Brittle-Ductile Transition and Hoek–Brown mi Constant of Low-Porosity Carbonate Rocks

The mechanical behavior of low porosity carbonate rocks is investigated by a series of conventional triaxial compression tests performed at room temperature, at various confining pressures up to 70 MPa and at a constant strain rate of 5 × 10 −5 s−1. Aiming at an improvement of the accuracy and quality of the constant mi of the non-linear Hoek–Brown criterion for jointed rock, four dense, high strength and low-porosity carbonate rocks were tested in conventional triaxial testing, under confining pressures over the entire brittle field, from σ3 = 0 to the brittle-ductile transition. Intact, fresh and dry specimens from limestones and two marbles were tested using a standard NX Hoek triaxial cell. The results indicate that the average brittle-ductile transition pressure and the value of mi determined by the experimental data over the entire brittle field, were approximately twice as high for limestones as for marbles. With the inclusion of the results from five well-known carbonate rocks published by other researchers, it was found that, for the total number of nine carbonate rocks, the ratio of the critical principal stress ratio at the transition (σ1/σ3) was equal to 5.84 irrespective of rock type, transition pressure, grain size, and mi value. Moreover, the transition pressure decreases logarithmically with the average rock grain size and the ratio of the transition pressure to the unconfined compressive strength σci.