Geoscience Reference
In-Depth Information
4.4 POTENTIAL FAILURE MECHANISMS AND SAFETY
CONSIDERATIONS
For the design of a structure using geotextile mattresses assessment of the following
potential failure mechanisms must be made:
￿
Rupture of the geotextile during handling and placement of the geotextile
mattress;
￿
Rupture of the geotextile as a result of anchoring - the additional tensile loads
have not been taken into account;
￿
Rupture of the geotextile through insufficient seam strength;
￿
Instability of the geotextile mattress due to wave attack;
￿
Instability of the geotextile mattress due to water flows parallel to the structure;
￿
Shear failure of the subsoil;
￿
Liquefaction of the subsoil;
￿
Vandalism and recreational damage.
For the design or construction phase, normally a safety factor of 1.1 to 1.2 is
applied to the thickness of geotextile mattresses in relation to the design value. During
construction, extra attention must be paid to safety when the pulling and hoisting of
mattresses is required.
The following section deals with the design formulae associated with the above
listed potential failure mechanisms.
4.5 DESIGN ASPECTS
Once the overall dimensions of the structure, the construction procedure and the
size of the elements have been determined, the detailed design is carried out. The
structure is assessed in respect of the stages shown in the design procedure, includ-
ing the required tensile strength for the geotextile and the stability requirements for
waves and water flows. If the stability requirements are not fulfilled, a heavier geo-
textile mattress must be selected, having a larger volume and higher geotextile tensile
strength. If the required tensile strength of the geotextile is not cost-effective, a lighter
and thinner geotextile mattress and/or different operating method may be selected.
The geotextile mattress is then checked against the stability requirements for waves
and water flows, and for geotechnical stability. Detailing at the edges of the geotex-
tile mattress is important to ensure no localised instability can occur and in many
instances this can be the critical part of the design.
4.5.1 Material choice
Most mattresses are made of polypropylene geotextile materials, using heavier geotex-
tiles to limit the effects of UV radiation and mechanical damage from vandalism, etc.
This is why the top geotextile layer commonly consists of a combination of geotextile
materials (to resist long term UV exposure and provide resistance to vandalism, etc.).
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