Trapdoor Model Tests on Multi-Layer Geosynthetic Reinforcement for Subsidence Mitigation

Localized subsidence remediation in narrow working areas is often constrained by limited reinforcement width and anchorage length. This study investigates the load-transfer and deformation behaviour of multi-layer geosynthetic reinforcement under such conditions through 1:5 scaled trapdoor tests and PFC2D simulations. A three-layer geotextile system was tested by varying anchorage length and interlayer spacing, and a soil arching load-transfer ratio, η, was introduced to quantify the contribution of soil arching. The results show that increasing anchorage length from 0.5B to B increased η from 74.82% to 86.51%, whereas a further increase to 1.5B raised η only slightly to 89.02%. Over the same range, surface settlement and the midspan deflection of the lowest reinforcement decreased by 26.84% and 16.46% from 0.5B to B, but only by a further 1.55% and 2.53% from B to 1.5B. At L = B, η reached 61.89%, 86.51%, and 78.75% for s = 0.3B, 0.2B, and 0.1B, respectively, indicating that the intermediate spacing was most favourable for soil arching load-transfer. The DEM results further showed diminishing returns with increasing layer number, with the three-layer configuration achieving about 95% of the five-layer performance. Overall, high and stable load-transfer performance was obtained at L/B = 1.0–1.25 and s/B = 0.15–0.2.