Geoscience Reference
In-Depth Information
The following can be assumed for geotextile containers with a cross-section of
10 m
2
when the wave period
T
=
3 s and when using sand with 20% particles finer
than 63
μ
m, thus
D
20
=
63
μ
m:
Given:
drainage distance:
d
=
2 m;
permeability of the fill material
: k
=
10
−5
m/s;
porosity:
n
=
0.4;
porosity reduction under constant wave load:
Δ
n
=
0.01;
one-dimensional compressibility of the grain skeleton during discharge:
α
c
=
3
⋅
10
−8
m
2
/N;
generation of pore-water pressure upon undrained load:
mean initial effec-
tive stress divided by the time needed for a number of waves to cause complete
liquefaction in undrained conditions
ψ
0
=
≈
(
ρ
-
ρ
w
)
⋅
g
⋅
d
/(30
⋅
T)
≈
20000 N/m
2
/
(30
⋅
3 s)
≈
200 N/m
2
s.
2777 s,
which indicates that complete liquefaction is unlikely, but that significant excess pore
pressures may occur in this situation.
Substituting these values in formulae 6.22 and 6.23 gives
T
d
=
117 s and
T
n
=
6.5.9 Placement accuracy
Before geotextile containers reach the bottom they may travel laterally (known
as 'sailing'). Since this phenomenon can be explained easily in terms of flow
mechanics, it is possible to simulate this with a mathematical model. The geotex-
tile container does not fall straight downwards due to an initial oblique angle of
the split barge or through currents or waves. During the fall the container is sub-
jected to a horizontal force and moment, causing the container to move sideways.
The extent of this movement (mean and standard deviation) increases with water
depth, flow velocity and wave height. Currents tend to have more influence than
waves. On an uneven bottom, for instance, when dropped on other geotextile
containers the geotextile container comes to rest more quickly; on a flat bottom
it takes a little longer and the potential horizontal shift is greater. Accuracy of
placement is enhanced by a larger container mass and faster opening of the split
barge.
Model tests were performed in the model facilities of Deltares (see Figure 6.13)
as well as a prototype field demonstration [6]. These tests have provided insight into
the drop process. Although uncertainty remains about the placement accuracy of geo-
textile containers dropped from a split barge, especially where this occurs at a water
depth of more than 15 m. At a water depth of less than 10 m good placement accuracy
can be achieved when dropping geotextile containers of approximately 300 m
3
under
the influence of certain currents and waves in order to build a stacked container struc-
ture with a side slope of 1(V):2(H).
At a water depth of more than 10 m the placement accuracy in currents and
waves decreases markedly and it is possible that the centre of gravity of the container
may move several metres horizontally during the fall.
