Size effects on strength and deformability of artificially jointed hard rock

An extensive experimental study is presented including around 140 triaxial compressive strength tests on artificially jointed hard rock. The experiments were performed on cylindrical and jointed specimens with either 1 sub-vertical and 2 sub-horizontal (1 + 2) or 2 sub-vertical and 3-sub-horizontal (2 + 3) joint sets. The specimen diameter consisted of 38, 54 and 84 mm and the rock was Blanco Mera granite. The confining pressures ranged between 0.2 and 15 MPa and such a testing program complements the previous study in this regard with about 100 triaxial tests on intact rock specimens. A complete set of stress-strain curves were obtained from the experiments followed by extracting the peak and residual strengths as well as indicative deformability parameters (elastic and Poisson's ratio). For the jointed specimens, the indirect strain measurement was corrected to implicitly calculate the elastic parameters following the energy-based concept. It was found that the peak strength is joint set dependent as per the conventional rock mass classification systems. For jointed specimens a moderate strength size effect was observed in which the peak strength tended to increase with size from 38 mm to 54 mm and then decreased with a rise in specimen diameter. Also, it was concluded that the effect of size on peak strength become less prominent in cases with more joints and the influence of additional jointing becomes less significant for larger specimens; both are novel findings which have not been explored before. The approximated variable elastic parameters of jointed specimens exhibited size independency but joint and confinement dependency, while the variable Poisson's ratios specifically, appeared to be size and joint set independent. The presented stress-strain data from granite specimens suggests that the mechanical parameters of jointed rock are largely controlled by the rock structure where the specimen size can play a significant role, particularly at low confinements.