A devastating sinkhole occurred in Florida on February 28, 2013, raising questions and concerns about this incredible phenomenon. Around 20% of the U.S. lies in areas susceptible to sinkhole events, highlighting the need for research and to be informed about this hazard.

What is a Sinkhole?

Geologically, a sinkhole is a depression in the ground that has no natural external surface drainage. Basically this means that when it rains, all of the water stays inside the sinkhole and typically drains into the subsurface.

Sinkholes are most common in what geologists call, “karst terrain.” What’s that? These are regions where the type of rock below the land surface can naturally be dissolved by groundwater circulating through them. Soluble rocks include salt beds and domes, gypsum, and limestone and other carbonate rock. Florida, for instance, is an area largely underlain by limestone and is highly susceptible to sinkholes.

When water from rainfall moves down through the soil, these types of rock begin to dissolve and spaces and caverns develop underground. Sinkholes are dramatic because the land usually stays intact for a period of time until the underground spaces just get too big. If there is not enough support for the land above the spaces, then a sudden collapse of the land surface can occur.

Keep in mind though that while collapses are more frequent after intense rainstorms, there is some evidence that droughts play a role as well. Areas where water levels have lowered suddenly are more prone to collapse formation.

Areas Most Susceptible

About 20% of our country is underlain by “karst terrain” and is susceptible to a sinkhole event. The most damage from sinkholes tends to occur in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania.

To read more from this article please visit USGS.gov

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Madrid Engineering Group, Inc. was then retained by the City of Winter Haven to prepare an emergency subsurface repair program designed to address the cause of the collapse in the ground surface near the pilot hole. As part of the work to prepare such a remediation program, MEG completed an SPT boring about 50 feet away. In this boring there was an apparent cavity in the ground surface extending from about 162 to 174 feet bgs. After promptly moving the drill rig off of the borehole, MEG noted that an oval-shaped depression in the ground surface near the borehole.

While coordinating directly with the City of Winter Haven, MEG prepared a set of specifications and recommendations to address the conditions around the second borehole, and assisted the City in selecting a reputable, experienced contractor to address the subsurface conditions. Over the next two weeks, MEG oversaw the placement of approximately 143 cubic yards of low-mobility compaction grout using the surface casing that was set in place during completion of the boring.

Ultimately, MEG provided the City of Winter Haven with a recommendation of a “phased” approach to address the subsurface conditions, based primarily on the value the City places on their facility and their perceived risk to their infrastructure. Currently, MEG is awaiting approval from the City to begin implementing a series of recommendations to stabilize the original pilot hole along with what appears to be an approximately 10-foot to 15-foot zone of very loose sediments from about 157 to 180 feet in close proximity to the two 0.5-MG raw water storage tanks and two high-capacity municipal water supply wells.

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The coastal area from Texas to Maine is home to tens of millions of people with $9 trillion worth of insured property that is exposed to the threat of hurricanes. Building science research has identified the areas of a home most at risk from hurricane-force winds and rains. The following information offers guidance for strengthening these areas, which will lead to a reduced risk of damage, fewer repairs, and also may qualify your home for a designation through the IBHS FORTIFIED for Existing Homes™* program.

* IBHS FORTIFIED for Existing Homes™ program offers three levels: Bronze, Silver and Gold designed to help strengthen existing homes through retrofit techniques that will ward off damage from specific natural hazards. Hurricane damage usually starts with the roof, so that is the place to start to make your home less vulnerable. If the roof is old or showing signs of damage or decay and needs to be replaced, re-roofing will significantly strengthen your home against hurricanes. The roof cover should be removed down to the sheathing. Sheathing should be checked for any damage and replaced as needed. Choose a high-wind rated roof cover and make sure ridge and off-ridge vents also are rated for high winds. Prepare to be able to shutter any gable end vents before a storm strikes or replace them with products rated for resistance to wind-driven rain intrusion. Soffits are vulnerable during a hurricane, so check the attachments. If your home has a gable end with a roof overhang greater than 12 inches, have the structure of that overhang checked and, if needed, braced. Find specific recommendations for how to carry out these retrofits at www.DisasterSafety.org/FORTIFIED.

For the complete pamphlet, please visit Disastersafety.org

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Geologic hazards of one variety or another occur over many parts of the United States, and represent a danger to both human welfare and public and private property. Generally, there are two categories of geologic hazards, including those that form on the order of less than 30 seconds to several days and tend to be more dramatic, to those that form gradually, taking decades to manifest themselves. Notably, not all geologic hazards are associated with erosional activity. Examples of instantaneous geologic hazards in the United States include volcanic eruptions, earthquakes, landslides, and sinkholes. Geologic hazards that form over the course of several months to several decades include ground settlement (such as the densification of loose surficial soils), aquifer compression, active (shrink-swell) soils, beach erosion / shoreline migration, and hill creep.

Winter Park Sinkhole
On May 8, 1981 the ground surface to the northwest of the intersection of Fairbanks Avenue and Denning Street in Orlando, Florida gave way, forming a sinkhole that ultimately measured about 350 feet wide and about 75 feet deep (left-hand photo). As the ground surface collapsed, a house fell into the sinkhole, along with several cars and part of a municipal swimming pool. In its present-day configuration (right-hand photo), the sinkhole is the centerpiece of a municipal park that includes “Lake Rose”, in honor of the homeowner who lost her house.

Active (i.e. “Shrink – Swell”) Soils
Certain varieties of clay minerals have an inherent property that expand or contract as water enters or leaves the molecular lattice much in the way that a dry sponge will expand as it absorbs water. In addition to natural changes in moisture content in the soil typically associated with seasonal variations in rainfall, the presence of large trees may also exaggerate the effects of “shrink – swell” soils. Recent literature reports that damage nationwide due to the effects of these soils is as high as $7 billion.

Hillside Creep
Although not common in Florida because of the generally flat terrain, hillside creep represents a long-term condition that may affect structures, roadways, and fencelines over several decades. This condition is caused by the long-term, gradual downhill movement of soil under the force of gravity. Conditions that greatly affect the presence and/or speed of “creep” include grain size (i.e. relative sand and clay content), vegetative cover, and moisture. Some sources do not classify “creep” as a hazard due to the long period of time associated with the condition. Even so, given enough time, creep has the potential to affect roadways and foundations.

Landslides / Mudslides
Landslides and mudslides are forms of “mass wasting”, a broad geologic term that is associated with instantaneous erosional events that also include avalanches and debris flows. Due to the relatively flat terrain in most of Florida, such events are quite unusual, although they may still occur when the slope of the ground surface is destabilized by rainfall. In the photograph below the ground surface gave way along the side of SR 50 in Clermont, Florida resulting in the soil from the embankment flowing into a duplex. No one was home at the time.

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Here is a fascinating presentation on the challenges of deep foundation installations in Central Florida, including individual case histories. Below you will find a snippet of the full PDF.

Central Florida is among the fastest growing urban areas in the United States. The Central Florida region addressed by this paper is roughly defined in Figure 1. The region includes Orange, Osceola, Seminole, Lake, Brevard, Volusia, Marion, Sumter and Flagler counties. This portion of Florida contains the metropolitan Orlando area, the Kennedy Space Center, Daytona Beach, Walt Disney World and the other themed entertainment attractions known throughout the world. Central Florida’s rapid development to accommodate its expanding economy, exponential population growth and tourism has required the steady construction of various types of civil engineering works. This paper addresses the challenging geologic/geotechnical conditions of this region of Florida and presents an overview of the local deep foundations practice.

Geologic Perspective

The geology of Central Florida is quite variable in terms of the thickness and elevations of the primary geologic units. In general, the northwest area has thinner overburden soils and rock at higher elevations. Conversely, in the south and east areas, the rock tends to be deeper and the overburden thicker.

Deep Foundations Practice

Deep foundations work involved in bridge construction is generally governed by the Florida Department of Transportation’s three main documents:

• Soils and Foundations Handbook
• Structures Design Guidelines  for Load and Resistance Factor Design
• Standard Specifications for Road and Bridge Construction

In addition, project-specific plan notes and special provisions are developed to address items such as minimum pile tip elevation, predrilling depth,  jetting, capacity testing, and soil setup.

Visit Pile.com to Read More.

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This paper provides an overview of the Mycenaean civil engineering projects
with emphasis in geotechnical engineering. Archaeological findings and
literature resources are used in this study. While there are difficulties in this
approach, it becomes evident that the Mycenaean engineers were competent
builders with extensive experience in geotechnical construction that included
fortifications, roads, bridges, embankments as well as underground construction
and tunnels. The method of construction for most of these projects is largely
unknown but detailed study of the remains could yield useful information on the
construction practices of this civilization.

Retaining Walls
Retaining walls were extensively constructed as part of road construction
projects, in other facilities, and as already discussed, in vaulted graves. All
retaining structures are gravity-type, i.e. the stability of the supported
embankment is ensured by the large weight of the blocks that form the wall.
Many examples of this type of construction exist. Figure 3 shows a large
embankment supported by a vertical gravity wall consisting of large, roughly
hewn blocks. Retaining walls were commonly constructed in road projects as
discussed in the following section.

Roads & Pavements
The Mycenaeans had a complicated transportation system that consisted of a
primary network of roads and many secondary roads. In the vicinity of Mycenae,
an extensive network of roads intended to serve the local needs is still
preserved and has been studied by several researchers (Steffen, 1884, Jansen
2002, Iakovidis and French, 2003).

Read More from this paper at Geoengineer.org

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The skate park south of LRMC closed late last year after the city sold the property to the adjacent hospital for an expansion project. Although there is still a gate open, most of the park is not suitable for skateboarding. The skating community unified and after much lobbying and fund raising are looking forward to the new skate park.

The project consists of the construction of a new concrete-paved skate park. Site work for this project included cut and fill operations and storm water drainage. MEG provided CEI where engineering technicians performed field density tests, earthwork inspection and concrete field and laboratory tests. MEG worked closely with Rodda Construction (General Contractor) and Team Pain to make this project a success.

A neat aspect of this project was the (re)use of extra concrete test cylinders (“hold” cylinders) along the top edge “coping” of several features throughout the skate park. Team Pain asked to incorporate the cylinders because the smooth surface of the concrete cylinders is desirable for skate tricks involving grinding the deck or trucks of the skate board along the coping. The cylinders are normal recycled along with the rest of the broken test cylinders from the concrete lab, however MEG was pleased to see them reused in this fashion.

Team Pain was involved with the design and construction of the new Lake Bonny Skate Park. It is a 22,000 square-foot skateboarding facility at Lake Bonny Park designed by Team Pain. Grand opening is set for May 18, 2013, please visit Team Pain for updates.

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part of the slow, natural process of erosion in Florida’s limestone/karst terrain that
forms over thousands of years. These common geologic phenomena generally occur where the
limestone is within a few hundred feet of the land’s surface.

So… How does this start?

Rainfall percolating, or seeping, through the soil absorbs carbon dioxide and reacts with
decaying vegetation, creating a slightly acidic water. Water continues to move through spaces
and underground cracks, slowly dissolving limestone and creating a network of cavities and
voids. As the limestone dissolves, pores and cracks are enlarged and provides a large passage
for more acidic water. Sinkholes are formed when the land surface above collapses or sinks into
the cavities or when surface material is carried downward into the voids.

Drought, along with resulting high groundwater withdrawals, can make conditions favorable for
sinkholes to form. In addition, heavy rains after draughts trends faster towards the karst layer
and the process to create a sinkhole begins.

If such sinkhole forms in an urban or suburban area, sinkhole can be fatal, as it recently
happened in Seffner.

The whole State of Florida is geologically hazardous. Sinkholes usually don’t collapse at once,
it happens in slow motion. You can see walls crack, strain, and complain as the earth begins
to slowly give way under the house. In many cases, residents have enough time to gather
valuables and evacuate to the safety.

“Losing a house to a sinkhole is very common, losing life is uncommon,” said retired University
of Florida geologist Tony Randazzo. “Most people will have some time warning of the pending
doom or catastrophic collapse. But there apparently were no warning signs of what happened at
the Bush house. That would be very scary.”

It is not yet clear what caused the Seffner sinkhole, however, but geologists say the area, which
is part of heavily populated I-4 corridor that crosses Florida’s midriff from Tampa to Daytona, is
particularly prone to sinkhole collapses.

A brochure issued by the Southwest Florida Water Management District lists several sinkhole
warning signs, including slumping trees or fence posts; the formation of small ponds in areas
where water has not collected before; wilting of small, circular areas of vegetation; and structural cracks in walls. www.swfwmd.state.fl.us/hydrology/sinkholes/brochure.pdf

Despite the efforts to spot sinkholes before they occur, there are still plenty of unpleasant – and
occasionally tragic – surprises, and there is no way of ever predicting where a sinkhole is going
to occur. 1

1Huffingtonpost.com

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GEO-FRONTIERS 2011 © ASCE 2011

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.

Click Here for Full Article

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; larrymadrid@madridengineering.com

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Larry Madrid, PE, president of Madrid Engineering Group, Inc. and a Florida sinkhole expert, was requested
on March 1, 2013 to come to the site of a sinkhole at 240 Faithway Drive in Seffner, Florida after Jeff
Bush had fallen into the collapse feature the night before. The Florida sinkhole was initially identified by Bill
Bracken, a structural engineer who is also trained as a first responder with Hillsborough County Fire
and Rescue who were some of the first on the scene. Bracken had initiated efforts to determine the
extents of the sinkhole when Hillsborough County asked for a third-party sinkhole expert to review
the methodology. Madrid reviewed CPT (cone penetrometer test) logs, electrical resistivity transects
and ground penetrating radar results, along with reports from rescue personnel who had indicated the
approximate size and shape of the hole at various times within the previous 18 hours. Soils information
indicated very loose sand to a depth of about 25 feet, underlain by very soft clay and clayey sand to a
depth of 57 to 67 feet where limestone was presumed to have been tagged by the CPT equipment.

After receiving a thorough briefing of the engineering data that had been gathered to that point,
Mr. Madrid participated in a private update of information to the immediate family and neighbors,
followed quickly by a press release to the media during the 6 o’clock news hour. The update consisted
of presentations by the Assistant County Manager Mike Merrill; Fire Chief Ron Rogers; Deputy Douglas
Duvall who had rescued Jeremy Bush (Jeff’s brother); Mr. Bracken and Mr. Madrid.

This tragedy is the only known death of a person being swallowed by a Florida sinkhole. In all previous
cases where a house or other building has been damaged by a collapse sinkhole, it is believed that
warning signals of ground movement and the reaction of the building to the movement have resulted
in sufficient time for those in the building to move to safety, or that the building was unoccupied at the
time of the collapse event.

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