Geoscience Reference
In-Depth Information
Figure 6.13
Model tests at Deltares on the dropping of geotextile containers.
To be able to place geotextile containers as accurately as possible, a thorough
drop plan must be established. In view of the increasing difficulty of accurate place-
ment at greater water depth, it is this area that demands most attention.
In constructing a stacked container structure it has to be accepted that a number
of units may fall outside the profile. The first layer of containers is the most difficult
to place ('sailing effect'). Taking account of the currents and waves a position will
have to be established from where the first container is dropped. At substantial water
depth the bed width of the profile of the structure to be built also increases. This wid-
ens the drop space for the first container (see Figure 6.14). After the first container is
dropped, its resting position must be measured immediately. The position of the split
barge during the drop and the position of the first container on the bed will indicate
the placement accuracy. Following this, the second unit can be placed. After each drop
the unit must be checked for its exact position since the placement accuracy is random
in nature. During the drop of the initial layer, the containers to be dropped will be
increasingly influenced by those already dropped, which helps the placement accuracy.
For the second and subsequent layers the placement accuracy increases significantly,
as stated previously. Field measurements and experiments have shown that the place-
ment of the second and subsequent layers, despite the presence of currents and waves,
can be twice or even four times as accurate as the dropping of the initial layer.
In summary, it can be concluded that placement accuracy increases under the fol-
lowing conditions:
the lower the water depth;
the lower the wave height and/or flow velocity;
the greater the cross-section of the geotextile container and split barge;
the greater the density of the geotextile container;
