Geotechnical News • December 2016
23
GEOTECHNICAL INSTRUMENTATION NEWS
deforms, the active waveguide de-
forms, generating AE that can propa-
gate along the waveguide. A transducer
coupled to the waveguide at the ground
surface converts the AE to an electrical
signal, which is processed by the AE
sensor. The AE sensor amplifies the
signal and attenuates frequencies out-
side of the 20 to 30 kHz range, remov-
ing low frequency (<20 kHz) environ-
mental background noise (e.g. traffic
and construction activity). The sensor
records the number of times the wave-
form crosses a pre-programmed volt-
age threshold level within pre-set time
intervals; ring-down counts (RDC) per
unit time (AE rates). The developed
AE monitoring system is called Slope
ALARMS (Assessment of Landslides
using Acoustic Real-time Monitoring
Systems).
Interpretation of AE
An increasing rate of displacement
generates an increasing number of par-
ticle-particle/particle-waveguide inter-
actions in the active waveguide. Each
interaction generates a transient AE
event, which combine and propagate
along the waveguide where they are
monitored at the ground surface. Hence,
AE rates produced and measured by the
system are proportional to the velocity
of slope movement. The coefficient
of proportionality is a measure of the
systems sensitivity (i.e. the magnitude
of AE rates produced in response to
an applied velocity) and is dependent
on a number of variables related to the
AE measurement system, such as: the
sensor sensitivity controlled by signal
amplification and voltage threshold;
the depth to the shear surface, which
influences the magnitude of AE signal
attenuation as it is transmitted from the
shear zone to the ground surface by
the waveguide; and active waveguide
properties such as the tube geometry
and backfill properties. The magnitude
of AE rate responses produced by each
measurement system will depend on
these factors, in addition to the rate of
slope displacement.
Warning messages
AE rates recorded in each monitor-
ing interval are compared to thresh-
old levels, which are derived for the
order of magnitude slope displace-
ment rate classifications (e.g. Cruden
and Varnes 1996); ‘slow’ (e.g. 1 mm/
hour), ‘moderate’ (e.g. 100 mm/hour)
and ‘rapid’ (e.g. 10,000 mm/hour). If a
sensor detects RDC within a set time
period that exceeds a trigger warning
level, the sensor transfers this to the
communication system through a wire-
less network link. The communication
system subsequently sends an SMS
message to responsible persons so that
relevant action can be taken (e.g. send
a suitably qualified person to inspect
the slope, stop traffic or other relevant
action). The absence of generated SMS
messages means that slope displace-
ment rates are lower than the minimum
threshold set. Automatically generated
daily health SMS messages provide in-
formation on the status of the system,
demonstrating it is operational. The
system therefore provides continuous
real-time informa-
tion on slope dis-
placement rates
with high tem-
poral resolution
(i.e. monitoring
periods are typi-
cally 15 or 30
minutes). Figure
2 shows an op-
eration schematic
of the AE early
warning system.
Installation
Active
wave-
guides are typi-
cally installed in
130 mm diam-
eter boreholes,
although smaller
diameter bore-
holes can be used
(e.g. down to
50mm as detailed
below). A minimum depth of approxi-
mately 2 m below existing or antici-
pated shear surface(s) is advisable. The
waveguide typically comprises lengths
of 50 mm diameter 3 mm thick steel
tubing connected with screw threaded
couplings. The annulus around the
steel tubing is backfilled with compact-
ed angular 5-10 mm gravel. The top 0.3
m of the borehole is backfilled with a
bentonite grout plug to seal against the
ingress of surface water. The steel tube
extends 0.3 m above ground level and
is encased in a secure protective cham-
ber. The AE sensor is located inside the
protective cover. A piezoelectric trans-
ducer is attached to the waveguide and
linked to the sensor via a cable. Wave-
guides can also be installed in incli-
nometer casings as detailed below.
Proof that it works
Comparisons with ShapeAccelArray
(SAA) measurements
SAAs installed at Hollin Hill, a shal-
low reactivated landslide in North
Yorkshire, UK, have allowed the com-
parison of continuous AE with continu-
Figure 2. Schematic of operation of the AE monitoring
and communication system.
