Modeling the Time-Dependent Behavior of Geosynthetically Reinforced Soil Structures with Cohesive Backfill - Reinforced Soil Engineering: Advances in Research and Practice - page 75

elasticity with a Hill yield criterion with isotropic hardening and an associated
flow rule. The creep response is accounted for through a strain hardening form of
a power law for uniaxial response. Although Perkins' model is more refined than
the simpler models described above, it lacks generality in that it preassumes creep
response of the geosynthetic.

7.2 Models Proposed to Simulate Relaxation

Compared to creep models, relatively few formulations have been proposed to
simulate the relaxation response of geosynthetics. Koerner et al. (1993) presented
the following two-parameter “in-house” formula for stress relaxation of
geomembranes.

ct 2 b Þ

where t denotes time, and b and c are constants. This type of behavior has been
referred to as “physical stress relaxation,” as opposed to chemical relaxation
(Debnath, 1985).

s ð t Þ¼ ð 9

7.3 Models Proposed to Simulate Both Creep and
Relaxation

In a recent paper, Sawicki (1998) proposed rheological models for predicting the
creep or relaxation response of specific geogrids. However, the models are
predicated on the a priori knowledge of the specific type of response. That is, it
must be known whether the geogrid will undergo creep or relaxation response; in
the course of loading, the response mode cannot change. Thus, Sawicki's models,
though more general than the basic rheological models discussed above, still lack
true generality.
Zhang and Moore have presented a more general model that accounts for
the elastic-viscoplastic response of geosynthetics (Zhang and Moore, 1997). This
multi-axial model, which is based on the unified theory of Bodner and Partom
(1975), has been shown to realistically simulate various aspects of geosynthetic
response with good agreement between numerical predictions and experimental
results.

7.4 Concluding Remarks Concerning Modeling

As evident from the overview presented in the previous section, the mathematical
modeling of the time-dependent behavior of geosynthetics has typically been
realized using formulations specifically designed to simulate creep response, or
those specifically designed to simulate relaxation. With the exception of