SUBMITTED BY
JENNIFER PELLERIN, FINAL YEAR GEOLOGICAL ENGINEERING STUDENT AT UNB
AND
DR. KARL BUTLER, P.GEO./P.ENG., PROFESSOR, UNB EARTH SCIENCES/GEOLOGICAL ENGINEERING
UNB GEOLOGICAL
ENGINEERING
STUDENTS
WIN
national award for earthquake study
n the afternoon of Tuesday,
August 23, 2011, people were
evacuated from several swaying
office buildings in downtown Fredericton
as the city was shaken by seismic waves
from a magnitude 5.8 earthquake that
occurred in Virginia 1500 km to the south.
The anomalously strong shaking, relative to
that felt elsewhere in the Maritimes, can be
attributed to amplification of seismic waves
in the thick, soft sediments underlying
the downtown area. Coincidentally, this
very phenomenon was the topic of an
award-winning study that had been
completed just months earlier by a group
of six geological engineering students
from the University of New Brunswick.
O
Earlier this fall, with support from
APEGNB and the UNB Engineering
Endowment Fund, five of the six students
embarked on a road trip to Toronto to
accept their award for first prize in the
2011 Group Undergraduate Student
Report Competition organized by the
Canadian Geotechnical Society (CGS).
The award, presented during the Pan-Am
CGS Geotechnical Conference from
October 2 to 6, 2011, included a $750
Students at the Pan-Am CGS Geotechnical Conference this fall. (L to R) Brad Copping,
Sonia Hachey, Jennifer Pellerin, Brandon Love, John Nichols. Sean Legassie was
unable to attend.
prize, a year’s membership in the CGS,
and registration at the conference. Students
took advantage of the opportunity to
attend conference presentations and
evening events as well as meet practicing
geotechnical and geo-environmental
engineers from across the country.
The students were recognized for their
report titled “Seismic Amplification and
Resonance Effects in Fredericton, New
Brunswick: Geological Origin and
Geotechnical Significance”. It was the
end-product of a team design course
completed in winter 2011 under the
supervision of Dr. Tom Al, P.Geo., and
Dr. Karl Butler, P.Geo./P.Eng., with the
support of Dr. James Hunter from the
Geological Survey of Canada (GSC).
This study highlighted and quantified
estimates of seismic amplification and
resonance to be expected in downtown
Fredericton as a consequence of the thick,
soft glaciolacustrine and fluvial sediments
on which it lies. Amplification occurs
in areas with decreasing seismic velocities
towards the surface. Resonance effects
are significant in areas where thick, soft
sediments lie directly upon material with
Figure 2: Horizontal
to vertical spectral
ratio curve for
ambient ground
vibrations at one
point in Fredericton
(Fred-077)
identifying a
resonant frequency
at 1.98 Hz. The two
outer lines represent
two standard
deviations from
the mean.
38
ENGENUITY FALL-WINTER / AUTOMNE-HIVER 2011-2012
UNB geological engineering students win
for Fredericton within the 2005 National
Building Code of Canada (NBCC), was also
completed for three locations where shear
wave velocity profiles were available.
The lowest values in the resonant frequency
map were found to correspond with the
thickest clay cover. The velocity contrast
responsible for most resonance was
determined to lie at the interface between
glacial till and the overlying soft sediments
composed of sand and clay/silt.
The building analysis found that the
7-storey TD Bank building on Westmorland
Street in Fredericton, NB, had a resonant
frequency of approximately 1.95 Hz, which
is not very close to the estimated soil column
resonance (~1.5 Hz) at that location.
Figure 3: Map of estimated resonant frequency variations in the downtown Fredericton area
based on HVSR analysis of ambient vibrations measured at nearly 100 locations (white
dots). Locations of selected boreholes are also shown.
markedly higher shear wave velocity.
Both effects increase the intensity of
shaking. Resonance also increases the
duration of shaking caused by earthquakes
and is of particular concern in areas where
the so-called “site period” of the soil
column is close to the natural frequency
of oscillation of multi-storey buildings.
Resonant frequencies were estimated using
a special three-component seismometer,
the Micromed Tromino®, on loan from
the GSC. Ambient vibrations from a
range of sources, including ocean waves for
the lower frequencies and nearer activity
for the higher frequencies, were measured
and analyzed for their frequency content.
• evaluate the resonant frequency of
two existing buildings
• create two detailed transects where
there was more subsurface data
available (from geophysical surveys
and borehole data).
One-dimensional shear wave modelling,
to predict expected ground motions for
earthquakes comparable to those used to
specify the probabalistic seismic hazard
In contrast, the 9-storey BMO/NB Power
building on King Street was found to have
a resonant frequency of approximately
1.2 Hz which is rather close to the resonant
frequency of the soil at this location
(~1.35 Hz) indicating that soil-building
interaction and increased building shaking
is to be expected. The 1D modeling (SHAKE
91 modelling) yielded amplification spectra
that showed resonant frequencies in
agreement with those estimated from the
ambient vibration measurements.
Modelling also suggested that estimates
of spectral acceleration, based on the
seismic hazard and soil class approach
specified within the 2005 NBCC, may
not be sufficiently conservative to account
for resonance effects in parts of downtown
Fredericton. The students acknowledge,
however, that this is a complex issue which
deserves more comprehensive investigation
given the important implications for building
design, safety and construction costs.
When the ratio of the horizontal to vertical
vibrations over a range of frequencies is
taken, the resonant frequency can be
estimated by the peak (see map). This
technique was used to:
• create a resonant frequency map of
the soil column in downtown
Fredericton (Figure 3)
ENGENUITY FALL-WINTER / AUTOMNE-HIVER 2011-2012
39