Dilation of anisotropic rock salt: Evidence from Mount Sedom diapir

Abstract

The mechanical behavior of bedded rock salt is studied using constant strain rate biaxial tests performed on 24 bedded rock salt samples from Mount Sedom diapir. Careful measurements of elastic parameters in unload‐reload loops indicate that they are influenced by confining pressure but are not sensitive to anisotropy. The yield stress is also influenced by confining pressure but not by anisotropy. The stress‐strain curve therefore indicates isotropy up to the yield stress. Initiation of dilation marks the onset of dependence of volumetric strain on anisotropy. Major principal compressive stress at dilation (σ 1,d ) is at maximum when compression is normal to bedding planes (β=0°), the stress decreases with increasing values of β, and minimum values are measured when compression is parallel to bedding planes (β=90°). Beyond onset of dilation point the relative dilation with respect to stress difference ( V = |Δε ν /Δ(σ 1 ‐ σ 3 )|; σ 1 > σ 1,d ; MPa −1 ) increases with decreasing confining pressure and with increasing β; namely, dilation is intensified with decreasing confining pressure and as the major principal compressive stress direction becomes parallel with bedding plane orientation. Current models for compression‐dilation boundary ignore anisotropy and therefore provide site specific solutions but fail to describe general rock salt behaviour. A new empirical model for compression‐dilation boundary in anisotropic rock salt, developed for a data set from Mount Sedom, predicts that stress at onset of dilation decreases with decreasing confining pressure and with increasing value of β. The rate of change of σ 1,d with respect to β decreases with decreasing confining pressure, and the rate of change of σ 1,d decreases with increasing confining pressure for all values of β.