With a length of 6.8 kilometers and a width of 46 meters, the two-way eight-lane tunnel will be the largest and widest in the world upon completion. Its tunnel will be 1.2 meters longer and two lanes wider than the HZMB.

Yang said the immersed tunnel used the steel shell tube, the first of its kind in China, and there were many other new structures and technologies, which brought lots of challenges, such as lack of technical standards in the country.

He said the tunnel will have 32 tubes and each tube will have about 10,000 tons of steel, 165 meters long and with a displacement of about 80,000 tons. Each tube has more than 2,500 separate chambers.

After tubes are built in factory, they are floated to the construction site, and the most difficult process is to sink and connect, Yang said. His company has invited experts to deliberate in rounds on the cement pouring, sealing and connection.

The Shenzhen-Zhongshan Bridge is another important infrastructure project of the country's 13th Five-Year Plan (2016-2020). The construction started on Dec 29, 2016. It is expected to put into operation in 2024.