This paper examines the effect of clay fraction and roughness on the relationship between the ratio of the CPT end resistance (qc ) to unit shaft friction (τf ) for displacement piles in sand. Tension load tests were performed on smooth and rough model piles that were jacked into sand, sand-clay mixtures and clay. Parallel constant normal load and constant normal stiffness direct shear interface tests were conducted to assist interpretation and allow separation of effects of interface friction angle and dilation on the capacities. It is shown that even a small amount of clay within a sand mass can have a major impact on the available shaft friction. The effects of pile roughness are important for developing the pile shaft capacities in these soils.