Elastic properties of saline permafrost during thawing by bender elements and bending disks

The elastic properties such as small-strain Young's, shear moduli and Poisson's ratio of permafrost are important parameters for analyzing the stress distribution and thaw-subsidence of permafrost and deformation of well casing embedded in permafrost. However, the deep natural permafrost warms up and even thaws due to energy extraction and its elastic properties change significantly with temperature. This paper presents elastic properties obtained by conducting laboratory testing of permafrost samples from the North Slope of Alaska. The testing specimens were conditioned from − 10 °C to − 2 °C at an interval of 2 °C, and to 20 °C for examining the temperature effects. The bending disks and bender elements were used to measure the compressional wave (P-wave) and shear wave (S-wave) velocities, respectively, for evaluating the small-strain elastic properties during thawing. Measurements were done in both horizontal and vertical directions for assessing anisotropy. The results show that the P- and S-wave velocities from this study partially overlap with in situ measurement data reported in the literature, and tend to be lower due to high salinity and lack of overburden effects, which is stronger on sandy permafrost. Results reveal anisotropy in the wave velocities in all types of permafrost tested, and the small-strain elastic properties of clay and silt permafrost exhibit no sharp change when the temperature crosses 0 °C, likely due to non-uniform freezing point depression caused by the varying and high salinity among specimens. Finally, parameters were provided for predicting the confining pressure dependent small-strain Young's modulus, and regression equations were proposed for predicting the modulus number and exponent in Janbu's model based on permafrost dry density and moisture content.