Geoscience Reference
In-Depth Information
seams, the degree of filling and resulting shape of the geotextile tube after filling has
to be established. The degree of filling depends on the following factors, amongst
others:
whether filling below or above water;
filling pressure.
The relationship between the shape of a geotextile tube and the tensile load
in the geotextile (as a result of the pressure exerted by the fill material) may be
derived using the “Timoshenko method”, the background of which can be found in
Appendix E and [44]. This method may be presented as analytical solutions, but
given their complexity can only be solved using numerical methods, e.g. the com-
puter programs GeoCoPS [21] and “Simulation of Fluid Filled Tubes for Windows”
(SOFFTWIN) by Palmerton [13].
The analysis method is based on the assumption that there is no transfer of shearing
stresses by the fill material on the surface of the geotextile in that part of the tube where
there is no contact with the foundation and that the geotextile tube has no flexural stiff-
ness. Under these conditions the tensile load in this part of the geotextile is constant.
The relationship between internal pressure, tensile load and curvature of the
geotextile is:
T
=
p
⋅
r
(5.6)
where:
T
=
tensile load in the geotextile [kN/m];
p
=
pressure in the fill material [kN/m
2
];
r
=
radius of curvature at a random point on the geotextile skin [m].
For the design calculation of a geotextile tube structure several numerical calcula-
tions are made (based on the Timoshenko method) to determine the tensile load in the
geotextile, the results of which are shown in Figures 5.8 to Figure 5.10. If the geotex-
tile tube is to be laid entirely under water the following steps must be followed:
Choose a desired height of the geotextile tube (in this example
h
=
2.0 m);
Use Figure 5.9 to determine the corresponding circumference of the geotextile (in
this example
S
10.4 m), the degree of filling with respect to the area is 82% and
the height ratio compared to the theoretical diameter is 62%;
=
Use Figure 5.10 to determine the corresponding theoretical tensile load in the
geotextile skin (in this example
T
=
16 kN/m).
For a geotextile tube of 2 m height above water a corresponding theoretical tensile
load of 26.5 kN/m is found. In this case the degree of filling is 72% and the height
ratio with respect to the theoretical diameter is 52%.
A safety factor has to be applied to the calculated tensile strength to account for
the influence of creep, ageing and the presence of seams. The theoretical required
tensile strength, as shown in Figure 5.10, is therefore increased in the design by a
factor of 3.5 (see Section 5.4).
