Abstract: Interlayer soil in railway substructures is characterized by a fine/coarse soil mixture. Considering that the resilient modulus M_r of the mixture is influenced by the microstructure of fine soil, it is worthwhile to investigate this aspect further. In this study, the microstructure of fines was explored by mercury intrusion porosimetry (MIP), and its influence on the M_r of the mixture was studied by multi-stage dynamic triaxial tests with varying deviator stress amplitudes σ_d. The results showed a fine matrix fabric obtained at water contents of fine soil w_f=17.6% and 13.7% (>the plastic limit of fine soil w_p=12%), and a fine aggregate fabric identified at w_f=10.6% (<w_p=12%). Interestingly, the influences of w_f and σ_d on the M_r of the mixture were observed: the rise in σ_d contributed to a decline in M_r when w_f>w_p but to an increase in M_r when w_f<w_p. It was concluded that, for the fine matrix fabric (w_f>w_p), increasing σ_d induced a reduction in M_r, while for the fine aggregate fabric (w_f<w_p), increasing σ_d gave rise to the growth of M_r. The distinct M_r-σ_d behaviors for these two fabrics were explained by the competing influences between soil hardening upon loading and soil rebounding upon unloading. For the fine matrix fabric (w_f>w_p), considering its high deformability, the rebounding effect on M_r outweighed the hardening effect, and thus a decline in M_r occurred with the growth of σ_d. Conversely, for the fine aggregate fabric (w_f<w_p), the rebounding effect on M_r was secondary compared with the hardening effect based on the consideration of its low deformability, and thus an increase in M_r was observed with rising σ_d.

Key words: Railway engineering; Resilient modulus; Unsaturated soil; Microstructure

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