24
Geotechnical News • December 2016
GEOTECHNICAL INSTRUMENTATION NEWS
ous subsurface displacement measure-
ments. A series of reactivated slope
movements occurred in response to pe-
riods of rainfall that produced transient
elevations in pore water pressure along
the shallow shear surface (1.5 m deep)
in January 2014 (Figure 3). These com-
parisons confirm that AE rates gener-
ated by the system are directly propor-
tional to the rate of displacement.
Retrofitting inclinometer
casings
Retrofitting inclinometer casings with
the AE system has two key benefits:
the provision of continuous real-time
information on slope movements; and
continued operation beyond displace-
ments that would normally be sufficient
to render inclinometer casings unus-
able (i.e. not allow the torpedo probe to
pass the shear surface). To trial this ap-
proach, an inclinometer casing was ret-
rofitted with an AE system at the Hol-
lin Hill landslide; results from this trial
for a period of movement are shown in
Figure 4, which demonstrate that incli-
nometer casings retrofitted with active
waveguides can provide continuous
information on slope displacements.
As the inclinometer casing diameter
is only 70 mm, waveguide tubing with
smaller diameter (25 mm diameter and
2 mm wall thickness) and sand back-
fill (sub-angular 0.6-2 mm) were em-
ployed. Active waveguides retrofitted
inside 50 mm diameter standpipe cas-
ings have also been shown to work ef-
fectively.
Further information
Multiple references to publications
about AE monitoring of slopes can be
found at
, in-
cluding further details of the system,
laboratory studies and detailed case
study information. Slope ALARMS
sensors can be purchased from Lough-
borough University along with associ-
ated technical support and organisa-
tions interested in collaborating to
further commercialise Slope ALARMS
are invited to discuss opportunities
with the authors (full contact details
are given at
. A
very low cost version of the sensor has
been developed for use in low and mid-
dle income countries to help protect
vulnerable communities, field trials are
in progress and details will be available
in the next 12 months, and will be sub-
mitted for publication in GIN. Other
sources of AE monitoring systems and
services are:
•
•
•
However, it should be noted that these
do not currently have equipment opti-
mised for continuous slope monitoring
in remote locations or experience of
such applications.
Summary
For soil slopes, the field evidence from
multiple long-term trials, supported by
controlled laboratory studies, prove
conclusively that AE rates measured
using an active waveguide system are
proportional to slope displacement
rates. AE rates can show when the
slope is stable, accelerating or deceler-
ating. Therefore, when employed with
user defined thresholds, AE monitoring
can provide a warning of instability. In
addition, the AE monitoring technique
has been shown sensitive to small mag-
nitudes of movement and very slow
slope displacement rates, which means
that it can provide early information
on the occurrence of slope movements
and changes in the rates of these move-
ments. This information is automati-
cally communicated in real-time to
nominated parties so that appropriate
actions can be taken. Monitoring of AE
has been in progress at example sites
for over five years with very few false
alarm events, giving confidence in the
robustness of the approach.
AE monitoring of rock slopes employ-
ing grouted waveguides is showing
potential to provide information on
Figure 3. Time series for reactivated slope movements at Hollin Hill land-
slide: Rainfall, cumulative AE and cumulative SAA displacement.
Figure 4. SAA measured displacement, retrofitted inclinometer AE and rainfall
time series for a period of reactivated slope movements at Hollin Hill.
