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expected benefits. Also sometimes measures that might be
needed as an outcome of OM are inefficient during
construction. This for instance is described by (Schmitt and
Schlosser, 2007) for the case of an excavation in Monaco
where huge stays bearing on the bottom of the excavation
would have caused major consequences for the completion
time of the project.
Although it might seem that OM through this weakness is
more beneficial in larger projects than in smaller, this not
necessarily is the case. For example small embankments lend
themselves often for the use of OM.
6. Communication between site and design office. Application
of OM requires direct communication between site and
design office, being responsible for direct analyses of the
measurements. If these different cultures do not find each
other easily in a project, this may cause delays in go – no go
moments or even proceeding of the work on site without
commitment of the design office. However, if
communication is planned carefully it can even be considered
a strength of OM that is brings design and construction close
to each other. Projects where the culture is based on
individual profit and loss opposed to mutual benefits, with
extremely low bid or difficult market conditions are not
suitable for the application of the OM.
Figure 1. Example project with application of OM in Amsterdam, Rokin
Station
Opportunities
Opportunities for the use of the OM are present at projects with
the following characteristics:
1. Presence of risks with low, but unacceptable a priori
probability of exceedance and significant consequences. For
the use of OM it is necessary that the full range of possible
behaviour is assessed and that it is shown that there is an
acceptable probability that the actual behaviour will be
within the acceptable limits (Eurocode 7). OM is suitable if
the probability is higher than acceptable for a standard
design, but is small enough to still have a large chance of
successfully completing the project without necessary
measures. This also requires the consequences to be large
enough to justify the additional costs. Examples can be the
impact of vibratory installation of sheet pile nearby a pipeline
or possible damage by vibrations to old monumental
buildings during driving of piles. The vibrations will most
likely be present, but the probability of exceedance might be
low enough to use the OM, in order to avoid a priori costly
measures in design.
2. Stakeholders. OM lends itself perfectly for good
communication with stakeholders involved in the project. For
instance a critical attitude of a project’s neighbours can be
addressed with a proper explanation of the project risks and
the way the project is organized to react pro actively if risks
seem to occur. It is shown at the North South Line in
Amsterdam during the application of OM in the final
excavation of Rokin Station that the stakeholders were
reassured by the extensive risk based OM approach. Also the
application of OM at the A2 Maastricht proved to be a very
good way for communication with the stakeholder (Grote and
van Dalen, 2012) The uncertainties related to the strength of
the limestone and the subsequent response of the excavation
wall, see Figure 2, made application of the OM suitable for a
good communication strategy. However, it should also be
mentioned here that miscommunication of the use of OM is a
threat for the project, since it can easily be interpreted
wrongly by stakeholders as a way of window-dressing a risky
project.
3. Best way out. Although the authors of this paper think OM
should be used ‘ab initio’, OM has proven many times to be a
very good opportunity in case unwanted events are (nearly)
happening, for instance observed from geotechnical
monitoring. Because the original design already is ‘in place’
and can not easily be changed, an OM approach can still save
the project.
Threats
Threats for the use of the OM are present at projects with the
following characteristics:
1. Quickly changing loads. One of the major and most well
known threats is the possibility of quickly changing loads
(causing brittle failure) such as deterioration of soils caused
by intrusion of groundwater. Also external loads such as
rainfall induced ground water surges or burst water mains as
well as the risk for liquefaction all are potential threats for
the use of the OM.
2. Unwillingness of authorities. Another type of potential threat
may be the willingness for authorities to allow the method,
even though according to Eurocode 7, the method is now
regulated. Use of OM almost inevitable requires efforts on
communication with the authorities in order to explain what
OM is, why it is used, and how is ensured that a safe and
sound construction will take place. This especially is the case
in countries with little experience with OM, such as the
Netherlands.
3. Time restrictions. Making an OM design requires more effort
in the design phase. If the design capacity is not adapted this
may lead to a longer design period. Projects with high
planning demands can therefor be impractical for the use of
OM, especially if it is expected that OM will not lead to time
savings during construction.
4. Calculation methods and tools do not always allow for proper
use of OM, in this case related to the necessary inverse
modelling. A large amount of data becomes available during
construction and needs to be processed. For instance for
settlement prediction software, modules exist in which fitting
between model parameters and measurements can take place
in order to make better forecasts for stages to come.
However, for other mechanisms such as deformations of
retaining walls or designs using finite element models this is
not easily done. Many calculations may need to be performed
in advance in order to use OM properly during the
construction. This might lead to inefficient use of OM,
causing high design costs or even (if mechanisms happen
outside the design expectations) the fact that OM can not be
used quickly enough during construction.
It can be concluded from all of the above SWOT conditions that
the observational method is best suited for projects that are
governed by the serviceability limit states. It is applicable, but
less suited, for designs governed by the ultimate limit states
with ductile behaviour, and it is unsuitable for the ultimate limit
states if brittle behaviour takes place.
