Residual strength and failure-mode transitions of loess–steel interface under large-displacement ring shear

Abstract

Shear behaviour at the loess–steel interface governs load transfer in steel pile foundations and retaining structures in collapsible loess regions. However, the large-displacement behaviour of compacted loess–steel interfaces, particularly the evolution of residual strength and failure mode, remains insufficiently understood. To address this issue, 62 large-displacement ring shear tests were conducted under constant normal stress to investigate the effects of shear velocity, normal stress, and compaction degree on the shear response of a compacted loess–steel interface. The results show that shear velocity has only a limited effect on the magnitude of residual shear stress, but it markedly affects vertical strain, failure morphology, and the partitioning of residual strength parameters. Increasing normal stress and compaction degree significantly enhances interface shear resistance, while higher normal stress promotes the development of grooved and blocky damage features. The results further suggest a transition in the dominant near-interface shearing mechanism, under the present test conditions, from friction-controlled sliding to bonding-enhanced soil-involved shearing. This transition provides a consistent explanation for the observed changes in deformation characteristics and residual strength parameters. Overall, the study establishes a mechanistic link between shear localisation, failure-mode transition, and residual strength evolution, and provides a useful basis for evaluating loess–steel interface behaviour in compacted or structurally disturbed loess.