Evaluating surface and subsurface conditions for marine foundations (bridges,
causeways, terminals, etc.) is often done with a limited amount of data.
Subsurface information for these projects is primarily acquired using traditional
methods, such as boreholes, test pits or divers, which tend to be expensive and
limited in the area of coverage. A number of geophysical methods, including
precision bathymetry, sidescan sonar, and seismic reflection or subbottom
profiling are extremely cost-effective and obtain significantly more data than can
be acquired using traditional intrusive methods. These geophysical methods
provide detailed surface images and continuous profiles of the subsurface
stratigraphy. This information can be invaluable for selecting the best location for
geotechnical boreholes and to map potential geohazards such as submarine
slides and filled or open scour holes. Offshore investigations conducted for a
proposed highway bridge (Knik Arm Bridge, Cook Inlet, AK), the retrofit of two
existing bridge (Antioch and Dumbarton Bridges, CA) and a proposed cruise ship
terminal (Ketchikan, AK) used a combination of geophysical methods to map
water depth, detect and map debris on the seabed, map the thickness of
unconsolidated sediment, determine the depth to bedrock and identify potential
geohazards such as scour holes. The results of the geophysical investigations
will be used to select locations for obtaining geotechnical borings, to extrapolate
geologic information between existing or proposed boreholes, evaluate the
potential depth of scour and design a pile support pier. Geotechnical engineers,
offshore contractors and bridge designers are realizing the importance of the
early use of offshore geophysics for (i) site characterization, (ii) to reduce the
uncertainty of geotechnical conditions thus avoiding designs that may be overly
conservative and thus more expensive and (iii) to assist in selecting locations for
geotechnical borings.
Author: Richard E. Sylwester, Golder Associates, Inc.
Publisher: DFI