Geoscience Reference
In-Depth Information
3.5.6 Stability of geotextile bags placed in mounds
The stability of a mound structure of geotextile bags is dependent on the following
potential failure mechanisms:
￿
stability of the mound subject to wave attack;
￿
shear failure of the subsoil;
￿
liquefaction of the subsoil.
Stability of the mound under wave attack
During wave attack, if geotextile bags are formed into a mound, a pressure difference
may occur between the water pressure within the mound and the external water pres-
sure, and this may cause shearing failures in part of the mound and thus pose a threat
to stability. For geotextile containers this phenomenon has been studied in small-scale
model tests such as those reported in [6]. In section 6.5.7 the stability of a mound
subject to wave attack is discussed in more detail.
Shear failure of the subsoil
If geotextile bags are used for a slope revetment, they must also be checked for poten-
tial shearing of the subsoil. A substantial part of this section is extracted from [31, in
Dutch].
A wave load on the top layer of a slope penetrates into the subsoil. The load is
dampened and delayed due to the water in the porous space being elastically com-
pressible. This creates fluctuating water movements and oscillations of hydrostatic
pressure in the subsoil, and thus corresponding oscillations of effective stress. This
phenomenon is called elastic recovery and can result in shearing of the subsoil. The
stability of the subsoil can be endangered if the elastic recovery causes the effective
stress to reduce to the extent that there is insufficient shearing resistance in the subsoil
to prevent failure.
The criterion for shearing resistance of the subsoil is considered to be fully
1-dimensional. This criterion is considered to be conservative because extra support
can be provided by subsoil elements below the element with the highest loading.
The criterion is:
sin
tan
α
δ
cos
(3.17)
Δ≤
(
Δ
+ Δ
)
α
max
t
k
s
where:
ΔΦ max
=
maximum head difference in the level of rise under the top layer of geo-
textile bags [m];
t
=
relative density of the filled geotextile bags, see formula (3.5) [
];
s
=
relative density of the subsoil ((
ρ subsoil -
ρ w )/
ρ w ) [
];
z
=
depth of the subsoil [m].
Design charts have been made of the subsoil shearing criterion for the most com-
mon situations in Appendix C. These show the maximum allowable wave height versus
 
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