Screw spike lateral restraint performance of grouting-repaired timber sleepers: an experimental and numerical study

Developing effective remedial strategies for railway timber sleepers is a critical issue, particularly at spike-hole locations where failures most commonly initiate. To clarify the reinforcement mechanisms and failure evolution of timber sleepers at spike-hole locations, this study investigates the lateral restraint performance of grouting-repaired timber sleepers through integrated experimental testing and 3D finite element (FE) analysis. The results of this study demonstrate a reliable repair method for used timber sleepers, which is particularly important for reducing maintenance costs and mitigating environmental impacts. A total of 24 samples were tested across 8 distinct configurations, accounting for timber condition (new vs. used), repair status, and loading direction (parallel vs. perpendicular to grain). The results show that polyurethane reinforcement significantly restores and enhances lateral restraint capacity. For new sleepers, the lateral restraint capacity at 5.1 mm displacement increased by 57.8% and 63.4% in the parallel to grain and perpendicular to grain directions, respectively. For used sleepers, capacities increased by 37.8% and 56.2%, respectively. The NR-L (new, repaired, parallel to grain) configuration exhibits the highest lateral restraint load of 21.3 kN at a displacement of 5.1 mm, and an ultimate lateral restraint load of 46.3 kN at failure. Notably, repaired used sleepers even outperform unrepaired new sleepers. The high-strength adhesive interface shifts the failure mode from localized timber crushing and interlaminar splitting to expansive shear failure. Numerical simulations reveal that the adhesive reduces the maximum stress by approximately 7–9% and the maximum equivalent plastic strain by 20–40% through interfacial stress homogenization, thereby mitigating localized stress concentrations and plastic damage accumulation.