Geoscience Reference
In-Depth Information
of a geotextile in the same manner as it is done for the fill material. However, in
practice, it is common to express the permeability of a geotextile as permittivity:
q
H
(A.2)
=
Ψ
Δ
where:
Ψ
=
permittivity [s
−1
];
q
=
specific discharge [m/s];
Δ
H
=
water head differential across the geotextile [m].
The permittivity is the ratio between the specific discharge and the water head
differential across the geotextile. By using permittivity, geotextile thickness is not
required to be known. Conversely, when using geotextile permeability, geotextile
thickness is required to be known and this can give rise to significant errors because
of the inaccuracy in measuring geotextile thickness.
In Europe, geotextiles are supplied according to CEN standard methods of test
(the European Norms that govern geotextile methods of test) by the manufacturer.
The CEN method of test for geotextile water flow is based on the discharge through
the geotextile at a constant water head differential of 50 mm which results in the
determination of a “velocity index” parameter. While this velocity index parameter
has the units m/s it should not be confused with the geotextile permeability value,
which also has the units m/s. However, the velocity index can be used to determine
both the permittivity and the permeability of the geotextile.
For permittivity, the following formula (A.3) applies:
q
H
(A.3)
Ψ
20
V
=
=
⋅
50
Δ
Where
V
H
50
is the velocity index determined in accordance with EN ISO 11058.
This relationship is only valid if
V
H
50
is given in m/s. To calculate the permeability,
the thickness of the geotextile must also be known. Table A.2 gives typical permittiv-
ity values for different geotextiles.
The permittivity of much of the geotextiles used for geotextile-encapsulated sand
elements (woven polypropylene) is in the range of 0.1-0.6 s
−1
at a specific discharge
of 0.01 m/s. The openings of geotextiles, and thus the permittivity, is limited by the
sand density requirement.
Table A.2
Permittivity values for various geotextiles [24].
O
90
[mm]
Ψ
[s
−
1
]
Type of geotextile
Monofilament
0.1-1
1-5
Tape yarns
0.05-0.6
0.1-1
Woven
0.2-1
0.05-0.5
Nonwoven
0.02-0.2
0.01-2
