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
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
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)
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
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
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
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 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 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