Geoscience Reference
In-Depth Information
the thickness of the top layer for different slope angles. The design charts apply where
geotextile bags are located directly on a sandy subsoil, and are given for two wave
crests (
s
op
=
0.03 and
s
op
=
0.05). The wave crest steepness can be determined by:
H
L
2
H
π
H
(3.18)
s
H
s
s
==
0
op
2
gT
T
The design charts in Appendix C list an equivalent thickness of the top layer.
If the system is placed directly on sand, the equivalent thickness is equal to the real
thickness of the top layer, i.e.
D
eq
D
k
. If the system is placed on a granular filter layer,
then the weight of the filter layer has a positive effect on the stability of the subsoil.
This effect can be expressed in terms of an equivalent thickness, calculated by:
=
Δ
f
(3.19)
b
DD
D
k
=+
⋅
f
eq
k
Δ
t
where:
D
eq
=
equivalent thickness of geotextile bags [m];
f
=
relative density of the granular filter layer [-];
f
=
thickness of the granular filter layer [m].
Liquefaction
A cyclic load causes a sand layer to compress, which reduces the pore space. The water
in the pores pressurises and flows out. Initially, excess hydrostatic pressures occur,
causing the contact pressure between the sand grains to decrease and consequently,
the resistance to shearing is lowered. In the extreme condition, the excess hydrostatic
pressures may be so large that the contact pressure between the sand granules reduces
to zero. This condition is known as liquefaction.
Liquefaction can occur during earthquakes as well as during wave attack on
slopes. With regard to the latter occurrence, the following design rules apply for struc-
tures with reasonably well-compacted subsoil:
For a top layer on sand there is no danger of liquefaction if the slope:
-
is less than or equal to 1V:3H, or;
-
is less than 1V:2H and the significant wave height
H
s
is less than 2 m, or;
-
is less than 1V:2H and the subsoil is well-compacted;
For a top layer on clay there is no danger of liquefaction;
For a top layer on a granular filter there is generally no danger of liquefaction.
3.6 CONSTRUCTION ASPECTS
From the previous sections, a number of construction aspects warrant attention:
Filling geotextile bags can be a labour-intensive process, which has consequences
with regard to the materials used and the number of people employed.
