Experts Determine Geologic Cause of Structural Damage

Two areas of expertise are needed to determine whether sinkhole activity is the cause of structural damage and how it can be repaired: Geology/Geotechnical Engineering and Structural Engineering.

Because there is so much variability in local geological formations, there is no standardized set of testing and analysis steps, and professional judgment is important.  However, there are generalized steps, processes and tools that help to inform professional judgment.

Initial Data Gathering

The first step is to collect background data on the site, including the area’s geomorphology, or types of layers and formations.  Also helpful is information on elevation, topography, and drainage.  Soil conditions in the area and the area’s history of sinkholes should be part of the background information.  Aerial photographs of land features may be telling.  Recent rainfall events should be considered.  Maps and elevations of nearby water bodies are potential factors.  Finally, background data should include the construction date of the house and of any additions or modifications should be noted.

Preliminary Site Inspection

It’s important for professionals to visit the site of structural damage and discuss with the owner or resident nature and timing of damage to the structure and any noteworthy facts about the property, e.g., wells, buried debris, removed tree stumps, etc.  The land itself should be inspected for any suspicious surface features, slopes, walls, water bodies, and utilities.  Data should be collected concerning widths of cracks, stressed vegetation, soil erosion, leaning walls or fences, location of septic tank, orientation of gutters and downspouts, etc.  New construction in the area should be noted.  Photographs should be taken.  It should also be determined to what degree access is available to conduct geophysical and geotechnical tests.

Geophysical Site Characterization

With background data and a preliminary site inspection, it should be possible to develop a hypothesis of subsurface geology and then undertake appropriate geophysical investigations to confirm or modify the characterization.  Among the tests that may be used are the following:

  • Ground Penetrating Radar, using radar pulses to image the subsurface and clarify shallow soil and rock conditions.
  • Profiling of Electrical Resistivity, measuring resistance of the soil to electrical current to identify the different materials that might be present.
  • Micro Gravity Surveys, measuring changes in earth’s gravity as small as one part in 1 billion to aid in characterizing geological formations.
  • Magnetometer Testing and other testing to determine where utilities may be buried.
  • Surface Wave Measurements, using vibrations to characterize subsurface features.

Floor Mapping

Floor Mapping is the use of precise tools to determine where a structure’s floor is depressed or elevated beyond tolerances specified in building codes.  Through these tests, a contour map of the structure’s floor may reveal insights about what is happening beneath the structure.

Subsurface Geological Testing

Experts may conduct standardized tests of soil penetration to determine soil characteristics.  Samples from boring into the ground may be collected and analyzed.  Cone penetration tests push a pipe with a cone on the tip into the ground at a controlled rate to delineate soil layers and materials.  A number of different subsurface tests may be conducted.

Laboratory Testing

Soil samples may be collected and tested for moisture content, gradation or grain size through laboratory testing.

Structural Inspection

The affected structure will be inspected for deterioration, modifications, and construction defects.

The Final Report

The final report that clarifies whether structural damage is caused by a sinkhole should include the expert opinion on what the cause of the damage is within a reasonable degree of probability.  It should include all of the evidence used to arrive at the expert opinion.  It should include the raw, uninterpreted data in the appendices of the report.  All photographs, notes, and other documents, as well as samples collected at the site, should be retained by the professional for an appropriate time.

For more information, refer to this report. http://www.dep.state.fl.us/geology/publications/sp/sp57d.pdf