Geoscience Reference
In-Depth Information
The variable
b
may be approximated as
b
=
(1.1 to 1.2)
⋅
D
.
where:
D
=
theoretical diameter of a tube [m].
When the crest layer is composed of two tubes connected structurally the equiva-
lent width is equal to 2
b
, Without structural connections two tubes have no greater
stability than one tube [41].
Despite geotextile tubes being able to have a relatively large length (100 m is
feasible) it is recommended in formula 5.7 that the length used in the calculations (in
the case of the tubes installed parallel to the direction of wave attack) does not exceed
twice the average thickness or height (
D
k
) of the (upper placed) geotextile tube [22].
⋅
Stability in water current over the top of the structure
Little information is available on the stability of geotextile tubes under the influence
of currents. The equilibrium of forces on a single element suggests the following
theoretical stability requirement [24] (see also 3.5.5):
u
1.
(5.8)
cr
≤
g
D
t
D
where:
cr
=
critical water flow velocity [m/s];
g
=
acceleration due to gravity (
g
=
9.81) [m/s
2
].
In [9, in Dutch] the results of experiments with 'sand sausages' under various
hydraulic loads are shown, with the conclusion that geotextile tubes are more stable
than given by the formulae in [22], which found:
μ
cr
(5.9)
05
10
.
≤
.5 to
1
g
D
t
t
k
This last value (0.5 to 1.0) is recommended as a preliminary design value. If the
stability of the geotextile tube is critical with respect to current, it is advisable to do
(large scale) model tests.
5.6 CONSTRUCTION ASPECTS
From the previous sections the points listed below are relevant to construction:
As a result of the permeability of the geotextile, the tube cannot be 'pumped up'
with water alone. First some sand will have to be pumped in to restrict the pores.
Following this, some pressure build up is possible with the geotextile tube taking
up a rounded shape;
