Jökull - 01.01.2011, Blaðsíða 51

Reviewed research article
Shear wave velocity in surface sediments
Bjarni Bessason and Sigurður Erlingsson
Faculty of Civil and Environmental Engineering, University of Iceland, Hjarðarhagi 2–6, 107 Reykjavík, Iceland
Corresponding author: bb@hi.is
Abstract — Surface sediments of different nature are common in Iceland. Natural soil sites and man-made
fillings commonly serve as foundations for different types of structures. In Civil engineering work it is funda-
mental to know the geotechnical properties of these materials in the upper 20–30 m. A seismic method called
Spectral Analysis of Surface Waves (SASW) has been used in recent decades in Iceland to measure and evaluate
shear wave velocity at different natural sites as well as in man-made fillings. The method is fast and involves
low cost equipment. It gives reliable results down to 20 m depth by using sledge as a seismic source and copes
with both soft and stiff soil sites. Furthermore, the technique can be applied at coarse grained gravelly sites
where it can be difficult to use borehole and penetration methods. We describe the methodology used in these
projects and review all SASW measurements carried out in Iceland. The soil strata at all test sites are classified
based on sieve analysis when possible. Natural sites and man-made fillings are kept separated. A database
and an open web site are introduced where all the SASW results can be viewed and shear wave profiles for
different soil types and unlike sites can be compared. The main aim with the database and the webpage is to
give scientists and engineers access to this data and enable them to compare stiffness at different sites.
INTRODUCTION
Background
Large plains of loose sand and sediments are common
in Iceland. They are created by ordinary fluvial and
Aeolian processes, by glacial outwash, by sub-glacial
outburst floods, by coastal sediment transport and by
ash fall from volcanic eruptions. In many cases these
have been formed in a relatively short time period dur-
ing a catastrophic event and can therefore have loose
compaction. All these processes are due to natural
forces and the sediments may be from a few centime-
ters up to tens of meters in thicknesses.
For civil engineering purposes it is common to
use all kinds of compacted soils in man-made fill-
ings, such as foundation pads, embankments, dikes,
dams, road construction, etc. Different soil materials
may be used in these earth works and the compaction
level can be highly diverse depending on compaction
method and effort.
In planning and designing buildings, bridges,
dams, roads and other structures it is fundamental to
know the geotechnical properties of the sediments and
fillings which will constitute the foundation, such as
their thickness, stiffness, degree of compaction and
damping properties. The shear wave velocity which
can be used to compute the stiffness of a soil layer is
a key parameter in this sense. In areas where earth-
quakes are of concern it is used when evaluating liq-
uefaction potential, estimating soil amplification and
defining earthquake loading. For instance in Eurocode
8, the European standard for design of structures
in seismic zones, a parameter reflecting the average
shear wave velocity in the uppermost 30 m at a given
construction site, called VS30, is used to define a site-
specific design spectrum (CEN, 2004 p. 34).
In the U.S. development of a next generation
of ground-motion attenuation models has been in
progress since October 2002 (Power et al., 2008).
This has been called the NGA project and it is a mul-
JÖKULL No. 61, 2011 51