Modeling the Time-Dependent Behavior of Geosynthetically Reinforced Soil Structures with Cohesive Backfill - Reinforced Soil Engineering: Advances in Research and Practice - page 68

(a strain criterion). For example, in the case of polyester geogrids, long-term
design strength is usually governed by tensile rupture; for polyethylene, the long-
term design strength may be governed by either strain or rupture (Ingold et al.,
1994).
Recently, a limited number of geosynthetically reinforced soil structures
with cohesive backfill have performed favorably. In particular, the results of
experimental and full-scale tests demonstrated that both the short-term and long-
term strength of cohesive soil might be increased by grid reinforcement (Jewell
and Jones, 1981). Bergado et al. (1993) reported that appropriately compacted
cohesive soils could generate pullout capacities comparable to those associated
with granular soils. This indicates that such structures have the potential of being
used in lieu of granular backfill, with the possibility of significant savings in
construction costs.
The rational analysis of geosynthetically reinforced soil structures with
cohesive soil requires a time-dependent representation of not only the
reinforcement, but also of the backfill. The latter, which is typically not an
issue for granular soils, tends to complicate the analysis. As an aid to analysts,
this paper presents a critical review of the state of the art in time-dependent
modeling and analysis of geosynthetically reinforced soil structures with
cohesive backfill. It is tacitly assumed that the analysis will be carried out
numerically using the finite-element method.
The numerical aspects of time-dependent finite-element analyses are well
understood. Details pertaining to such aspects are, however, beyond the scope of
this paper. The interested reader is directed to references on the subject, such as
Zienkiewicz and Taylor (2000).
2 GENERAL PROBLEM FORMULATION
The use of cohesive soils, possibly with low permeability, as backfill has the
potential of complicating the problem formulation. In particular, if the backfill is
largely saturated, the issues of excess pore pressure and flow of pore fluid become
significant. Consequently, the problem must be cast in the framework of a
coupled deformation-flow (“Biot”) formulation. In such a mixed formulation, the
primary dependent variables are typically displacements and pore pressures.
If excess pore pressures are not a concern, an irreducible formulation with
displacements as primary dependent variables is sufficient. Provided that nearly
incompressible material idealizations are avoided, a rather wide range of
irreducible elements can be used in the analysis. These are briefly discussed in the
next section.