ABSTRACT
In the structure of old railway lines in France, a blanket layer was created mainly by interpenetration of subgrade soil into the ballast layer under train action since the construction of the lines. The hydraulic proprieties of this layer are essential in evaluating the stability of the old railway platform after heavy rainfalls upon train circulations. In the present work, the blanket layer soil taken from an old railway platform situated in Sénissiat (near Lyon, France) were characterised in laboratory. The Atterberg limits, methylene blue value were determined on the fine-grained portion, whereas analysis of the grain size distribution covered the whole grain elements of different dimensions for both the blanket layer soil and the subgrade soil. The blanket layer soil is constituted of large grain elements and of fine grains from subgrade. Estimating that mainly the fine-grained fraction governs the permeability of the blanket layer, the unsaturated hydraulic properties of the blanket layer soil passed through 2 mm sieve were studied. The water retention curve was measured by the filter paper method. Unsaturated hydraulic conductivity was determined by the instantaneous profile method using an infiltration column of 50 mm in diameter and 200 mm high. Further developments planned for the analysis of the properties of the blanket layer soil are discussed.