1884
Proceedings of the 18t
h
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
to have some sort of a variation between parts or stages of the
construction. This is essential to make it possible to learn
from previous behaviour, which is the essence of the OM.
Both projects that are multistage or that are executed
according to an incremental construction process are suitable
for application of the OM.
Multistage projects
for example include a staged excavation
or staged application of loads. These provide good
possibilities for the OM. Subsequent stages of loading can be
based on results measured in previous stages. Examples
include the excavation after collapse of the Heathrow
terminal described by Hitchcock (2003) and the raising of the
embankment of the Betuweroute Cargo Rail on very soft
soils as described in the Geotechnet report by Huybrechts
(2000). Using the multi-stage construction process, reliability
can be controlled by interpreting monitoring of the previous
stages and by taking subsequent actions if necessary. Also
excavations that progress in depth, for which the struts can be
pre-stressed according to the rate of deformation, or when
additional struts or soil nails can be installed depending on
the deformations, may possess good characteristics for the
use of the OM.
Another strength characteristic is present in projects with an
incremental construction process
. These projects are flexible
in the speed with which they progress or consist of several
steps. An example may be in NATM tunnelling work, or
vibratory installation of (sheet) piles, where the rate of
advancing can be controlled based on the monitoring results.
Also projects with a long length (in similar soil conditions)
for example line infrastructure projects such as roads and rail
can provide a good basis for the OM, such as for example
described for the Limehouse Link by Glass and Powderham
(1994).
2. Short project duration in relation with beneficial short term
behaviour of soil. In some cases short term soil behaviour
may be a strength,, such as when the undrained strength of
soils is larger than the drained strength and only short term
loading conditions are applicable which have diminished
before drainage takes place. Here also the NATM method
could be mentioned.
3. Displacements as leading design characteristic. Projects
where displacements govern the design are by nature often
suitable for the use of the OM. Deformations can usually be
monitored accurately and extensively and provide good
indication of the mechanisms that have to be controlled.
When deformations of adjacent buildings are important,
projects can be suitable for the OM, but it must be mentioned
that the possible measures and variations might be limited to
a specific and tight range of acceptable expected impact, thus
giving less space for its application. It can however be
considered a strength in the sense of this SWOT if a project
relates to existing structures or conditions that are difficult to
assess, such as the stability of an existing embankment (Lee,
2012) or old existing structures with unknown response
(Chapman and Green, 2004). The application of the OM in
those cases might solve otherwise unknown response of the
structure. In general projects where epistemic uncertainties,
which originate from insufficient knowledge of a property,
can be decreased by the use of monitoring might be suitable
(Nossan, 2006).
4. Integrated responsibility for both design and construction.
Cases where a strong connection exists between design and
construction teams and in which good communication
between parties is assured, have a strong case for the use of
the OM. The OM works well with an alliance contract in
which risks (and opportunities) are shared between client and
contractor, see section 3 of this paper.
5. Flexible and risk based culture. It can also be considered a
strength if the culture of each organization involved is open
to some flexibility but also very strict with regard to risk
management and monitoring. If staff members are
sufficiently experienced and had proper training, preferably
related to the use of the OM, this is a main benefit. A
management commitment to implementing the OM approach
at all levels is also an organizational strength.
6. High ground heterogeneity and or uncertainty in failure
mechanism. In cases with high uncertainty a ‘standard’ (non
OM) design approach forces the designer to make
conservative design assumptions, leading to costs that
possibly are not necessary and can be avoided. This leads to a
potentially high cost differences between a ‘standard’ design
and an OM design. It is the advantage of using the OM to
justify a set of more favourable assumptions leading to a
more cost effective design. This for instance can be the case
when a decision needs to be made between a shallow
foundation and a piled foundation, as has been experienced
by GeoImpuls participants for a LNG terminal with high
demands for dissimilar settlements, or in geological
heterogeneous areas (for instance close to rivers).
Two combinations of variability are especially suitable for
the OM. First, the soil strength or stiffness is not well known
or has a large spread, but the load that will be presented is
relatively well known (for example in NATM tunnels or deep
excavations as described by Kamp (2003) or the railway
example by Lee (2012). Secondly, if the opposite is the case
and the load is relatively unknown but the soil strength is
well known, for example in the case of deep foundations and
embankments described by Peck (1969) and many others, the
method could also work well. If both are known, or both are
unknown, the OM is not suitable and this should be
considered a threat.
Weaknesses
Opposite to the benefits are of course also weaknesses for the
application of the OM. If any of the following characteristics
exist, application of the OM may result in additional challenges
or may not be suitable.
1. Too little time between measurements and measures. A
major weakness exists if mechanisms involved in the project
reveal themself quicker than measures can be implemented.
In the case of brittle failure monitoring may not provide
previous warning. Brittle failure is a no go for the OM, while
late appearance may make application of the OM inefficient
since savings of necessary reinforcements can not be decided
early enough, such as described by (Korevaar, 2012).
Examples are non-ductile failures of structural members such
as struts/waling connections in multi-propped basements as
described by Patel (2007) or the vertical equilibrium of deep
excavations.
2. Measurements that cause failure. In some mechanisms, for
example related to the pull out capacity of (micro)piles or
anchors, monitoring would require failure of the system,
which is not acceptable.
3. Failure mechanism/parameter can not be measured. It can
also be problematic if the monitoring system is not able to
capture the correct mechanism or relevant parameters. This is
often the case as stiffness and strength of soils are only
weakly correlated, meaning that deformation measurements
do not always indicate a possible failure of the strength of a
material.
4. Change of failure mechanism during construction. Other
weaknesses could be that during the construction process, the
failure mechanisms change, for example if shallow failures
become deep failures, primary consolidation becomes creep
etc.
5. Costs for changes during construction are higher than profits
minus costs for monitoring. The use of OM inevitably
requires usually costly continuous measurements that have to
be taken, interpreted and analysed during construction.
During the design more scenarios need to be calculated
together with analysis of other cases/experiences in order to
know what to expect. These costs needs to be balanced with
