Geoscience Reference
In-Depth Information
on the field data measurements at a water depth of 15-22 m for still water without
current (the average horizontal displacement is then zero):
(
)
(H.1)
s
h
32
h
−
h
>
h
p
3
where:
p
=
standard deviation of the placement accuracy [m];
h
=
water depth [m].
This result shows a somewhat greater variability for the situation where full-
scale geotextile containers are placed than in the model study by Deltares. These
results demonstrate that accurate placing on a sandy bottom at larger water depths
is limited.
In the literature, model research has been carried out (scale 1:60) into the place-
ment accuracy of geotextile containers in water currents [34]. The water current
direction was parallel to the container placement. The drop depth varied from 30 to
48 m, current velocities varied from 0.5 to 2.9 m/s (prototype values). The authors
describe that geotextile containers with a length of up to 15 m (which is relatively
small in relation to what is common) oscillate some-what in their descent through the
water. For elements in the shape of sandbags, the fall was of a spiral in nature. It was
assumed that all the variability in placement was caused by the water current and thus
the relationship between the theoretical displacement of
x
m by the water current and
the placement accuracy as a standard deviation is given by 0.25
x
m.
The placement accuracy of geotextile containers can also be compared to that
of rocks dropped in water. This has been studied by Van Gelderen and Vrijling [28].
They observed that for several rocks the placement accuracy can be compared with a
Rayleigh distribution where around 40% of the rocks have a standard deviation that
can be stated as:
⋅
50
(H.2)
chD
s
c
⋅
p
where:
p
=
standard deviation of the placement accuracy [m];
c
=
constant (approximately
=
0.7) [
−
];
h
=
water depth [m];
D
50
=
nominal diameter of the rock [m].
Although this formula is derived on the basis of rubble experiments and not for
geotextile containers, it is important to note that according to this formula the stand-
ard deviation of the placement accuracy increases with increasing
D
50
, thus a large
rock has a relatively larger potential deviation. For geotextile containers the deviation
can also be considerable. The
D
50
value of a geotextile container is difficult to deter-
mine but it is certainly several metres (3 m or so). According to formula (H.1) a fall
depth of 20 m would result in a standard deviation of 4.6 m (this corresponds well
with the value found for the deviation using formula (H.2) of 4.8 m).
