DIKE ENGINEERING - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 302

assessment of an economically optimal design is achieved by the application of the
probabilistic approach, which involves a close inspection of water levels and
corresponding failure mechanisms, their probability and their coherence. This
approach has gained support in the last twenty years and has become the accepted
design philosophy of modern dike design and maintenance in the Netherlands.
Dike-ring approach
The strength of the chain of defences is never greater than their weakest link.
This anticipates a safety norm of an area protected by a sequence of dikes, referred
to as a dike-ring. For each zone surrounded by a dike-ring the safety norm is
related to the economic, social and environmental values assigned to the protected
area. Hence, the probability of local dike failure is connected to the likelihood of
damage to the protected assets, i.e. risk = probability of failure x consequences.
Dike technology is therefore based on three major elements:
- The safety philosophy; the dike-ring approach, application of probabilistic
methods, and research on economically and ecologically sound ways to retain
the water, in balance with the protected values.
- The design and the control of functions of water-defence systems; control-
guidelines, technical elaboration of various failure mechanisms, and research on
new ways of realisation of structural elements.
- Rational maintenance; methods for preventive maintenance, methods for error
detection and monitoring, and efficient techniques of maintenance.
Safety philosophy
Disasters due to technical failure seem to be socially less acceptable than natural
catastrophes. An important question is whether disasters due to technical failure
can be avoided and whether damage due to natural catastrophes can be minimised.
An observation is that in a society full with sophisticated technical systems
disasters are unavoidable. Accidents are the logic consequence of a society that
"lives" on technology. The discussion about safety mainly deals with probability
theory and social acceptance. In a society that is full of technical systems, there
exists an apparent subjective safety during a period of absence of accidents. Safety
assessment therefore comprises the evaluation of risk sensation involving objective
personal and social acceptance and the analysis of probability of failure involving
various realistic events. The new Water Retainment Act provides the possibility to
base the safety philosophy on likelihood of inundation. Three risk-components
determine the relation between safety level and a probability of failure or
malfunctioning: (1) personally acceptable risk, (2) optimised economic risk, and
(3) socially acceptable risk.
The indivudually acceptable risk is the probability of a casualty during an action
while considering the personal factor (voluntary or involuntary activity). The
optimised economic risk is the probability of exceeding expected cost of
damage/maintenance, which may include the number of casualties in case of failure
(inundation). The socially acceptable risk is related to a balance of social costs and
benefits, including cultural, ethical and environmental aspects. The most critical or