Imaging of crushable grains when shearing against rough interfaces

Previous studies have highlighted the propensity for rough interfaces, such as those used for hot on-bottom oil pipelines to damage the natural soil structure. This paper presents the results of a series of modified direct shear tests on marine soils containing naturally-formed, crushable invertebrate faecal pellets. Macrophotography of pellets shearing against a rough interface shows evidence of pellet breakage and crushing, especially on the first cycle of shearing. Unjammed particles within the shear zone show evidence of local bursts of pore flow following sudden compaction events. After a large shearing displacement, a clearly defined shear zone is observed comprising crushed fragments. During cyclic shearing, significant particle rearrangement and compaction is apparent when the shear direction is reversed, before jamming of both intact and crushed particles causes dilation after a few millimetres of shearing in the reverse direction. Tests are undertaken at both fast and slow shear rates to identify the difference in behaviour under undrained and partially-drained conditions. The observations made in these tests challenge the suitability of small-amplitude cyclic direct shear testing to simulate monotonic large-amplitude shearing displacements, but highlight the importance of conducting cyclic tests where these are relevant to the design.