Combined effects of discontinuities and intact rock on weak rock at various scales

Discontinuity characteristics significantly influence the behaviour of rock masses. Despite advances in experimental techniques, the combined effects of intact rock strength, joint spacing, length, orientation, and size under uniaxial loading are rarely reported in the literature. This study addresses these gaps by employing the Design of Experiment method in laboratory experiments on weak rock. Results reveal that weak rock mass strength is primarily influenced by discontinuity persistence, followed by intact material strength, with specimen size and joint orientation having secondary impacts. In addition, the strength reduction factor for weak rock masses exhibits an inverse trend as strength decreases, resembling soil-like behaviour, which existing downgrading systems fail to capture. In addition, the study examines weak rock specimens with two discontinuous joint sets, which are seldom reported in the literature. These specimens demonstrate higher strength reductions due to the additional joint sets facilitating crack coalescence. Furthermore, the jointed specimens exhibit a size effect similar to that observed in intact rocks, showing an initial increase followed by a decrease in strength. Analysis of material particle size suggests that the size effect in brittle rocks is closely linked to the micro-properties of specimens under laboratory conditions. These findings have significant implications for understanding rock mass behaviour. The outcome of this work can be used to improve rock mass classification and calibrate numerical modelling in weak rock.