Geotechnical News • March 2012
25
GEOTECHNICAL INSTRUMENTATION NEWS
are less susceptible to the requirement
for interpretation, however, they may
not result in any impact or damage to
a structure. For instance if an entire
building settled by 25mm or rigid body
tilt occurred to the structure, the struc-
ture may not be damaged in any way.
A common solution to this issue is
to set triggers on parameters that are
easier to report such as settlement and
tilt, then only calculate and carry out
interpretation on parameters such as
curvature once the Amber trigger has
been reached. There remains some
residual risk with this approach and
therefore it is prudent for interim
checks to be carried out by the engi-
neer responsible for interpreting the
monitoring data prior to an Amber
trigger being breached.
Trigger values for compensation
grouting
Following on from the previous sec-
tion, further consideration is required
regarding triggers where compensation
grouting is proposed. Specifying limits
on just settlement can lead to signifi-
cant amounts of grout being unneces-
sarily pumped into the ground and
consequently additional cost. However
it is considered prudent to specify trig-
gers relating to heave movements to
check against inappropriate operation
of the grouting system. Where settle-
ment occurs, it is recommended that
triggers are specified relating to limits
on imposed gradient and potentially
deflection ratio, if agreement can be
made among the project participants
of how to calculate the latter.
Instrument reading accuracy and
triggers
Care should be taken when selecting
instruments to ensure they can be read
to sufficient accuracy and precision.
Accuracy in this article is defined as
a measure of how close the measured
value of the parameter is to the true
value, while precision is the repeat-
ability of a measurement when there
is no real change in the parameter
being measured. Trigger levels should
be at least several times larger than
the accuracy of measured changes.
Account should also be made of any
diurnal trends that could take place
and these should be identified from
baseline readings. If the calculated
displacements are small (for example
only a few millimetres) and toler-
able values are considerably larger,
it is prudent to set the Amber trigger
at a displacement higher than the
calculated value and in keeping with
the general recommendation that
they should be at least several times
larger than the accuracy of measured
changes. This represents an alternative
to setting trigger values close to calcu-
lated values identified earlier.
Identifying trends of data
As the construction work progresses, it
is important to review trends of move-
ment even if the readings are within the
Green zone and haven’t exceeded any
trigger values. Trends within the Green
zone can give useful forewarning. A
pro-active approach is therefore recom-
mended for reviewing monitoring data.
Review of the data and trends must be
made with knowledge of the con-
struction progress and any important
environmental factors. In determining
trigger levels and defining the process
initiated once they are exceeded, con-
sideration should be given to the time
needed to instigate any pre-planned
response to a developing trend.
Conclusions
This short article has identified some
of the key considerations for setting
trigger values relating to monitoring
displacement and deformation. The
article has highlighted the require-
ment that the person setting triggers
must have intimate knowledge of the
design. Guidance is also given relating
to which deformation parameters to
set trigger values on and appropriate
review of monitoring data relative to
triggers during the setup of a monitor-
ing system and during construction.
Mike Devriendt
Associate, Arup, 13 Fitzroy Street,
London, W1T 4BQ, England.
T: +44-20-77552163,
E:
Figure 1. Setting trigger levels for a building subject to settlement from
tunneling.
