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MASW SEISMIC SURVEYS FOR DETERMINING IBC SITE CLASSIFICATION
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A multi-channel analysis of surface waves (MASW) seismic survey can be conducted to
determine approximate shear wave velocities of the subsurface soils and bedrock.
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The weighted average
value of the shear
wave velocity data
within the depth of 0
to 100 feet can be
used to determine the
International Building
Code (IBC) site
classification.
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A typical MASW survey for IBC classification consists of one spread of twenty-four surface geophones spaced 3 to 5
feet apart and connected to a seismograph. The spread is placed across a portion of a proposed building footprint
and the source of the seismic wave consists of a drop weight.
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The seismic data is
downloaded to a computer and
processed using the SurfSeis
program developed by the
Kansas Geological Survey.
The SurfSeis program
develops a dispersion curve
(frequency range vs. travel
time) from the fundamental
mode of the surface wave data.
An inversion of the dispersion
curve data produces a depth
vs. shear wave velocity profile.
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Although the N-values from SPT data are commonly used to determine IBC site classification, the SPT method
does not benefit from the higher shear wave velocities that may be found within the depth interval of boring
refusal to 100 feet. Consequently, conducting an MASW survey may yield a higher IBC classification than the
SPT method resulting in a building design and construction cost savings.
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An MASW survey for IBC
classification can be conducted in
one field day and the results can be
determined within 24 hours after the
fieldwork is completed. Depending
upon site conditions, an MASW
survey can also be conducted to
map the top of bedrock similar to the
results obtained from a standard
seismic refraction survey.
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