This work describes the effect of triaxial compressive confinement on the strength of columnar S2 ice loaded under conditions leading to brittle failure. Cubes of columnar S2 ice were tested at {dollar}rm{lcub}-{rcub}10spcirc C{dollar} under triaxial (proportional) loading where the ratio of normal stresses, applied perpendicular to and parallel to the long axes of the columnar-shaped grains, was varied in a systematic manner using a true multiaxial loading system. The strain rate in the direction of the greatest stress was {dollar}rm 6times10sp{lcub}-3{rcub} ssp{lcub}-1{rcub}.{dollar} Tests conducted using columnar S2 saline ice were used to construct a brittle failure surface which is faceted and reveals four regimes: (I) of lower across-column confinement where the along-column confinement has no significant effect on the major across-column stress at failure, but where the minor across-column stress raises the strength; (II) of higher across-column confinement where along-column confinement now raises the major across-column stress at failure; (III) of predominantly along-column loading where the along-column failure stress increases in proportion to the smaller of the two across-column confining stresses; and (IV) of higher confinement where failure is triggered through an end-crushing mode where the failure stress is independent of the loading path. Tests conducted using columnar S2 fresh-water ice revealed similar regimes of behavior and similar strengths. Regimes I, II, and III are characteristic of the ice, and each shows a high sensitivity of strength to the appropriate confining stress. Within these regimes the behavior is explained in terms of the frictional crack sliding-wing crack mechanism of brittle compressive failure. In general, the strength of columnar S2 saline ice and fresh-water ice loaded within the brittle regime depends on both the degree and the direction of the confining stresses.
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