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GPR, Floor Elevation, and the SPT: What they are and How do they Help Confirm Sinkhole Activity in Florida. By Michael Laurato

GPR, Floor Elevation Surveys, and the SPT:  What are they and How do they Help Confirm Sinkhole Activity in Florida. 

By Michael V. Laurato, Esq.

            Under current law, insurance companies are required to conduct certain statutorily compliant scientific testing, if, after inspecting a home, there is reason to believe the damage to the home is consistent with sinkhole activity.  To meet the requirements ofFlorida’s sinkhole law, the insurance company is, first, required to contract with a qualified professional, such as a geologist or geotechnical engineer to conduct a subsurface investigation of the soils below the property.   Next, these insurer-retained professionals are statutorily required to conduct sufficient testing to either confirm or exclude sinkhole activity as a cause of damage, within a reasonable degree of professional probability.  Finally, the insurer-retained professional must produced a statutorily compliant report, which contains the findings and the cause of the damage. 

            At first glance, this sinkhole determination may seem like a straightforward application of scientific or geologic knowledge.  Because sinkhole activity deals with subsurface conditions that  may not be readily or objectively verifiable, professionals in the field have developed certain generally accepted scientific methods or tests to confirm or deny the existence of sinkhole activity at a particular site.  Three such tests are ground penetrating radar (or GPR), the floor elevation survey, and the standard penetration boring (or SPT). 

Each of these tests is aimed at acquiring different data, which, in turn, is added into the sinkhole “calculus,” to either support or detract from a particular conclusion about whether sinkhole activity is present.  Although there are other tests available to the professionals involved in sinkhole detection—and reasonable professional disagree as to the efficacy of each such test in detecting the presence of sinkhole activity—these three types of tests are commonly used in sinkhole investigations.  Accordingly,  in order to analyze the particular findings relative to the existence of a sinkhole, it is important to understand the findings of the GPR, floor elevation survey, and the SPT borings.

Ground Penetrating Radar, or GPR, is almost an intuitive concept.  Simply, it is radar signal which penetrates the ground and then reflects upward to be captured and measured by a radar machine.  Similar to the radar technology used by law enforcement to ticket speeding, GPR, as opposed to be “shot” horizontally towards an approaching vehicle, is transmitted vertically down through the soil strata.  The radar signals used in this process are capable of penetrating the ground to various depths.  The depth of penetration of GPR signal varies from place to place, depending on the soil properties present, such as grain size, mineralogy, and moisture content.  Therefore, the actual usefulness in conclusively finding sinkhole activity with GPR is minimal, because the actual depth of signal penetration is generally limited to the upper levels of the soil strata, due to the presence soils, such as clays, which do not conduct the GPR signal.  With that being said, however, the GPR is helpful in localizing “anomalous” areas in the upper level of the ground, that may indicate areas where the upper level soils are “down warping” into lower levels.  If the GPR locates an area in the upper soils that appears to be dipping downward, that may mean that the soils are dipping into a sinkhole at a lower soil level, which the GPR cannot penetrate.  In the event the GPR reveals one of these “anomalies,” further investigation is required, as the GPR has indicated an appropriate area from additional evaluation.  Thus, the GPR is useful identifying areas where sinkhole activity may be located.

Like GPR, the floor elevations survey, is another tool in sinkhole detection, but not a conclusive one.  In a similar fashion to GPR, the floor elevation survey, by its name, describes exactly what it is:  a method to measurement differences in the elevation of the floor.  The purpose of the floor elevation survey is to find unacceptable shifts in the foundation of the home, under the assumption that if the foundation has shifted, something significant, like a sinkhole, might be the cause.  Using a gas-filled device, precise measurement of the entire floor are taken at set distances.  After all the measurements are taken, a view of the highest and lowest points of the floor are  ascertained.  If the maximum elevation difference falls outside of acceptable standards for concrete, the test is considered as indicative of shifting of the home’s foundation.

Standard Penetration Tests are aimed at uncovering soil conditions deep below the surface—much deeper than the GPR or of the foundation of the home.  The aim of the standard penetration borings is to actually drilled down to the limestone itself, taking soil samples all along the way.  To accomplish this task, large drilling rigs, equipped with a metal rod and 130 pound hammer are used.  The hammer is raised to a height of three feet above the metal rod, which is inserted into the ground.  In theory, the metal rod will advance into the ground, under both the weight and force of the falling hammer strike.  Inserted onto the tip of the metal rod is a sampling spoon, which captures soil samples at various depths and intervals.  The soil samples are collected, classified, and bagged for later laboratory analysis.  In this fashion, the SPT tests accomplish two separate goals.  The first is actually capturing a sample of the soil on the property.  The second is obtaining data on the soil density.  In optimum, non-sinkhole conditions, the soil should become denser at greater depths, requiring more blows of the hammer to advance the rod the same distance as it progress deeper into the ground.  By measuring the hammer blows required to advance the rod at different depths, the professional can extrapolate the density of the soil and located ravel zones of weak soil that should be competent and required more hammer blows.  A low number of hammer “blow counts” at greater depths, or the rod advancing under its own weight without the necessity of a blow from the hammer,  is a finding consistent with sinkhole activity.

These three tests are some of the major probes conducted in any sinkhole investigation.  Interpreting these tests is complex and requires some scientific knowledge.  If there is an issue surrounding the methodology employed or the interpretation of the data collected by anyone of these tests, a homeowner should consult with a qualified professional geologist or geotechnical engineer for further information.