North China's first immersed tube undersea tunnel, the Dalian Bay Undersea Tunnel, opened to traffic on Monday in Dalian, Liaoning province. The 5.1-kilometer tunnel consists of 18 tubes, each weighing about 60,000 metric tons, and accommodates a two-way six-lane urban expressway.

The 7-kilometer road connecting the tunnel also opened on Monday, reducing travel time from one hour to five minutes and linking the less developed area in the north of the bay to the prosperous business area in the south.

"Ten years have passed since it was determined to be built with the immersed tube process in 2013. It has filled a number of industrial gaps and created a number of records in the field of cross-sea immersed tunnel construction," said Qian Qihu, an academician from the Chinese Academy of Engineering.

"In the past 10 years, the construction of China's submerged tube tunnel has changed from a 'following' to a 'staying abreast' and finally to a 'leading' position," he said.

Song Jiangwei, an engineer with the project builder from the First Harbor Engineering Co of China Communications Construction, said: "Local temperatures might drop below — 20 C in winter. Compared with the construction of the Hong Kong-Zhuhai-Macao Bridge, the geology is much more complex here.

"For example, precision underwater blasting was needed to blast away underwater rocks. But a new problem arises: Once the rock is cracked, the slope risks destabilizing after the leveling ship pitches the piles."

In order to ensure installation quality and avoid safety risks, the technical research team developed the full floating leveling process — to lay and level the gravel foundation bed in a full floating state without inserting or pulling out piles.

During the four years of construction, the project team overcame many difficulties, successfully cracked technical difficulties, realized the localization of some core equipment and technology, and applied a number of new structures and technologies.

"The project has formed core technology with independent intellectual property rights, and has strongly promoted the progress of China's offshore submerged tube tunnel construction technology. It has also boosted the country's strength in submerged tube tunnel construction," said Lin Ming, an academician from the Chinese Academy of Engineering.

As a new north-south expressway for the port city, it is expected to help alleviate traffic congestion and expand the space for urban development in Dalian.