QINGDAO - Construction of the longest undersea subway tunnel in China was completed Monday in Qingdao, a coastal city in Shandong province.

The 5.4-km-long tunnel, between the stations of Dayang and Qingdao North, is part of the Subway Line No 8 project in Qingdao.

The eastern side of the tunnel was dug by the slurry shield method, while the western side by the mining method, said Wang Zili, chief engineer of the project with the China Railway Group, the company responsible for the construction.

Multiple measures, including the utilization of a robotic arm, were adopted to avoid water and mud bursts, Wang said.

The tunnel has surpassed the cross-sea tunnel of the Qingdao's Subway Line No. 1, which has 3.49 km of the line undersea, becoming the longest undersea subway tunnel in China, according to the China Railway Group and the Qingdao Metro Group, operator of the subway lines.

The Subway Line No 8, which will run 61.4 km with 18 stations designed, will connect the international airport of Jiaodong and railway stations of Jiaozhou North, Hongdao and Qingdao North, linking the city proper of Qingdao, the economic zone of Hongdao and the city of Jiaozhou.