1886
Proceedings of the 18t
h
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
3 CONTRACTUAL ASPECTS
When discussing the possibilities of the Observational Method
in geotechnical engineering, it becomes obvious that contract
requirements should facilitate or, to say the least, should not
obstruct its use. In The Netherlands projects are awarded based
on the so called UAV (Uniform Administrative Conditions) or
the UAV-gc (Uniform Administrative Conditions for integrated
contracts). In contracts where the UAV-applies, the client is
responsible for the design and the contractor is responsible for
the execution of the works (Traditional contract). Since it is
virtually impossible to make a design with the Observational
Method without expert knowledge of construction methods,
there are limitations to the use of the OM in this kind of
contracts. Once the contract is awarded, for instance when a
contractor is selected based on general conditions and unit
prices, it is possible to change the design using the expertise of
the contractor.
If the UAV-gc applies the contractor is responsible for both
the design and the executions of the works (Design and
Construct contract). All though the possibilities for the use of
the OM as a design method are significantly greater than
compared to the UAV type of contract, there are still a number
of challenges to overcome. One of the main challenges is that in
order to get the contract awarded, the contractor first has to be
selected. Since the only award criterion that is deemed truly
objectively is price, a problem arises in selecting the best offer
for the works. The cost price resulting from a design based on
OM will vary around the cost price of the most probable way of
execution of the works. By nature of the method, it is
impossible to submit such a price in a bid. In the Netherlands it
was concluded that in order to use the OM as a design method
from the start, it is strongly advised and beneficial to execute
the project in an alliance between client and contractor.
In this kind of contract client and contractor share a common
objective, for example the execution of the project in a safe and
cost-effective manner with a minimised risk for the
surroundings. All the unknowns in a project that is designed
using the OM can be a shared responsibility. Both client and
contractor will be fully involved in all decision making and will
have an equal part in any additional costs or benefits. Part of the
Betuwelijn Cargo Rail Line (Huybrechts, 2000) has been
successfully constructed in this way. The challenge of selecting
the most qualified contractor remains. One of the suggestions to
overcome this challenge is to have a “beauty contest” and a
known budget price for the total works. In this way the client is
able to select a contractor based on value (best value
procurement). Contractors are asked to present themselves not
only with reference to the projects they carried out in the past
(track record), but also with respect to the proposed method of
cooperation with the client. The staff that the contractor wants
to deploy for the project will be judged not only on their
technical know-how, but also on their “soft skills”, since
cooperation is the key-word in an alliance type of contract.
Over the last years several projects in the Netherlands have
been awarded in this manner. For the OM to be used within
such contracts, all other requirements for the successful use of
the OM should also be fulfilled. However, an alliance type of
contract comes close to the ideal contract framework that was
described as being “utopia” in CIRIA Report 185 (Nicholson et
al., 1999).
4 PROJECT ASPECTS
Some project examples are given in this section with their
relative appropriateness to the use of the OM. It must be
mentioned that each project should be considered in their
specific settings, both physically and organizationally. In the
examples, only the most common aspects have been considered.
For deep excavations the use of the OM is usually limited to
the focus on the settlements in the surrounding structures or
soil. In some cases, struts can be optimized but it may not
always be possible to decide in time whether a strut layer
actually can be omitted. If long cut and cover lengths are
present, the subsequent sections may learn from earlier sections.
Chapman describes several cases where the use of the OM was
successful; whereas Karlsrud and Andresen (2008) state that the
OM is not particularly suitable for deep excavations. It can be
dangerous if sudden increases in water pressures may happen,
accidents such as strut failure or unforeseen loads next to
excavation happen. It is rather difficult to apply the OM to
assure the vertical equilibrium of deep excavations, although
this was actually done in Rokin station for the Amsterdam
North South Line, as best way out, see Figure 1. Usually this
aspect is considered as a potentially brittle behaviour, but in this
specific case the behaviour was expected to be more ductile
since the water carrying sand layer causing the possible uplift
was very thin.
For deep foundations the method is usually difficult to apply
because strength at failure often governs the design. There are
however good examples that for the re-use of existing piles
(Huybrechts, 2000) the OM shows some good possibilities. For
TBM tunnelling the OM is often used to control the settlements,
and for example not to design the tunnel lining, where
standardization is always more efficient than optimization over
shorter lengths. For NATM tunnelling the method is often
mentioned and it should be possible if a safe base design is
present. (Muir Wood, 1990) and (Kovari and Lunardi 2000)
state however that the OM for NATM is actually not working in
a correct way.
Embankments are usually well suited for the application of
the OM. Examples are mainly related to settlement control and
staged construction, but also include the control of stability
(Lee, 2012). In a similar way especially suited for the OM seem
projects where a surcharge is placed, a tank is filled or similar
loading of soil with storage takes place. The flexible use of
(pre)loading has proved very efficient in many cases.
Other types of projects suitable for the application of the
OM are pipelines when deformation limits are very strict,
because the allowable values are difficult to assess in design.
Environmental projects (contaminated sites) have been
presented by Morgenstern (1994) and drainage works by
(Roberts and Preene, 1994).
Very simple structures (‘in the backyard’) are usually not
suitable for the OM, because the costs of the additional
monitoring are often larger than the benefits for the project.
In all types of projects where buildings are present at short
distance, the method may be beneficial because they can be
strictly monitored. On the other hand, much more flexibility in
the system is present if no such buildings/structures are present.
Usually in dealing with stringent deformation limits it is
necessary to have a more robust design, which reduces the
effectiveness of the use of OM.
5 CONCLUSIONS
Conclusions in this paper are given in the form of Go/No Go
items for clients and project initiators, as well as designers in a
very early stage of the project, to determine whether or not the
OM could be a wise approach in their specific project, given the
specific circumstances. These Go/No Go items are listed by
importance, based on the opinion of the authors. Some issues
form the Go/No Go list may be given facts for a project, some
may be project choices that may benefit (or contradict) the use
of the OM. Some items should be taken merely as reminders of
how to organize the project most efficiently. These items are
labelled in the third category ‘To overcome’.
Go:
Multistage projects and/or projects with an incremental
construction process.
Presence of risks with low, but unacceptable a priori
probability of exceedance and significant consequences.
