In this study,a fouled ballast taken from the site of Sénissiat,France,was investigated.For the hydraulic behaviour,a large-scale cell was developed allowing drainage and evaporation tests to be carried out with monitoring of both suction and volumetric water content at various positions of the sample.It was observed that the hydraulic conductivity of fouled ballast is decreasing with suction increase,as for common unsaturated soils.The effect of fines content was found to be negligible.For the mechanical behaviour,both monotonic and cyclic triaxial tests were carried out using a large-scale triaxial cell.Various water contents were considered.The results were interpreted in terms of shear strength and permanent axial strain.It appeared that the water content is an important factor to be accounted for since any increase of water content or degree of saturation significantly decreases the shear strength and increases the permanent strain.Constitutive modelling has been attempted based on the experimental results.The model in its current state is capable of describing the effects of stress level,cycle number and water content. In this study, a fouled ballast taken from the site of Sénissiat, France, was investigated. For the hydraulic behavior, a large-scale cell was developed allow drainage and evaporation tests to be carried out with monitoring of both suction and volumetric water content at various positions of the sample.It was observed that the hydraulic conductivity of fouled ballast is decreasing with suction increase, as for common unsaturated soils. the effect of fines content was found to be negligible.For the mechanical behavior, both monotonic and cyclic triaxial tests were carried out using a large-scale triaxial cell .Various water contents were considered.The results were interpreted in terms of shear strength and permanent axial strain .It that the water content is an important factor to be accounted for be any increase of water content or degree of saturation greatly decreases the shear strength and increases the permanent strain .Constitutive modeling has been attempted based on the experimental results. The model in its current state is capable of describing the effects of stress level, cycle number and water content.