Geoscience Reference
In-Depth Information
of the formulae gives the required tensile strength of the geotextile (using a safety
factor 1):
⋅
⎛
⎜
⎞
⎟
J
C
DV
bS
ρρ
−
(3.3)
T
2
n
w
⎛
⎝
ρ
g
=⋅
⋅
⋅
⋅
ρ
d
w
where:
T
=
tensile load in the geotextile [N/m];
b
=
width of the geotextile bag [m];
S
=
circumference of the geotextile bag (perpendicular to the longest axis) [m];
J
=
tensile stiffness of the geotextile, see formula 2.3 [N/m];
d
=
drag coefficient (for which 1 may be assumed) [
−
];
ρ
=
density of the geotextile-encapsulated sand element [kg/m
3
];
ρ
w
=
density of water (1,000 for freshwater 1,030 for saltwater) [kg/m
3
];
g
=
acceleration due to gravity (
=
9.8) [m/s
2
];
V
=
volume of the geotextile bag [m
3
].
This formula applies when the maximum velocity is reached in water. This is the
case for dumping when the water depth is more than 10 m. However, if the bags fall
partly through air (for example when dumped from a side stone barge), they quickly
acquire speed and can achieve terminal velocity at smaller water depths. Several fac-
tors that could lead to a lower required tensile strength, such as the cushioning effect
of the sand-fill are not included here. Consequently, the overall factor of safety (
γ
) of
1.0 can be applied.
In chapter 6 a calculation method is given in which the cushioning effect of the
sand-fill is also taken into account. This is applied to geotextile containers but can,
in principle, also be used for geotextile bags. Because the shape of geotextile bags is
different (relatively rougher) compared to geotextile containers, the method described
above is recommended for geotextile bags.
3.5.3 Stability in waves
The stability under wave attack of structural units (blocks, armour stone, gravel, etc.)
used in hydraulic engineering is commonly shown in terms of the ratio of wave height
to the weight of the structural units. The stability criterion is often a function of the
breaker parameter:
H
f
ξ
(3.4)
s
()
=
Δ
D
t
D
where:
1
ρρ
−
(3.5)
Δ=
(
)
s
s
w
n
−
⋅
t
ρ
w
This formula is different to what is normally used in conventional hydraulic structures. The reason is that
the stability of geotextile-encapsulated sand elements as a whole is of importance, and not the stability
of the individual grains in the element. The weight of the elements depends on the porosity of the system
and thus the term (1 -
n
) is included in the formula.
1
