When Borings Aren't Enough
By Donald J. Sipher, PE, Regional Vice President,
Froehling & Robertson, Inc., and Ted Dean of ATS International
Conventional subsurface exploration methods (i.e., test borings)
are fairly effective in developing a general picture of a site's
subsurface conditions, particularly for foundation design purposes.
But, these methods are not particularly effective in quantifying
those subsurface conditions for cost-estimating purposes. That's
where geophysical methods can be used to good advantage. Clients
and owners everywhere are carefully considering site development
costs and are seeking ways to maximize up-front investment in
subsurface exploration while minimizing the risk of cost overruns
because of unexpected conditions. Although no true substitute
exists for a thorough subsurface exploration using conventional
test borings, other tools are available to help fill in the
gaps between borings and even find the small anomaly borings
might otherwise miss. These tools include electrical resistivity
imaging, electromagnetic induction, and ground-penetrating radar.
Resistivity defines the characteristic of a material to limit
or resist the flow of an electrical current. Resistivity surveys
are conducted by inducing a low-voltage current into the ground
between two electrodes and measuring the potential at other
electrodes. Rocks and soils have different resistivities that
often are dependent on their level of water saturation and differences
in lithology. Higher water saturation results in greater current
flow (low resistivity) because of dissolved ions in the water.
Dry soils and rocks usually have higher resistivities, and void
spaces are completely resistant.
Electromagnetic induction (EM), also called terrain conductivity,
is a measure of how well the subsurface materials conduct electric
current. The conductivity of these materials is a function of
their physical properties, namely porosity, permeability and
the nature of the fluid within the pores. The EM equipment consists
of a portable control module and a hand-held boom. The control
box sends a current to a transmitter coil at one end of the
boom that induces eddy currents in the material below the instrument.
This generates secondary electromagnetic fields that are proportional
to the intensity of the current flowing in the material. A receiver
coil located at the other end of the boom intercepts the secondary
EM field and produces a voltage proportional to the subsurface
conductivity. The value is a measure of the bulk conductivity
from the surface to the effective depth of operation. Unlike
electrical resistivity, EM instruments do not require ground
contact. Therefore, data can sometimes be obtained very rapidly.
Some equipment can be operated by only one person and data can
be collected continuously or at discreet points. EM generally
has lower resolution than does electrical resistivity because
of the configuration of the transmitter and receiver coils.
Ground penetrating radar (GPR) operates by transmitting pulses
of ultra high frequency radio waves (microwave electromagnetic
energy) into the ground through a transducer or antenna. These
waves travel through the ground until they reach an electromagnetic
contrast, which causes some waves to be reflected back to surface
where they are detected by a receiver antenna. By moving the
antennas across the ground, an image of subsurface reflections
A few examples where these methods have been implemented will
illustrate the value of geophysical techniques.
- An assisted living facility recently was planning a major
expansion. The site under consideration would require extensive
excavation into an existing hillside. While rock outcrops
were not visible on the hillside, rock is present in the surrounding
area. Further, the local bedrock is known for extreme irregularity.
As the project was in preliminary planning, the client wanted
a cost-effective means to assess the risk of rock excavation.
A combination of a few test borings with an electrical resistivity
survey provided the needed information. The test borings suggested
no significant rock, and the electrical resistivity survey
confirmed that a deep soil profile did exist between the test
borings with the presence of near-surface rock just beyond
the boring locations. The project is proceeding without the
risk of unexpected rock excavation costs.
- A major hospital complex was planning a significant addition
on a sloping hillside containing several tall retaining walls
held by tieback anchors. The new building foundations would
need to be installed without cutting the anchors. A conventional
test boring program was adequate for characterization of subsurface
conditions for foundation design but could not help locate
the tie-back anchors. For this task, an electromagnetic induction
survey was performed. This survey provided indications of
where the anchors were located and has been utilized to direct
additional examinations of the anchor locations.
- A storm water detention pond located in a sinkhole-prone
area required a new lining to allow the pond to adequately
retain water. Associated with this effort was a concern for
potential sinkholes within the pond area; incipient sinkhole
conditions can easily be missed by visual observations. Again,
the combining of standard test borings with an electrical
resistivity survey provided the needed information. A previously
unidentified potential sinkhole feature was identified, thus
allowing for appropriate engineering design.
- An athletic center addition was proposed for a site where
test borings indicated a potential for encountering rock in
the required excavation. An electrical resistivity survey,
correlated to the test borings, assisted in quantifying the
expected volume of rock removal, thereby allowing an accurate
These examples illustrate new technologies that are available
in the geotechnical engineering world. Properly applied, they
provide cost-effective solutions to evaluate uncertain subsurface
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