Combined Influences of Water Content and Coarse Grain Content on Shear Strength of Unsaturated Soil Mixture

An interlayer existed between the ballast layer and subgrade in the conventional railway substructure. Considering that the shear strength tau of the interlayer soil was influenced by the changes in the ballast grain content and water content, this aspect was explored in the present study. Monotonic triaxial tests were fulfilled, which considered five coarse grain contents fv and three water contents of fine soil wf. The results showed that the growth in fv contributed to an increment in tau of the soil mixture under both saturation and unsaturation. Conversely, in previous studies, the growth of fv induced an increment in tau under saturation, but a decline in that under unsaturation. This was explained by the competing influences of fv and suction psi: in previous studies, increasing fv induced a decline in the dry density of the fine soil fraction rho d-f, which contributed to a decline in psi. When the negative influence of declining psi outweighed the positive influence of the incrementing fv, the tau of the soil mixture decreased. Meanwhile, modelling of the tau-psi relationship in the soil mixture with varying fv was performed. This proposed model was examined using the test results from both the present and previous studies, which shows its reasonably good performance.