True-triaxial compressive strength of Maha Sarakham salt

• The effect of σ2 on salt strength can be described best by the modified Wiebols and Cook criterion.
• The elastic parameters of the salt tend to be independent of σ2.
• The Coulomb and Hoek and Brown criteria cannot describe the salt strengths beyond the condition where σ2=σ3.

True triaxial compressive strengths of the Maha Sarakham salt are experimentally determined by using a polyaxial load frame. The salt specimens with nominal dimensions of 5.4×5.4×10.8 cm3 are loaded to failure with the minimum principal stress, σ3, varying from 0 to 7 MPa, and the intermediate principal stress, σ2, varying from 0 to 80 MPa. The major principal (axial) stress is increased at a rate of 0.5–1.0 MPa/s until failure occurs. Based on the Coulomb criterion the internal friction angle determined from the triaxial loading condition (σ2=σ3) is 50°, and the cohesion is 5.0 MPa. The elastic parameters of the salt tend to be independent of σ2 for the applied stress range. The effect of σ2 on the salt strength can be described best by the modified Wiebols and Cook criterion with a mean misfit of 3.5 MPa. The (power law) Mogi criterion underestimates the salt strength, particularly under high σ3 values. The modified Lade and 3-D Hoek & Brown criteria overestimate the strength at all levels of σ3. The Coulomb and Hoek & Brown criteria cannot describe the salt strengths beyond the condition where σ2=σ3, as they cannot incorporate the effects of σ2. Both circumscribed and inscribed Drucker–Prager criteria severely underestimate σ1 at failure for all stress conditions, showing the largest mean misfit of 19.5 MPa.