Mechanism of structural defects in cut-and-cover tunnels within high-fill regions and gradient pile foundation reinforcement technology

Tunnel traverses high-fill soil-rock mixtures regions will often be susceptible to structural defects, which necessitate strata reinforcement. To mitigate the extent of these defects, this study employs a combination of model tests and numerical simulations to investigate the stability of tunnel structures. The mechanisms of structural defects were clarified, the influence of different soil-rock mixture strata shapes on structural defects was investigated, and a gradient pile foundation reinforcement technique was proposed based on this engineering case. The findings reveal that the primary manifestations of structural defects are various types of cracks, which developed in regard to the depth of the high-fill soil-rock mixture at the tunnel base, the self-settlement of the mixture, and the layer-by-layer backfilling at the tunnel vault. The differences between the model test and numerical simulation results are minimal, indicating that the shape of the soil-rock mixture stratum can significantly influence structural defects. The gradient pile reinforcement technology reduced tunnel settlement by up to 73.0%, while the foundation’s bearing capacity met the required standards, effectively controlling both tunnel settlement and structural defects. In terms of reinforcement for non-horizontal strata, gradient pile reinforcement technology demonstrates good adaptability, providing a feasible solution for similar engineering projects.