Geoscience Reference
In-Depth Information
crest on the waterline and
H
s
H
s
under water) or in areas
with larger waves if the crest is 2
H
s
or more under water. If these criteria are not ful-
filled it is possible that there will be sand migration in the container thus leading to
undue deformations. It should be noted that this conclusion depends on the degree of
filling. The authors have used a low degree of filling (
<
1.1 m with the crest 0.75
⋅
44%) for the basis of their con-
clusions. With a higher degree of filling a container should be able to withstand higher
wave attack, but usually it is difficult to dump a geotextile container with a higher
degree of filling because of the opening width of the barge. The wave attack criterion
presented by van Steeg and Klein Breteler is rarely a limiting factor since the use of a
split barge for container installation requires that the top of a geotextile container be
located several metres below the water line.
∼
6.5.6 Stability in currents over the structure
In [19] the stability of geotextile containers under current loads was modelled at a
1:20 scale. The results apply to currents perpendicular to the axis of a stack of geo-
textile containers. Four situations were modelled:
situation 1:
6 m high 4-3-2 stacking with crest at 0.56 m under water:
instability at a flow velocity of 0.57 m/s in front of the structure;
situation 2:
5 m high 4-3-2 stacking with crest at 3.50 m under water:
instability at a flow velocity of 1.42 m/s in front of the structure;
situation 3:
5 m high 4-3-2 stacking with crest at 4.70 m under water:
instability at a flow velocity of 1.70 m/s in front of the structure;
situation 4:
5 m high 3-2-1 stacking with crest at 3.50 m under water:
instability at a flow velocity of 1.34 m/s in front of the structure.
For the critical flow velocity across the crest of the stacked geotextile containers
a design formula is given in [22, see paragraph 5.4.11]:
u
.5 to 1.0
(6.15)
cr
05
<
g
D
t
D
where:
cr
=
critical flow velocity across the crest of the structure [m/s];
g
=
acceleration due to gravity [m/s
2
].
Currents parallel to the axis of stacked geotextile containers allow much higher
flow velocities. If the stacking is constructed of geotextile containers of a certain
length in the sea or in a river or an estuary, then the current will veer from the struc-
ture. To determine the stability, an assumption has to be made for the flow velocity
component perpendicular to the stacked containers upstream and/or above the crest
(see figure 6.10). The upstream values can be comparable to the critical values from
[19], with the values above the crest comparable to formula (6.15).
