Evolution characteristics of seepage-stress fields and optimization of waterproof-drainage system during the operation period of urban small interval tunnels

The long-term structural safety issues of water-rich tunnels in complex hydrogeological conditions during operation period and their mutual interaction mechanism with groundwater environment are receiving increasing attention. To study the evolution process of seepage and stress field of urban small interval tunnels and propose reasonable waterproof-drainage methods, seepage model tests and numerical simulation studies are carried out based on the Kexuecheng Tunnel in Chongqing City. The evolution process of water and earth pressure on the lining structure is revealed. Subsequently, a proactive control technology for groundwater environmental protection based on collaborative regulation of drainage and structural safety is proposed. The results indicate that the precipitation funnels formed by the left and right holes of urban small interval tunnels partially overlap and influence each other in space. The water pressure exhibits a W-shaped distribution in the horizontal direction, gradually evolving into a concave arc shape as the burial depth increases or decreases. The distribution of earth and water pressure in small interval tunnels shows significant asymmetry. The asymmetry of water pressure weakens with the increase of groundwater level, while the asymmetry of earth pressure is the opposite. The influence range of the seepage field and stress field for a small interval tunnel in water-rich environments is up to 80~120 m. The grouting circle has good water blocking and pressure bearing capacity, and can share 20~40% of the earth and water load. But the reduction effect of water pressure and surrounding rock pressure weakens with the increase of water head height. Under heavy rainfall conditions, the evolution process of water and earth pressure on the lining can be divided into three stages: initial infiltration phase, rapid growth phase and gradual stability phase. The secondary lining cracking and water inrush disasters caused by high water pressure are mainly concentrated at the haunch, arch foot and tunnel bottom. For high water pressure sections of urban small interval tunnels, it is recommended to add drainage blind pipes at the inverted arch. In ecologically sensitive areas, the active regulation technology should be adopted for groundwater discharge in tunnels to protect the ecological balance of groundwater environment.