Control Mechanism of Large Deformation in Soft Rock Tunnel Based on Polyurethane Foam Buffer Layer

ABSTRACT Large deformations of the surrounding rock often occur in tunnels excavated in soft rocks with high geostress, which can lead to the destruction of the lining structure. Therefore, the adoption of an appropriate support structure to control soft rock deformation is particularly important to ensure the safety of the tunnel in the preconstruction and post‐operation. In this paper, a yielding support system containing polyurethane (PU) foam energy‐absorbing buffer layer is proposed, and based on the yielding support mechanism, combined with numerical simulation and model experiments, the thickness of the energy‐absorbing buffer layer and the location of the arrangement are specifically studied. The findings indicate that the yielding layer, with a PU foam energy‐absorbing buffer layer, exhibits a substantial yielding phase compared to a rigid support structure. The strain‐dependent adjustment of the elastic modulus method can better simulate the stress characteristics of the yielding layer in numerical calculations than the case of a constant modulus of elasticity. In this study, the optimum thickness of the PU foam energy‐absorbing buffer layer was 20 cm, and the optimum location was between the initial support structure and the secondary lining structure. Scale model testing further corroborates these results, demonstrating that the rigid support system's secondary lining structure develops cracks at the arched roof and elevated arch locations under a 2.0‐MPa load. In contrast, the yielding support system absorbs energy from surrounding rock deformation through compression, maintaining stability even under an 8.0‐MPa load, thereby enhancing the overall performance of the support structure system. The scale model test analysis serves as additional validation for the numerical simulation results.