Geoscience Reference
In-Depth Information
At the transition between the fill material and geotextile the specific discharge is
constant and thus:
(A.4)
Hk
=
ΨΔ
i
⋅
s
where
k
s
is the hydraulic conductivity of the soil [m/s] directly adjacent to the geotextile.
No excess pore pressure will be created if the pressure fall across the geotextile is
sufficiently small. A safe assumption is therefore:
ki
s
H
Δ
001
.01 m
(A.5)
≤
=
Ψ
The permeability of a geotextile will vary as a result of its interaction with the
adjacent fill material. When adjacent fill material is present, this interaction will
always result in a lower measured geotextile permeability than if the geotextile was
tested in-isolation. When adjacent fill is present three phenomena may occur, namely,
blinding, blocking and clogging.
Blinding occurs when the fill material has grains that are large compared to the open-
ings in the geotextile. An example is a geotextile that is blinded by the presence of adja-
cent armour stone. Here, the armour stone lies on the geotextile and no water can flow
through the geotextile (where this blinding occurs) so the average permeability of the
geotextile is lower.
If the particles of the fill material are moved by the flow of water and deposited in
the openings of the geotextile blocking occurs, a phenomenon that can occur in thin
materials and exists when the following factors are present at the same time:
a uniform opening size exists in the geotextile;
a uniform grain-size distribution exists in the fill material;
O
90
/D
90
lies between 0.5 and 1.0.
grain
geotextile
flowing water
Figure A.1
Blocking mechanism.
