  Flow-induced failure of granular materials is relevant to a broad range of
geomechanical applications. Plasticity, which is the inherent failure mechanism
of most granular materials, enables large deformations that can invalidate
linearised models. Motivated by fluid injection into a borehole, we develop a
steady-state model for the large deformation of a thick-walled,
partially-permeable, elastic--perfectly-plastic annulus with a pressurised
inner cavity. We account for pre-existing compressive stresses, as would be
present in the subsurface, by subtracting a compressed initial state from our
solutions to provide the additional disturbance due to fluid injection. We also
introduce a simple parameter that allows for a smooth transition from an
impermeable material (i.e., subject to mechanical loading at the inner wall) to
a fully permeable material (i.e., subject to an internal pore-pressure
gradient), which would be relevant to coated boreholes and
very-low-permeability materials. We focus on the difference between poroelastic
and poroelasto-plastic deformations, the role of kinematic and constitutive
nonlinearity, and the transition from impermeable to fully permeable. We find
that plasticity can enable much larger deformations while predicting much
smaller stresses. The former makes model choice increasingly important in the
plastic region, while the elastic region remains insensitive to these changes.
We also find that, for a fixed total stress at the inner wall, materials
experience larger deformations and generally larger stresses as they transition
from impermeable to fully permeable.
