Effects of fines and water contents on the mechanical behavior of interlayer soil in ancient railway sub-structure

In the ancient railway sub-structure in France, after years of operation, the inter-penetration of fine particles of sub-grade and ballast has created a new layer referred to as the interlayer. As it was naturally formed, the fines content and water content of the interlayer vary considerably. In this study, the effects of the fines and water contents on the mechanical behavior of interlayer soil were investigated by carrying out large-scale monotonic and cyclic triaxial tests. The results of the monotonic triaxial tests show that adding more fines in the interlayer soil does not significantly change the shear strength in the dry condition (water content w=4% and 6%), but drastically decreases the shear strength parameters (friction angle and cohesion) in the nearly saturated condition (w=12%). The cyclic triaxial tests were performed at various deviator stress levels. By considering the permanent axial strain at the end of application of each stress level, it was found that the higher the fines content in the nearly saturated condition (w=12%), the larger the permanent axial strain. In the case of lower water content (w=4% and 6%), the opposite trend was identified: adding fines decreases the permanent axial strain.