The map of the to-be-built Ningbo-Zhoushan railway with an undersea tunnel shows the route between Ningbo and Zhoushan. (Photo provided to chinadaily.com.cn)

The technical proposal for the Ningbo-Zhoushan Railway, China's first high-speed railway line with an undersea segment, has been completed, the Ningbo Evening News reported on Saturday.

After two years' preparation, the core technical proposal for the project's 16.2-kilometer-long undersea tunnel, the longest in the country, passed assessment by a panel of experts headed by Qian Qihu, an academician from the Chinese Academy of Engineering, on Thursday.

Connecting the Zhoushan Islands in Zhejiang province with the mainland, the 77-km Ningbo-Zhoushan railway line will run through the tunnel and over several bridges from Ningbo to Zhoushan via Jintang Island.

The project will allow high-speed trains and vehicles to travel across the sea via separate tunnels.

Unlike the 6.7-km immersed tube tunnel that is part of the Hong Kong-Zhuhai-Macao Bridge, the undersea tunnel on the Ningbo-Zhoushan railway line will be built using a shield and be deeper, creating multiple difficulties for the construction crew.

At a maximum optimal depth of 78 meters and with a diameter of 14 meters, the undersea tunnel's shield will be 10.87 km long upon completion, setting a new world record, according to Tang Xiongjun from China Railway Siyuan Survey and Design Group, who is in charge of the undersea tunnel project.

When it is completed, trains traveling at 250 km/h will be able to go from Ningbo to Zhoushan in just 30 minutes. Driving between the two places, made possible by a series of bridges, currently takes about three times as long.

Trains using the tunnel will be able to travel from Hangzhou, the capital of Zhejiang province, to Zhoushan in one hour and 20 minutes.

Officials said construction of the line is likely to start this year, with the project to take six to seven years to complete.