Load Transfer Directionality of Snakeskin-Inspired Piles during Installation and Pullout in Sands

Deep foundation applications, such as foundations for jacket structures and reaction piles, may benefit from larger shaft resistances during tensile compared with compressive loading. A suitable analog for designing surfaces whose load transfer depends on the direction of loading is the skin of snakes. Snake scales reduce friction when they move forward (i.e., caudal direction) and increase friction when they move backward (i.e., cranial direction). A series of centrifuge load tests were conducted on instrumented piles in medium-dense sand to investigate the load transfer behavior of piles with snakeskin-inspired surfaces. Load tests were performed on three bioinspired piles, a reference rough pile, and a reference smooth pile at two embedment depths each. The results present distributions of axial load and shear stresses along the pile length and head load–displacement and local shear stress–displacement relationships. During both installation and pullout, the cranial shaft friction mobilized similar shear stress magnitudes and shear resistance distribution with depth to that of the rough pile, while similarities were observed between the caudal shaft friction and that of the smooth pile. The results show that the bioinspired piles mobilize directionally-dependent shaft capacities, where the ratio of capacities during cranial pullout to caudal installation was measured to be 1.6 to 2.1, as evidenced by the computed $\beta$ coefficients. Variations in relative density in the centrifuge models were quantified by means of cone penetration test (CPT) soundings, and the pile test results were corrected for potential scaling effects to obtain $\beta$ coefficients that are more representative of field conditions.