A photo shows the entrance to the Gokarla Tunnel, a key section of the Lhasa-Tsethang Highway, in the Tibet autonomous region. [Screengrab/CCTV News app]

After seven years of construction, work on the world's longest high-altitude road tunnel was completed on Wednesday in the Tibet autonomous region.

Connecting Dechen township of Dagze district of Lhasa, and Samye township of Zhanang county in the region's Lhokha city, the Gokarla Tunnel is a key section of the Lhasa-Tsethang Highway. The highway is one of the important measures in creating a one-hour economic circle in the central part of the region, according to China Communications Construction Company, which built the project.

The altitudes at the entrances of the tunnel are 4,248 meters and 4,117 meters above sea level. With one lane of the tunnel stretching 12,790 meters and the other 12,780 meters, the tunnel has been hailed as the world's longest at such an altitude, according to the company.

Zhou Xiaobing, a leader of the project, said the average altitude of the tunnel is 4,300 meters, and the geological conditions along the route are extremely complex.

"During construction, we faced difficulties in terms of the sheer length of the tunnel, ventilation, large inclines, lack of oxygen and frigid cold weather," said Zhou, who added that excessive soil moisture and dangerous gases also proved hazardous.

Zhou said that if the project were not located in the mountains it should have only taken a maximum of four years to complete, but when the average altitude is above 4,000 meters, things get complicated.

To cope with the construction difficulties, the workers used advanced equipment such as three-arm drilling trolleys, wet spraying manipulators, electric loaders and electric excavators.