Geotechnical News • September 2017
21
GEOTECHNICAL INSTRUMENTATION NEWS
System Checks/Validations
A practical approach for displacement monitoring
Isabella Ramaccia and David K Cook
What?
When a monitoring system is installed
a System Check should be undertaken
on instruments to confirm that the data
collected are correct, correctly identi-
fied and correctly transmitted to and
received by those needing to review
that data. Reference can be made to
“
Monitoring Underground Construc-
tion, A best practice guide
” published
by the British Tunnelling Society,
where it is described as “a process for
ensuring that the value obtained for a
measurement is a true reflection of the
actual change in the parameter being
monitored”. The most comprehensive
form of System Checking is the whole
System Check. This involves artifi-
cially inducing a known displacement
to an instrument and testing whether
the expected result is reported.
This differs from a pre-installation
acceptance test undertaken to verify
that the instrument is operating cor-
rectly and not, for example, damaged
in transit.
The purpose of this article is to
describe how System Checks have
been undertaken for a number of
instrument types so that methods can
be determined for future implementa-
tion with these and other forms of
instrumentation. This concept has
developed over time, as instrumenta-
tion becomes more complex so that
confidence can be established before
the monitoring data is used for deci-
sion making.
Whilst this article uses displacement
monitoring examples the general
principles can be applied to all forms
of instrumentation.
Other terms such as “Validation
Check”, “Verification Check”,
“Validation Process” and “Acceptance
Tests” have also been used to describe
this work element.
Why?
A System Check provides the neces-
sary confidence that instrumentation
is measuring parameters to the correct
magnitude and direction.
When a monitoring system is speci-
fied it should be unnecessary for
the specifying organisation to know
how each component of the instru-
ment, communications system and
visualisation software operates and is
interconnected. For example valid data
production will be dependent on the
following elements being correct:
• Instrument location
• Instrument orientation
• Wiring instrument to data logger
• Transmission to processing location
• Import to data management pack-
age
• Identification of instrument within
the data management/visualisation
package
• Calibration factors input
• Sign convention
• Use of environmental corrections
(such as temperature and pressure)
• Instrument operation (at time of
installation)
If a calibrated displacement is input at
the instrument, the resulting data can
be compared with the direction and
magnitude of that displacement at the
output software. If it doesn’t match,
within reasonable limits for the param-
eters being checked, then the system
should not be considered as commis-
sioned and therefore not accepted until
the faults are clearly identified and all
discrepancies satisfactorily resolved.
Note this is not an accuracy check, it
should be considered a reality check.
This will provide an indication that the
instrumentation system meets specifi-
cation in terms of operation.
A common error with certain types of
monitoring systems is that the com-
bined response of instrumentation
and software is not tested before the
actual effect of the works is detected.
This can result in erroneous read-
ings and a need for corrective action.
For example, more than one settle-
ment monitoring system has initially
reported heave, instead of settlement,
simply because instruments had been
connected the wrong way round or an
incorrect sign convention programmed
into the processing or visualisation
software.
The System Check will ensure validity
of the data (at the time of the check)
and confirm the system configuration,
which will depend on site constraints.
As an example for instruments con-
nected in a chain (i.e. electrolevel
beams) validation of the system
configuration is also based on the
continuity of the chain installed along
a structure.
If novel or unproven technologies are
proposed then provision of a System
Check will provide confidence to the
parties involved.
When?
A System Check is most easily under-
taken at the time of installation, as
part of the commissioning process. If
undertaken retrospectively the System
Check is likely to disrupt the read-
ings being recorded. As many proj-
ects require a period of background
monitoring it is important to have
confidence in data obtained from the
beginning of that period or it may not
be possible to use it for the project.
How the System Check is to be
implemented must be considered prior
to installation so that it is undertaken
at an appropriate point in the process.
The monitoring designer must con-
