Abstract - Laboratory tests of rocks indicate that strength, angle of internal friction and stiffness decrease with increasing porosity. The Particle Flow Code (PFC3D) is used to investigate elasticity, strength, friction angle and porosity relations. Models were generated using the particle insertion method, a constructive method, and the dynamic specimen genesis procedure widely used by PFC3D users. For the particle insertion method ‘seed’ particles are first generated within the specimen domain. The specimen is then filled up by iteratively inserting particles in such a way that each new particle touches four neighbours. The number of particles and the final porosity that can be achieved with that method are a function of the predefined particle size range; a maximum to minimum particle radius ratio of 10 gives porosities of ~ 23%. In contrast, the dynamic specimen generation procedure, where particles with a uniform size distribution are randomly generated within the model domain and then inflated/deflated in order to achieve a low isotropic stress, typically yields model porosities of ~35%. Model porosities between these two end-member models were achieved using two different methods: (i) Systematically deleting the smallest particles from a model with 23% porosity, and (ii) using the constructive method to insert additional particles into a given model with an initial porosity of 37%. A series of confined triaxial extension and compression tests was performed on samples that were generated with these different particle packing methods. The 3D DEM models demonstrate that Young’s modulus, strength and friction angle decrease with increasing porosity, whereas Poisson’s ratio is (almost) porosity independent. The particle size distribution has an impact on the elastic properties, but does not affect strength and friction. All our 3D DEM model property relations (elasticity, strength, friction angle) are consistent with continuum mechanics predictions and empirical rock property relations.
Abstract of talk given to:
FLAC/DEM Symposium on Nunmerical Modelling, Minneapolis, August 2008.