Buried Karst Creates Hazards and Project Delays

by Jim Olson, P.G.

October 18, 2015

Random SPT Borings Can Miss Potentially Hazardous Buried Karst Features and Important Compositional Boundaries

Simple Solutions

Construction projects have little time for costly mistakes and an easy one to avoid is hazardous, active karst features.  We get pulled into projects when there is a concern about subsurface geologic hazards, such as sinkholes, near-surface clays and organics, and areas of weathered and fractured limestone.

Using Electrical Resistivity Image (ERI) to perform geophysical surveys, our Professional Geologists characterize the subsurface geology and further identify potential geologic and geotechnical hazards that exist below ground.  Sinkhole development is very common in Florida hence assessing the potential for sinkhole development is an issue that cannot be overlooked.

Electrical resistivity

Electrical resistivity measurements involve the passing of an electrical current underground and measuring its resistance to flow. Different earth materials, such as clay, sand and limestone, and subsurface cavities resist the flow of electrical current differently. Substantially greater contrasts in the degree of resistance are used to identify and locate boundaries among different materials as well as the presence of cavities.

The ERI data can be collected using multiple types of arrays. The level of subsurface clarity and the depth limits of the modeled ER data are primarily dependent on the type of array and the total spread of the electrode array.

Measurements of ERI are made with Advanced Geosciences SuperSting R8 8-channel Resistivity Meter with an incorporated switchbox and a passive electrode cable system.  The resulting data is processed utilizing EarthImager 2D or 3D, a computer program that produces two-dimensional or three-dimensional vertical cross section models of the subsurface.

Blog_EarthImager 2D_Construction 1120px

This image of processed ERI data depicts the mapped boundary between the sand/clay overburden and the variable upper limestone surface.  SPT borings were completed in anomalous zones to confirm depth to limestone and possible sinkhole activity.

ERI surveys can be completed in a relatively short period of time.  Typically 4 to 5 traverses can be measured in a day and projects covering large areas can be completed in less than a week.

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