Modeling of small-diameter rotary drilling tests on marbles

In this paper, continuum mechanics and discrete modeling are applied to investigate numerically rotary drill cutting experimental results on four marbles. Rock-cutting tests were performed by a new portable rotary microdrilling tool currently employed in practice for the quasi-non-destructive characterization of strength properties of rocks. The objectives of this research work are twofold, namely: (a) to gain insight in the cutting mechanism of cohesive-frictional rocks, and (b) to examine the comparability of numerical models predictions with experimental results by solving the forward problem. In the first type of model, a plane strain continuum calculation is done with a non-hardening, elastic-plastic, linear Mohr-Coulomb model with non-associative flow rule. In the second type of numerical model, discrete element calculations are done on a simulated plane strain sample of 540 discs. In both models, estimations are made on components of force applied to the cutting face of the bit and are compared with measurements taken with the data-acquisition system of the portable microdrilling tool during specially designed tests on marbles. It is found that the predictions of the continuum model are in full accordance with measured forces during drilling. It is also shown that the cohesion and internal friction angle are the most important parameters affecting the rock drilling resistance, as is depicted by the limit analysis theory of plasticity. Moreover, the calibration of the discrete element model on the experimental data permits the approximate estimation of the mode-1 fracture toughness for each type of marble. (c) 2006 Elsevier Ltd. All rights reserved.