Mechanism and Design Optimization of Geosynthetic-Reinforced Subgrades for Highway Widening Based on an Improved Soil-Reinforcement Interface Model

Geogrid reinforcement is an effective subgrade treatment technique that plays a critical role in improving structural stability and controlling deformation in highway widening projects. In this study, the reinforcement mechanisms and performance of geosynthetic-reinforced embankments with varying heights were systematically investigated using finite element simulations conducted in ABAQUS. An improved nonlinear soil-reinforcement interface model was incorporated and implemented through a user-defined FRIC subroutine, allowing for a more accurate representation of nonlinear shear behavior at the soil-geosynthetic interface and providing deeper insight into the reinforcement mechanism within the subgrade structure. The results indicate that bottom-layer reinforcement offers the most significant improvement in overall stability and deformation control. Although multi-layer reinforcement configurations (top-middle-bottom or middle-bottom) further enhance performance, their additional benefits are limited for low embankments. Tensile strain within the reinforcement decreases with increasing distance from the existing slope, with the bottom geosynthetic layer exhibiting the most uniform strain distribution and playing a dominant role in settlement control. Considering both structural performance and reinforcement efficiency, a “sparse-top and dense-bottom” reinforcement configuration is recommended. Specifically, single bottom-layer reinforcement is suitable for embankments ≤ 3 m in height, double-layer reinforcement (bottom-middle) is optimal for embankments 3–7 m high, and triple-layer reinforcement (top-middle-bottom) is recommended for embankments exceeding 7 m, in combination with ground improvement, compaction control, and slope protection measures to ensure overall stability. The reinforcement optimization strategy proposed in this study provides a scientific basis and practical guidance for the structural design and performance enhancement of highway widening projects.