Performance Properties of Geogrids - Reinforced Soil Engineering: Advances in Research and Practice - page 26

5 PARTIAL FACTORS

The P char must be reduced because damage to the geogrid as a result of
construction activities such as compaction of fill layers occurs. This damage
results in a reduction of the strength of the material. Also, environmental
conditions during the service life of the structure influence the strength.
The mechanical damage factors are product- and application-related. Most
products have been tested under controlled field conditions, and specific factors
are quoted by manufacturers. With a view to, for example, BBA certification,
independent checks have been made to certify these values. The values are
typically in the range of 1.05-1.7 depending on the type of fill and the type of
geogrid (BBA, 1997).
Allen and Bathurst (1994) have evaluated 3500 index tests on different
geosynthetic reinforcement products. They concluded that damage results in
reduction in strength and in reduction in strain. The secant modulus remains the
same.

A chemical environment factor must also be applied. All polymer materials
are vulnerable to certain specific chemical conditions. It is therefore useful to
determine whether these conditions will occur in the design case and what the
concentration of chemicals will be. Polyester, for example, is sensitive to an
alkaline environment with pH levels higher than 9.5 combined with a temperature
above 50

8

C.
Under normal soil conditions, the phenomena of hydrolysis will not have
an impact on the allowable strength (BBA, 1997;Schmidt et al., 1994). Special
high-alkaline conditions, continuing for most of the design life of the structure,
must be evaluated more closely. The results of three years of recent testing at high
temperatures in Germany confirmed these conclusions (Wilmers, 1997). Thermo-
oxidation is a process that influences the ductile behavior of polyolefins like
HDPE and PP. Also, this itemmust be evaluated if the lifetime conditions include
the existence of chemicals that may lead to dissolution of additives. These
plasticizers or other additives in the polymer composition are evaporated or
slowly washed away from the polymer. Extensive testing in various laboratories
has shown that the reduction in strength and other properties of the geogrids is
negligible when the material is used in normal soil conditions. Also the CEN
committee TC 189 is developing standard test methods as index tests for
chemical attack. HDPE material is sensitive to environmental stress cracking.
However, extensive studies by ERA, Small and Greenwood (1993) have not
revealed such a sensitivity in the HDPE geogrids tested.
Resistance to fungi and other biological attacks has to be evaluated. As far
as we know, all geogrids are capable of withstanding this type of attack without
loss of strength.