After the 1995 Hyogo-ken Nanbu Earthquake in Japan, rock structures, including rock slopes and underground cavities, were required to be evaluated for seismic resistance. Understanding the dynamic deformation properties of hard rock is essential for performing dynamic response analysis of large deformations of rock structures. This paper discusses the effect of confining pressure on the strain level dependence of the Young’s modulus and damping ratio of water-saturated sandstone based on the results of cyclic triaxial tests. The relationship between the axial strain and equivalent Young’s modulus ratio is different for quasi-elastic deformation below 2,000 με and elastoplastic deformation above 2,000 με and hence can be approximated by two logarithmic functions. The relationship at the quasi-elastic strain level can be approximated by the same function regardless of the confining pressure. However, at the elastoplastic strain level, the slope of the logarithmic function decreases with increasing confining pressure. Furthermore, the relationship between the axial strain and damping ratio is distributed on the same curve regardless of the confining pressure in the axial strain range below 4,000 με. However, when the axial strain exceeds 4,000 με, this relationship deviates from the curve and the damping ratio rapidly increases with decreasing confining pressure. The experimental results for water-saturated sandstone were compared with those for dry sandstone presented in a previous paper. At the quasi-elastic strain level, the relationship between the axial strain and equivalent Young’s modulus ratio can be approximated by the same function regardless of the water content. However, at the elastoplastic strain level, the slope of the logarithmic function is greater for water-saturated sandstone than for dry sandstone. Meanwhile, the relationship between the axial strain and damping ratio, regardless of the water content, is distributed on the same curve in the range from quasi-elastic to elastoplastic strain.
