Consistent trends in the true triaxial strength and deformability of cores extracted from ICDP deep scientific holes on three continents

Abstract

An extensive true triaxial testing program was carried out on core samples from three ICDP-sponsored deep scientific boreholes, KTB (Germany), SAFOD (United States), and TCDP (Taiwan). The three rocks differ in the processes that formed them and in many of their mechanical properties. However, all three rocks exhibited similar failure mechanism, in which induced or reopened microcracks are primarily aligned with the σ 1-σ 2 plane, and the developed fault is steeply inclined in the σ 3 direction. Rock strength in all tested rocks increases with σ 2 when σ 3 is kept constant. Thus, the common Mohr-type criteria, which ignore the effect of σ 2, typically underestimate rock strength. Rather a 3D criterion that involves all three principal stresses represents well experimental results. Fracture plane slope for the same σ 3 steepens as σ 2 rises, contrary to Mohr-type criteria. With respect to deformation, the onset of dilatancy increases with σ 2. In conclusion, true triaxial tests conducted on cores from three scientific boreholes, revealed important details of mechanical behavior not otherwise observed in conventional triaxial tests. In addition, they show mechanical behavior similarities as related to σ 2 effect regardless of rock type.