Testing the Strength of Buried Surface Hoar Weak Layers Under Combined Compression and Shear Loading

Abstract We experimentally determined the multiaxial strength of weak layers of buried surface hoar using displacement‐controlled tests under combined normal and shear loading on a custom testing machine. We tested 63 samples from three different weak layers, with strengths ranging from 1 to 10 kPa with considerable scattering. While shear strength was lower than compressive strength (typically about one to two thirds), we did not observe the very low shear strengths implied by a Mohr‐Coulomb‐cap type of failure envelope. Instead, our data were better described by elliptical failure envelopes, consistent with numerical predictions for other weak layer types. Density and microstructural parameters were quantified from micro‐computed tomography. Bulk density alone did not predict strength or envelope shape, but the data are not yet sufficient to derive robust quantitative structure–property relations. The envelope shape also appeared to be unrelated to the elastic properties, indicating that different microstructural aspects might govern elasticity and strength.