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Investigation of the mechanical behaviour of fine/coarse soil mixture

Abstract : In the French conventional railway tracks, under the decades of traffic loadings, an interlayer was created by interpenetration of ballast grains and subgrade fine soil. Considering its high bearing capacity related to its high dry density (2.4 Mg /m3), the French national railway company (SNCF) decided to keep it as one part of the substructure during the renewal program. As the interlayer soil corresponded to a mixture of ballast grains and fine soil, the mechanical behaviour of such fine/coarse soil mixture was investigated in this study.The water retention properties of soil mixture were investigated under varying coarse grain contents and dry densities of fine soil. The filter paper method was adopted to measure the matric suction. The mercury intrusion porosimetry tests were performed for the microstructure observation of fine soil. Results showed that the soil-water retention curve of soil mixture was independent of coarse grain content, and only dependent on the dry density of fine soil. Moreover, the soil-water retention curve in low suction range was governed by macro-pores of fine soil, while in high suction range by micro-pores of fine soil.The effect of microstructure of fine soil on the mechanical behaviour of soil mixture was studied. The microstructure of fine soil was measured by mercury intrusion porosimetry tests, and its effect on the mechanical behaviour of mixture was examined by monotonic triaxial tests. Results showed that the developed sample preparation approach, which is to compact the soil at the Optimum water content and then modify it to the desired value by either wetting or drying, allowed the effect of microstructure of fine soil on the mechanical behaviour of mixture to be minimized. Using this sample preparation approach, the effect of water content on the mechanical behaviour of mixture with varying coarse grain contents was investigated by monotonic triaxial tests. A characteristic coarse grain content was identified, separating the fabric of mixture into two zones: a fine matrix macrostructure when the coarse grain content was smaller than the characteristic value, and a coarse grian skeleton fabric when the coarse grain content was larger than the characteristic value. Moreover, the characteristic coarse grain content increased with the decrease of water content, which was explained by the shrinkage property of fine soil.The effect of water content on the permanent strain, the resilient modulus and the damping ratio of soil mixture was investigated by cyclic triaxial tests. A long-term multi-stage loading procedure was performed for studying permanent strain, and a shor-term multi-stage loading procedure for studying resilient modulus and damping ratio. Results showed that increasing water content led to an increase of permanent strain and a decrease of resilient modulus owing to the decrease of matric suction, and an increase of damping ratio owing to the increase of soil viscosity.A constitutive model was proposed for the permanent strain, allowing the effects of the number of loading cycles, deviator stress and coarse grain content to be accounted for. Furthermore, the effect of matric suction was taken into account by incorporating soil-water retention curve. Another constitutive model was developed for the resilient modulus, allowing the description of the resilient modulus variation with changes in deviator stress, matric suction and coarse grain content
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Yu Su. Investigation of the mechanical behaviour of fine/coarse soil mixture. Géotechnique. École des Ponts ParisTech, 2021. English. ⟨NNT : 2021ENPC0015⟩. ⟨tel-03467834⟩

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