Investigating mechanics of conglomeratic rocks: influence of clast size distribution, scale and properties of clast and interparticle cement

Conglomeratic rocks are composed of macro-particles (varying in size from gravels to cobbles and boulders with diameter greater than 2 mm) bonded by fine-grained cement particles with diameters of < 2 mm. Determination of strength and deformability of such materials in the laboratory is challenging due to sampling and testing constraints induced by the strength and stiffness contrast of the particles and the cement matrix. Alternatively, the mechanics of such materials has been studied by laboratory testing on a synthetic conglomerate composed of steel spheres and Portland cement. The physical modeling and testing was supplemented by numerical simulations in PFC^3D by preparation and testing of equivalent numerical models. This paper presents the results of numerical investigations conducted on conglomerates with the objective of finding the effect of particle size distribution, scale effects and the sensitivity of variable mechanical parameters of clasts and cement. Two different clast particles were considered, namely sandstone and granite, both with Hertzian contacts in addition to steel particles. For each clast type, the following bed matrices were modeled to glue the clasts by deriving necessary micro-mechanical parameters from published literature; Portland cement, argillaceous cement and arrenaceous (quartzitic) cement. The mechanical responses of the numerical conglomerates were examined in uniaxial, triaxial and Brazilian tensile testing. Macroscopic behavior of each combination of conglomerate was then analyzed. The results show that particle size distribution affects both the mechanical response and damage pattern in the specimen, while scale, i.e. the effect of specimen size, keeping the micro-structure the same, was found to influence the strength and elastic response of the conglomerates similar to other natural rocks. The properties of clast and cement were also found to influence mechanical response of conglomerates in numerical simulation. The peak strength and the stiffness of the conglomerate increases with the increase of the stiffness and the strength of the cement matrix in uniaxial or close to uniaxial stress states, while negligible dependence on the stiffness and strength of the clast material. In contrast, the mechanical properties of clast materials were found to increase the strength and stiffness of the conglomerate in triaxial stress states, while there was negligible dependence on the strength and stiffness of the cement matrix.