Geoscience Reference
In-Depth Information
function occur? The overall safety factor thus comprises a collection of partial safety
factors that all reduce the design value of the strength of the geotextile. In 2.4 these
partial factors are examined more closely. Fixed partial factor values are difficult to
provide because they are dependent on the required economic lifetime. Currently,
the following partial factors for the material are being used: weakening by seams 2,
damage during construction 1.25, loss of strength through creep 1.4 to 4 (dependent
on material type). These default values must be used with caution, with a minimum
overall safety factor of 3.5 to 4.0 being applied.
The stability relationships used in this manual for the loads, e.g. waves, currents, etc.,
are empirical in nature based mainly on model research. For these relationships, safety
is implicitly included since a stability relation, based on the performed research, will be
drawn underneath all points where instability was measured. Therefore, no additional
safety factors to these stability relationships have been applied. Thus, the application of
safety factors here only relates to the strength properties of the geotextile to be used. It
should be noted that this is only valid for the conditions tested in the performed research.
The designation of the appropriate safety factors for the different geotextile-
encapsulated sand elements are detailed in the respective sections entitled “Failure
mechanisms and safety considerations” of chapters 3, 4, 5 and 6.
2.3.3 Potential failure mechanisms
The designer must have an understanding of the potential failure mechanisms that could
occur in hydraulic structures. Experience has shown that whenever a structure fails, it
tends not to be as a result of underestimating the loading but is more often due to a fail-
ure mechanism that was not considered during design. It is, however, outside the scope
of this manual to detail all possible potential failure mechanisms for different hydraulic
structures, e.g. wave overtopping, piping and geotechnical instability during the con-
struction and in-situ phases. We refer to the various existing publications such as [11].
The potential failure mechanisms considered here all relate to the grouping (with
the exception of geotextile mattresses) of geotextile bags, geotextile mattresses, geo-
textile tubes and geotextile containers, as used in hydraulic structures. The individual
chapters in this manual examine these potential failure mechanisms for each type of
element, related to both the construction and operational phases, such as:
instability due to wave and current loadings;
instability due to tilting, rolling, slipping or overturning;
washing out of fill material through the pores of the geotextile;
puncture of the geotextile as a result of the impact of sharp-edged armour stone;
failure of seams and/or geotextile through tensile rupture;
liquefaction of sand in the structure leading to adverse deformation.
The cause and effect of each potential failure mechanism, as well as the interrela-
tionship with other potential failure mechanisms can be illustrated by a fault tree that
defines an undesired event, e.g. the failure of a bank protection made of geotextile
bags. The branches of the fault tree show the various causes that could lead to this
event. The probability of the occurrence of these individual potential failure mecha-
nisms and the relationship between them generates the probability of this undesirable
