Study on the Settlement Characteristics and Treatment Methods of the Foundation for Utility Tunnel in Collapsible Loess Area

To explore economically reasonable seepage control measures at the excavation-filling interface and foundation treatment methods for utility tunnels in collapsible loess areas. The infiltration and collapsibility characteristics of the combined site with three types of interface control infiltration and five types of foundation treatment measures were studied through on-site immersion tests of the utility tunnels in this study. The results show that the seepage control measures at the excavation-filling interface can mitigate concentrated seepage and delay the onset of foundation wetting. Surface compaction proves to be the most cost-effective method for seepage control. The model of foundation settlement can be described as an approximate linear and hyperbolic function segments. The h_ult is the ultimate settlement of the combination of 5 types of utility tunnel sites, which can be used as the allowable settlement values for the design of utility tunnel and pipelines. The deformation rate ’a’ of the foundation, caused by the upper compacted soil wetting, exhibits an approximately linear correlation with the h_c of the foundation treatment by vibration compaction. A larger h_c suggests more severe damage to the original loess beneath the foundation and a greater measured settlement of the utility tunnel, indicating that vibration compaction is unsuitable for utility tunnel foundation treatment. The thin cushion layer significantly affects settlement control, while the bentonite waterproof blanket effectively reduces differential settlement and provides foundation waterproofing. Consequently, the combined methods of "surface compaction for seepage control at the excavation-filling interface, foundation anti-seepage using geotechnical materials, and either a thin cushion layer or overturning compaction" is the most cost-effective in controlling the deformation of utility tunnel foundations in collapsible loess areas. The research results offer theoretical support for the foundation treatment of utility tunnels in collapsible loess sites.