Geoscience Reference
In-Depth Information
2.4.6 Damage during installation of gravel layers
For all applications where geotextiles are used, and thus also for applications with geo-
textile-encapsulated sand elements, the installation process is critical in respect to instal-
lation damage. Distinction can be made between major installation damage (tearing,
weakening of seams, etc.) which can be attributed to design and construction errors, and
minor installation damage. An example of minor installation damage is where a covering
layer of armour stone on a geotextile may result in the geotextile being punctured at vari-
ous locations and this may lead to a loss of fill material. In these cases it may be desirable
to apply a cushion layer of smaller gravel material directly on the geotextile prior to
applying the armour stone. It is important that the diameter of the stones used directly
on top of the geotextile be sufficiently small, resulting in limited point loads on the
geotextile-encapsulated sand elements (more information on this can be found in [11]).
2.4.7 Durability
The lifetime of structures constructed of these geosystems is mainly governed by the durability
of the geotextile used. The properties of geotextiles may change over time due to:
UV radiation;
oxidation;
hydrolysis;
chemical and biological exposure;
mechanical damage;
creep and/or relaxation.
In Table 2.6 a qualitative summary is provided of the inherent resistance of geo-
textiles to various kinds of exposure.
As geotextiles age, oxidation can occur through a combination of temperature,
UV radiation and environment, and over time this can result in the geotextile becom-
ing brittle. To combat this, anti-oxidants and UV stabilisers are added to the basic
material of the geotextile. These govern the long term quality of the geotextile.
When polyester geotextiles are placed in a hydraulic environment a chemical reaction
known as hydrolysis may reduce their strength over time. The rate of hydrolysis is very
slow, but it may be accelerated at increased pH levels. For geotextiles manufactured from
high molecular weight, high modulus polyester polymer the rate of strength reduction
due to hydrolysis is negligible in hydraulic conditions (where there is no UV exposure).
Chemicals and bacteria in the vicinity of the geotextile may also be detrimental in
certain circumstances. Polyester is, for example, susceptible to a strong alkali environ-
ment and polypropylene is sometimes adversely affected by strong oxidising agents.
Another time-associated phenomenon is creep, where the geotextile continues
to deform under an applied load. This deformation can be such that the material
ultimately fails. In Table 2.7 the modified allowable tensile strength allowing for the
effects of creep is shown for various geotextile materials. Polypropylene (PP) and
polyethylene (PE) can weaken due to creep at medium tensile stresses after a rel-
atively short time period. Polyester shows good creep resistance and thus may be
used effectively for long-term load carrying applications. However, for applications
using geotextile-encapsulated sand elements tensile loads only occur during element
