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Original map element courtesy of PytyCzech

Building on advances at similar tunnels in China, the Dalian Bay Undersea Tunnel team had to solve numerous technical challenges and adapt to cold site conditions to complete the first cross-sea immersed tube tunnel in the northern reaches of the country. To ensure safety and high quality production, the team says it developed many first-of-its-kind methods and technologies—and built the most advanced and highest production capacity dry dock for an immersed tube prefabrication yard in Asia.

The six-lane tunnel creates a new transportation artery, eliminating a lengthy C-shaped detour around the bay to get from north to south Dalian and cutting travel time from an hour to just 5 minutes.

Stretching more than 5 kilometers, the tunnel embeds onto a sea floor comprised of an uneven mix of rock and clay. Just over 3 km of immersed tube connects to a 1.5-km cut-and-cover section, 341 m of connecting road, and 250 m of open section.

The immersed tube portion of the tunnel consists of 18 reinforced concrete elements, including 13 straight 180-m-long elements and five 148-m-long curved elements.

South entrance to the tunnel threads in between existing structures.
Photo courtesy CCCC First Harbor Engineering Co. Ltd.

Frigid Sea

The approximately $1-billion project commenced in May 2019 and wrapped in April 2023. During the four winter seasons in the project’s duration, temperatures dropped as low as -6° F (-21° C). The conditions prompted specific adaptations for materials.

“The team conducted an investigation on offshore concrete structures ranging from 15 to 86 years naturally exposed in the cold sea areas of China,” says Sun Zhu, deputy chief engineer with CCCC First Harbor Engineering Co., the main contractor on the project. “Based on this research, a theoretical model was established to predict the 100-year service life of concrete structures in the marine environment of Dalian’s cold region.” As a result of the studies, the team utilized 400,000 tons of machine-made sand for the high-performance concrete adapted for use in a marine environment, which eliminated the environmental impacts of mining natural sand.

Five tubes feature a horizontal curvature required by the tunnel’s route. Each curved tube has 7 segments, each of which was formed by a continuous placement of concrete for the full cross-section.
Photo courtesy CCCC First Harbor Engineering Co. Ltd.

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Photo courtesy CCCC First Harbor Engineering Co. Ltd.

On-site Laboratory

To study other aspects of the construction and operation of the tunnel, the team established a laboratory at which more than 20 scientific and technological research and process experiments were performed, focusing mostly on the marine environment and foundations.

“Notably, we have investigated the propagation characteristics of underwater blasting vibration waves and their impact on surrounding existing structures,” Zhu says. “This includes determining the influence zones of underwater blasting at varying magnitudes. Based on these findings, protective measures were implemented to safeguard the previously installed immersed tube sections during the underwater rock foundation blasting construction.”

Crews prepped rock foundations for 13 of the tube elements, with the remaining elements sitting on clay, requiring soil replacement to maintain a smooth stiffness transition. The tunnel route had a maximum water depth of 32 m.

The traditional method to lay and level the riprap layer that the tubes would rest on employs spud barges that are firmly moored to either side of the foundation trench and then leveled. However, the team deemed it too risky because of karst caves, clay soils and cracking introduced into the rock from blasting. Instead, the team developed a method to allow the ship hull to float on the sea surface, stabilized by anchor cables, Zhu says. The team then developed advanced control systems for automatic positioning and hull leveling. “This method achieved a leveling accuracy of plus or minus four centimeters and demonstrated improved operational efficiency compared to the conventional procedure,” Zhu says.

BIM and other integrated technologies were used extensively, including modeling the immersed tubes and installation vessels.
Model Ccurtesy CCCC First Harbor Engineering Co. Ltd.

Tube Factory

Due to restrictions on the available real estate along both shores of Dalian Bay where the tunnel would terminate, the tunnel required a significant curve. As a result, the five curved tube sections were designed with a horizontal curve radius of 1,050 m, making it the tightest curve of any immersed tube tunnel in China, says Chen Zhengjie, design manager with Shanghai Tunnel Engineering & Rail Transit Design and Research Institute.

Each straight tube element, weighing in at 60,000 tons, is divided into 8 segments, while the curved elements are divided into 7 segments, for a total of 119 segments. In another first for China, the design specifies a “fully flexible segmental element scheme … which enables effective reduction in longitudinal forces on the element structure, and allows for better adaptation to uneven foundation settlement,” Zhengjie says.

Each segment prefabricated in the dry dock was formed by one-time pouring of the full cross-section to avoid adverse effects of construction joints.

“The segment joint can transmit compression, but not tension,” Zhengjie says. “Furthermore, the segment joint can transmit vertical and horizontal shear forces through the special shear bar distributed on the top slab, bottom slab, side wall and middle wall.”

Before each element was floated, it was tensioned using temporary pre-stressed cables to connect longitudinally all of the segments within the element. “After the element is sunk into position for a period of time, the temporary pre-stressed cable at the segment joint is cut to release the pre-stressing force, so that the segment joint becomes a flexible joint that can adapt to certain deformation,” Zhengjie adds.

The project team built the highest capacity dry dock in Asia, able to prefabricate, store and outfit six tubes at once.
Photo Courtesy CCCC First Harbor Engineering Co. Ltd.

All of the temporary pre-stressed cables for each element were cut as a batch, after ensuring that backfill coverage had been completed and settlement had stabilized. Crews used sensors to monitor any changes before and after the cutting to ensure settlement did not exceed 1 mm.

The team applied BIM, IoT and other digital platforms to monitor and guide the entire construction process, which will carry over into operations and maintenance. “We developed comprehensive models for the immersed tube structure, its outfitting components and the construction vessels using BIM,” Zhu says. A spectrum of other technologies, including measurement, guidance and control systems, pulling and joining systems, ballast water systems, underwater visualization and detailed weather forecasting became integrated into the overall platform.

Crews continually refined the prefabrication and installation process over the course of the project, to where they were able to install three immersed tubes in 40 days, and all 18 tubes in 20 months. For one of the tubes, installation took just 12 hours. The level of precision and speed of delivery helped save nearly $14 million, the team says.

“The technological research and development, as well as the preparatory work undertaken in the early stages, played a pivotal role” in the project, Zhu says. “It underscores the importance of advanced planning and prioritizing technology in engineering projects, particularly for large-scale undertakings.”

The project has received numerous awards in China for its sustainability, safety and IT and BIM applications. A total of 179 intellectual property rights have been authorized, including 43 invention patents, 132 utility model patents and four software copyrights, according to CCCC First Harbor.

“Our team will continue to explore advancements in intelligent construction, as well as in the areas of low carbon technologies and energy conservation,” Zhu says.