Experimental and Numerical Investigations of Flat Approach Slab–Soil Interaction in Jointless Bridge

In jointless bridges, a grade flat approach slab (GFAS) with the same elevation as the pavement can transfer the girder’s longitudinal deformation to the backfill. However, any cracks and settlement of the pavement usually occur at the end of the GFAS. To address this shortcoming, the buried flat approach slab (BFAS) horizontally embedded at a depth in the backfill was proposed. The complicated flat approach slab–soil interaction (FASSI) of the BFAS has not been systemically investigated. To address this gap, the influence of the FASSI on the mechanical performance of the approach slab and the backfill deformation was investigated in this research to understand the mechanism of the FASSI in absorbing one part of the girder’s longitudinal deformation and transferring the rest to the soil. Experimental tests on the FASSI with different embedded depths under longitudinal displacements were conducted. Numerical parametric analyses were carried out by considering the embedded depths and slab lengths as the parameters based on a finite element model verified using the test results. The results show that load–displacement curves of the FASSI comprise three stages: the elastic stage (approach slab’s displacement was absorbed by sand), the elastoplastic stage (sand deformation was observed), and the failure stage (overall shear failure of the sand was found). The longitudinal displacement transfer mode and vertical deformation distribution mode of the sand were affected by the embedded depth and slab length. With an increase in the embedded depth or a decrease in the slab length, the sand deformation decreases, which is beneficial for avoiding pavement crack risks and improving the pavement evenness. Finally, a simplified calculation formula that can be used to predict the load–displacement curves of the FASSI was proposed. This research provides the theoretical basis for the design and construction of the flat approach slab in jointless bridges.