SITE EFFECTS MODELING BY HYBRID TECHNIQUE: THE CASE STUDY OF SOFIA, BULGARIA 27 o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October.

SITE EFFECTS MODELING BY SITE EFFECTS MODELING BY HYBRID TECHNIQUE: HYBRID TECHNIQUE:

THE CASE STUDY OF SOFIA, BULGARIATHE CASE STUDY OF SOFIA, BULGARIA

27o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY

UNIVERSITA’ DEGLI STUDI DI TRIESTEDIPARTIMENTO DI SCIENZE DELLA TERRA

BULGARIAN ACADEMY OF SCIENCE CENTRAL LABORATORY FOR SEISMIC MECHANICS AND EARTHQUAKE ENGINEERING

BULGARIAN ACADEMY OF SCIENCE

IVANKA PASKALEVA

PETIA DINEVA

CRISTINA LA MURA

GIULIANO F. PANZA

Main Topics Main Topics

HYBRID TOOL: a brief introduction; the viscoelastic isomorphism of Bardet

model: advantages and implementation in the hybrid technique;

simulation study: effect of the Bardet model, application to Sofia, Bulgaria;

conclusions


Review….Hybrid ToolsReview….Hybrid ToolsSite Effect: difference in earthquake ground motion between two nearby sites!

Lateral heterogeneities may give rise to complex wave patterns, so high performance methods have to be developed for simulating seismic wave propagation in complex media.

27o CONVEGNO GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY

In hybrid computational schemes the numerical method is applied in the laterally inhomogeneous part of the medium,whereasthe laterally homogeneous part is treated with analytical method.

Combination of MSM and BIEM is the hybrid technique

used in this work.

Review…..MSM - BIEM…Review…..MSM - BIEM…

• lateral inhomogeneity • surface topography

• water saturation in soils • seismic source properties

MSM is applied for numerical simulation from the sourceposition to the local region of interest, while the BIEM isused inside the local multilayered geological region.


It has the potential to investigate the combined effects of different physical phenomena, like

VISCOELASTIC ISOMORPHISM…VISCOELASTIC ISOMORPHISM…In order to describe the poroelastic properties of the layers, the viscoelastic isomorphism of Bardet is used.

A two phase solid - fluid system (saturated system) is used its main components are:

• dry rock (dry soil)

• solid grain

• fluid

n = Vp/V is the porosity V is the total volume of the solidVp is the pore volume

Kg g Kf f Kdry dry =(1 - n)g

bulk modulus and density


VISCOELASTIC ISOMORPHISM…VISCOELASTIC ISOMORPHISM…Elastic properties of the saturated system

sat = dry + nf = (1-n)g + n fdensity

shear modulus = dry = sat =

governing equation of motion in the frequency domain are

describing the wave propagation in fluid saturated soils Biot (1956)


VISCOELASTIC ISOMORPHISM…VISCOELASTIC ISOMORPHISM…

Q and R are the Biot’s elastic constants for saturated soils

….BIEM has a limited usage in dynamic poroelasticity dueto the multiphase nature of the model…… an equivalent viscoelastic model is proposed (Bardet 1992)

saturated soil is considered as a viscoelastic material


VISCOELASTIC ISOMORPHISM…VISCOELASTIC ISOMORPHISM…Bardet introduced the viscoelastic isomorphism by equating

Wave numbers in Biot’sporoelastic model

Wave numbers inviscoelastic model

from this operation an equivalence between the poroelastic material and the linear Kelvin - Voigt viscoelastic material

is established


so a two phase poroelastic dynamic behavior is approximated by the dynamic response of one-phase

viscoelastic material

SIMULATION STUDY…..SIMULATION STUDY…..FORMULATION OF THE PROBLEMLet us consider a finite local saturated geological region imbedded in a vertical inhomogeneous half space where a seismic source is buried

all soil layers in Ωo are isotropic, anelastic and homogeneous

the non-parallel layers in Ω are water saturated


SIMULATION STUDY…..SIMULATION STUDY…..The main aim is to obtain synthetic seismograms at sites along the free surface of the region Ω.

TWO SUB-PROBLEMS

• external sub-problem: wave propagation from seismic source to the boundary

• internal sub-problem: wave propagation in the finite local geological region Ω

The two problems are combined through the fulfillment of thecompatibility and equilibrium conditions at the boundary


SIMULATION STUDY…..SIMULATION STUDY…..

2 and 3 are water saturated, 1 is not saturated, points S’s are the sites where solutions are computed


SIMULATION STUDY…..SIMULATION STUDY…..EARTHQUAKE SCENARIO

two earthquake scenarios have been used, they do differ for the value of the magnitude M

strike angle 21o

dip angle 44o

rake angle 309o

source depth 12kmsrc/rec dist 20km

for both

Scenario a) M= 3.7 Scenario b) M= 6.5


SIMULATION STUDY…..SIMULATION STUDY…..EFFECT OF THE BARDET MODEL

peaks are shifted to lower frequencies

main peak decreases forsaturated soils

earthquake scenario a)







main peak decreases forsaturated soils

duration of the signal isreduced in saturated soils


one station calledREDUTA from theresidential district

it is located in;here there is the Geo Milev Park together with a

medical center, a library and

one of the mostprestigious schools

of Sofia.

CASE STUDY: SOFIACASE STUDY: SOFIA


CASE STUDY: SOFIACASE STUDY: SOFIA


another calledMLADOST4 for

the same reason as the former;

in this district the Business Park

Sofia has been

constructed and theairport is not so far

CASE STUDY: SOFIACASE STUDY: SOFIAThe parametric study of site effects in two different water

saturated geological profiles, corresponding to twodifferent zones of the city, is made in

terms of acceleration response spectral ratio, i.e.

RSR = RS from signals along the laterally varying section

RS from signals for the bedrock

the single - degree - of - freedom damped oscillator used for the computation of the response spectra is with

5% damping

the earthquake scenario is b), so M = 6.5


CASE STUDY: SOFIACASE STUDY: SOFIAREDUTA

0

5

10

15

20

0 0.2 0.4 0.6 0.8 1

Period (sec)

RSR

Site S 1 S2 S4

0

5

10

15

20

0 0.2 0.4 0.6 0.8 1

Period (sec)

RSR

Site S1 S2 S4

dry soil saturated soilhorizontal component

vertical component

0

9

18

27

0 0.2 0.4 0.6 0.8 1

Period (sec)

RSR

Site S1 S2 S4

0

9

18

27

0 0.2 0.4 0.6 0.8 1

Period (sec)

RSR

Site S1 S2 S4 27o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY

CASE STUDY: SOFIACASE STUDY: SOFIAMLADOST4

dry soil saturated soilhorizontal component

vertical component


CASE STUDY: SOFIACASE STUDY: SOFIAdifferent response for different sites: there are site effectsdue to the mixed influence of lateral inhomogeneity andsaturation of soil

there is a shift of the peaks toward the higher periods forsaturated soils the main peaks of saturated soils decreases with respectto dry soilsthe fluid acts as stiffener of the material: the pore pressureinduced by the seismic load resists the compression andstiffens the rockfluid acts as a viscous damping element: a diffusion processdominated by viscous effects


CONCLUSIONSCONCLUSIONS

The hybrid MSM - BIEM technique, combined with theviscoelastic isomorphism of Bardet, is applied for the

investigation of the site effects phenomena

due to several different physical properties like lateral inhomogeneity and water saturation in soil.

The hybrid technique allows to study simultaneouslythe combined effects, hence a better

estimation of the seismic risk is provided.

The simulation has been performed for the city of Sofia; Soil mechanical properties and geotechnical parameters are those more common in the central part of the city.

THANKS for the ATTENTION!!!27o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY

SITE EFFECTS MODELING BY HYBRID TECHNIQUE: THE CASE STUDY OF SOFIA, BULGARIA 27 o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October.

Documents

o convegno gruppo nazionale

geofisica della terra

o source depth

viscoelastic isomorphism

panza slide

o dip angle

o rake angle

equivalent viscoelastic