True triaxial strength, deformability, and brittle failure of granodiorite from the San Andreas Fault Observatory at Depth

► Two series of true triaxial tests conducted on the SAFOD granodiorite. ► In both tests rock strength increases with raising σ2 for a given σ3. But unjacketed rock strength is 50% lower than jacketed one for the same σ2 and σ3. ► Jacketed sample failed typically by forming a through-going shear fracture. But unjacketed sample failed by forming extensile cracks parallel to the σ3 faces.