Abstract: Roughness is one of the key factors governing the shear mechanical behavior of pile-soil interfaces. Regular groove structure, as a geometrically controllable form of roughness, exerts a significant influence on the shear mechanical behavior and deformation characteristics of the interface. Therefore, a series of visualized direct shear tests was conducted on interfaces between regular grooved structural plates and silty clay using an improved visualized direct shear device. The shear mechanical properties of the interface under different groove widths, groove depths, and their combinations were investigated. Digital image correlation was employed to elucidate how groove parameters influence the shear deformation characteristics and mechanical mechanisms of the interface. The results show that, with increasing groove width and depth, the cohesion of the pile-silty clay interface first increases and then decreases, whereas the interface friction angle varies only slightly. The regular groove structure significantly enhances the interface shear strength. The shear strength increases first and then decreases with groove width, and increases first and then tends to stabilize with groove depth. An optimal groove width of 8 mm and an optimal groove depth of 4 mm were identified, at which the interface shear performance reaches its maximum. Increasing groove width leads to shear localization. Shear deformation gradually transitions from being controlled by a single passive zone to a zoned pattern, forming successively a passive zone, transition zone, and active zone along the shear direction. Under the optimal groove width, the passive zone reaches its maximum extent of 12.25 mm, with a vertical expansion height of 2.31 mm. The contribution of groove depth to interface shear strength exhibits a pronounced size effect. Under the optimal groove depth, the soil within the grooves is fully mobilized to participate in interface shearing, producing the most significant strengthening effect. Reasonable selection of grooved parameters is therefore crucial for fully mobilizing the shear resistance of pile–soil interfaces with regular grooved structures. The findings provide a theoretical basis for the rational selection and optimization design of surface structural parameters of threaded piles or ribbed piles in engineering.

Key words: pile-soil interface, roughness, digital image correlation, grooved parameters, direct shear test, shear behavior