Keywords: blanket layer, railway platform, large scale triaxial test, cyclic behaviour, water content effects.

In the structure of old railway platforms in France, a blanket layer was created naturally, mainly by the interpenetration of the sub-grade soil and ballast subject to train action. In order to respect the new requirements of train circulation (including heavier and faster trains), numerous old railway lines have been modernized, repaired or rehabilitated. Preliminary investigations conducted on different sites shows that poor drainage was the major cause for track deterioration. It has been observed that the variation of the water content greatly influences the platform of an old railway. Therefore, to propose a rational recommendation for the modernization, it is necessary to better understand the hydro-mechanical behaviour of the blanket layer.

Large scale triaxial tests are usually carried out in the laboratory to analyse the mechanical behaviour of soil specimens with a diameter of 300mm and a height of 600mm. A set of monotonic triaxial tests were performed at a water content of 4% and 12% (corresponding to the saturated condition). For the cyclic test, the stress paths defined are based on the results from the monotonic tests. Three samples with three water content value (4%, 6% and 12%) were studied. The soil sample was first loaded at a confinement pressure of 30 kPa, corresponding to the in-situ condition. The deviator stress was varied from 0 to 45 kPa, 90 kPa, 140 kPa and 200 kPa. Except for the three first steps where 30000 cycles were applied for each step, 9x10⁵ cycles were applied for the last step under a maximum deviator stress of 200 kPa.

At the first cycles of each stress level, the axial strain increased quickly. By increasing the stress level the axial strain also increases, but in general it tends to stabilize at the end of each stress level. The influence of water content on the mechanical characteristics of the soil blanket layer was found to be significant. The results show that the permanent axial strains are larger for higher water contents. For the saturated condition rising rate of the axial strain is higher than in unsaturated conditions. At the last stress level, the saturated specimen reached failure while failure did not occur for the other unsaturated specimens.

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