On November 27, the TJ-04 section of the Second Jiaozhou Bay Subsea Tunnel in Qingdao, Shandong Province, currently under construction, was successfully advanced to its maximum design depth—115 meters below sea level, which is also the deepest position known to have been reached in domestic subsea construction.

It is understood that the service tunnel works of the TJ-04 section run from mileage FWK8+795 to FWK13+967, with a total length of 5,172 meters. During construction it serves as an important construction access passage, and after operation it will serve as an emergency firefighting and daily maintenance and inspection passage as well as a cross-sea pipeline corridor.

Wang Junjie, project manager of the Jiaozhou Bay Second Tunnel project of China Railway No. 2 Engineering Group, introduced that the service tunnel is designed to be constructed entirely by the drill-and-blast method, designed as a two-lane tunnel with a standard excavation cross-section about 8.6 m wide and about 8.6 m high. The design geology is dominated by granite and diabase, with well-developed joints and fissures in the rock mass and multiple fault fracture zones. The deepest point of the tunnel floor is 115 m below sea level. The construction geology is complex and the construction risk is high. It is a key schedule-control line for the construction of the Jiaozhou Bay Second Tunnel and is a high-risk tunnel control project of the group company.

Since construction began in September 2022, the project has carried out careful organization and planning, continuously optimized the construction scheme, effectively increased the input of manpower and equipment resources, successively organized capable and efficient management teams and strong construction crews to mobilize on site, promptly established a dedicated grouting and water-stopping task force, strengthened technical research on construction of a world-class subsea tunnel, adopted multiple technical measures for effective organization and management, overcame construction difficulties, and ensured smooth progress of construction.

In response to the complex geological conditions, technologies such as SAP, transient electromagnetic surveying, advanced drilling, and borehole imaging were used to detect adverse geology and groundwater conditions ahead through multiple levels and means. In response to the actual conditions of many water-rich sections, large water volumes, and high water-plugging standards, a dynamic grouting control system was established. Based on borehole water volume, water pressure, and water inflow locations, the grouting scheme was optimized in real time, and combined with technologies such as rapid sealing of the borehole opening with membrane bags, the efficiency of advanced curtain grouting was improved.

To address the difficulty of long-distance single-head ventilation, the project adopted high-power intelligent fans, large-diameter air ducts, and intelligent air distribution valves. At the same time, it was equipped with high-head, high-power water pumps and high-pressure-resistant drainage pipes, and drainage was organized in two stages to solve the difficulty of long-distance, large-slope reverse-slope pumping and drainage. The tunnel successfully passed through the F9-F11 fault zones, and construction smoothly reached the deepest point of 115 meters below sea level. At present, it has fully entered the construction of the subsea pump room.

The main line of the Second Jiaozhou Bay Subsea Tunnel project is about 17.476 km long (including 3,681 m in the Huangdao land section), and the main tunnel has a total length of 14.376 km (9,983 m in the sea section + 4,393 m in the land section). It is the world's largest subsea highway tunnel under construction and is positioned as a two-way, six-lane urban expressway focusing on passenger transport while also accommodating small and medium-sized freight. The project is of great significance for optimizing the urban spatial structure, promoting the integrated development of the east and west banks of Jiaozhou Bay as one city, and alleviating traffic pressure. It will enhance the overall competitiveness of the Jiaodong Peninsula urban agglomeration and help Qingdao achieve higher-level development.