Geotechnical News • March 2018
29
Geological engineering -
“
Smart
”
ground support:
Continuous strain monitoring using fiber optics
Nicholas Vlachopoulos and Bradley Forbes
The need for continuous strain
sensing of ground support
elements
A rising demand for underground
transportation and resource manage-
ment has led to the development of
many more subterranean projects
(deep foundations, tunnels, utility
corridors etc.) which are constructed
at larger scales, over greater distances,
increased depths, and within proxim-
ity to sensitive urban environments
(i.e. reduced tolerances with respect to
adjacent infrastructure). For such proj-
ects, engineering design of support is
primarily based on the stress and strain
that are developing within the support
structures as a result of the surround-
ing ground conditions. These ground
loads are distributed continuously and
spatially and as such, an improved
understanding of the continuous strain
profile would provide better insight
into the true behaviour of such support
elements. Research currently being
conducted at the Royal Military Col-
lege of Canada focuses on such micro-
scale geomechanical mechanisms and
interactions with a view to determin-
ing the overall design implications for
full-scale (Figure 1) support design for
tunnels (for example).
The use of fiber optics within the
Geotechnical / Geological Engineering
field is not a new concept. There are
multiple projects that have utilized a
particular type of fiber optic technol-
ogy in the past, ranging from their use
to monitor the construction and perfor-
mance of embankments, tunnels, piles,
mining operations and other geotech-
nical works. It is important to note
that not all fiber optic technologies are
similar as each type has their unique
strengths and limitations. Historically,
monitoring of such ground support
members has been limited to electrical
and mechanical techniques (e.g. foil-
resistive strain gauges, inclinometers,
linear variable displacement transduc-
ers). Such techniques provide discrete
measurement points, implying that
many sensors are required to obtain a
full strain profile along the length of
the support element.
These techniques provide a limited
spatial resolution along the element,
making such methods prone to mis-
interpretation, underestimation, and
possibly omission of support response.
For example, it not uncommon to
observe a ‘failed’ rock bolt that has
been subjected to both axial loads
as well as bending (i.e. transverse
loading(s))
Optical fiber technology,
specimen preparation and
methodology developed
An optical frequency domain reflec-
tometry (OFDR) technology using
low cost single mode optical fiber was
investigated as a potential distributed
strain monitoring technique for ground
support members. What makes this
OFDR technology particularly attrac-
tive for monitoring the aforementioned
support elements is the capability to
monitor strain with a spatial resolution
of 0.65 mm along the length of the
optical fiber sensor. As well, the opera-
tional accuracy is quite acceptable
(better than +/- 10 microstrain).
COMPUTING IN GEOTECHNICAL ENGINEERING
From the GS Board
Figure 1. Macroscale testing: fiber optics installed in an active mine within
Northern Ontario.
