Discussion on strength analysis of salt rock under triaxial compressive stress

Through analyzing the salt rock deformation under triaxial compressive stress condition, the large compressive deformation properties of salt rock under higher confining pressure are shown. The problems of the axial stress-strain obtained by axial load divided by the initial cross-sectional area of the specimen are pointed out for triaxial compressive tests. Thus the engineering strain and logarithmic strain are analyzed and compared; and the applicable conditions of these two strains are explained. Finally, the triaxial compressive tests for different confining pressures are carried out and the test results are compared. The research results shows that the logarithmic strain should be used to analyze the large deformation property and the modification of axial stress for salt rock when the larger deformation occurs. The deformation obtained by logarithmic strain and engineering strain for salt rock could be expressed as a linear function of confining pressure. When the confining pressure is 20 MPa, the axial compressive deformation is 3.09 times that of 5 MPa. The higher the confining pressure is, the larger difference of the maximum axial stress is between obtained by the initial cross-sectional area and the logarithmic strain corrected area, and the latter is less than the former. When the confining pressure is up to 20 MPa, the maximum axial stress for the logarithmic strain correction area is only 63.85% of that for the initial cross-sectional area.