A modified triaxial test for characterising soil–structure interface behaviour

A modified triaxial test (MTT) was developed to investigate soil–structure interface (SSI) behaviour under controlled triaxial conditions. The modification integrates custom-fabricated material pieces and a specialised loading cap, enabling direct vertical shearing of the soil interface within a standard triaxial apparatus. A dedicated calibration procedure was implemented to ensure accurate determination of soil interface shear response. Comparison with the modified direct shear test (MDST) showed that the MTT produced interface friction angles comparable to those obtained from the MDST, with differences of only 0.6–4.6% for the peak friction angle and 0.1–5.5% for the residual friction angle across the investigated interface roughness conditions. The MTT results were considered potentially more representative because the proposed configuration promotes a more even mobilisation of interface shear resistance than the MDST, while also providing improved control of the stress state and boundary conditions within a triaxial testing environment. Complementary FLAC3D simulations were conducted to examine the spatial distribution of stresses along the interface, indicating that shear stresses are mobilised in a broadly uniform manner over the majority of the interface length. Taken together, the proposed MTT provides a reliable, practical, and more versatile alternative for characterising SSI behaviour.