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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 is developed.

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 budget projection.

These examples illustrate new technologies that are available in the geotechnical engineering world. Properly applied, they provide cost-effective solutions to evaluate uncertain subsurface conditions.

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