Geoscience Reference
In-Depth Information
4.2 INSTALLATION PROCEDURE
Geotextile mattresses are filled at ground level or in-situ on the slope. Filling can be
carried out pneumatically, mechanically or hydraulically. These methods are further
described in [22, section 5.2.2].
Most commonly, mattresses are filled hydraulically. By hydraulic filling the high-
est possible filling density can be achieved. With this method the fill pressure of the
sand-water mixture plays a role in achieving maximum filling density. During filling,
the pressure in the mattress must be prevented from rising too high, especially if the
geotextile silts up during the filling process.
Preferably, geotextile mattresses should be filled in-situ, which minimizes the ten-
sile loads generated in the geotextile. In case in-situ filling is not possible, the geotex-
tile mattresses can be filled at ground level and transferred by a pontoon and installed
in place using a clamping system, or be lifted into place with a crane. In both cases
significant tensile loads will be generated in the geotextile, in which case the geotextile
used must take into account these additional installation tensile loads. It should be
noted that a well-filled mattress is important in order to prevent the loss of sand in
the mattress when hoisted.
The method of construction chosen is determined by the size of the mattress, the
available space in which to work, accessibility from the bank and from the water,
economic feasibility and the experience of the installer.
4.3 GEOMETRIC DESIGN
In Figure 2.1 a general design procedure is given for geotextile-encapsulated sand ele-
ments. The first step in the design procedure is to establish the basic functional and
technical requirements. This is an area that falls outside the scope of this manual. As
already discussed (see 2.2), it is assumed that the designer is already at the design stage
and has a clear understanding of the functional requirements, has a draft design of the
overall structure and wants to go through the detailed stage.
The overall dimensions of the structure are established first. This is followed by
the size of the elements and the construction of the structure based on experiential
data, constructability, economic feasibility and application area. Where possible, this
is done in consultation with the installer.
The common width of standard geotextile roll materials is 4.50 to 5.30 m. By
sewing more strips to each other larger continuous sheets can be made, thus reducing
the amount of overlap. The strip width is limited by the amount of space available
at the construction site and by handling and placement issues. If the mattress is filled
in-situ, there are less limitations regarding dimensions.
There are several different types of geotextile mattresses [22]. The most common
are listed below (see also Figure 4.2):
The woven mattress.
This involves two geotextiles, one above the other, woven
together on a loom where the upper and lower geotextiles are woven together at
regular intervals. This process requires special production equipment.
