Geoscience Reference
In-Depth Information
approach for practical purposes. Preliminary results are compiled in a concept
control-guideline for a try out.
Failure mechanisms or malfunctioning
For a typical dike cross-section various failure mechanisms (limit states) can be
distinguished (Fig 17.5). Traditionally, much attention is given to the mechanism
of overtopping and wave overtopping. By the selection of a proper crest height
with respect to the exceedence of design water levels and wave heights a safe dike
profile can be accomplished. Breaches in the past have proved that overtopping
was not always the cause of inundation. Piping, micro-instability and slope-slides
have induced flood disasters. Also situations that did not happen so far could be
added, such as collision during high water by a large ship. A similar consideration
applies to structures in the dike (sluices, floodgates, locks, pumping stations), when
they participate in the water-retaining function of the dike. Excess loading may
also exceed the bearing capacity of the foundation causing large unacceptable
displacements. History shows that structures in dikes present a major threat. An
aspect of special concern is the human factor, i.e. the manual operation of sluices,
locks and floodgates during high waters. In the analysis, the probability of the
human error should be considered. For a dike-ring the probability of failure of any
dike section and of any structure may lead to inundation of the area, as the total
strength is not greater than its weakest link. The probability of inundation increases
with the length of dikes in a dike-ring. Usually, a dike, which is well designed and
performs the required functions, also satisfies the safety conditions of various
failure mechanisms. The function "water-retaining" demands a suitable "crest
height", a suitable "water tightness" of the dike body and subsoil, a suitable
"stability" to transfer the water level induced shear force, and a suitable
"resistance" of the dike to all forces which are related to its existence (current
forces, wave loading, tourist and cattle damage). If the principal elements are well
designed, i.e. the crest height, the outer slope, the core, the inner slope, and the
subsoil, the principal limit states or failure mechanisms are dealt with
automatically.
Dealing with uncertainties
The safety of a certain mechanism is controlled by investigation of the critical
situation, the limit state
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, where loading and mobilised strength is in equilibrium,
and any increase of loading causes collapse. Sometimes a limit state cannot be
assessed by calculations. Physical tests and pilot tests or engineering experience is
used. The probability method, in fact, elucidates and quantifies the relative
importance of uncertainties. It is here, that conventional deterministic calculation
methods apply as a cornerstone in an integral approach. Uncertainties are classified
as intrinsic (related to nature: time, stochastic water levels, and space, soil
heterogeneity) and epistemic (related to models: statistics, incomplete data,
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A distinction is made between ultimate limit state and service limit state. The former is the
extreme situation when breaching occurs, the latter when any designed function is being
jeopardised.
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