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

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 σ1-σ2 plane, and the developed fault is steeply inclined in the σ3 direction. Rock strength in all tested rocks increases with σ2 when σ3 is kept constant. Thus, the common Mohr-type criteria, which ignore the effect of σ2, typically underestimate rock strength. Rather a 3D criterion that involves all three principal stresses represents well experimental results. Fracture plane slope for the same σ3 steepens as σ2 rises, contrary to Mohr-type criteria. With respect to deformation, the onset of dilatancy increases with σ2. 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 σ2 effect regardless of rock type.

Research Highlights
► Intermediate principal stress s2 affects mechanical behavior in all rock types.
► Mohr-Coulomb criterion is only a conservative estimate of rock strength.
► For constant minimum stress, fault angle rises with σ2.
► The onset of dilatancy rises with σ2.
► 3D testing of ICDP cores reveals consistent σ2 effects on strength and deformation.