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Correlations between the shear wave velocity profile and the response spectrum
based on SASW tests
Corrélation entre le profil de vitesse d’ondes de coupe et le spectre de réponse
basé sur l’essai SASW
Gonzalez L.
CMGI; Civil Engineering Department, University of Chile
Pinilla C.
IDIEM, University of Chile
Peredo V.
Golder Associates, Chile
Boroschek R.
Civil Engineering Department, University of Chile
ABSTRACT: This work analyses the seismic response of six sites, where seismic stations were located, during the 2010 Maule
earthquake (Chile), correlating the ground acceleration response with the shear wave velocity profile and the parameter Vs30. In order
to obtain the shear wave velocity profile, the SASW method was implemented, reaching a depth of 30 m. Trial tests were conducted
where shear wave velocity profiles were already measured with Downhole tests. The results showed reasonable agreement between
the velocity profiles. The obtained shear wave velocity profiles in most of the sites are consistent with the geology and stratigraphic
information. The main conclusions of this work are that i) in soils with a shear wave velocity profile that does not increases
monotonically with depth, the acceleration response spectrum presents more than one peak, which should be incorporated in the
design spectrum with a wider plateau to cover all the peaks ; ii) no unique correlation was observed in all the sites between the Vs30
and the pseudo acceleration, which implies that this parameter is not sufficient by itself to seismically classify the soils, in particular if
they have different geological formations.
RÉSUMÉ : Ce travail analyse la réponse sismique de 6 stations sismographe pendant le sisme du Chili du Maule dans l’année 2010,
en reliant l’accélération de réponse en surface avec le profil de vitesse d’ ondes de coupe et le paramétre VS30 pour chaque
emplacement. Pour obtenir le profil de vitesse des ondes de coupe, la méthode SASW a étée mise en oeuvre jusqu’á des profondeurs
au moins de 30 m. Des essais d’épreuve ont été effectués dans des emplacements où on avait préalablement mesuré la vitesse d’ondes
de coupe á travers l’essai Downhole, en montrant les résultats un accord raisonnable entre les profils. Les principales conclusions de
ce travail sont: i) Dans des sols avec un profil de vitesse d’ondes de coupe non monotonement croissant en profondeur, le spectre
d’ondes de coupe d’accélération présente plus de un peak, lequel pourrait être intégré dans le spectre design vers d’un plateau plus
large qui obtient couvrir tous les peaks; ii) Il n’y a pas été observée une seule corrélation dans tous les emplacements mesurés entre la
VS30 et le spectre de réponse de pseudo-accélération, ce qui implique que ce paramètre n’est pas suffisant par elle seulement pour
classer sismiquement le sol.
KEYWORDS: shear wave velocity, Vs30, SASW, response spectrum.
1 INTRODUCTION.
The 27
th
of February of 2010 the 8.8 Maule earthquake hit the
central-south region of Chile, causing hundreds of deaths. Even
though most of the casualties were caused by the tsunami that
hit the coast along hundreds of kilometers, damage to relatively
new buildings opened a debate about the way the Chilean
seismic code was facing some important issues. One of these
issues was related to the soil classification system. Before this
earthquake, the most important parameter to classify seismically
the soil was the Nspt. However, after the debate, the Vs30 took
place as the most important parameter to seismically classify the
soil. This change, along with other modifications, was
implemented in the Chilean seismic code.
This article presents the main results of the work conducted
by the authors, in order to analyze if the shear wave velocity
profile up to a depth of 30 m and the Vs30 are good parameters
to classify the soil.
In Chile the main methods to measure or estimate the shear
wave velocity profiles were the Downhole, Crosshole and
Refraction tests. The first two methods are relatively expensive
and are time consuming, since they require one or more than
one drillholes, respectively. The refraction test, on the other
hand, measures the compressional wave velocity. Besides, this
technique does not work if the shear wave velocity decreases
with depth. Therefore, the SASW appeared as a fast and
inexpensive method to estimate shear wave velocity profiles,
taking into account that the Vs30 parameter is now much more
required than in the past.
The SASW method was implemented to obtain shear wave
velocity profiles up to a depth of at least 30 m in six sites where
seismic stations recorded the Maule earthquake. The seismic
response of these sites, like acceleration response spectrum and
peak ground acceleration, were analyzed and correlated to the
shear wave velocity profile and the Vs30 parameter.
2 METHODOLOGY.
The SASW (Spectral Analysis of Surface Waves) method was
initially implemented using a 10 kg hammer and a 63.5 kg steel
mass, dropping it from a height of approximately 0.3 m, where a
depth of 15 m was reached (Peredo 2011). In order to reach
deeper layers and estimate the shear wave velocity up to a depth
of at least 30 m and therefore estimate Vs30, beside the 10 kg
hammer, the 63.5 kg steel mas was lifted up to a height of 4.5 m
using a tripod (Pinilla 2012).
Each test consisted in the placement of two 4.5 Hz
geophones, located on the ground surface, 2, 4, 8, 16, 24, 32 and
