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GEOTECHNICAL
METHODS OF TESTING AND WHY MANY VALID SINKHOLE CLAIMS ARE WRONGLY
DENIED:
There
are a variety of methods that are commonly utilized in the
investigation of possible sinkhole activity. Some of them have
significant disadvantages, and in certain locations are completely
ineffective. Briefly stated, the more common ones are:
1. Ground Penetrating Radar (GPR):
An electromagnetic signal is transmitted into
the subsurface and is reflected wherever there is a distinct change
of sediment or water content. It produces a cross-sectional image of
the subsurface by transmitting pulses of ultra high frequency radio
waves into the ground. GPR data can be collected continuously or at
station intervals. A major disadvantage of this method is that it
can only read down to about 25 feet (at best) and does not work on
conductive overburden, such as clay. It is also not recommended
unless the surface is relatively smooth. This method is usually used
in conjunction with the Standard Penetration Test (SPT) by
identifying “areas of interest” to be drilled. Still yet, we
sometimes see the drilling in areas of “non-interest”, which usually
finds “no sinkhole activity.” Another disadvantage is that there are
often many inaccessible areas in which it cannot get to, such as
close to the house or building because of existing plants or shrubs.
That prevents other possible “areas of interest” from being
detected.
2. Site Drilling:
There are a variety of drilling techniques available. They include
shallow drilling (sometimes referred to as “hand auger”), which
removes soil samples from the depths of approximately eight feet.
This type of drilling has limited value, as it does not penetrate to
a sufficient depth. It is, however, often used in conjunction with
other methods, and is basically used to determine the strength of
the soil the foundation is resting on. Sinkhole activity usually
occurs much deeper than this method allows. Another common form of
drilling is the “standard penetration testing (SPT drilling)” and
can reach depths of over 100 feet. SPT drilling has drawbacks, one
of which is that it is only one-dimensional.
Unfortunately, sometimes an insurance company makes a determination
that there is no sinkhole activity by only utilizing the SPT method,
which gives only a one-dimensional subterranean view. For example,
the sinkhole (cavern) may be underneath a structure, and by drilling
on either side of the structure, they will find no evidence of
sinkhole activity. The SPT can also miss the raveled soil or cavern
by just a short distance. Also, part of its interpretation is based
on averaging the “N” value (the number of hammer strikes it takes to
move the drill through the ground). An especially thin or heavy
layer of the subsurface can easily skew that average.
Two and three-dimensional views of the sub-surface can be obtained
by several different methods of testing. One such method is referred
to as “cross-bore hole acoustic tomography.” In basic terms, the
multi-dimensional image of the sub-surface is obtained by lowering
in one hole a transmitter and in another hole a receiver. Sound or
radio waves are emitted from the transmitter to the receiver at
various depths, usually continuous. By using more than two holes, a
three-dimensional sub-surface view is obtained. This method works
because various types of soils, rocks, and other material have
different densities, which can be interpreted via the sound
readings. Computers translate the echoes and draw an image of the
sub terrain.
3. Electrical Resistivity (ER);
Electrical Resistivity Imaging (ERI); Multi-Electrode Electrical
Resistivity (MER):
These are various types of surface geophysical methods of mapping
the subsurface geology by placing multiple electrodes in the ground.
They measure the natural resistance of soils and other subsurface
material to the flow of electricity. In other words, they measure
the bulk resistivity of the subsurface to determine geologic
structure and/or physical properties. They yield a two or
three-dimensional view of the sub terrain and can provide the
equivalent information that would take hundreds of SPT’s to provide.
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THREE
DIMENSIONAL
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TWO DIMENSIONAL |
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