UNCERTAINTY AND RELIABILITY - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 119

- When prediction of geotechnical behaviour is difficult, it can be appropriate to
apply the approach known as the observational method, in which the design is
reviewed during the construction.
The Observational Method (OM) in ground engineering is a continuous, managed,
integrated, process of design, construction control, monitoring and review that
enables previously defined modifications to be incorporated during or after
construction as appropriate. All these aspects have to be demonstrably robust. The
objective is to achieve greater overall economy without compromising safety
(CIRIA). Hence, at the start, the basis is (after Powderham) a design with all
possible uncertainty (risks) with sufficient margins (risk analysis) in accordance
with actual building codes. The OM then allows adjusting the earlier estimated
risks by using an adequate and suitable monitoring (observation) following a
predetermined protocol. The purpose is minimising time and cost without danger,
and the obtained benefit is distributed amongst partners according to a predefined
agreement. For more information see Nicholson et all.
D RISK APPROACH AND VISUALISING THE UNSEEN
In many cases on-site soil investigation is limited and the real subsoil condition
remain unseen, which can sometimes lead to risky situations. It is up to the
geotechnical engineer to create proper awareness about such circumstances and to
suggest and design adequate monitoring and alertness procedures. The expert was
traditionally on the site in early times, but nowadays he is more likely to be remote
from the site or not involved at all. We should be aware of risk exposure and
juridical aspects and bring the expert back to site: the remote observational
specialist.
Dealing with risks
Risks are categorised according to the certainty of their occurrence and related
consequences (van Staveren). They represent a range with at one end completely
uncertain risks and at the other end unambiguously well-defined risks. To risks
with a well-defined probability, based on sound scientific proof, the concept of
own responsibility or solidarity applies. In juridical terms, the principle of
prevention prevails. Somewhere in the range, the uncertainty of the probability and
consequence of a risk is so large that it is not possible to have sufficient control. To
these risks the principle of precaution prevails. This implies that a proactive policy
should be adopted, which is based on presumptions about possible consequential
damage without sound scientific proof. Finally, unknown risks exist for which
preparation is not possible.
Three aspects are distinguished in the application of the principle of precaution
for human activities in the subsoil: scientific uncertainty, damage threshold and
reversal of the burden of proof. From a scientific view, natural processes in the
underground are uncertain because of complexity, variability, limited transparency
and limited knowledge of long-term effects. Therefore, the type and gravity of
related damage is unsure. Before mitigating measures can be or (in a juridical
sense) must be taken, the potential damage threshold must be determined. Here, the
concept of non-negligible damage with regard to the natural environment prevails,