January/February 2021 - 76

Geophysics integration throughout engineering life cycle (credit: e4sciences)

subsurface conditions at the site. That
allows them to work with the engineering team to select an appropriate
geophysical investigation where realistic expectations and methodological
limitations are understood.
The benefits of engaging with
geophysicists early in the project engineering life cycle were discussed. This
early interaction allows geophysics to be
incorporated throughout the life cycle of
the engineering investigation wherever
is most appropriate - from planning to
design and construction to monitoring.
Two case studies were presented that
illustrated the successful use of marine
and land geophysics for geotechnical
subsurface characterization during
different phases of engineering projects.
The first involved the implosion of the
Champlain Bridge that connects New
York and Vermont over Lake Champlain.
Marine geophysical methods, including
magnetic and sub-bottom profiling,
were utilized for phased clearance
surveys to locate bridge debris from the
implosion that could have impacted
construction of piles for the replacement
bridge. Marine seismic reflection was
also used to map the top of bedrock for
the design of the replacement bridge
foundation elements.

76 * DEEP FOUNDATIONS * JAN/FEB 2021

Bedrock topography
developed from land
and water-based
seismic surveys
(credit: Hager-Richter,
e4sciences)

The second case study involved
replacing Rourke Bridge over the
Merrimack River in Massachusetts.
Geophysical surveys were conducted
during the project planning phase and
involved the integration of marine and
land-based geophysics along multiple
alignments for the proposed bridge and
approaches. The geophysical methods
included marine seismic reflection,
sub-bottom profiling and magnetic
imaging; land-based seismic refraction
and shear wave velocity testing were
also used. The marine geophysical
work was conducted to identify
obstructions and to map the top of till
and bedrock. Both assisted in selecting
the proposed bridge alignment. Landbased seismic refraction was
conducted to determine the depth and
configuration of the bedrock surface to
assist in the selection of the locations
for the proposed bridge approaches,
whereas shear wave velocity testing
provided data used for site-specific
seismic analysis.

The case studies provided successful
examples of using surface geophysical
methods to supplement rather than
replace standard geotechnical
subsurface characterization techniques
like borings and test pits. That is,
geophysical methods can fill data gaps
and help focus consideration of where
additional data, point or geophysical
information may be needed to have
complete subsurface characterization
investigations. Borehole geophysical
methods were also discussed as a
means of providing high-resolution,
focused characterization of the
subsurface for both natural materials
and manufactured structures.
By integrating results from an array
of methods, including marine, land and
borehole geophysics with standard
investigation methods, a clear and more
accurate subsurface characterization
can be reported. This allows for less
uncertainty, and therefore, greater
project cost savings during design and
construction.
January/February 2021

Table of Contents for the Digital Edition of January/February 2021
