UC Davis

The interface between the soil and structural material is the region where load is transferred for many geosystems, including deep foundations, soil nails, modified soil earth walls, and tunnel walls. Specifically, the frictional resistances generated by piles depend on a number of factors, such as the sand density and friction angle, effective stress acting on the pile surface, and the surface roughness of the pile. Certain types of piles shafts may benefit from resistances that are different in one direction of loading compared to the other, including reaction piles that generate lower installation resistance compared to the pullout resistance or offshore foundations subjected to tensile bias loading during a storm surge. Engineering the surface of such foundations to produce a directionally-dependent shaft resistance could result in a reduction of the required installation depth, and therefore reducing material and installation costs. A pile shaft that results in directionally-dependent resistances may be designed by structuring the shaft surface with asymmetric asperities similar to a ratchet. In order to narrow the parametric space of possible solutions, a bioinspired approach was employed in this work. Biogeotechnics has gained traction as a field in recent years due to its utility in providing efficient solutions to engineering problems based on designs already existing in nature. A number of morphologies exhibit directional-dependent resistances, including the ventral scales of snakes, which were considered in these studies. Particularly, cranial shearing is induced when the scales are displaced against the soil, resulting in mobilization of larger frictional resistances. In contrast, caudal shearing is induced when the scales are displaced with the soil, resulting in smaller frictional resistances. A number of geometric profiles based on the ventral scales of snakes were used to study pile shaft interface behavior, both as planar surfaces in laboratory interface shear testing and as piles with custom-machined surfaces. Constant normal stiffness (CNS) laboratory interface shear testing on sand was performed on reference rough, smooth, and snakeskin-inspired surfaces to study the interface shear at the unit resistance level under both monotonic and cyclic loading, while centrifuge pile load testing was performed to assess the analogous field-scale behavior in terms of installation and pullout resistances as well as stability under cyclic loading. The CNS laboratory tests showed that the snakeskin-inspired surfaces mobilized directional-dependent shear resistances under monotonic loading, which generally agreed with results from the pile shaft resistances measuring during installation and pullout. The laboratory tests also showed that the failure envelopes associated with the snakeskin-inspired surfaces were nonlinear compared to reference rough and smooth surfaces. Potential mechanisms underlying this trend were investigated using particle image velocimetry and it was found that the trend is correlated with the shear strains at the interface at different normal stress conditions. Pile load tests were conducted on internally instrumented piles with reference rough, smooth, and snakeskin-inspired pile shaft geometries to assess the evolution of load transfer with depth. The results indicate that piles displaced in the cranial direction gradually shed load compared to piles displaced in the caudal direction due to the greater magnitudes of mobilized skin friction of the former. In addition, cranially displaced piles require more displacement to reach the maximum resistance compared to caudally displaced piles. When the piles were subjected to cyclic loading with a tensile bias, it was found that the cranially pulled piles failed in fewer cycles than the caudally pulled piles when normalized by the total shaft resistance but could resist greater absolute loads. The results highlight the effect of both cranial and caudal directions on cyclic stability of pile shafts. Cyclic interface shear tests were conducted using CNS conditions to better understand the mechanics governing the pile shaft behavior. The results indicate that under symmetric loading conditions, pile shaft elements fail in the caudal direction in a brittle manner. However, when a mean load bias is introduced in the cranial direction, the behavior changes and the interfaces fail by a progressive accumulation of displacements in the caudal direction. The results also provide insight to the effects of initial normal stress, loading amplitude, and boundary stiffness on the number of cycles to failure for both reference rough, smooth, and snakeskin-inspired surfaces. These studies show the applicability of snakeskin-inspired geometries in piling applications. The agreement between laboratory and centrifuge pile tests indicate that trends observed in laboratory tests may inform field scale behavior. The use of the snakeskin-inspired surfaces readily exhibits directional-dependent load transfer behaviors which may be used to reduce or increase resistances in compression or pullout depending on the design requirements.