Role of grain size and shape on undrained monotonic shear, liquefaction, and post-liquefaction behaviour of granular ensembles

This paper presents a fundamental study to explore the independent effects of grain size and shape on the preliquefaction, liquefaction, and post-liquefaction shearing behaviour of granular ensembles through a series of multi-stage constant volume simple shear tests. Three different granular materials (glass ballotini, river sand, and manufactured sand) of three different sizes (fine, medium, and coarse) with distinct shape descriptors were chosen for the study. Shape parameters of the granular materials, including roundness, sphericity, regularity, and angularity, and their grain level kinematic behaviour were determined from the microscopic images using image analysis algorithms. Experiments were carried out on reconstituted specimens prepared at different relative densities of 15, 30, and 45%, and the results were interpreted in the light of the critical state framework. Undrained monotonic shear tests showed that ensembles with similar grain shapes exhibit a unique critical state line (CSL) and also exhibit a unique phase transformation line (PTL), irrespective of the grain size. However, ensembles with different grain shapes exhibited different CSLs and different PTLs. Irrespective of grain shape, an increase in grain size increased the liquefaction resistance because of an increase in the tendency for dilation. Irrespective of grain size, an increase in particle angularity and irregularity increased the liquefaction resistance due to an increase in the interlocking tendency at grain contacts. Grain size and shape significantly affected the post-liquefaction shear strength of the granular ensembles. The post-liquefaction initial and secondary shear moduli were found to be highly dependent on the grain size and shape.