Geoscience Reference
In-Depth Information
Table 2.3
Recommended design retention criteria for geometrically closed
geotextiles.
Sand (D
>
60
μ
m)
Stationary hydraulic load (current)
O
90
<
5
D
10
C
1/2
and
O
90
2
D
90
Dynamic hydraulic load (wave attack)
O
90
<
1.5
D
10
C
1/2
and
O
90
<
D
90
O
90
=
pore size of the geotextile that corresponds to the average diameter of the sand fraction of
which 90% remains on the geotextile (in the wet sieving method).
D
x
=
sieve size through which x% fraction of the sand material passes.
C
u
=
uniformity coefficient of the sand (
D
60
/
D
10
).
on the sand grains. In this case, washing out of fines will be limited because the sand
acts as a natural filter. However, if the geotextile can move and there is not always a
confining stress present a natural filter cannot be formed and hence a considerable
amount of fine particles can pass through the geotextile. For such a situation stricter
retention criteria are necessary as presented by Heibaum [35].
2.4.3 Permeability
The permeability of a geotextile is of limited significance for the filling operation
of geotextile-encapsulated sand elements unless hydraulic fill is used. In hydraulic
structures, geotextile permeability can, however, be important. The permeability of
the geotextile must not hinder groundwater flow through the structure, e.g., in slope
protection where inadequate permeability can lead to excess pore pressures that result
in possible instability of the slope.
A commonly used empirical relationship is that the permeability of the geotextile,
as measured by an index test (see 2.4.8), must be a factor of 10 times higher than the
permeability of the soil being protected. A more detailed discussion of geotextile per-
meability and its relevance to soil filtration is given in Appendix A.
Application of the empirical relationship that the permeability of the geotextile
has to be 10 times the soil permeability is sometimes awkward given that the perme-
ability of the geotextile is a function of its thickness, and the thickness of the geotex-
tile incorporated in a structure is often unknown. In other instances there may be little
hydraulic pressure drop through the geotextile. Consequently, use is then made of the
permittivity property of the geotextile, see also Appendix A.
2.4.4 Tensile strength and strain
Normally the highest load is exercised on geotextile-encapsulated sand elements dur-
ing the construction phase. The tensile strength of the geotextile and the strength of
the seams must be sufficient to resist the loads imparted during the filling, transport-
ing and placement of the elements. Depending on the application and construction
method, the tensile strength, strain and the base geotextile material used can all be
important considerations. Woven geotextiles generally have a relatively high tensile
strength and a low maximum strain, while nonwoven geotextiles have a relatively low
