Geoscience Reference
In-Depth Information
where:
α
=
slope angle of the structure [deg];
δ
=
friction angle between the geotextile bag surface and the subsoil [deg].
Various tests have been performed to determine the friction angle between geo-
textiles and different subsoils. The results show a large range of friction angles:
δ
. For applications under water, or where a subsoil is comprised of wet
clay, it is recommended to use a friction angle of
=
20
°
-40
°
. If the calculations result in
low stability under longitudinal currents due to a low surface friction angle between
the bags and the subsoil slope, it is recommended that actual testing be performed to
determine the appropriate geotextile bag/subsoil friction angle. Additionally, a toe
structure can be used to prevent the lowest geotextile bags sliding from the slope.
Another aspect that has to be considered when using formula (3.11) is the pos-
sibility of sand movement within the geotextile bag. With a continuous external water
flow the geotextile bag may be internally stable, but if the sand in the geotextile
bag moves, this could result in deformation of the bag and lead to bag instability.
This phenomenon was first reported in 1968 [30] in one of the earliest studies into
geotextile-encapsulated sand elements. This early small-scale study into the stability
under current flows showed that at a flow velocity of more than 1.5 m/s, internal sand
movement occurred in the geotextile bag. Moreover, geotextile bags placed on a slope
as low as 1:8 would become unstable at a flow velocity of 2.5 m/s because of internal
sand movement irrespective of the size of the geotextile bags. Recent research on geo-
tubes showed that also the degree of filling has an influence [41], [33].
δ
=
20
°
3.5.5 Stability when subject to overtopping currents
For some structures, such as soil protection dykes, groynes and underwater dams, the
stability when subject to current attack must be checked. The following dimensionless
relationship, formula (3.16), between the external current acting on the structure and
the resistance (weight) of the geotextile bags can be used.
u
(3.16)
F
cr
≤
gD
t
D
where:
cr
=
maximum allowable flow velocity over the crest of the structure [m/s];
F
=
stability factor [
−
].
Various values of the stability factor
F
have been documented:
F
=
1.2 [22];
F
=
0.5 - 1.0 [9, in Dutch];
F
=
0.9 - 1.8 [19].
0.9 for either where an external current
is perpendicular to the axis of the structure or where geotextile bags are placed in
mounds.
It is recommended to use a value of
F
=
