Directional friction in load transfer across soil–structure interfaces. (a) Monotonic and (b) cyclic interface shear tests on snakeskin-inspired surface and sand showing the larger stress ratio and friction angle mobilised during cranial shearing (data from O'Hara & Martinez (2020)); δ, interface friction angle. (c) Centrifuge load tests on snakeskin-inspired piles highlighting the greater forces mobilised during cranial installation and pullout (data from O'Hara & Martinez (2020))