Geoscience Reference
In-Depth Information
strength. With low maximum breaking strains (
20%) relatively little deformation
occurs upon loading. This is an advantage since the shape of the tube is maintained
after filling. Relatively low maximum breaking strains may also mean that tubes may
be damaged when subjected to larger deformations, e.g. due to local scour or insta-
bilities. Small diameter tubes are sometimes made of nonwoven geotextiles that allow
greater deformation, however, since the tensile strength of nonwovens is lower than
that of woven material, large diameter tubes cannot be made and the shape of the tube
cannot be maintained over time when using a nonwoven.
The geotextile rolls have a maximum width of 4 to 6 metre. Strips with the width of
the rolls and the length of the desired circumference of the tube and are sewn together
along the edges to form the tube. Moreover, at regular distances, filling and draining
ports are sewn into the geotextile tube structure (see Figure 5.7). Manufacturers have
their own proprietary techniques for the precise shape of these ports that govern the
maximum pressure that can be applied during the filling of the geotextile tube. In most
cases this is approximately 5 kPa. The opening of the drainage port is then 0.5 m or less
(depending on the sand concentration in the fill) above the top of the geotextile tube. In
the calculations in this chapter 5 kPa is used as the pressure on the upper side of the tube.
This pressure determines the shape of the tube and also the maximum degree of filling.
The seams are often the weakest point of the geotextile tube. In sewing the geo-
textiles together a butterfly seam or (double) J-seam is normally used (see 2.4.5).
Proprietary seaming techniques may also be used in order to achieve higher seam
efficiencies. The strength of the seams made in the factory is approximately 50% to
80% of the maximum strength of the geotextile materials.
<
5.5.2 Required tensile strength
The geotextile has to be sufficiently strong to resist the loads that occur during the
filling of the geotextile tube. To determine the ultimate load on the geotextile and the
Figure 5.7
Drainage port in action during filling a geotextile tube for Delta flume tests [38].
