Detection and measurement of strain localisations in clay using particle image velocimetry

Strain localisation width is an important characteristic for failure assessments in a range of boundary value problems, as it affects the rate of strength reduction in strain-softening soils. Although image-based deformation measurement techniques have been used to measure strain localisation widths in sands, there have been fewer investigations in clays and measurement accuracy remains unquantified. In view of this, this paper investigates the feasibility of using image-based deformation measurement techniques to measure strain localisation widths in clays. A simple method, based on the measured displacement field, is proposed to ascertain boundaries of localised deformation regions. This method is then applied to analyses using synthetic images with prescribed localisations to establish the optimal measurement point size (patch size) and spacing and the minimum measurable localisation width. The paper then applies the technique to measure the localisation width that develops during undrained pipe penetration in normally consolidated clay. A strain localisation is seen to form close to the pipe and then extend away from the pipe due to progressive failure and narrow as penetration increases before becoming inactive. The measured localisation widths are above the minimum measurable width, indicating that accurate measurement of strain localisation using image-based deformation measurement techniques is feasible.