Observations of strain localisations during pipe penetration into clay

Strain localisation widths in sand are known to be controlled by the particle size, such that their magnitude can be readily determined. However, there is less certainty on how to select appropriate widths for localisations in clays. This poses problems for numerical analyses involving strain softening, as the numerical results are dependent on the localisation width. This paper addresses this uncertainty through a combination of experiments that allowed localisations to be detected and their widths measured, and finite-element analyses in which the localisation width was controlled using the ‘non-local’ method. Both the numerical modelling and the experiments considered vertical penetration of a horizontal pipe into normally consolidated clay. Strain localisations in the experiments are seen to narrow with strain level, but also as penetration velocity decreases. This dependence on penetration velocity is interpreted as being due to contraction attributable to drainage from within the localisation, which also increases soil strength at the boundaries of the localisation, leading to further thinning to avoid mobilising the stronger soil. Although these aspects cannot be implemented in total stress (undrained) numerical analyses, the global response can be reproduced accurately, provided that the analyses employ a non-local approach to avoid periodicity in the resistance.