Quantifying the influence of sleeper spacing on lateral resistance using a novel experimental method

Lateral resistance, principally dictated by the interaction between sleepers (ties) and ballast, is a fundamental property of ballasted railway track which is critical to overall track stability and safety. This study investigates the influence of sleeper spacing on lateral resistance to support overall track system design and track safety management. A novel multi-sleeper push test (MSPT) methodology, utilising both a kinematic restraint to better replicate track conditions and a state-of-the-art digital image correlation (DIC) system to measure surface ballast movement, has been used to evaluate the lateral resistance behaviour of two different sleeper types (concrete and steel) under seven different sleeper spacing conditions (550–850 mm). This is the first direct comparison of modern concrete and steel sleeper variants under these conditions. It is found that both sleeper types exhibit similar lateral resistance behaviour at small displacements (less than 3 mm) but, regardless of the spacing used, steel sleepers have significantly greater lateral resistance beyond this point. Although it depends on the assessment method used, concrete sleepers demonstrated an optimum spacing of approximately 675 mm, whilst steel sleepers showed highest resistance at approximately 706 mm. However, concrete sleepers showed less sensitivity to changes in spacing compared to steel sleepers, which retained high resistance over a narrower range of spacings. These findings contribute to track safety management by improving buckling mitigation strategies and component selection, whilst unlocking the ability to assess system-level impacts to aid the pursuit of more sustainable railway infrastructure design.