response ü Nonlinear site response 2

Introduction and General Concepts 3

Site Response • Observed Records = (Source) + (Path) + (Site) Assumption • Site response is Linear • Source effects are common to each recorded data • Path effects are common to all recorded data 4

Grenoble example, France Ø Complex geology (3D) Ø Mountain basin Ø Fluvial deposits Ø Glacial deposits Ø Important urban development 5

Surface observations of site effects ü Amplification of the ground motion ü Increment of the signal duration ü Ground motion variability 6

San Francisco Bay (USA) 7

Garner Valley - USA 8

Site effect definition Output signal (sediment) Input signal (bedrock) Soil deposit Site effect = Output / Input (deconvolution) 9

Site response quantification • The response is broadband • The mean value is stable 10

Site effects Ø Definition: Influence of local geology on the seismic wave propagation. Site response is measured using the so-called transfer function Ø Linear site effects: the transfer function is independent of the input Ø Nonlinear site effects: strong feedback between the input and the medium Engineers design earthquake resistant structures including site effects (if present) 11

Empirical Evidence of Site Effects 12

Northridge M6.7, 1994 (USA) 13

Kobe M6.9, 1995 Japan • Near source effects • Site effects • Bad design 14

Theoretical Computation of Site Response 15

Site ¡effects ¡-­‑ ¡physical ¡basis Effect ¡of ¡the ¡local ¡geology ¡on ¡the ¡ground ¡mo9on – Refrac9on, ¡diffrac9on, ¡focaliza9on – Trapped ¡waves • ver9cal ¡reverbera9ons • horizontal ¡reverbera9ons ! + nonlinear soil behavior ! Consequences – Construc9ve ¡interference: ¡amplifica9on ¡ – Trapped ¡waves: ¡increase ¡of ¡the ¡seismic ¡dura9on – Resonance ¡of ¡fundamental ¡and ¡harmonic ¡modes Bard ¡(2006) 16

Where ¡does ¡the ¡amplifica9on ¡come ¡from? Bard ¡(2006) 1) ¡1D ¡case: ¡ 2) ¡2D ¡/ ¡3D ¡case: ver9cal ¡reverbera9ons Lateral ¡reverbera9ons ρ 1 , β 1, ζ 1 h ρ 2 , β 2, ζ 2 f o Broadband ¡effect f o =Vs/4H amplification amplification frequency frequency 17

Simplified Computation (1D) Amplification V 1 , ρ 1 H Soil Resonance Frecuency Rock V 2 , ρ 2 18

Why do we have site effects? Classical Effects New Effects (Taiwan) 19

Empirical estimation of site effects Spectral ratios (earthquakes) H/V (noise) Resonance frequencies and related soil amplification 20

Site effects in complex media (movies) 21

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Site Effects and Urban Planning 24

A structure has a fundamental period of vibration 10 stories T= 0,2 s T= 1,2 s T= 6,5 s 25

Amplification: Los Angeles at T=3s 26

Soil Classification Ground Type Parameter Vs30 (m.s^-2) [EC8] A >800 B 360 – 800 C 180 – 360 D <180 E C or D layer, underlain by stiffer material with Vs>800 m.s^-2 Vs : average shear wave velocity in the first 30 meters 27

Site Response and Ground Motion Attenuation 28

Chi-Chi M7.2, 1999 (Taiwan) 29

PGA – Distance Distribution 30

PGA – Distance Distribution 30

Nonlinear Site Effects 31

Chi-Chi M7.2, 1999 (Taiwan) Formation of a waterfall: 8 m 32

Chi-Chi M7.2, 1999 (Taiwan) Vertical displacement (~10 m) of the dam 33

Chi-Chi M7.2, 1999 (Taiwan) 34

Examples of landslides Tottori, Japan 2001 Chi-Chi, Taiwan, 2000 35

Liquefaction examples (free field) 36

Liquefaction of the soil foundation 37

Soil liquefaction of the bridge’s soil foundation 38

Kobe: Jan. 1995, M6.9 Vertical Settlement Lateral Spreading 39

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Hysteresis model Initial loading Backbone 40

Hysteresis model Initial loading Backbone 40

Hysteresis model Initial loading Backbone Unloading branch 40

Hysteresis model Initial loading Backbone Unloading branch 40

Hysteresis model Reloading Initial branch loading Backbone Unloading branch 40

Modulus degradation and damping curves Ø The shear modulus decreases G for increasing deformation levels Ø The damping increases proportionally to the deformation 41

How is the transfer function affected? 1. The shear modulus is computed as G= ρβ 2 2. The fundamental frequency of the soil is f 0 = β /(4H) 3. If G changes, so does β : if G(-) ---> β (-) ---> f 0 (-) ü Deamplification: the damping increases (pay attention) ü Increase of the signal duration (long period waves arrive later) 42

The effect of depth Initial conditions σ y σ y0 = ρ H g σ x0 = K 0 σ y σ x σ m0 = (2 σ x0 + σ y0 )/3 τ max = σ m0 sin( φ ) G/Gmax More nonlinearity at shallow depths More linear at depth 43

Numerical solution Why? Ø There is no analytical solution Ø Finite differences, spectral elements, finite elements methods Boundary conditions: Ø Surface: free surface effect Ø Bedrock: elastic boundary conditions (transmitted waves) or rigid boundary conditions (complete reflection) 44

Nonlinear Effects: TTRH02 Station (Japan) Site amplification is different for strong ground motion 45

Why do we have this difference? Load duration Deformation amplitude Note that we do not make any difference between large or small events 46

EPRI modulus reduction and damping curves 47