Evaluation of sleeve friction measurement in soft soils using rate interface tests under different boundary conditions

Sleeve friction (f s ) measurements in soft organic soils are influenced by interface rate dependency and large-deformation shearing. The evaluation of f s measurement during piezocone soundings (CPTu) is done with rate-dependent interface shear tests conducted under different boundary and drainage conditions. Contact behaviour between soft soil and smooth steel was investigated in ring shear apparatus, and direct shear box device with pore water pressure measurement. In undrained regime, the peak interface shear strength under constant height (CH) and constant normal load (CNL) conditions are coherent, whereas the residual interface undrained shear strength under CNL mode is approximately two times higher than under CH mode. It is demonstrated that normally consolidated soft soil–steel interface exhibits negative viscous behaviour. The f s variability is primarily influenced by the interface boundary conditions. It was observed that the f s measurements fall within interface shear strength measured under CNL and CH conditions, exhibiting a tendency leaning towards CNL mode. The CH condition is barely meet in CPTu sounding. The widely held assumption that f s is equal to the remoulded undrained shear strength is merely coincidental. The presented research provides further insights into soft soil–smooth steel interface behaviour and enhances the CPTu data interpretation.