Core Viewpoint - The successful completion of the Nanjing Metro Line 4 Phase II underwater tunnel marks a significant milestone in the construction of urban rail transit, achieving a record depth of 68.58 meters below the Yangtze River surface, enhancing the connectivity of Nanjing's cross-river transit network [1][3]. Group 1: Project Overview - The Nanjing Metro Line 4 Phase II project starts from Zhenzhuquan East Station and runs approximately 10 kilometers, featuring six underground stations [3]. - The underwater tunnel spans 3,062.6 meters and employs a single-tube double-track design, utilizing a large-diameter slurry balance shield tunneling machine with a diameter of 11.64 meters [3][4]. Group 2: Technical Challenges - The construction faced significant challenges, including traversing 2,200 meters of the Yangtze River's middle section, characterized by high water permeability and extreme underwater pressure of nearly 90 meters [4]. - The tunnel had to pass through 13 different geological strata, with the most challenging being a 788-meter section of moderately weathered mudstone, which has a quartz content of 90% and poses risks such as increased tool wear and reduced tunneling efficiency [4]. Group 3: Technological Innovations - The project utilized a custom-designed shield machine, "Xin Zheng Cheng Hao," equipped with advanced systems for pressure balance and high-pressure flushing, enhancing cutting efficiency and tool lifespan [5]. - The application of synchronous double-liquid slurry technology was a first in domestic metro underwater tunnels, effectively controlling the segment uplift to within 5 millimeters and ensuring high-quality tunnel formation with no leakage [6].

Core Viewpoint - The successful completion of the Nanjing Metro Line 4 Phase II underwater tunnel marks a significant advancement in urban transportation infrastructure, enhancing connectivity across key districts in the city [1][3]. Group 1: Project Overview - The total length of the Line 4 Phase II project is approximately 10 kilometers, starting from Zhenzhuquan East Station and extending eastward, ultimately connecting with the first phase at Longjiang Station [3]. - The underwater tunnel section, measuring 3062.6 meters, is a critical control node for the entire line and is part of the strategic planning for the Yangtze River's crossing channels [3]. - The tunnel features a long-distance crossing of 2200 meters through the main section of the Yangtze River, with the lowest point reaching 68.58 meters below the water surface, setting a record for the deepest underwater metro tunnel in the Yangtze River Delta region [3]. Group 2: Impact on Urban Transportation - This is the third underwater metro tunnel completed in Nanjing, which will further enhance the urban rail transit network and significantly improve cross-river travel efficiency for residents along the route [5].