Abstract: River-crossing tunnels in mountainous cities face numerous challenges, including significant terrain elevation differences, complex geological conditions, and densely built urban environments. The authors examine the Yangtze river tunnel of the Chongqing-Hunan high-speed railway in Chongqing, China, focusing on the project′s topography, hydrogeology, and surrounding urban setting. A multidimensional technical comparison and innovative design process are undertaken to optimize the route alignment, excavation scheme, shield type selection, construction organization, operational drainage, maintenance, and ventilation. As a result, a key technological integration system is developed for river-crossing tunnels in mountainous cities. The findings are as follows: (1) The slurry balance shield effectively accommodates geological conditions of 0.9 MPa high water pressure and sandstone-mudstone interbedding, thereby preventing water and mud inrush and enhancing tunneling efficiency. (2) In-tunnel shield assembly and disassembly are implemented using launch and reception tunnels with diameters of 8.5 m and 4.0 m, respectively, successfully mitigating the risks associated with conventional ultra-deep shaft construction and reducing urban disruption. (3) A mushroom-shaped cross-section with an ultra-large area of 613 m² is adopted, and multi-section stepped excavation with composite support using rib-plate-anchor retaining walls ensures construction stability. (4) A segmented operational drainage system and an under-rail corridor ventilation layout are established, and slurry transport is conducted via shipping, addressing challenges related to high water pressure, seepage, and environmental protection in urban areas. Engineering practice confirms that the proposed integrated technology system effectively reduces construction risks and urban impact while improving the quality and efficiency of tunnel construction in mountainous city environments.

Key words: mountainous city environment, river-crossing tunnel, route scheme, shield type selection, in-tunnel shield assembly and disassembly, mushroom-shaped section