Geoscience Reference
In-Depth Information
Placement accuracy
for geotextile containers
The placement accuracy is based on an average lateral displacement along with a
corresponding lateral standard deviation. The magnitude of the average placement
variability can be largely compensated during installation by taking appropriate pre-
cautionary measures, such as upstream dropping. The lateral standard deviation is
random.
Small-scale tests at Deltares studying the placement accuracy for a drop depth of
15 m on a scale 1:20 showed rather large lateral displacement, see Table H.1, espe-
cially when the containers were dropped in water with a current. Theoretical studies
confirm that such displacements are possible. In [20, in Dutch] the results of both the
model study and the theoretical modelling are described.
The large horizontal displacements are probably the result of a smooth bed, which
can form a water cushion between the bed and the (nearly fully descended) geotextile
container. The geotextile container acquires a little lift from this cushion and the hori-
zontal current velocity increases along with the horizontal displacement. When the
bed is rough there is much less chance of a water cushion forming.
Based on the substantial horizontal displacements found in the Deltares model
study, it was decided to perform field studies to analyse the placement accuracy data
during the construction of the Kandia dam. A sonar scan was carried out both before
and after the dropping of each container so that the position of the geotextile con-
tainer on the bottom could be accurately determined and compared with the position
of the split barge at the moment of drop. In [4] a design formula is presented, based
Table H.1
Average lateral displacement [m] (difference between horizontal drop
position and position where the container comes to rest) and lateral
standard deviation [m] for the dropping of geotextile containers at
various current velocities and significant wave heights at a fall height of
around 16 m (recalculated from small-scale tests). The large standard
deviations in these tests lead to an additional study. See also text.
Average lateral displacement
Lateral standard deviation
H
s
=
0 m
H
s
=
1.2 m
H
s
=
0 m
H
s
=
1.2 m
u
=
0 m/s
0.8
4.6
0.8
4.0
u
=
0,5 m/s
10.6
16.6
6.4
7.0
u
=
1,0 m/s
22.6
24.6
3.6
1.2
