Geoscience Reference
In-Depth Information
CHAPTER 7
FIELD SEISMIC TESTING IN GEOTECHNICAL
EARTHQUAKE ENGINEERING
Kenneth H. Stokoe, II
Department of Civil, Architectural and Environmental Engineering,
The University of Texas at Austin, U.S.A.
k.stokoe@mail.utexas.edu
Abstract.
Field seismic testing is an active and growing area in geotechnical earthquake engi-
neering. The primary purpose of the field tests is to develop compression-wave velocity
(
V
p
)
and
shear-wavevelocity
(
V
s
)
profiles.Theseprofilesareusedtorepresentthestiffnessesofthegeologic
materials in the small-strain range. Many seismic methods are available for shallow investigations
as discussed in the paper. Shallow investigations are defined as profiling to depths less than 75m.
Developmentsareoccurringinprofilingtointermediate(75to225m)anddeep(greaterthan225m)
depths. The seismic methods used for deeper profiling are the downhole, suspension logging, and
surface-wavemethods.Examplesofdeeperprofilingarepresented.Inaddition,fieldseismicmeth-
ods are being developed to perform parametric studies in situ. Examples are presented that show
in-situ measurements of the effects of: (1) stress state on V
s
and V
p
, (2) nonlinear straining on
shear modulus, and (3) cyclic loading leading to liquefaction.
1. Introduction
The starting point when evaluating the response to earthquake shaking of critical facil-
ities founded on or embedded in the earth is small-strain stiffness profiles, expressed
by the variation of compression-wave velocity
with
depth (Kramer, 1996). Profiles of V
p
and V
s
are measured in the field using seismic
methods. The seismic method or combination of seismic methods employed in field
investigations depends upon: (1) the geologic profile, (2) the maximum profiling depth,
(3) the size of the investigation area, and (4) the critical level assigned to the structure
or facilities. In many instances, the field investigation is limited to the top 30m at one
to three locations. The purpose of the investigation is to evaluate the average V
s
over
the top 30m
(
V
p
)
and shear-wave velocity
(
V
s
)
for use in a code-based design. Both intrusive and nonintrusive
seismic methods are used in these shallow investigations and only one field method
is employed. Intrusive methods used in such investigations are the crosshole, down-
hole, seismic cone penetrometer (SCPT), seismic flat-plate dilatometer, and suspension
logger (P-S logger). The nonintrusive methods are surface-wave methods and are based
onmeasuringRayleigh-typewaves.Thespectral-analysis-of-surface-waves(SASW)and
multi-channel-analysis-of-surface-waves (MASW) methods are two active-source meth-
ods employed in shallow investigations.
(
V
s30
)
