Geoscience Reference
In-Depth Information
The design formulae for the stability of a stack, presented in this section, are
mostly based on a limited number of tests and therefore must be used with the requisite
caution. From tests on tubes (which are more numerous than tests on containers) it is
known that the way the tubes are stacked and the position with respect to the water
line has an influence. The way containers are stacked is not taken into account in the
formulae presented here. Furthermore, it will be shown later that the placing accu-
racy of geotextile containers is limited, and this may lead to instability at lower wave
heights than according to the formulae presented below.
Small-scale model tests have studied the wave conditions under which instability
will occur [15]. The study used normally filled geotextile containers (46%) and, addi-
tionally, filled geotextile containers (70%) on a scale of 1:20. The slope of the stacked
containers was 1(V):3.1(H) and 1(V):1.7(H) respectively.
The pressure difference over the outermost layer of geotextile containers on the
seaward side appeared to be mainly dependent on the wave height and the water level
in relation to the stacked crest. The critical condition occurs when there is a wave
trough on the seaward side of the stack. At this point there is low water pressure
on the seaward side and high water pressure on the landward side of the stack. This
results in an outward water pressure gradient in the container stack which is greater
the narrower the width of the stack. Based on the test results, the following empirical
relationships have been established for maximum hydraulic heads over the outermost
layer of the geotextile containers:
⎛
⎝
⎞
⎟
D
H
(6.16)
⋅
0
.
24
l
D
t
t
0
04
0
.
77
f
b
b
b
D
k
6
+
⎞
⎠
+
D
k
≅
=
⎛
⎜
⎛
⎝
t
+
0
.
k
B
tot
s
⎛
⎝
⎞
D
H
(6.17)
04
0
.
31
l
⎛
D
t
t
0
1
.
00
f
b
b
D
k
3
5
+
⎠
+
b
D
k
≅
=
⋅
⎛
⎜
⎛
⎝
t
+
.
0
.
k
B
tot
s
where:
Φ
=
expected maximum hydraulic head difference over the outermost layer of
the geotextile containers [m];
D
t
=
thickness of the shear-susceptible layer of the geotextile containers - see
Figure 6.11 [m];
tot
=
total width of the layer of the geotextile containers under consideration -
see Figure 6.11 [m].
To establish stability requirements for container stacking, the calculation model
of Nurmohamed [2, in Dutch] is used. For the angle under which the geotextile con-
tainers can shear (see Figure 6.12):
β
=
α
−
ψ
(6.18)
where:
β
=
shearing angle of the outer geotextile containers [deg];
α
=
stack slope [deg];
ψ
=
fill dilatancy angle [deg].
