Physical and numerical investigation of conglomeratic rocks

Abstract

This thesis investigates the mechanics of clast supported conglomerates, through physical and numerical simulations on idealised specimens with spherical clasts and a homogeneous cement matrix. In the physical experiments synthetic conglomerate specimens were prepared from steel spheres as clasts and Portland cement paste as the cement matrix. The mechanical parameters of these specimens were measured in ISRM standard tests. Numerical specimens were prepared in PFC3D using measured and known micro parameters, and tested in conditions as equivalent as possible to the physical experiments. In order to validate the numerical simulations, the responses of the numerical and synthetic conglomerates were compared. Although the numerical tests reproduced many features of the physical tests, some significant differences were observed which were attributed to the presence of cement matrix in the synthetic conglomerate. After a achieving a reasonable calibration between the physical and numerical conglomerates, the simulations were extended to investigate the sensitivity of the cement matrix and the clast properties, effect of specimen size and size distribution of the clasts in controlling the mechanical response of a clast supported conglomerate. The study showed that the mechanical response is sensitive to the strength and stiffness of the cement matrix in uniaxial conditions only. Similarly, clasts’ strength and stiffness was found to significantly influence the mechanical response in triaxial loading but not in uniaxial loading. The specimen size was found to influence the mechanical response of conglomerates, similar to natural rocks. In the clast size distribution study, the peak strength and stiffness of the conglomerate was observed to decrease as the maximum to minimum clast size ratio is increased. A micro mechanical investigation using PFC2D was conducted to explore the clast-cement interaction by modelling the cement matrix as an aggregate of micro particles. It was observed that the properties of the clast-cement interface significantly affect the failure mechanism and peak strength in various modes of deformation. Similarly, the role of the cement matrix was also investigated. It was found that the cement matrix acts as a stress riser and a relation was proposed to estimate the cement induced stress effect, named, the Cement Wedge Effect