Analytical solutions and a vast majority of numerical ones for fracture propagation in saturated porous media yield smooth behavior while experiments, field observations and a few numerical solutions reveal stepwise crack advancement and pressure oscillations. To explain this fact, we invoke selforganization of rupture observed in fracturing solids, both dry and fully saturated, when two requirements are satisfied: i) the external drive has a much slower timescale than fracture propagation; and ii) the increment of the external load (drive) is applied only when the internal rearrangement of fracture is over. These requirements are needed to obtain clean Self Organised Criticality (SOC) in quasi-static situations. They imply that there should be no restriction on the fracture velocity i.e. algorithmically the fracture advancement rule should always be independent of the crack velocity. Generally, this is not the case when smooth answers are obtained which are often unphysical. Under the above conditions hints of Self Organized Criticality are evident in heterogeneous porous media in quasi-static conditions using a lattice model, showing stepwise advancement of the fracture and pressure oscillations. We extend this model to incorporate inertia forces and show that this behavior still holds. By incorporating the above requirements in numerical fracture advancement algorithms for cohesive fracture in saturated porous continua we also reproduce stepwise advancements and pressure oscillations both in quasi-static and dynamic situations. Since dynamic tests of dry specimens show that the fracture advancement velocity is not constant we replicate such an effect with a model of a debonding beam on elastic foundation. This is the first step before introducing the interaction with a fluid.

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A number of discrete element analyses of undrained triaxial shear tests on crushable assemblies are performed using 3-dimensional particle flow code (PFC3D). The undrained shear of the samples is simulated by assigning different velocities at the boundaries to control the constant volume of the samples during the shear. Particle breakage is studied using the octahedral shear stress breakage criterion and the fragment spawning mode satisfying an Apollonian sphere packing. The microparameters of the soils are obtained by simulating the test results reported on a Nansha calcareous sand. The effects of particle breakage and drainage condition on the critical state behaviours of the sand are intensively examined. It is found that particle breakage and drainage condition do not affect the slope of the critical state p′−q line. In the ecs−(p′cs/pa)0.7 space, the critical state line translates downward and rotates clockwise when particle breakage is considered, whilst drainage condition does not affect the critical state line. The intrinsic macro–micro correlations underlying the critical state behaviours are explored by analysing the evolution of representative micromechanical indicators such as coordination number, redundancy ratio, and fabric and force anisotropies.

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