 Last October, several families near downtown Lake Wales, FL, awoke to find their homes
were sinking! Because the building’s concrete block walls were separating and the front door was
jammed shut, one of the families could only escape through a window. By the end of the day, one
of the homes had sunk two feet, and hairline fractures were found in other nearby structures.
What happened? The occurrence was a ‘surface collapse feature,’ commonly known as a
sinkhole, which measured approximately 150 feet in diameter and one to two feet in depth. And
although this wasn’t a typical Sunday morning event, according to information on the University
of Florida website, Florida has more sinkholes than any other state in the U.S.
 MCD of Central Florida, Inc. in. Lake Wales completed a ground penetrating radar survey
of the area, and then contracted Direct Push Services, LLC (DPS) to complete five CPT borings
as part of an emergency response
situation for the City of Lake Wales.
DPS used a 6625CPT machine and
CPT tooling from Geoprobe Systems® to
complete the borings which were used to
further evaluate the
subsurface conditions
in the area of the
collapse feature.
In five hours, the DPS field
team, consisting of Todd Ives,
Operations Manager for DPS,
and Ron Wilson, Project Manager
for MCD, completed the five
CPT borings to depths of up
to approximately 120 feet and
totaling 548 feet. Matthew Cain,
MCD Vice President and Senior
Geologist, touted the expediency
and safety factors involved with
selecting DPS and cone technology for this investigation. “In situations
where you have unstable ground conditions and need to make a quick
determination of subsurface conditions, there is no better choice than CPT.
Did You Know? |
Qattara, located west of Cairo, Egypt, is the largest natural sinkhole in the world measuring 50 mi. long by 75 mi. wide, and is 436 ft. deep. (80 km x 120 km x 133 m) Source: www.webecoist.com |
DPS was not only responsive but also knowledgeable of the intent of our
study. Their performance exceeded our expectations and provided us and our
clients excellent service.” Matthew further noted, “In reality there was no
choice between completing SPT borings using rotary wash methods versus
using CPT. We couldn’t risk the safety of the drill crew nor the surrounding
structures by disturbing the soils with rotary wash methods.”
 Data was evaluated the following day and a report generated for MCD.
MCD was pleased with the depth capabilities of the 6625CPT machine
which is one of the reasons this wasn’t the first time the company had
contracted with DPS. “Since we’ve been using Direct Push Services and
their 6625CPT machine, mostly in conjuction with SPT borings,” Matthew
added, “we have routinely achieved depths of 100 feet or more. The selfanchoring
system provides a great push platform and little disruption of the
work area, which generally is residential lawns.”
DPS is one of the newest Geoprobe® customers
to join the ranks of the ‘100 Club’. |
 
There were notable differences in the results of the CPT borings between the borings completed
outside of the surficial feature and the boring completed near the center of the feature. CPT-1, completed east of the surface
collapse feature, encountered generally sandy soils throughout, with generally increasing strength (equivalent SPT ‘N’ values)
throughout until the termination depth of approximately 96 feet below ground surface. A rapid increase in the tip resistance
and change in the tilt angle, along with increased equivalent SPT ‘N’ values was interpreted by MCD as the limestone surface.
This was later confirmed by MCD using SPT borings. CPT-4, completed near the center of the surface collapse feature, encountered
a much more variable stratigraphy, consisting of sands and silty sands with several units of soils interpreted as clays,
clayey silts, and silty clays throughout. Also of note were the much lower equivalent SPT ‘N’ values at depths below approximately
40 feet. In addition, CPT-4 did not encounter the limestone surface to the maximum depth of the boring. |
|
 1981 WINTER PARK, FL SINKHOLE
Photo courtesy
of Thomas Scott,
Ph.D., P.G., Assistant
State
Geologist, Florida
Geological Survey.
Aerial view of the
Winter Park, FL,
sinkhole photographed
in 1981
which measured
320 ft wide and 90
ft deep (98 m x 27
m). The sinkhole
destroyed a
home, vehicles,
half of the city’s
Olympic-sized
swimming pool,
and a portion of
the street. The
sinkhole backed
up on a German car shop which explains the lost vehicles including a couple of Porsches. Three of the vehicles were
retrieved by crane. Today, the sinkhole is known as Lake Rose and resembles a small fishing pond, but with one
exception ... a Porsche rests at the bottom of the lake. The Winter Park sinkhole and the Lake Wales sinkhole both
formed on the ridge system in the heart of Central Florida where sinkhole lakes are common. |
GPR1 ... Area of downwarped subsurface soil reflections probably representing a geologically
younger area of subsidence. The most recent reactivation of this feature is believed responsible
for damage to the duplex on this site.
GPR2 ... Area of very shallow downwarped subsurface soil reflections probably representing a
low area that was filled to elevate and level the ground surface prior to the construction of the
intersection. Possibly representing a moderately-aged or younger area of subsidence in the
geologic past.
GPR3 ... The largest and most speculative geologic feature. Appears to be delineated by
southward or southwestward dipping reflections around 130 to 160 ft. north along east side
of street, and westward dipping reflections along avenue in front of the recently damaged
duplex. Possibly represents the northeast quadrant of a larger and geologically older area
of subsidence. A similarly large or larger area is supported by a sequence of nearly closed
contour lines on the USGS topographic map (Lake Wales 7-1/2 min quadrangle).