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 m i 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 σ 3 = 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 m i 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 (σ 1 /σ 3 ) was equal to 5.84 irrespective of rock type, transition pressure, grain size, and m i 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 σ ci .