Bull Earthquake Eng (2011) 9:1877–1898 DOI 10.1007/s10518-011-9274-2 ORIGINAL RESEARCH PAPER
Proposal for a soil classification based on parameters alternative or complementary to Vs,30 L. Luzi · R. Puglia · F. Pacor · M. R. Gallipoli · D. Bindi · M. Mucciarelli
Received: 25 January 2011 / Accepted: 12 April 2011 / Published online: 3 May 2011 © Springer Science+Business Media B.V. 2011
Abstract The selection of specific elastic response spectra according to soil categories is the easiest way to account for site effects in engineering projects and general-purpose hazard maps. Most of the international seismic codes make use of the average shear wave velocity of the upper 30 m (Vs,30 ) to discriminate soil categories, although some doubts arose about the capability of Vs,30 to predict actual soil amplification. In this work we propose two soil classifications in which the soil fundamental frequency (f0 ) becomes either an alternative or a complement to Vs,30 . The performance of the derived categorizations is achieved through the estimation of the standard deviation associated to ground motion prediction equations of acceleration response spectra, considering recordings extracted from the Italian strong motion data base. The results indicate that there is a significant reduction of the standard deviation when the classification is based on the couple of variables Vs,30 –f0 , although a classification based of the single f0 also leads to satisfactory results, comparable with those obtained assuming a classification scheme based on Vs,30 . Keywords Soil classification · Vs,30 · Fundamental frequency · Ground motion prediction equation
L. Luzi (B) · R. Puglia · F. Pacor · D. Bindi Istituto Nazionale di Geofisica e Vulcanologia, Milan, Italy e-mail:
[email protected] M. R. Gallipoli Istituto Metodologie Analisi Ambientali CNR, Tito Scalo, PZ, Italy D. Bindi GFZ-German Research Centre for Geosciences and CEDIM, Potsdam, Germany M. Mucciarelli Università della Basilicata, Potenza, Italy
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Bull Earthquake Eng (2011) 9:1877–1898
1 Introduction Significant increase in damage has been directly ascribed to the effects of local site conditions during recent earthquakes, such as 1985 Mexico City, 1989 Loma Prieta, 1994 Northridge, 1995 Kobe, 1999 Chi-Chi, among others. The selection of specific elastic response spectra according to soil categories is the easiest way to account for site effects in engineering projects and general-purpose hazard maps. Ground motion prediction equations often consider a soil categorization, in order to quantify the variation of ground motion due to the presence of soil layers of different depths and nature. The works of Borcherdt and Glassmoyer (1992) and Borcherdt (1994) were the first to propose the adoption of the Vs,30 parameter (average shear wave velocity of the upper 30 m) as a tool to discriminate soils with similar seismic response. Many seismic codes are based on this parameter, such as the 1997 NEHRP Provisions and 1997 Uniform Building Code (BSSC 1998) or the Eurocode 8 (ENV 1998, Eurocode 8: Design of structures for earthquake resistance, hereinafter referred to EC8, see Table 1). In Italy, the Italian code Norme tecniche per le costruzioni (NTC 2008; Chap. 3 Azioni sulle costruzioni, Par. 3.2 Azione sismica) proposes the same soil categories as identified in the EC8 code. Nevertheless, after Borcherdt (1994), some doubts arose about the capability of Vs,30 to predict amplification outside the region where the method was developed (southern California). Steidl (2000) found a poor correlation between site class and site amplification and
Table 1 EC8 classification (ENV 1998) Subsoil class Description of stratigraphic profile
A
B
C
D
E
S1
S2
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Rock or other rock-like geological formation, including at most 5 m of weaker material at the surface Deposits of very dense sand, gravel, or very stiff clay, at least several tens of m in thickness, characterised by a gradual increase of mechanical properties with depth Deep deposits of dense or medium-dense sand, gravel or stiff clay with thickness from several tens to many hundreds of m Deposits of loose-to-medium cohesionless soil (with or without some soft cohesive layers), or of predominantly soft-to-firm cohesive soil A soil profile consisting of a surface alluvium layer with Vs,30 values of class C or D and thickness varying between about 5 and 20 m, underlain by stiffer material with Vs,30 > 800m/s Deposits consisting—or containing a layer at least 10 m thick of soft clays/silts with high plasticity index (PI > 40) and high water content Deposits of liquefiable soils, of sensitive clays, or any other soil profile not included in classes A–E or S1
Parameters Vs,30 (m/s)
NSPT (bl/30 cm) cu (kPa)
>800
–
–
360–800
>50
>250
180–360
15–15
70–250
