Geoscience Reference
In-Depth Information
The following section deals with the design formulae for these potential failure
modes.
3.5 DESIGN ASPECTS
Once the overall dimensions and the planned construction procedure of the structure
have been established, and the size of the geotextile bags has been determined, the
detailed design can be carried out. The structure is assessed for each of the potential
failure modes shown in the design chart, including the required tensile strength of
the geotextile and the stability requirements for waves and currents. If the stability
requirements are not fulfilled, a larger, heavier geotextile bag must be selected or
the geometry of the structure changed. If the required geotextile tensile strength is
too high, a smaller geotextile bag or different operating method can be selected.
The following sections investigate the various components of the design cycle in more
detail.
3.5.1 Material choice
In the Netherlands geotextile bags of woven polypropylene are normally used.
These materials have a greater tensile strength and stiffness than nonwoven geo-
textiles. A greater tensile stiffness can be an advantage because the bags hold their
shape better and do not deform as readily. The disadvantage is that at relatively
low strains high localised stresses may cause rupture of the bags. If maintenance
of shape is important, polyester geotextile materials may also be used, however,
polyester is a smooth material and therefore the bags may move over each other
due to the lower surface friction characteristics. Geotextile bags also need to be
robust enough to survive impact on the bottom during any dumping operation,
see 6.5.3.
3.5.2 Required tensile strength
In addition to filling, the method of placement (by positioning or dumping) governs
the required tensile strength of the geotextile used. If this is done utilizing lifting eyes
incorporated into the geotextile bags, the whole bag weight will be transferred via
the lifting eyes to the geotextile. As the calculation example in this chapter suggests
(see 3.7), this approach requires good quality seams and a specific minimum tensile
strength of the geotextile.
It is also possible to design and hold the geotextile bag during the filling and instal-
lation operation so that it is always fully supported within a mould (see Figure 3.2).
In this case the required tensile strength is not directly governed by the weight of
the bag.
If a geotextile bag is dumped under water, it must be robust enough not to be
damaged when it impacts the bottom. In [5] a calculation method has been developed
for geotextile containers, that can also be applied to geotextile bags, where first the
fall velocity is calculated and then the corresponding impact stresses. A combination
