Asphalt/ballast trackbeds for improved clearance beneath historical bridges for electrification works

Electrification of existing railway lines offers the potential to operate more sustainably with zero carbon emissions at the point of use by eliminating the need for diesel multiple units, diesel locomotives and/or dual electric/dieselvehicles. However, many existing lines are not suitable for placing overhead electrical lines continuously along the route because of low clearance beneath historical overbridges. These can be replaced, but unless the bridge is already life expired this is not usually economically viable. An alternative approach is to lower the track under the structure. However, this may have structural implications. To mitigate these the engineered trackbed thickness could be reduced so that less material is dug out during the lowering works. This research investigates the potential for reducing the trackbed thickness by using an asphalt layer with an overlying low thickness ballast layer. A series of tests were carried out using the Southampton Railway Testing Facility (SRTF), a laboratory representation of a single sleeper bay of track. Different thicknesses of ballast and asphalt with varied ballast gradations were placed into the apparatus for evaluation. Tests were carried out by applying a cyclic load representing a 20 tonne axle load at 4Hz for up to 3 million cycles (60 million cumulative tonnes). It was found possible to reduce the trackbed thickness; however, there remained minimum required thicknesses for both the asphalt and ballast layer needed to prevent stress localization, which manifested as damage to the asphalt surface and non-even resilient response along the sleeper length with cycles