  A dual-continuum model can offer a practical approach to understanding
first-order behaviours of poromechanically coupled multiscale systems. To close
the governing equations, constitutive equations with models to calculate
effective constitutive coefficients are required. Several coefficient models
have been proposed within the literature. However, a holistic overview of the
different modelling concepts is still missing. To address this we first compare
and contrast the dominant models existing within the literature. In terms of
the constitutive relations themselves, early relations were indirectly
postulated that implicitly neglected the effect of the mechanical interaction
arising between continuum pressures. Further, recent users of complete
constitutive systems that include inter-continuum pressure coupling have
explicitly neglected these couplings as a means of providing direct relations
between composite and constituent properties, and to simplify coefficient
models. Within the framework of micromechanics, we show heuristically that
these explicit decouplings are in fact coincident with bounds on the effective
parameters themselves. Depending on the formulation, these bounds correspond to
end-member states of isostress or isostrain. We show the impacts of using
constitutive coefficient models, decoupling assumptions and parameter bounds on
poromechanical behaviours using analytical solutions for a 2D model problem.
Based on the findings herein, we offer recommendations for how and when to use
different coefficient modelling concepts.
