<p>Engineering practices demonstrate that continuous flight auger (CFA) piles exhibit a higher bearing capacity than slurry-supported bored piles, as the former enhances the pile surface roughness and the latter contains a lubricated mud cake along soil-pile interface. This strengthening effect is very pronounced in weathered granitic soil, which is a special soil widely distributed in Asia. Against this backdrop, this study investigates the effects of surface roughness and mud cake on the interfacial responses between concrete pile and weathered granitic soil. Direct shear tests were conducted to imitate the soil-pile shearing process and to provide the fundamental insights. In the tests, the shearing behaviors between rough concrete and soil under varied normal stresses and soil moisture content were first examined. Then, the roughness levels of the specimens were varied from 0.1 to 1.0&#xa0;mm to revealed the associated effect. Experimental results indicate that pile roughness mainly enhanced the interfacial cohesion due to the soil-roughness interlocking. Finally, in-between mud cakes with thicknesses ranging from 1 to 3&#xa0;mm were introduced to investigate their effects, and a nonlinear negative relationship emerged between the shear strength and mud cake thickness. Based on the experimental observations, the material costs between CFA pile and slurry-supported bored pile are estimated and compared, providing a useful basis for optimizing pile foundation design in weathered granitic soil areas.</p>