ABSTRACT
Sinkhole activity is a covered loss under Florida’s current regulatory statutes
regarding property insurance, and several thousand subsidence investigations are
completed annually for homeowners with cracking distress and other possible
indicators of sinkhole activity. The majority of these insurance claims are located in
Florida’s gulf coast communities, north of and including Tampa. Here, the limestone
is shallow and is overlain by sedimentary and wind-blown sand and clay layers. In
addition, there is often, but not always, a vertical downward gradient from the
surficial aquifer to Floridan aquifer. In Florida’s geologic past, before the sand and
clay layers were laid down, changes in the ocean level caused weathering of the
limestone surface, creating an uneven surface where subsequent infills of loose sands
and soft clays have still not consolidated. This has created subsurface conditions that
appear to be sinkholes, but are not. Sinkhole formation is chiefly the result of
raveling of overlying granular soils into voids, cavities and caves within the
limestone. Proper analysis of these cases requires the geotechnical engineer to
understand the regional geology, and the geologist to understand the engineering
conditions under which damage can occur to a structure. The case studies presented
allow the reader to distinguish the subtleties between general subsidence and
subsidence due to true sinkhole activities.

INTRODUCTION
In Florida, the large number of sinkholes that have occurred over the past 30
to 40 years has been an impetus for the state government to require sinkhole
insurance coverage for residential and commercial properties statewide. West-central
Florida has recently been called the “sinkhole capital of the world,” owing to the
number of sinkhole insurance claims that are made in this region. Many of these
claims are valid, with damage to structures clearly due to sinkhole activity. For
example, a collapse sinkhole may be easily identified by the professional geologist or
geotechnical engineer, and the attendant damage easily visible including cracks to the
walls, skewed door and window openings, and visible sloping of the floors. In these
cases, where repairs are practical, the most common, successful remediation method
is low mobility grouting to seal the limestone surface and prevent further raveling.

Unfortunately, damages to structures can occur where cause of damage is not so
easily ascertained. Oftentimes the “damage” that initiates a sinkhole claim is minor
cosmetic cracking that can be attributed to a multitude of causes. Subsidence
sinkholes, as opposed to collapses, cause depressions to occur at the ground surface
that are sometimes not readily apparent, and damage to the structure may indicate
near surface ground movement sufficient to cause damage and the potential effect of
sinkhole activity. However, because there are numerous potential causes of damage
to structures, the engineer and geologist must provide forensic analysis of the damage
to determine the most probable causes.

The current state of Florida’s sinkhole insurance industry is one of increasing
claims, stretching statewide and including areas that are not predisposed to sinkhole
activity, based on geologic formation, hydrogeologic setting, and documented history
of sinkholes. Therefore, many insurance claims are being made with no engineering
or geologic basis; instead, claims are based on the fact that it is known within the
industry that it is difficult to tell whether or not there is sinkhole activity; and
sometimes even more difficult whether or not there is a link between sinkhole activity
and the distress observed. Much of the driving force behind insurance claims is legal
in nature, and related to the wording of Florida Statute 627.706 defining sinkhole
activity for insurance purposes. The ability to certify “elimination of sinkhole
activity as a contributing cause of the damage within a reasonable professional
probability” as required by this statute in many cases poses significant technical
challenges. In those cases where sinkhole activity cannot be eliminated as a cause of
damage within a reasonable professional probability, State law requires the insurance
company to pay out on the claim and/or make repairs.

The purpose of this paper is to examine the geologic setting in west-central
Florida where most of the sinkhole claims are made, to differentiate between sinkhole
activity and other geological processes, and to present the means by which they can
be differentiated. The most important of these is the geologic framework which, if
properly understood, provides the means to differentiate in most cases between
sinkhole occurrence or other causes of damage.

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By: L. D. Madrid, P.E., D.GE, MASCE; R. Stach, P.G., AIPG; and J. Delashaw, P.E., MASCE
Madrid Engineering Group, Inc., 2030 State Road 60 East, Bartow, Florida 33830; PH (863) 533-9007; FAX (863) 533-8997; [email protected]