Abstract - The elasto-plastic behaviour of porous rock loaded under axisymmetric
laboratory conditions and the resultant modes of localised deformation are investigated using
Distinct Element Method (DEM) numerical models in which rock is idealised as an
assemblage of spherical, unbreakable, cemented particles. These bonded particle
models are loaded axially, either in extension or compression tests, using distortional
periodic space, while a constant confinement is maintained laterally using a simulated
flexible membrane. The emergent elasto-plastic bulk behaviour observed in the
numerical simulations is qualitatively similar to that observed for natural rock and
includes (i) pressure and stress-state sensitive elasticity, (ii) a positive correlation
between friction coefficient and dilatancy factor, and (iii) yield/failure envelopes that
depend on the third stress invariant. The shear band orientations and kinematics in the
DEM models are quantitatively compared with localisation theory. Average shear
band inclinations, defined as the angle between the shear band normal and the greatest
compressive stress direction, are a few degrees greater than predicted by theory in
compression tests, and over ten degrees greater in extension tests, differences which
are attributed to differently shaped yield surfaces in model and theory. Nevertheless,
the transition from dilational shear bands at low confining pressure to compactional
shear bands at high confining pressure is successfully predicted by localisation theory.
The numerical models illustrate that the rich elasto-plastic behaviour and the pressure-
sensitive modes of localisation typical for porous rock emerge from relatively simple
particle-particle contact laws. Moreover, compactional shear bands can develop, at
least during localisation, in the absence of particle size reduction (i.e. crushing)
provided that the material can accommodate pore reduction by particle rearrangement.
International Journal of Rock Mechanics & Mining Sciences, 57, 75-88, 2013.