Study on Synergistic Settlement Control Method of Utility Tunnel in Thick Collapsible Loess: Combining Enlarged Foundation Base with Light Backfill

To address the collapsibility issue of utility tunnel foundations in thick collapsible loess, four types of enlarged foundation base and five light backfill schemes were selected. The variation laws of foundation settlement, foundation pressure, and structural stress under each scheme were investigated through numerical simulation, and a synergistic settlement control method combining enlarged foundation base with light backfill for utility tunnel was proposed. The results indicated that: adding a rigid bearing slab on the natural foundation significantly reduced the foundation pressure and its peak value; however, there exists an optimal width for the bearing slab, and a double-layer bearing slab more readily satisfied the requirements for settlement, deformation, and material strength. The foundation pressure under all working conditions was less than the site′s initial self-gravity stress. After implementing the enlarged foundation base and light backfill, the loess layer beneath the foundation was in an unloaded state. Among them, scheme 1 (EPS + A05 Foam Concrete) and scheme 4 (A03 + A05 Foam Concrete) exhibited the optimal pressure reduction effect, with the average foundation settlement controlled within 45.8-50 mm. The foundation pressure was reduced by 44.72%-61.77% compared to the initial self-gravity stress. Moreover, the stratum stress within the depth range of 6-13 m was significantly lower than the initial collapse pressure, indicating a low collapsibility risk for the loess within this depth range. Based on comprehensive analysis, a settlement allowable value of 50 mm is recommended for utility tunnels in collapsible loess regions, and scheme 1, scheme 4, type B (Natural Foundation + Bearing Slab), and type D (Natural Foundation + Double-layer Bearing Slab) are recommended as the optimal solutions.