Modeling the Time-Dependent Behavior of Geosynthetically Reinforced Soil Structures with Cohesive Backfill - Reinforced Soil Engineering: Advances in Research and Practice - page 76

the Zhang and Moore (1997) model, few simple yet robust models appear to have
been proposed that account for both creep and relaxation in a robust yet
reasonably accurate fashion.
In addition, simple mathematical models, rheological models, and integral
techniques all lack one fundamental characteristic that is necessary for their
implementation into finite-element computer programs. Namely, using any of the
aforementioned approaches, one cannot compute a consistent incremental

tangent modulus E r ¼ › :

The above shortcomings manifest themselves in the inability to perform
proper finite-element analyses. In particular, consider the approach used by
Helwany (1992) in his analysis of a geosynthetically reinforced wall with
cohesive backfill. Using an integral technique similar to that described by Eqs.
(5)-(8), he states:

s =› e

For each time increment Dt, the expected creep strains in all viscoelastic bar
elements are calculated. Equivalent nodal creep forces (corresponding to the
expected creep strains) are then calculated and applied at the nodal points of
each viscoelastic bar element. The response of the structure is then evaluated
through regular finite element procedure.

Such an approach is deficient for two reasons: First, it a priori assumes
creep response for the reinforcement [a condition that has been shown by
Dechasakulsom (2000) not to be true for the particular wall analyzed]. Second, by
assuming “expected” creep strains, this approach precludes a consistent finite-
element analysis from being performed. In such an analysis, the strains are
computed as secondary dependent variables from the displacements (the primary
dependent variables) and are not prescribed at the outset.

8 CHARACTERIZATION OF INTERFACES

One of the most important factors in accurately predicting the behavior of
reinforced soil structures is the ability to account for relative displacement
between the backfill soil and reinforcement and between the structural members
(e.g., facing) in contact with the soil. The possible ramifications of failing to
model the latter interfaces have been discussed by Kaliakin and Xi (1992), who
note that spurious results are very likely in such cases.
The interaction between the soil and the reinforcement and between the soil
and the structural members can be modeled by introducing suitable interface
elements. The proper kinematic response of such elements (Kaliakin and Li,
1995) is particularly important for geosynthetically reinforced soil structures
with cohesive backfill, as they are placed between the soil and reinforcement and
thus link these two time-dependent materials. Provided they are robust, standard