Measurement of soil deformations in vane shear tests on low-strength clays

Assessing the effect of a submarine slide impact on seabed infrastructure requires soil strength measurements across a range of strain rates at water contents that span the solid–fluid transition. Although vane shear tests have been used to make such measurements, quantifying the operational strain rate associated with the measured soil strength is somewhat uncertain as this will depend on how soil is assumed to move beyond the vane blades—specifically whether soil movement is assumed to be continuous (and diminishing) with increasing radius, or whether it is assumed to be confined within a strain localisation of finite width. In view of this, this paper applies image-based deformation measurement techniques to measure soil movements in vane shear tests on low-strength clays (with peak strength less than 1 kPa) at different vane rotations (and hence mobilised soil strengths), and at different rotational velocities. The experimental observations reveal a diffuse deformation zone before the peak shear stress is mobilised, which then shows the expected narrowing with increasing vane rotation due to strain softening. However, narrower deformation widths were observed at the circumference of the vane blades in tests with lower rotational velocities, which is interpreted as being due to a combination of strain rate effects and local drainage.