Geotechnical News December 2011
33
GEOTECHNICAL INSTRUMENTATION NEWS
For one test, a vertical force was
applied on a cable section by raising
the cable to different heights above the
floor of the trench in order to simulate
a highly localized ground deformation
event – see Figure 5.
Different forces were applied to the
cable during the test, to simulate differ-
ent levels of ground displacement, with
cable displacements of 15 cm, (6 in.),
30 cm, (1 ft), 61 cm (2 ft), and 1.2 m.
The test was repeated at several differ-
ent locations to evaluate the capability
of the system for reliably determin-
ing the exact location of ground strain
events. The system proved capable of
sub-meter accuracy, at multiple test lo-
cations along the 4 km-long cable. All
the tests demonstrated the proper func-
tioning of the system, both in terms of
ground deformation detection and alert
triggering with exact locations.
The recorded results and graphs
showed how the different amounts of
deformation of the cable can influence
the strain distribution along the sensing
cable. The data showed a coherent be-
haviour of the system at all of the test
locations – see Figures 6a and 6b.
Challenges Encountered
and Overcome, and Lessons
Learned
Some of the biggest challenges in the
development of an FO distributed
project can be field issues during
installation of the sensing cable.
Despite the overall relative ease of the
installation by conventional trenching
and horizontal boring, inconveniences
that can occur over such a wide
area, with a 4 km perimeter, must be
considered, including the need to divert
around buried obstacles; to modify
the cable path to avoid third party
properties; and to cross beneath roads
and surface water drainage features
using lined, horizontal borings. These
issues can usually be overcome because
FO sensing cables are relatively easy
to handle, when installed by trained
personnel, and, if necessary the cable
can be cut and spliced to facilitate the
installation. The capability to splice
provided the opportunity to install
the cable in several sections, greatly
simplifying the field modifications
needed to install the cable and bypass
or overcome obstacles. A challenge
that had to be met and overcome on the
Hutchinson project was the presence
of a particular type of rodent (pocket
gopher) that, in their feeding habit of
burrowing through the ground to eat
plant roots, were found to be damaging
the cable. Although the cable was being
installed inside a woven fiberglass
sleeve to deter such rodents, damage
was still being done. Fortunately the
damage was discovered by continuous
and scrupulous quality checking that
was on-going during installation. A
new, more robust, armored cable was
quickly designed, tested, and produced
at the factory. The new cable was
required to not only be rodent-proof,
but to still be sufficiently flexible
to serve the detection sensitivity
specifications of the project. The first
prototypes from the factory included
a precisely wrapped, flexible steel
ribbon-armored layer, plus a larger
cable diameter designed to exceed the
effective jaw spread of the rodents.
Prototypes of the new cable were
tested under laboratory conditions for
suitability of its mechanical and optical
characteristics before the subsequent
full production run, which then
produced all of the cable needed for
the project. The re-designed cable has
overcome the rodent issue.
Conclusions
Monitoring of the ground for the
earliest possible warning of incipient
or actual formation of a sinkhole due to
collapse of underground mine caverns
involves challenges that are uniquely
addressed by a fiber-optic system.
Since sinkhole formation resulting
from mine cavern collapse can occur
very rapidly, and possibly with little or
no prior warning, a monitoring system
that can run virtually continuously
is essential if an effective, earliest
possible warning is to be provided.
For the project discussed in this article,
the caverns are widespread across a
significant area, are near significant
infrastructure (including rail), and
will lead to sensitive ground strain
variation if their collapse is imminent.
A distributed FO system offers
significant advantages compared to any
other possible monitoring approach in
addressing all of these factors, and is
very well suited to this complex task.
The entire system was developed
to provide fully automatic and self di-
agnostic capabilities, no operator re-
quired; to dispatch alerts via telemetry
through both email and cell phone sms;
and to provide for remote control of the
system to increase troubleshooting ef-
fectiveness and system maintenance.
The ultimate value of the system is its
ability to allow a rapid and effective
response and intervention to the con-
sequences of potential rapid sinkhole
formation due to collapse of a cavern.
Reference
D.Inaudi, B.Glisic “Distributed Fiber-
optic Sensors: Novel Tools for the
Monitoring of Large Structures”
GIN September 2007, pp 31-35.
www. geo techn icalnews . com/
pdf/GeoTechNews/2007/GIN_
sept_2007.pdf
Bill Shefchik, Burns & McDonnell,
9400 Ward Parkway, Kansas City, MO
64114 USA,
email:
Reynold Tomes, Burns & McDonnell,
9400 Ward Parkway, Kansas City, MO
64114 USA,
email:
Riccardo Belli, SMARTEC SA, Roctest
Group, Via Pobiette 11, 6928 Manno
– Switzerland, Tel: +41 91 610 18 00,
email:
