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Geotechnical Definitions
The performance of geotechnical instruments is primarily defined in
terms of range, linearity, sensitivity, resolution, accuracy and
repeatability.
Not all of these will be appropriate in every case, and occasionally
there may be other factors quoted, such as temperature range.
These terms are defined below.
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Range:
Expressed in terms of the lowest and highest
reading the instrument can be expected to handle. The user of
the instrument will normally specify the range required for
their specific requirement
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Resolution:
The minimum increment between one reading and
the next highest or lowest (or the smallest division on the
readout scale). Normally varies inversely with the range, so the
larger the range an instrument has to cover, the larger the step
between readings. Determined primarily by the resolution of the
readout equipment used to read the instrument.
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Accuracy:
Defined as the closeness of approach of a
measurement to the true value of the quantity being measured.
This is often expressed in terms of a range within which the
value read may differ from the actual value, e.g. "±0.1% of full
scale", or "±0.5mm". The accuracy of an instrument is evaluated
during calibration, when the instrument is compared with
calibration equipment whose accuracy is known and verified.
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Repeatability or
Precision:
The value returned by the instrument is going to
vary slightly every time a reading is repeated. Repeatability is
expressed as a plus or minus value; indicating how close
repeated readings will be to the arithmetic mean.
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Linearity:
Defined as the closeness of a measurement to a straight line
drawn through the calibration data. This is often expressed in
terms of a range within which the value read may differ from the
straight line value, e.g. "±0.1% of full scale", or "±0.5mm".
The linearity of an instrument is evaluated during calibration,
when the instrument is compared with calibration equipment whose
accuracy is known and verified.
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Sensitivity: |
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Defined as the
number of reading units for a given applied unit e.g. 200mV/1°
or 100Hz/Kpa. |
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Instruments are used in all phases of a
project, from initial investigations, where factors such as soil
permeability, pore-water pressure and slope stability need to be
considered, to the construction process itself, and finally to
monitor the performance and safety of the finished structure.
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Geotechnical instrumentation in
use - examples
The appropriate use of geotechnical instruments is crucial to
ensuring the safety and success of a major engineering project
like a dam or tunnel. It is important to be aware of the kind of
behaviour that needs to be monitored, what the best instruments
are to do the monitoring, and the best ways to position them on
the site.
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