22
Geotechnical News • December 2016
GEOTECHNICAL INSTRUMENTATION NEWS
• What is soil mechanics?
In addition, John Burland has an out-
standing presentation style – a model
for us all. During the monitoring course
in Italy (see above), John will again be
telling us about his work to stabilize
the Leaning Tower of Pisa and to pro-
tect Big Ben in London while excavat-
ing for a new “Underground” (subway)
station alongside.
Closure
Please send an abstract of an article
for GIN to
co.uk
—see the guidelines on
www.
geotechnicalnews.com/instrumenta-
tion_news.php
Kasutta (“Let our glasses meet”) -
Greenland
An acoustic emission slope displacement rate sensor:
Comparisons with established instrumentation
Alister Smith, Neil Dixon, Daniela Codeglia, Gary Fowmes
What it can do
The following are lessons learned from
extensive laboratory experiments and
field trials of the Acoustic Emission
(AE) slope monitoring system:
• It provides information on slope
displacement rates continuously
and in real-time.
• It is sensitive to small displace-
ments and very slow displacement
rates.
• It is able to inform operators in
real-time that a slope is accelerat-
ing (or decelerating) with quan-
tification of changes in rates of
movement.
• It continues to operate at larger dis-
placements (at >500 mm of shear
surface displacement) than other
subsurface instruments.
• Inclinometer casings and standpipe
piezometer pipes can be retrofitted
with the AE system and converted
into continuous real-time displace-
ment rate sensors.
• Quantification of displacement
rates from detected AE is indepen-
dent of host slope soil.
• One sensor at a site can inform tim-
ing of site inspections and trigger
manual readings of inclinometer
casings.
• All sensor elements are located at
ground level for ease of mainte-
nance and reuse.
• Sensor costs are lower than current
continuously read in-place incli-
nometer systems.
• Low-cost materials are installed
in the borehole and are easily
reproducible (comparable instal-
lation cost to
inclinometer
casings).
How it works
Acoustic
Emission
AE are high-
frequency stress
waves that prop-
agate
through
materials
sur-
rounding the gen-
eration source. In
soil, AE is gener-
ated by inter-par-
ticle friction and
in rock by frac-
ture propagation
and displacement
along disconti-
nuities. Hence,
the detection of AE is an indication of
deformation.
System overview
The active waveguide (Figure 1) is
installed in a borehole that penetrates
existing or anticipated shear surfaces
beneath a slope. It comprises a metal
waveguide tube with a granular back-
fill soil surround. When the host slope
Figure 1. Schematic of an active waveguide installed
through a slope with an AE monitoring sensor connected
at the ground surface.
