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In the conventional French railway substructures, a layer namely interlayer was created naturally by the interpenetration of ballast and subgrade. The lower part of this interlayer is characterized by a matrix of fines with inclusion of coarse grains in it. Moreover, the coarse-grain content is decreasing over depth. In this study, the effect of volumetric content of coarse grains f v (volumetric ratio of coarse grains to the whole sample; grain size > 2 mm) on the mechanical behavior of the lower part interlayer soil was investigated by carrying out monotonic triaxial tests. The results showed that increasing the volumetric content of coarse grains f v led to more significant dilatancy. The shear strength also increased with the increase of f v and the increasing trend appeared to be much more significant when f v >20%. Examination of the variations of peak deviator stress with f v revealed the existence of a characteristic value of f v - cha  ≈ 27% that separates two zones with different coarse-grain effects: a fine-fine contact dominating zone when f v  ≤ 27% and a grain-grain contact dominating zone when f v  > 27%. To clarify the observed phenomena, X-ray microcomputed tomography (μCT) scans were performed on the as-compacted samples, allowing the coarse-grain distribution inside the as-compacted sample to be visualized. It was found that when f v ranged from 0% to 10%, the skeleton of the sample was mainly constituted by fines with the coarse grains floating in the fines matrix. By contrast, when f v reached 20%, the coarse grains formed some aggregates but the fine-fine contacts were still dominating. When f v increased up to 35%–45%, the skeleton of the sample turned to be dominated by the coarse grains. This suggests that the characteristic volumetric content of coarse grains identified through monotonic triaxial test results corresponds to the transition of soil fabric from fine-fine contacts to grain-grain contacts. [ABSTRACT FROM AUTHOR]