The Impacts of Surface Roughness on Cyclic Shearing Behavior of Interface between Crushed Mudstone and Steel cased Rock-Socketed Pile

Abstract In the waterway construction projects of the upper streams of the Yangtze River, crushed mudstone particles are widely used to backfill the foundation of rock-socketed concrete-filled steel tube (RSCFST) pile—a structure widely adopted in port constructions. In practice, steel-mudstone interfaces are subjected to complex loading conditions and the surface profile varies in a certain range during construction and operation. The changes in boundary condition and material profile significantly impact the behavior of interface when subjected to cyclic loads, such as ship impacts, water level fluctuations, and wave-induced loads. In this study, a series of cyclic direct shear tests were carried out to investigate the impacts of surface roughness on interface cyclic behavior of RSCFST pile-soil. The relationship between roughness and shear stress, shear stiffness, damping ratio, normal stress, and particle breakage ratio are discussed, respectively. The mechanical behaviors observed at the steel-mudstone interface are as follows: (1) Surface roughness amplifies the initial shear stress, stiffness, and anisotropy at the steel-mudstone interface. However, it results in a reduction of these parameters in their final stages. (2) The damping ratio of the rough interface exhibits an initial increase followed by a decrease, while the smooth interface demonstrates the exact opposite trend. (3) Volumetric strain demonstrates shear contraction and shear dilatancy, both of which become increasingly evident with enhanced roughness. (4) A significant increase in the particle breakage ratio is observed with increased roughness, with the majority of fragmented particles falling within the 0.075–0.25 mm size range.