Performance Properties of Geogrids - Reinforced Soil Engineering: Advances in Research and Practice - page 23

Figure 3 Typical creep graphs, Fortrac.

where

T d is the design strength.

T c is the characteristic strength above which the material fails in tension

from peak loading during the design life of the structure.

f m is a material factor f m ¼ f m1 ·f m2 ; where f m1 ¼ f m11 ·f m12 and f m2 ¼

f m21 ·f m22 :
The various factors f mxx are described in the BS 8006 and cover factors for
manufacture/control values, extrapolation of test data, susceptibility to damage,
and environment. The values of the above parameters differ for flexible and stiff
grids. Various loads factors f are used in the design to complete the ultimate
limit state design. This method complies with the approach followed in
Eurocode 7 for geotechnical designs. P char [t, T ]orT c for HDPE grids is derived
from a creep deformation limit of 10% during a design life (Jewell and
Greenwood, 1988). In the case of polyester (PET) grids, this value is obtained
from stress-rupture data produced by tests on yarns running more than 10 years
and tests on actual products running for more than 9 years now. All these data
are summarized in one stress-rupture graph, which is normalized as a
percentage of the short-term ultimate tensile strength, to give the so-called
stress-rupture graph,

in which a 95% confidence limit indicates the

characteristic levels (Voskamp, 1990).