Geoscience Reference
In-Depth Information
2.3 SAFETY CONSIDERATIONS
2.3.1 Introduction
From a design perspective, the structure must meet its functional requirements over
the design lifetime. This also applies to the probability of failure, which must be
lower than the pre-established target, which is sometimes defined per failure mode.
This means that the probability of a load on the structure exceeding its resistance
remains below a target value, which itself is established considering the potential con-
sequence of structural failure. The use of probabilistic reliability concepts is becoming
more and more popular in the civil engineering branch lately. There are four distinct
levels:
Level III: Fully probabilistic method;
Level II: Semi-probabilistic method;
Level I: Quasi-probabilistic method (partial safety factors);
Level 0: Deterministic method (overall safety factor).
These methods are described, and used, in detail in [10].
2.3.2 Deterministic method
For designing with geotextile-encapsulated sand elements the knowledge and experi-
ence has not yet developed to the extent that probabilistic methods can be used reli-
ably. Therefore, this manual focuses on deterministic methods only. For loads and
strengths, fixed design values are incorporated for each parameter separated by an
overall safety factor.
R
≥
γ
⋅
s
(2.1)
where:
R
=
resistance (strength);
s
=
loads;
γ
=
overall safety factor.
The overall safety factor used must have a value greater than or equal to 1.0
(in practice an overall safety factor between 1.1 and 1.5 is common). In many cases
there are no formal procedures available to determine the appropriate overall safety
factor. In practice, the established safety is that which is still available in respect of the
(characteristic) design value. It is thus visible to both the designer and the user what
safety is present in the structure in relation to the design values. This safety approach
is adopted in this manual.
Strictly speaking, overall safety factors are used to compensate for uncertainties
and coincidences. The method is also applied when taking account of the reduction
of the strength of the geotextile material, caused by UV radiation, thermal oxidation,
seams, creep or damage during construction. There is a close relationship with the
question: what is the desired (economic) lifetime, and when may a loss of structural
