Cyclic strain amplitude impact on soil dynamic parameters of conventional railways’ track-bed materials

Abstract

In order to improve the service provided by railway operators, infrastructure administrators seek to upgrade the service speeds on their conventional network. When train speed is upgraded, an increase of the cyclic strain amplitude caused by train’s axles is expected. The strain amplitude loading track-bed layers below ballast, as the interlayer, depends on the train speed, the rolling stock (axle’s load) and the mechanical state of the upper track-bed materials. The train loading, resulting in soil strain, is attenuated by ballast grains in depth when transmitted from rail to trackbed materials. Axle loads and train speed will play a significant role in the track-bed materials cyclic strains. An interlayer soil, fabricated in laboratory, was tested on a large scale triaxial cell (ø=300 mm). Different cyclic solicitations, controlled by strains amplitudes, were induced to the sample. The vertical strain amplitudes were included in the range 0.05 % - 0.1 %. This range of vertical strain amplitudes were taken in accounts from literature review and from in-situ measurements on conventional tracks. Moreover, different soil confining pressures were considered for the cyclic tests: from 25 kPa to 200 kPa. Cyclic loading was applied in compression and traction. Furthermore, as strain amplitudes were very small, they were measured using local strain gages able to measure strains up to 10-6 %. These gages were glued to the sample to determinate vertical and radial strains. Therefore, tests results show a decrease of soil’s resilient modulus similar when strain amplitude is increased. The decrease rate is similar for all the tested confining pressures. This decrease of resilient modulus could involve an increase of materials plastic deformation when train speed is upgraded or axle load is increased on a line.