LHASA, April 7 (Xinhua) -- Construction on the last two of the 47 tunnels on a 435-km railway linking Lhasa and Nyingchi in southwest China's Tibet Autonomous Region was completed Tuesday, marking huge progress for the mammoth project.
The Mainling Tunnel, located in the city of Nyingchi, is 11,560 meters long with an average elevation of 3,100 meters above sea level and a maximum burial depth of about 1,200 meters. It is believed to be one of the most difficult tunnels ever built.
The 8.7-km Zagar Tunnel in the city of Shannan traverses seven faults of complex geological structures, including one of the thickest water-rich moraine layers.
Though it was rainy on Tuesday morning, workers braved the high altitude anoxia and coldness and shouted happily when the construction of the Mainling Tunnel was officially completed.
"Rockbursts occurred in about 65 percent of the tunnel, or 7,500 meters, causing high safety risks," said He Xu, a director of the project. "During the construction of certain sections, the rocks burst like loaded guns, threatening the workers' lives."
Workers also frequently encountered noxious gases. A monitoring system was installed in the tunnel to carry out all-day automatic detection and alarming of sulfuretted hydrogen, methane and carbon monoxide. Ventilation devices were also installed to dilute harmful gases in the tunnel.
Yang Zeng, a chief engineer of the Zagar Tunnel, said it took over two years to overcome the engineering roadblocks brought by the 960-meter-thick water-rich moraine layer in the tunnel.
The completion of the two tunnels on the Lhasa-Nyingchi railway has increased the total tunnel length to 216.5 km, almost half of the railway length.
Meanwhile, 75 percent of the railway is bridges and tunnels, and over 90 percent of the railway is at over 3,000 meters above sea level on the Qinghai-Tibet Plateau.
Over 20,000 builders have been working on the railway since the end of 2014.
As of Tuesday, 119 bridges have been built, leaving just one bridge to be completed. About 115 km of tracks have been laid, and the rest will be completed by the year end.
As Tibet's first electric railway with a designed speed of 160 km per hour, it is expected to be completed and put into operation in 2021.