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Geotechnical News • March 2012
GEOTECHNICAL INSTRUMENTATION NEWS
having numerous other trigger levels
defining different actions. While fur-
ther trigger levels may have the benefit
of allowing more detailed planning of
escalating contingency responses, if a
trigger doesn’t result in a defined pro-
cess, it is proposed that there should
be no need for the trigger.
The following zones are commonly
defined:
• Green = OK, proceed
• Amber = Monitor more fre-
quently, review calculations and
start implementing contingency
measures if trends indicate the Red
trigger may shortly be reached
• Red = Implement measures to cease
movements and stop work.
Alternative words are also commonly
used to describe the Amber and Red
triggers. These include:
Amber = Threshold, Alert, Review,
Warning
Red = Limit, Maximum, Action,
Response, Tolerable limit
Prior to construction work starting
a process and timeframe should be
defined that project participants adhere
to once a trigger has been reached. It
is also recommended that consider-
ation is given, prior to construction
work starting, of the actions or mitiga-
tions that can be readily deployed once
trigger values are reached. This may
avoid scenarios where the program
is impacted due to cessation of work
once a Red trigger has been reached or
exceeded.
Consideration should then be given
to the rate at which movements are
likely to occur. For instance, the rate at
which movements take place around a
tunnel excavation formed using a tun-
nel boring machine is generally much
quicker than the rate of movement
around open or retained cut excava-
tions. This will influence project
participants’ views on what actions are
appropriate and possible as and when
trigger values are exceeded.
Defining trigger values
While the previous section provides a
framework for defining trigger levels,
the following approach is commonly
used for defining the value of the trig-
ger levels based upon earlier design
analysis:
• Amber trigger is set close to the
‘calculated’ displacement from
analysis;
• Red trigger is based on a tolerable
‘damage’ or deformation criteria.
When setting Red trigger levels, an
alternative definition is, “a conserva-
tive estimate of when a serviceability
limit state is likely to be exceeded”. In
this regard it is useful to consider the
Amber and Red trigger levels to be set
on two separate unrelated scales; one
related to calculated movements and
one relating to tolerable movements.
An example of how trigger values can
be set is provided in Figure 1.
Consideration should be given to the
degree of conservatism adopted in the
calculation to define the Amber trigger
level.
For assessing movements caused by
tunnelling and with reference to the
example provided in Figure 1, good
practice
suggests carrying out service-
ability limit state calculations using
a cautious estimate (or conserva-
tive) volume loss rather than a ‘best
estimate’. Therefore if setting the
Amber trigger at 80% of the calculated
movement, the actual movement can
be expected to be of similar magnitude
or less than the specified Amber trigger
level. Measured displacements greater
than the Amber trigger will therefore
identify that the movements are in
excess of calculated displacements
using ‘best estimate’ parameters and
should therefore prompt a review.
It is also common to relate contractual
requirements to trigger values with
respect to responsibility of causing
impact and requirements for repair to
third party structures. Commonly the
Amber trigger is used to define where
responsibility transfers from the proj-
ect client or promoter of a project to
the construction contractor. The Amber
trigger may therefore represent a level
that should not be exceeded provided
‘reasonable skill and care’ is adopted in
carrying out the construction work.
Further considerations
What movement is tolerable?
It was recommended above that Red
trigger levels should be based on a
tolerable damage or deformation cri-
teria. When assessing some third party
assets, tolerable deformations are not
always easy to calculate. An example
of where this could be difficult is
assessment of deformation of a tunnel
being used as part of an operational
urban metro system. Tolerable defor-
mations under this scenario can be
related to several elements:
1. Structural deformation;
2. Clearance of trains to tunnel lining;
3. Deformation of track within the tun-
nel; and
4. Deformation of services and utili-
ties within the tunnel.
Assessing the amount of deforma-
tion
that each of the above elements
can tolerate have varying degrees of
difficulty. Specifying trigger levels
on each of these factors is also chal-
lenging as it may result in a complex
range of trigger values for the same
3
rd
party structure. Where possible it
is advantageous to identify the critical
element(s) and base triggers on these.
On what parameters should you set
trigger values?
Consideration must be given regard-
ing which measured parameters to
set trigger values for. One particu-
lar challenge is that parameters (or
deformations) that cause damage such
as imposed curvature are not straight-
forward to calculate from monitoring
results. Interpretation is often required
to calculate an appropriate curvature.
The requirement for interpretation
may lead to disagreement between
project participants. Parameters that
are easier to report from monitoring
data results such as settlement or tilt
