Site characterization and seismic codes

Kyriazis Pitilakis, ProfessorEvi Riga, Civil Engineer, M.Sc.

Dr. Roula Roumelioti

Aristotle University of Thessaloniki, GreeceLab. of Soil Mechanics, Foundations and Geotechnical Earthquake Engineering

2015 ORFEUS Annual Observatory Coordination meeting

Bucharest, 23 September 2015

1

Aim

Importance of soil and site characterization in Earthquake Engineering and Engineering Seismology.

What do we mean with site characterization? and for what purpose?

Understanding ground motion?

Research oriented?

Seismic design of structures?

Codes?

Risk assessment?

2

Aim

Reduce uncertaintiesat least the epistemic ones!

Increase safety within reasonable margins

3

More data or better data

More data generally increase uncertainties

Better constrained and well focused data is what we really need

Good and sufficient records in various rock conditions are still very few worldwide!

4

5

Objectives

Soil-site characterization for seismic codes (EC8)

Seismic codes: Ordinary structures and “normal” soil-site conditions

Ground shaking characteristics for “normal” soil-site conditions

Basin and topographic effects

Seismic codes: Special soil-site conditions (beyond A,B,C,D soil classes of EC8)

Liquefaction, precarious slopes

Near field conditions

6

• Importance of geological information and data• Tectonics, active faults, fault mechanism, distance measure, azimuth effects• Near field conditions, long period pulses, fault normal-parallel motions• Hazard occurrence [NDP, HP, LP]

• Geometry of geological formations, lateral geological discontinuities, topography and basin geometries

• Most of our ideas and ways of tackling the problem of ground motion evaluation are stemming from 1D wave propagation theory

• Rock basement, depth, characteristics i.e. Vs, Vp

• Records on real rock basement or outcrop are limited

• Water table, saturation, seasonal variations, pore pressure

Miscellaneous

7

• Uncertainties in site characterization: How important they really are?

• Geotechnical parameters are practically measured locally. Their extrapolation to large 3D structures involves several uncertainties

• Laboratory and in situ tests and surveys (local or global?) • Good correlation of in situ geotechnical and geophysical surveying and

testing methods i.e. SPT, CPT, SASW etc with Vs and shear strength parameters: It is of paramount importance.

• The correlation of site characterization and soil classification parameters i.e. Vs, T0, strength, compressibility etc with different Intensity Measures IM used for seismic design and performance assessment of various structures is still rather poor.

• Appropriateness of IM for different structural typologies

Miscellaneous

8

We should also keep in mind that in engineering practice soil-site classification and the parameters describing this classification are also used for other design purposes like:• Earthquake induced settlements• Seismic bearing capacity of shallow and deep foundations• Seismic design of foundations to ground motions and permanent ground

displacements• Seismic design of retaining walls• Seismic design of underground structures and pipelines• Soil-foundation-structure interaction effects

Moreover the soil classification in EC8 and the proposed parameters should be conforming with EC7 soil parameters

Other issues

9

• Introduction- General comments

• Site – soil classification and site-dependent elastic response spectra• Is Vs,30 appropriate for site – soil classification?• New site – soil classification • New elastic response spectra and amplification factors• Demand spectra• Effects of subsurface geology – Basin effects• Soil strength parameters and G-γ-D curves• Topography effects• Liquefaction• Seismically precarious slopes• Summary of parameters needed for soil and site characterization

• EUROSEISTEST data base and portal

Outline

10

• For the seismic design of structures using the current seismic codes the site of interest must be classified into one of the soil categories adopted by the code. Based on the soil class the appropriate site-dependent design spectrum can be defined.

• Site categorization schemes of the seismic codes use different description of geological and geotechnical parameters to define the soil classes. The most commonly used parameter is the Vs,30, i.e. the average shear wave velocity of the top 30m of the soil profile.

Site – soil classification

11

Site – soil classification (U.S. seismic codes)

• U.S. seismic codes prior to 1994 (e.g. 1978 ATC provisions) proposed four soil types characterized by both qualitative and quantitative criteria, including type, thickness and shear wave velocity.

• In post 1994 U.S. seismic codes (e.g. the 1994 and 1997 editions of NEHR and the 2000 International Building Code) a new soil categorization scheme was introduced, which uses Vs,30 as the main categorization parameter. Standard penetration blow count NSPT and undrained shear strength Su may also be used to characterize the top 30m of the soil.

Soil class Description Vs,30 (m/s) NSPT Su (kPa)

A Hard rock >1500 - -B Rock 760-1500 - -C Very dense soil and soft rock 360-760 >50 >100D Stiff soil 180-360 15-50 50-100E Soft soil <180 <15 <50

F Soils requiring site-specific evaluations - - -

12

Site – Soil Classification in Seismic Codes

Code CategoriesQualitative

CriteriaSoil Stratigraphy Vs

AdditionallyCriteria

ATC3 4 Vs -

IBC2000 6 (5+1*) Vs,30 NSPT, Su

EC8-EN7

(5+2*) Vs,30 NSPT, Su

Japan 2001 3 Vs,T1, T2,

H Descr.

France 19905

(4+1*) Vs, Vp

NSPT, Su

Dr, Cc, etc.

Turkey 2007 4 Vs, H NSPT, Su, Dr

Norway 1998 3 Vs Descr.

New ZealandDraft-2000

5 (4+1*) Vs, Vs,30NSPT, Su, To

H< 100m

www.iaee.or.jp/worldlist.html

13

Site – soil classification (EC8)

• The first version of Eurocode 8 (CEN, 1994) proposed the use of site-dependent elastic response spectra for three soil classes A, B and C, which roughly correspond to hard, intermediate and soft soils.

• In the current version of EC8, Vs,30

parameter is used as the main classification parameter, following the U.S. practice, along with NSPT, plasticity index PI and cu.

• 5 main + 2 special soil classes are defined

14

Site – soil classification (EC8)• For the definition of ground type, in situ data from the same or close by areas

with similar geological characteristics (?) may be integrated.

• Vs profile may be estimated by empirical correlations with in-situ penetration resistance (SPT, CPT) and other geotechnical tests and soil properties.

• For important structures in high seismicity regions, site specific in situ measurements of the Vs profile should be used , especially for soil classes D, S1

or S2.

• “More detailed consideration of site effects to account for deep geology may be specified in the National Annexes. Unfortunately, this refinement rarely takes place” (Trifunac, 2012).

• “While the EC8 code uses the term ground types, it can be seen from the above that in fact they represent only five ranges of soil stiffness near surface, without any reference to the thickness of the soil layers or the geological deposits bellow.“(Trifunac, 2012)

15

Site-dependent elastic response spectra (EC8)

16

Site-dependent elastic response spectra (EC8)

maximum S

• Elastic response spectra depend on site class through:• Soil amplification factor S• Corner periods TB,TC,TD which define PGA-normalized response spectra

17

A, M ≤ 5.5

Soil classes B-C

A, M > 5.5

B, M > 5.5 C, M > 5.5

Soil class A

only 230 rather reliable records!

Pitilakis et al. (2012)

Site-dependent elastic response spectra (EC8)Validation of EC8 normalized response spectra

A, M ≤ 5.5

18

D, M ≤ 5.5 D, M > 5.5

E, M ≤ 5.5 E, M > 5.5

Pitilakis et al. (2012)

Site-dependent elastic response spectra (EC8)Validation of EC8 normalized response spectra

Soil class D

Soil class E

19

Validation of EC8 S factors

Pitilakis et al. (2012)

Logic tree approach

Site-dependent elastic response spectra (EC8)

20

r ij r ij,AB

r ij,CF

r ij,Zh

r ij,CY

(GM ) (T) 0.35 (GM )0.35 (GM )0.10 (GM )0.20 (GM )

= ⋅

+ ⋅

+ ⋅

+ ⋅

Main problem: Results depend on the reliability of the GMPEs prediction for rock

Site-dependent elastic response spectra (EC8)

Approach 1 (Choi & Stewart, 2005)

Pitilakis et al. (2012)

ij ij r ijS (T) GM /(GM )=

Validation of EC8 S factors

21

Site-dependent elastic response spectra (EC8)

Pitilakis et al. (2012)

Approach 2 (Rey et al., 2002)Validation of EC8 S factors

Main problem: Lack of reliable and numerous records for rock sites

0.01 0.1 1Period (s)

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

R*S

A ( k

m*c

m/ s

2 )

C, M=7.5-8, N=200A, M=7.5-8, N=6C, M=7-7.5, N=158A, M=7-7.5, N=5C, M=6.5-7, N=207A, M=6.5-7, N=39C, M=6-6.5, N=520A, M=6-6.5, N=47C, M=5.5-6, N=176A, M=5.5-6, N=28

0.01 0.1 1Period (s)

0

500

1000

1500

2000

2500

3000

3500

4000R

*SA

( km

*cm

/ s2 )

C, M=5-5.5, N=127A, M=5-5.5, N=36C, M=4.5-5, N=126A, M=4.5-5, N=36C, M=4-4.5, N=100A, M=4-4.5, N=33

(a)

(b)

22

Pitilakis et al. (2012)

Type 2 (Ms≤5.5)

Soil Class SHARE-DS1 SHARE-DS2 SHARE-DS3 EC8 Proposed

Ap.1 Ap.2 W.A. Ap.1 Ap.2 W.A. Ap.1 Ap.2 W.A.

B 0.90 1.55 1.23 1.51 1.37 1.44 - - - 1.35 1.40

C 1.93 2.54 2.23 2.19 2.12 2.16 - - - 1.50 2.10

D 3.36 3.07 3.22 2.92 2.00 2.46 - - - 1.80 1.80a

E 0.98 1.79 1.39 1.30 1.96 1.63 - - - 1.60 1.60a

Type 1 (Ms>5.5)

Soil Class SHARE-DS1 SHARE-DS2 SHARE-DS3 EC8 Proposed

Ap.1 Ap.2 W.A. Ap.1 Ap.2 W.A. Ap.1 Ap.2 W.A.

B 1.47 1.34 1.41 1.53 1.08 1.31 1.49 0.94 1.22 1.20 1.30

C 2.09 2.24 2.16 2.06 1.46 1.76 1.82 1.15 1.48 1.15 1.70

D 1.74 1.42 1.58 1.56 0.92 1.24 - - 1.35 1.35a

E 0.91 1.07 0.99 0.97 0.83 0.90 0.93 0.78 0.85 1.40 1.40a

(a) site specific ground response analysis required

Validation of EC8 S factorsSite-dependent elastic response spectra (EC8)

23

Site-dependent elastic response spectra (EC8)

Comparison of EC8 Type 1and 2 normalized response spectra for ground types A and C with UBC spectra and the standard spectral shape by Biot 1941 (from Trifunac, 2012)

Reference rock motionSite-dependent elastic response spectra (EC8)

SHARE project:www.share-eu.orghttp://www.efehr.org 25

Reference rock motionSite-dependent elastic response spectra (EC8)

• Very limited number of records from rock or rock-like sites (Vs,30>800m/s)

SHARE SM database, Yenier et al. (2010) 26

Reference rock motionSite-dependent elastic response spectra (EC8)

• Very limited number of records from rock or rock-like sites (Vs,30>800m/s)

1143

7

16

9

19

15

26 26

30

20

35

29 29

17

12

26

18

22

14

20

10118

13

86

9910

453

1

53

1 21214

1 22 1 21 1 1 2 1 10

5

10

15

20

25

30

35

40

45

50

20 60 100

140

180

220

260

300

340

380

420

460

500

540

580

620

660

700

740

780

820

860

900

940

980

1020

1060

1100

1140

1180

1220

1260

1300

1340

1380

1420

1460

1500

1540

1580

Num

ber o

f site

s

Vs,30 (m/sec)

AD BC

38242 184

15 57138

1820

1246

83379

A B C D EEC8 soil class

Number of stations / records(Total number: 536/3666)

SHARE-AUTH SM database, Pitilakis et al. (2013)27

Records on soil class A sites (SHARE-AUTH database)

0 20 40 60 80 100Time (s)

-400

-200

0

200

400

A cce

l era

ti on

( cm

/ s2 )

0 20 40 60 80 100Time (s)

-40

-20

0

20

40

A cce

l era

ti on

( cm

/ s2 )

0 20 40 60 80 100Time (s)

-80

-40

0

40

80

A cce

l era

ti on

( cm

/ s2 )

0 20 40 60Time (s)

-80

-40

0

40

80

A cce

l era

ti on

( cm

/ s2 )

0 10 20 30 40Time (s)

-80

-40

0

40

80

A cce

l era

ti on

( cm

/ s2 )

0 2 4 6 8 10Time (s)

-40

-20

0

20

40

A cce

l era

ti on

( cm

/ s2 )

0 10 20 30 40Time (s)

-400

-200

0

200

400

A cce

l era

ti on

( cm

/ s2 )

0 5 10 15 20 25Time (s)

-150

-100

-50

0

50

100

150

A cce

l era

ti on

( cm

/ s2 )

0 4 8 12 16Time (s)

-80

-40

0

40

80

A cce

l era

ti on

( cm

/ s2 )

0 20 40 60 80Time (s)

-80

-40

0

40

80

A cce

l era

ti on

( cm

/ s2 )

0 20 40 60 80Time (s)

-120

-80

-40

0

40

80

120

A cce

l era

ti on

( cm

/ s2 )

0 100 200 300Time (s)

-40

-20

0

20

40

A cce

l era

ti on

( cm

/ s2 )

0 10 20 30Time (s)

-80

-40

0

40

80

A cce

l era

ti on

( cm

/ s2 )

0 10 20 30 40Time (s)

-600

-400

-200

0

200

400

600

A cce

l era

ti on

( cm

/ s2 )

0 4 8 12 16 20Time (s)

-800

-400

0

400

A cce

l era

ti on

( cm

/ s2 )

Oshika station, NMiyagi Perfecture EQ, 2003

Ube station,Nw Off Kyushu EQ, 2005

Kamitsushima station, Nw Off Kyushu EQ,2005

Nishinoomote station,Kyushu EQ, 1996

Tolmezzo-Diga Ambiesta station,Friuli EQ, 1976

Gilroy array, Loma Prieta EQ, 1989

Seto station, W Tottori Prefecture EQ, 2000

Tarcento station, Friuli aftershock, 1976

Bisaccia station, Irpinia aftershock, 1980

Tarcento station, Friuli aftershock, 1976

Tolmezzo-Diga Ambiesta station,Friuli aftershock, 1976

Gilroy array, Morgan Hill EQ, 1984

Auletta station, Irpinia EQ, 1980

Bisaccia station, Irpinia EQ, 1980 Pacoima Dam station,

Northridge EQ, 1994

28

Records on soil class A sites (SHARE-AUTH database)

Bisaccia station , Irpinia EQ and aftershock, 1980

0 0.5 1 1.5 2 2.5T (s)

0

1

2

3

4

5

6

PSA/

PGA

EC8-Class A-Type 1EC8-Class A-Type 2

29

Records on soil class A sites (SHARE-AUTH database)

0 0.5 1 1.5 2 2.5T (sec)

0

1

2

3

4

PSA/

PGA

MEDIANPROPOSED

16th-84th percentile

N=18

A1&A2, M>5.5

0 0.5 1 1.5 2 2.5T (sec)

0

1

2

3

4

PSA/

PGA

MEDIANPROPOSED

16th-84th percentile

N=11

A1&A2, M<=5.5

0 0.5 1 1.5 2 2.5T (sec)

0

2

4

6

8

PSA/

PGA

MEDIAN16th-84th percentilesPROPOSED

N=18

A1&A2, M>5.5

0 0.5 1 1.5 2 2.5T (sec)

0

2

4

6

8

PSA/

PGA

MEDIAN16th-84th percentilesPROPOSED

N=11

A1&A2, M<=5.5

30

Is Vs,30 appropriate for site – soil classification?

• Advantages of Vs,30:• Simple and effective in practice• Requires little data: a simple N-SPT of 30m long or less is enough!

• Disadvantages of Vs,30:• It is not a fundamental (geotechnical) parameter• Could mislead grossly in different cases like: deep low stiffness deposits

lying on much harder rock; sites with a shallow velocity inversion; sites with velocity profiles which are not monotonically increasing with depth or do not exhibit a strong impedance contrast in the first dozen meters or in basin type structures.

• Can the single knowledge of Vs,30 quantify properly amplification, which is mainly due to the effects of impedance contrast?

• Proposal of different alternative parameters (T0, H, Vs,av, Vs,10, Vs,25)

31

00

1100

2200

3300

4400

SSMM--SSCCSSCC

SSMM

SSCC

MMLL

CCLL

SSMMsscchh

SSMM

sscchh

sscchh

00 440000 880000 11220000 11660000

VVss((mm//ss))

00 4400 8800 112200 116600NN3300--SS..PP..TT..

33888800

2244,,6600//1100

6600//1100

5511

1188,,6600//1155

4477

2244,,6600//1155

6600//55

8811

--1199..3300

00

1100

2200

3300

4400

CCLL

CCLL

MMLL

CCLL

CCLL--MMLL

CCHH

CCLL

00 440000 880000 11220000 11660000

VVss((mm//ss))

4400 8800 112200NN3300--SS..PP..TT..

3344110000//1100

6655

112277

110022

110000//1100

110000//1100

''110000//1155

--33..6600

00

1100

2200

3300

4400

CCLL

CCLL--MMLL

SSMM

SSMM--SSCC

MMLL

ttrraavv.. RR**

ttrraavv.. RR

00 440000 880000 11220000

VVss((mm//ss))

00 2200 4400 6600 8800 110000NN3300--SS..PP..TT..

3311//55

1100

88

>>110000

--22..5500

Representative soil profiles from strong-motion station sites in Greece classified as soil class B according to EC8

Is Vs,30 appropriate for site – soil classification?

32

Representative soil profiles from strong-motion station sites in Greece classified as soil class C according to EC8

00

1100

2200

3300

4400

SSMM

MMLL

CCLL

CCLL--MMLL

MMLL

SSMM

CCLL

CCLL

CCLL

CCLL

00 220000 440000 660000 880000

VVss((mm//ss))

00 2200 4400 6600 8800 110000NN3300--SS..PP..TT..

11112288

1122

44

55

11665544

7788

5500//1155

5500//1155

5500//55

5500//1122

5500//1155

5500//1122

''5500//1133

--55..7700

00

1100

2200

3300

4400

SSMM

MMLL

CCLL

CCLL--MMLL

MMLL

SSMM

CCLL

CCLL

CCLL

CCLL

00 220000 440000 660000 880000

VVss((mm//ss))

00 2200 4400 6600 8800 110000NN3300--SS..PP..TT..

11112288

1122

44

55

11665544

7788

5500//1155

5500//1155

5500//55

5500//1122

5500//1155

5500//1122

''5500//1133

--55..7700

00

1100

2200

3300

4400

CCLLCCLL

CCHH

SSCCCCLLMMLL

MMLL

CCLL

CCLLCCLL

CCLL

CCLL

CCLLCCLL

00 220000 440000 660000 880000

VVss((mm//ss))

00 4400 8800 112200 116600NN3300--SS..PP..TT..

33

55

1100

1100

1133

5533

5544

77228844

7799

110066

8899

8866

110055

112266

--22..0000

Is Vs,30 appropriate for site – soil classification?

33

34

Soil Profiles from Coastal Area

2nd International Conference on Performance-Based Design in Earthquake Geotechnical Engineering

0

10

20

30

40

Debris

Calc.SandSt.

MarlesMarl-

Stones

0 200 400 600 800

Vs(m/s)

0 20 40 60 80 100N30-S.P.T.

21

26

2121

2327

2920

2526

2735

30

36

0

10

20

30

40

GW-GCSandSt.

Calc.SandStone

Marls

0 200 400 600 800

Vs(m/s)

20 40 60 80 100N30-S.P.T.

59100

66100100100

100

10068

6180

5045

36

0

10

20

30

40

SC-GC

SandStone

Serp.

Serp.

0 800 1600

Vs(m/s)

0 20 40 60 80 100RQD

4

15

7

22

4

50

85

RQD

283 916427

Elevated marine terraces:gradual decrease of Vs with depth – large variability

Is Vs,30 appropriate for site – soil classification

Sites with identical Vs,30, but different layering, can have significantly different response

Is Vs,30 appropriate for site – soil classification?

Idriss (2011)

• Bucharest, Mexico City, other deep basin sites like the basin of Po in North Italy or even cities with archeological layers of considerable thickness like Rome or Thessaloniki, are among the most characteristic cases of not appropriateness of Vs,30

Is Vs,30 appropriate for site – soil classification?

36

Is Vs,30 appropriate for site – soil classification?Gradient shear wave velocity

0 0.2 0.4 0.6 0.8 1z/h

0

0.2

0.4

0.6

0.8

1

[ Vs( z

) -Vs ,

t op] /(

V s ,b o

t -Vs ,t o

p)

a=0.69

Fit Results

Equation Y = pow(x,a)a = 0.69

Number of data points used = 2988Average X = 0.446144Average Y = 0.516936

Residual sum of squares = 123.441Coef of determination, R-squared = 0.51

= + − ⋅

azV (z) V (V V )hs s,top s,bot s,top

Soil class C sites from SHARE-AUTH SM database with depth>30m

Riga (2015)37

New site – soil classification scheme (Pitilakis et al., 2013)

• Soil classes initially proposed based on theoretical 1D analyses of representative models of realistic soil conditions (Pitilakis et al., 2004, 2006)

• Further developed based exclusively on experimental data from the SHARE data base enriched where possible from other sites worldwide (Pitilakis et al., 2013)

• Main parameters:• Fundamental period of soil deposit T0

• Average shear wave velocity of the entire soil deposit Vs,av

• Thickness of soil deposit H• N-SPT, PI, Su

• More detailed geotechnical soil description and categorization

38

New site – soil classification scheme (Pitilakis et al., 2013)

Α1 Rock formations Vs ≥ 1500 m/s

Α2

Slightly weathered / segmented rock formations (thickness of weathered layer <5.0m )

≤ 0.2s

Surface weathered layer: Vs,av ≥ 200 m/sRock Formations:Vs ≥ 800 m/s

Geologic formations resembling rock formations in their mechanical properties and their composition (e.g. conglomerates)

Vs ≥ 800 m/s

Β1

Highly weathered rock formations whose weathered layer has a considerable thickness (> 5.0m - 30.0m)

≤ 0.5s

Weathered layer, Vs,av ≥ 300 m/s

Soft rock formations of great thickness or formations which resemble these in their mechanical properties (e.g. stiff marls)

Vs: 400-800 m/sN-SPT > 50 Su> 200 KPa

Soil formations of very dense sand – sand gravel and/or very stiff/ to hard clay, of homogenous nature and small thickness (up to 30.0m)

Vs,av: 400-800 m/s N-SPT > 50Su > 200 KPa

Β2

Soil formations of very dense sand – sand gravel and/or very stiff/ to hard clay, of homogenous nature and medium thickness (30.0 - 60.0m), whose mechanical properties increase with depth

≤ 0.8s Vs,av: 400-800 m/s N-SPT > 50Su > 200 KPa

Description Τ0 Remarks

39

New site – soil classification scheme (Pitilakis et al., 2013)

C1

Soil formations of dense to very dense sand – sand gravel and/or stiff to very stiff clay, of great thickness (> 60.0m), whose mechanical properties and strength are constant and/or increase with depth

≤ 1.5sVs,av: 400-800 m/sN -SPT> 50Su > 200 KPa

C2

Soil formations of medium dense sand – sand gravel and/or medium stiffness clay (PI > 15, fines percentage > 30%) of medium thickness (20.0 – 60.0m)

≤ 1.5sVs,av: 200-450 m/sN -SPT> 20Su > 70 KPa

C3

Category C2 soil formations of great thickness (>60.0 m), homogenous or stratified that are not interrupted by any other soil formation with a thickness of more than 5.0m and of lower strength and Vs velocity

≤ 1.8sVs,av:200-450 m/s N-SPT > 20Su > 70 Kpa

Description Τ0 Remarks

40

New site – soil classification scheme (Pitilakis et al., 2013)

D1

Recent soil deposits of substantial thickness (up to 60m), with the prevailing formations being soft clays of high plasticity index (PI>40), high water content and low values of strength parameters

≤ 2.0sVs,av ≤ 300 m/sN-SPT < 25Su < 70KPa

D2

Recent soil deposits of substantial thickness (up to 60m), with prevailing fairly loose sandy to sandy-silty formations with a substantial fines percentage (not to be considered susceptible to liquefaction)

≤ 2.0sVs,av ≤ 300 m/s N-SPT < 25

D3

Soil formations of great overall thickness (> 60.0m), interrupted by layers of category D1 or D2 soils of a small thickness (5 – 15m), up to the depth of ~40m, within soils (sandy and/or clayey, category C) of evidently greater strength, with Vs≥ 300 m/sec

≤ 3.0s Vs,av : 150-600 m/s

Description Τ0 Remarks

41

New site – soil classification scheme (Pitilakis et al., 2013)

Ε

Surface soil formations of small thickness (5 - 20m), small strength and stiffness, likely to be classified as category C and D according to its geotechnical properties, which overlie category Α formations (Vs ≥ 800 m/sec)

≤ 0.7sSurface soil layers, Vs,av ≤ 400 m/s

ΕX

Loose fine sandy-silty soils beneath the water table, susceptible to liquefaction (unless a special study proves no such danger, or if the soil’s mechanical properties are improved)Soils near obvious tectonic faultsSteep slopes covered with loose lateral depositsLoose granular or soft silty-clayey soils, provided they have been proven to be hazardous in terms of dynamic compaction or loss of strength.Recent loose landfillsSoils with a very high percentage in organic materialSoils requiring site-specific evaluations

Description Τ0 Remarks

42

New site – soil classification scheme (Pitilakis et al., 2013)

0 10 20 30 40 50 60 70 80100

200

300

400

500

600

700

800

DA

C

B

Ε

H (m)

Vs,3

0(m

/s)

EC8

Vs,a

v(m

/s)

t a s et a 0

B1 B2

A2

E

C2

C1

D3

0 10 20 30 40 50 60 70 80100

200

300

400

500

600

700

800

H (m)

C3

D1, D2

EC8 New CS

43

New site – soil classification scheme (Pitilakis et al., 2013)

Amplification factors S (at T=0sec)

Soil Class

Type 2 (Ms≤5.5) Type 1 (Ms>5.5)

Ap. 1 Ap. 2 Weighted Average Proposed EC8 Ap. 1 Ap. 2 Weighted

Average Proposed EC8

B1 1.28 0.99 1.13 1.20 1.35 (B)

1.03 1.03 1.03 1.10 1.20(B)B2 1.89 1.17 1.53 1.50 1.36 1.28 1.32 1.30

C1 2.02 1.46 1.74 1.801.50(C)

2.19 1.27 1.73 1.701.15(C)C2 2.08 1.39 1.74 1.70 1.35 1.15 1.25 1.30

C3 2.59 1.61 2.10 2.10 1.57 1.07 1.32 1.30

D 2.19 2.26 2.23 2.00a 1.80 2.03 1.79 1.91 1.80 a 1.35

E 1.54 1.30 1.42 1.60a 1.60 1.10 0.94 1.02 1.40 a 1.40a Site specific ground response analysis required

44

New site – soil classification scheme (Pitilakis et al., 2013)

Elastic acceleration response spectra (5%)

45

Period-dependent amplification factors

Pitilakis et al. (2012, 2013)

EC8 Improved EC8

New CS0 1 2 3 4T (sec)

1

1.5

2

2.5

3

S

Current EC8 - Type 1

BCDE

0 1 2 3 4T (sec)

1

1.5

2

2.5

3

S

Improved EC8 - Type 1

BCDE

0 1 2 3 4T (sec)

1

2

3

4

5

S

New CS - Type 1

B1Β2C1C2C3DE

46

Site – soil classification - The case of Thessaloniki

EC8 classification scheme New classification scheme

47

Demand spectra

• Performance-based design

48

Demand spectra

49

Soil strength parameters and G-γ-D curves (EC8)

• Soil strength parameters: • Undrained shear strength Su for cohesive soils• Cyclic undrained shear strength τcy,u for cohesionless soils• Angle of friction and cohesion (UU or CU conditions)

• Soil stiffness: • Maximum shear modulus G=ρ Vs

2 at very low strains• Dependence of G (and Vs

*) on the soil strains must be taken into account through proper reduction factors (EC8?) or better selecting appropriate G/G0-γ- D curves for all soil types

• Soil damping: • Soil damping should be estimated from laboratory or field tests (?)• The dependence of damping ratio on the soil strain level must be taken

into account (as for the soil stiffness)

51

Vs and G –γ - D

Field methods for Vs

Invasive: CH, DH, P-S log, SCPT

Non-Invasive: SASW, f-k, SPAC, ReMi, SWI, MAM

Several correlations with SPT, CPT

52

Vs and G-γ-D

Vucetic and Dobry (1991)

RC and CTX lab tests, or/andTypical set of curves from the literature (PI and Dr% for clayey and cohesionless soils are proposed in the literature)

53

Vs and G-γ-D values suggested in EC8

For Vs>360m/s?

54

Basin effects

• They can be taken into account through an aggravation factor AGF

• Parametric numerical analyses for idealized trapezoidal basins

=( )AGF TSpectral acceleration from 2D analysisSpectral acceleration from 1D analysis

Chávez-García and Faccioli (2004)

Material property Material 1 Material 2 Material 3

Sediments

S-wave velocity (Vs in m/s) 250 350 500Quality factor of S-waves (Qs) 25 35 50

P-wave velocity (Vs in m/s) 1600 1750 2000Quality factor of P-waves (Qp) 50 70 100

Density (ρ in kg/m3) 200055

Basin effects

56

Maximum aggravation factor along the surface of the basin(linear viscoelastic analyses)

Pitilakis et al. (2015)0 0.1 0.2 0.3 0.4 0.5

x/W

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

max

AG

F

w=2500mw=5000mw=10000m

h=120m, a1=a2=45o, Vs=250m/s

0 0.1 0.2 0.3 0.4 0.5x/W

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

max

AG

F

a1=a2=20a1=a2=45a1=a2=65

w=5000m, h=120m, Vs=250m/s

Basin effects

57

Effect of shear wave velocity gradient

Riga (2015)

• Detrimental (increase of AGF) effect of shear wave velocity gradient at the vicinity of the lateral discontinuity in particular for low Vs values

• Minor effect at the constant-depth part of the basin

0 250 500 750 1000 1250x

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

max

AG

F

homogenous - viscoelasticgradient - 0.1g

w=2500m, h=250m, a=45o, Vs,av=250m/s

0 250 500 750 1000 1250x

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

max

AG

F

w=2500m, h=250m, a=45o, Vs,av=350m/s

0 250 500 750 1000 1250x

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

max

AG

F

w=2500m, h=250m, a=45o, Vs,av=500m/s

Basin effects

58

Effect of soil nonlinearity

Riga (2015)

0 250 500 750 1000 1250x

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

max

AG

F

gradient - 0.1g gradient - 0.3g gradient - 0.5g

w=2500m, h=250m, a=45o, Vs,av=250m/s

0 250 500 750 1000 1250x

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

max

AG

F

w=2500m, h=250m, a=45o, Vs,av=350m/s

0 250 500 750 1000 1250x

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

max

AG

F

w=2500m, h=250m, a=45o, Vs,av=500m/s

• Consideration of soil nonlinearity for the sediments material does not affect the estimated aggravation factor significantly (small decrease of AGF far from the basin edge and minor increase close to the basin edge)

Basin effects - Summary

59

• Average AGF is varying from 1.1 to 1.5 (sometimes more!)• AGF is not uniform along the basin• AGF depends mainly on the following parameters

• Geometry (width, depth, slope)• Vs(z)and less on• Intensity of ground motion • Soil NL (G-γ-D)

Topography effects (EC8)

• Simplified period-independent amplification factors ST are proposed for slope inclination greater than 15o and height greater than H=30m.

• In the presence of a soft surface layer the amplification factor should be increased by 20%.

Not sufficiently complete and accurate: Further improvement is needed

60

Liquefaction (EC8)

• EC8 calls for an evaluation of liquefaction susceptibility for extended layers of loose sand with or without silt/clay fines beneath the water table level. For shallow foundations evaluation of liquefaction susceptibility may be omitted when the saturated sandy soils are at depths greater than 15m.

• Minimum required investigations for evaluation of liquefaction susceptibility: SPT or CPT (or CPTU) in-situ tests and grain size distributions. PI may be used as complementary information

• If liquefaction hazard may not be neglected, and the liquefaction susceptibility is high, well established methods of geotechnical earthquake engineering can be used.

• A simplified liquefaction analysis is proposed, which uses empirical charts correlating situ measurements (SPT blow-count, CPT resistance or Vs) with cyclic shear stresses

61

Liquefaction (EC8)

• Simplified liquefaction analysis

FS = CRR / CSR

cyclic resistance ratio CRRfrom empirical charts based on SPT blowcount, CPT cone resistance

or Vs

cyclic stress ratio:

CSR=0.65 (amax/g) (σv0 /σ΄v0) S

If FS = CRR / CSR ≤ 1,25, the soil is considered as susceptible to liquefaction

62

Liquefaction (EC8)

Detailed liquefaction analysis needs detailed knowledge of the soil properties and local geology

• Analysis under effective stresses• Pore pressure build-up• Estimation of the permanent ground settlements and lateral spreading

Question: in case of a record with liquefaction evidence:Is it rational to compute an elastic response spectrum for the entire recordand if this is acceptable can this spectrum be used as a design spectrum?

63

Seismically precarious slopes• Displacement-based approaches are preferred.• The yield acceleration coefficient ky is used to represent the overall

resistance of the slope. (Newmark 1965)• ky depends primarily on the dynamic strength of the material along the

critical sliding surface and the structure’s geometry and unit weight.• EC8 does not provide any relationships for ky. In the literature there are

analytical equations, e.g. Bray and Travasarou 2007, Pitilakis et al., 2015

Shallow sliding Deep sliding

Bray et al., (2007)64

New predictive relationships (Fotopoulou and Pitilakis, 2015): Numerical

The optimal scalar and vector IM are identified through regression analyses correlating the numerical seismic slope displacements (D) with various IMs:

-Peak ground acceleration (PGA)-Peak ground velocity (PGV)-Arias intensity (Ia) -Mean period (Tm)-Spectral acceleration at period at 1.5Ts [Sa(1.5Ts) ] -ky/PGA: ratio of critical or yielding acceleration ky to PGA

Seismically precarious slopes

In(D)= -9.891+ 1.873·ln(PGV) - 5.964·ky + 0.285·M ± ε·0.65In(D)= -2.965 + 2.127·ln(PGA) - 6.583·ky + 0.535·M ± ε·0.72

In(D)= -10.246 - 2.165·ln(ky/PGA) + 7.844·ky + 0.654·M ± ε·0.75

In(D)= -8.076 + 1.873·ln(PGV) + 0.200·ln(Ia) - 5.964·ky ± ε·0.61

In(D)= -8.360 + 1.873·ln(PGV) - 0.347·ln(ky/PGA) - 5.964·ky ± ε·0.64

where D is in m, PGA in g, PGV in cm/s and Ia in m/s

The free field ground surface intensity parameters (i.e. PGA, PGV, Ia) could be used in the equations without any modification with depth

Scalar models

Vector models

Seismically precarious slopesNew predictive relationships (Fotopoulou and Pitilakis, 2015)

Summary of soil-site classification parameters

• Site classification: Vs,30, NSPT, Cu, PI, H, Vs,av, T0

• Soil profile and soil properties description (soil type, PI, Dr%, etc) • Soil strength: Cu, τcy,u (φ, c under UU or CU conditions)• Ground water level• G-γ-D curves• Liquefaction: NSPT (or CPT or Vs), ρ, granulometry• Topographic effects: slope inclination angle, H • Basin effects: basin morphology and dimensions, (width, slope and depth)

sediments properties [mainly Go(z)], location along the basin surface• Geology• Tectonics, fault proximity and fault type/characteristics• SFSI: G(γ), ν• Slope stability evaluation of the slope displacements: c, φ, ρ• Settlements: E, ν (except liquefaction)• Foundation bearing capacity: Cu or τcy,u, ρ, c’, φ’, E

67

EUROSEISTEST database (http://euroseisdb.civil.auth.gr)

Station Dedicated webpage

68

EUROSEISTEST database

1. General Information• Station Code• Network• Instrumentation• Power Supply• Housing

2. Geographical Information / Geomorphology• Location• Elevation from sea level• Station coordinates• Projection system• Site morphology

3. Geological Information• Surface geology• Reference for geological map• Existence of boreholes in the proximity of the site (yes/no)

Available data and metadata

69

EUROSEISTEST database

Available data and metadata

4. Geotechnical Site Characterization (in graphical and/or ascii form)• Sampling borehole(s) • Standard Penetration Test (CPT)• Cone penetration test (CPT)• Laboratory tests (classification, strength and compressibility, RC and

CTX: G-γ-D curves, etc.)• Geotechnical technical reports

5. Geophysical Site Characterization (in graphical and/or ascii form)• Shear-wave velocity profile • Compression-wave velocity profile• Quality factor, Q• CH, DH, SASW, Microtremor array measurements, etc

6. Site Response (in graphical and/or ascii form)• Standard spectral ratios• H/V ratios

70

EUROSEISTEST databaseExample station metadata (TST)

71

EUROSEISTEST databaseExample station metadata (TST)

Geotechnical Site Characterization

72

EUROSEISTEST databaseExample station metadata (TST)

Geophysical Site Characterization

73

EUROSEISTEST databaseExample station metadata (TST)

Site Response

74

EUROSEISTEST databaseStation Description Sheets

75

EUROSEISTEST databaseTime histories of the 12/9/2005 earthquake (Μ~5, R~8 km)

as recorded in the down-hole accelerographic array at the center of the valley (TST)

0m

21m

40m

72m

136m

196m

Vertical component

Radial component Transversal component

0m

21m

40m

72m

136m

196m

0m

21m

40m

72m

136m

196m76

EUROSEISTEST databaseExample

Waveforms

77

EUROSEISTEST database

SW-NE direction

Down-hole configuration at TST

TST_196

GRA

STETST_73

SW NE

NNW

SSE

PRR

TST_40TST_18

TST_136

W03W02W01TSTE01 E02 E03

PRO

PRO_033

FRMBUT

STC

Example Waveforms

78

EUROSEISTEST databaseOngoing: Processing of the homogenized data set – Azimuthal variation in input motion

Events:• Doirani, 2009-05-24• East of Sithonia, 2008-12-27

Same magnitude (Mw=4.1)Similar epicentral distance (~80km)

Commonly recorded at stations PRR and KOK

79

EUROSEISTEST database

H (m) Vs (m/sec)

10 185

20 260

40 340

50 468

90 625

90 622

100 730

1100

Station: KOK

H (m) Vs (m/sec)

13 300

25 475

35 590

>750

Station: PRR

Ongoing: Processing of the homogenized data set – Azimuthal variation in input motion

80

Aim

Importance of soil and site characterization in Earthquake Engineering and Engineering Seismology.

What do we mean with site characterization? and for what purpose?

Understanding ground motion?

Research oriented?

Seismic design of structures?

Codes?

Risk assessment?

81

Thank you for your attention

82