An interlayer soil was identified in the French conventional rail tracks, corresponding to a mixture of subgrade fine soil and ballast grains. Experimental results show that the permanent strain of such fine/coarse soil mixture can be at stable or unstable state, depending on the stress level with respect to the plastic shakedown limit σdth (the maximum stress level for the stable permanent strain). As the water content of soil mixture changed, the matric suction changed subsequently, leading to a change of σdth. In this study, a model incorporating the soil–water retention curve was proposed to describe the variation of σdth with for the soil mixture at varying coarse grain content fv. This proposed model implied the plastic shakedown limit under saturated state σdth-sat, the plastic shakedown limit under a reference state σdth-ref, the matric suction corresponding to the reference state ref, and the parameters related to the soil water retention curve. The model was validated using three series of studies in literature. Based on an existing approach implying a representative permanent strain rate 1/as, the σdth under varying and fv values was determined for these three series. For each series, the obtained σdth was divided into two groups: the first group was used to determine the model parameter B in the relationship between σdth and, while the second group was adopted to validate the proposed model with the previous determined parameter B. A reasonably good agreement was obtained between the measurements and the calculations for these three series of studies. A comparison between the proposed model and a representative existing model was made, showing the performance of the proposed model in describing the variations of σdth with for soil mixture at varying fv.