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Geotechnical News • September 2017
GEOTECHNICAL INSTRUMENTATION NEWS
sider how the behaviour of the system
can be verified and any false alarms
trapped as part of the specification
requirements of the System Check. A
system of testing should be considered
to verify that the monitoring system
(including data processing) correctly
reports the nature of changes before
critical works commence. The System
Check process should be detailed in
the Inspection and Test Plans (ITP)
– see the Glossary at the end of this
article.
If an Instrumentation and Monitoring
system is altered it may be necessary
to undertake at least a partial System
Check to maintain confidence in the
data output for those elements that
have been replaced or repaired.
How?
System Checks are specific to instru-
ment type and the requirement must
be clearly defined in the specification
regarding scope and inclusion as part
of the commissioning process. The
method used to undertake the System
Check should be described in the
Method Statement (see the Glossary
at the end of this article) and agreed
between all relevant parties with the
system commissioning not considered
as completed before satisfactory com-
pletion. This differs from a laboratory
calibration test and may not achieve
the same accuracy, but does provide a
practical check.
Examples of System Checks
The following were undertaken
when technologies were first being
implemented but the need for System
Checks may remain for future proj-
ects, for these instrumentation types.
Electrolevels
A System Check was carried out
on electrolevel chains installed to
structures predicted to be affected by
a major tunnelling project. The system
was designed so that a calibrated
shim could be placed at one end of
an installed electrolevel beam and
the magnitude and direction of that
displacement confirmed in the data
visualisation software. This deter-
mined whether the entire monitoring
system (including instrument, loggers,
transmission elements and reporting
software) correctly reported both the
magnitude and direction of the change.
It was used to confirm that electrolev-
els were correctly wired to the multi-
plexer/data logger, the data correctly
referenced and processed and correct
calibration factors used. Discrepancies
found were investigated and remedied
before commissioning was considered
complete.
In-place Inclinometers (IPI)
Sometimes IPIs are installed but
when construction influences occur
the data indicates displacement in the
opposite direction to that expected.
At that point, usually at a particularly
inconvenient time, it may be necessary
to retrieve the IPIs to verify correct
installation with consequent project
delays.
On one project a calibrated frame
was constructed and the fully wired
up chain of inclinometers arranged so
that each IPI was placed in the frame
immediately prior to installation in
the casing. Whilst in the frame the IPI
was tilted in the plane of interest and
displacements recorded (in both mag-
nitude and direction) within the data
management/visualisation software.
This provided confidence to the Proj-
ect Owner regarding output from the
IPI system before construction works
commenced.
Automatic Total Stations (ATS)
In the early days of ATS, prior to
major implementation (72 instru-
ments) on a large infrastructure
project, it was necessary to provide
confidence to the Project Owner
before committing to the major invest-
ment required that the instruments
would perform as required. A trial
was undertaken and an ATS installed
(which would be required as part of
the full installation) with reference tar-
gets and a number of the prisms to be
monitored. The location of one prism
was capable of adjustment by cali-
brated distances. This one instrument
system was set up, the bugs sorted
and a System Check undertaken. The
adjustable prism was moved by known
distances in x, y and z directions. The
data visualisation software was then
interrogated to determine the displace-
ments the ATS was measuring relating
to that prism displacement. Following
successful completion of this trial and
operation of this reduced system for a
period of months the full ATS system
was ordered and installed.
Reflectorless Automatic Total Station
(RATS)
The use of RATS was proposed on
the ATS project described above, as
a replacement for manual levelling in
trafficked areas along the centre line
of each tunnel, to reduce risk to survey
teams. There was a need to demon-
strate the system capabilities prior to
an investment in the number of RATS
to supplement the ATS installation.
An RATS was installed to its pro-
posed location and its reading circle
on the ground determined at a num-
ber of locations, based on the angle
of sight from instrument. At each of
these locations discs approximating
to the reading circle were applied and
the changes in x, y and z for those
thicknesses recorded by the RATS.
Comparisons were undertaken in dry
and wet conditions and on differ-
ent materials to determine whether
the reduced accuracy (compared to
manual levelling) was acceptable to
the project. Another part of the check
was to determine the apparent hori-
zontal displacement of the reflectorless
monitored point (and its effect) due to
the change in level of that point and
mitigation methods implemented.
In addition the total time from reading
to data availability including cycle
time (the time taken for the instru-
ment to physically take a round of
readings from all the reference prisms
and monitored reflectorless locations),
data transmission, processing through
to availability of data for review, was
verified before project-wide imple-
mentation.
