Coupled effects of temperature gradients and differential settlement on damage and debonding in ballastless track at subgrade-tunnel transitions

The subgrade–tunnel transition section (STTS) is critical to railway lines and is prone to differential settlement and longitudinal temperature gradients, threatening operational safety. Existing studies have mostly examined the mechanical responses caused by differential settlement or temperature loading separately, while their coupled effects on concrete damage and interlayer bonding degradation in ballastless tracks remain insufficiently clarified. To investigate these coupled effects, a finite element model of CRTS Ⅰ double-block ballastless track in the STTS was established, incorporating concrete damaged plasticity and interlayer bonding. The deformation, damage distribution, and interlayer contact state under coupled loading were analyzed. Results show that increasing longitudinal temperature gradients weaken the bond between the track slab and backfill layer, with damage initiating at slab edges and propagating inward. A settlement wavelength of 5 m minimizes rail upward arch deformation because the track slab ends straddle the settlement trough. Shorter wavelengths and larger amplitudes are more likely to induce interlayer gaps and structural damage. At a wavelength of 5 m, a settlement amplitude of only 15 mm can cause transverse penetrating cracks on the lower surface of the supporting layer. These findings support the safety assessment and maintenance of ballastless tracks in STTSs.