Geoscience Reference
In-Depth Information
5.4 FAILURE MECHANISMS AND SAFETY
CONSIDERATIONS
In designing a structure with geotextile tubes the following failure mechanisms have
to be taken into account:
Rupture of the geotextile tube through excessive pressure during filling;
Rupture of the geotextile tube through inadequate strength of the seams;
Instability (rolling) of the geotextile tube during filling;
Instability of the geotextile tube when subjected to wave attack;
Instability of the geotextile tube when subjected to current flowing over the structure.
The design formulae for these failure mechanisms are discussed below.
In the design process a safety factor of 1.1 to 1.2 is normally used for geotextile
tubes. In addition, strength-reduction material factors are used. The strength-reduction
material factors to be applied can best be derived from the material data provided by
the manufacturer and the tables contained in this publication.
During the filling of the geotextile tube, the geotextile may be weakened by the
abrasion of the sand-water mixture. In practice, a strength-reduction material factor
of 1.25 is used. This chapter uses the following (commonly used) values for the
strength-reduction factors:
γ
for strength of the seams
=
2
γ
for creep
=
1.4
γ
for abrasion damage related to the above
=
1.25
This leads to an “overall” strength-reduction material factor of 3.5.
5.5 DESIGN ASPECTS
Once the main dimensions and the construction of the structure as well as the size of
the elements have been determined, the detailed design is carried out. The structure
is assessed in respect of the components shown in the design chart (see Figure 2.1),
including the required tensile strength of the geotextile and the stability requirements
for waves and water currents. If the required tensile strength is too high for the geo-
textile and seams to be supplied at an acceptable cost, a smaller geotextile tube or
different operating method may be selected, provided the overall design requirements
can be fulfilled. Following this, the geotextile tube is checked for stability require-
ments against waves and water currents, and geotechnical stability associated with
any tube stacking. If the stability requirements are not fulfilled either a larger geotex-
tile tube must be selected or supplementary measures taken. The next section looks
more closely at the various components of the design cycle.
5.5.1 Material choice and fabrication
A standard geotextile tube is made of woven geotextiles composed of polypropylene
or polyester. Woven geotextiles readily retain sand and exhibit a relatively high tensile
