Geoscience Reference
In-Depth Information
Table 2.2
Some properties per raw material for geotextiles.
Unit weight
[kg/m
3
]
Tensile strength
[N/mm
2
]
Elastic modulus
[N/mm
2
]
Maximum strain
[%]
Material
Polyester (PET)
1380
800-1200
12000-18000
8-20
Polypropylene (PP)
920
400-800
2000-8000
6-25
Polyethylene (PE)
900-930
350-600
600-6000
10-30
PP and PET are the most commonly used materials for application in
geotextile-encapsulated sand elements, fulfilling separation, filtration and contain-
ment functions. If the geotextile remains under tension in the structure, the use of
polyester could prove advantageous because of its better long-term load carrying
capabilities.
When using PP and PE under water, account must be taken of the specific gravity
of these materials (between 0.9 and 0.95), which is lower than that of water, so the
materials will float.
The following sections deal with the sand tightness, the permeability and the
strength, and following this is an appraisal of the quality of the seams, damage dur-
ing installation and the durability of geotextiles. Finally, the main aspects of the CE
(Conformité Européene) Marking are stated.
2.4.2 Sand tightness
When using geotextiles for filtration purposes, to prevent the erosion of fine par-
ticles through the geotextile in hydraulic structures a distinction is made between
geometrically open and geometrically closed geotextiles. The pores in geometrically
closed geotextiles are so small that the sand is physically blocked, so it will prevent
erosion for all hydraulic gradients. Geometrically open geotextiles act as a filter
over a limited gradient range. At higher gradients the sand could be washed out.
In geotextile-encapsulated sand elements only geometrically closed geotextiles
are used because high gradients can occur. The basis for determining the required
pore size (
O
90
) of the geotextile is that only a very small amount of sand may be
allowed to erode, otherwise the encapsulated sand elements will deform too much.
Table 2.3 shows the recommended design criteria for sand retention [22].
During the filling of geotextile-encapsulated sand elements there will also be
a dynamic load if the elements are filled hydraulically. Some initial loss of sand is
not serious during the filling process, however, there must be no loss of fill material
over time.
The above retention criteria are conservative and are based on an uncovered
encapsulated sand element with a high hydraulic load. For a steeper sand gradation
curve (
C
u
<
2), and moderate load, the criteria can be relaxed to some extent, with
the size of the openings a little larger, for example,
D
90
D
90
. Large-scale
experiments in the Delta flume of Deltares have revealed that these criteria are not
valid when the sand can still move around in the structure behind the geotextile.
The criteria presented in Table 2.3 assume there is a certain confining stress exerted
<
O
90
<
2
⋅
