On the Effect of Lateral Confinement on Rock-Cutting Tool Interactions

Previous investigations of rock-cutting tool interaction by normal indentation of rocks were mostly conducted at ambient pressure, a condition not representative of the lateral stresses at the excavation face. Despite recent advances in understanding rock-cutting tool interactions with lateral confinement, working out the details of the relationship between rock fracture due to indentation and confinement still requires experimental investigations and theoretical analyses. We conducted a series of normal indentation tests on samples of Gildehaus Bentheim sandstone at lateral confinement up to 20 MPa using a set-up that allowed us to monitor lateral sample deformation and acoustic-emission activity. Experimental observations show that lateral confinement has a significant effect on the load responses and suppresses sample dilation; furthermore, the propagation path of the macroscopic fracture deviates from the indentation direction as confinement increases. The peak indentation pressure increases with confinement and its occurrence is accompanied by significant dilation and acoustic emission activity indicating that it coincides with initiation of macroscopic tensile fracturing. A cavity-expansion-based theoretical model, that accounts for the increases in compressive strength and fracture toughness with lateral confinement, captures the trend of increasing indentation pressure with lateral confinement. In addition, the good agreement between theoretical predictions and experimental data indicates that lateral confinement promotes the growth of the damage zone under the indenter preceding potential macroscopic tensile failure. The model correlates thrust required to break rocks in-situ with rock strength parameters, tool shape parameters, and lateral confinement, thus providing a starting point for optimizing the design of cutting tools. The set-up for normal indentation tests with lateral confinements allowed for monitoring the apparent lateral strain of specimens, providing information on the failure process during indentationIndentation pressure exhibits a peak in contrast to force at all confinements investigated and the peak value in indentation pressure is suitable for an indicator for fracture initiationThe strengthening effect with confinement, evidenced by the increase in force that is required to break the rock, is associated with decreasing density and length of pre-existing flawsTheoretical models are proposed to integrate the influence of confinement on rock indentation, the predictions of which agree well with experimental dataThe strengthening effect can be accounted for by the increase in the compressive strength and in the fracture toughness as confinement increases, the former controls the damage-zone formation and the latter ultimate macroscopic failure