At 10:21 on January 10, the subsea tunnel on the section from Dalian Metro Line 5's Railway Station Station to Suoyuwan South Station achieved safe breakthrough. Invested in, constructed and managed by China Railway North Investment, surveyed by China Railway Bridge Survey and Design Institute, and built by China Railway First Group, it is the world's first large-diameter shield subsea tunnel in karst strata, China's longest large-diameter shield tunnel in hard rock strata, and China's largest-diameter metro subsea tunnel.

This marks China's successful conquest of the "world-class challenge" of large-diameter shield tunneling beneath the sea through karst geology, accumulating successful experience for global "river-crossing and sea-crossing" tunnels involving subsea karst strata and long-distance hard-rock large-diameter shield construction.

▲During this year's New Year's Day holiday, CCTV produced a special report on the sea-crossing project of Dalian Metro Line 5▲

It is understood that Dalian Metro Line 5 has a total length of 24.484 kilometers and 18 stations. The large-diameter shield section from Railway Station Station to Suoyuwan South Station, a controlling project of the entire line, is 2,882 meters long, with 2,310 meters passing beneath the waters of Dalian's Suoyuwan Bay.

Four Major Challenges, None Easy

The section was launched on January 18, 2019. During sea-crossing construction, it faced four major challenges—"long, large, high and risky"—which brought unpredictable difficulties to construction and were characterized by industry experts as an unprecedented "world-class challenge." "Long":

The tunneling distance in high-strength hard rock strata is long, with the shield required to continuously pass through 2,882 meters of hard rock strata.

"Large":

High environmental risk. The large-diameter shield passes beneath important structures and traffic facilities such as navigation channels and wharves, encountering weathered troughs, fault zones and mixed hard-soft strata, with limestone, diabase and calcareous slate widely distributed. Rock compressive strength is high, with a maximum of 118 MPa, posing major safety risks.

"High":

Great tunnel burial depth and high water pressure, with a maximum water head pressure of 0.5 MPa, equivalent to the pressure of three people standing on an area the size of an egg.

"Risky":

The tunnel passes through 1,538 meters of an intensely developed karst zone. Exploration found more than 1,000 karst caves of various sizes, with the largest 29.8 meters high. Detecting and handling the caves is extremely difficult and risky, and shield tunneling must proceed as cautiously as "mine clearance."

Major National Equipment Deployed

Since the start of construction, China Railway has actively implemented General Secretary Xi Jinping's important instruction on the "three transformations," carefully explored a new "five-in-one" project construction management model integrating design, general contracting, construction, equipment and manufacturing, and made every effort to solve the "world-class challenge" of long, large underground tunnels beneath deep water.

China Railway First Group and China Railway Industry jointly developed "Haihong," the world's most functionally complete and technologically advanced shield machine (a 12.26-meter-diameter air-cushion slurry pressure-balance shield machine) for sea-crossing construction. They also pursued technological and management innovation, building an intelligent information management system to guide on-site construction in real time. In particular, in response to high rock strength and severe cutter wear during tunneling in karst strata, the builders developed a shield cutter-change system with analytical functions.

▲"Haihong" large-diameter slurry shield machine▲

Scientific Research Safeguards Construction

The builders also developed and applied information technologies including a BIM+GIS three-dimensional dynamic simulation management platform, a comprehensive safety and quality information control platform, a remote monitoring and intelligent decision support system for the shield cluster, and a tunnel alignment control system using a segment arrangement line inversion algorithm, ensuring the safe and smooth passage of the large-diameter shield through the karst cave cluster.

At the same time, the project department established the innovation studio of national model worker Liang Xijun, vigorously carried out technical research, and overcame risk sources including the large-diameter shield passing through the karst development zone beneath the sea, shield sealing safety, high-pressure hyperbaric chamber operations, cutter management for long-distance hard-rock tunneling, and settlement control for structures beneath the large-diameter shield. Construction proceeded safely and smoothly, achieving a domestic record of 212 meters of monthly tunneling in long-distance hard-rock strata beneath the sea, and ultimately bringing the "super sea-crossing" to a successful conclusion. After completion, Dalian Metro Line 5 will form a north-south urban express corridor, shortening travel time between Suoyuwan and the Railway Station from more than 30 minutes to less than 3 minutes, effectively easing existing traffic pressure and creating a new coastal urban pattern of "one bay, two shores."