Gushing water causes delays to China’s longest tunnel21 December 2021
Progress on what will become China’s longest tunnel – and the longest in the world – is being hampered by strong gushing water, despite being located in one of the driest regions on Earth, reported The South China Morning Post.
The ambitious Xinjiang water supply project aims to convey meltwater from the Altai Mountains to the northern Xinjiang region – of which more than 60% is taken up by the Gobi Desert. However, high groundwater levels are causing strong and frequent inrushes of water said to be able to fill a swimming pool in an hour. These threaten to seriously hamper excavation progress, according to Deng Mingjiang, a professor with the Chinese Academy of Engineering.
By the end of June 2021, after two years of construction, nearly 60% of tunnel works had been completed, reported the engineers. But when inflows occur, TBMs must be stopped and pulled back to avoid serious damage. Workers must also evacuate the tunnel. TBM advance rates in the wet areas have therefore been affected, reduced to only 200m/month – roughly half of what would be expected in the drier areas of Xinjiang.
Xinjiang’s deserts are said to have ‘extremely complex’ geologies and, to make matters worse, many of the estimates derived from geological surveys turned out to be wrong, according to a paper written by Mingjiang. Although GPR can detect the presence of water in front of the TBM, it typically does so when the machine is stopped. To deal with the challenge, Chinese engineers have come up with a new type of seismic detector that is mounted on the TBM and uses vibrations to detect water and other impediments behind the tunnel face.
A total of 20 TBMs have worked simultaneously on the Xinjiang water supply project’s network of deep tunnels. The longest of the three mega tunnels is the Kashuang, which at 280km will be more than twice as long as New York’s Delaware Aqueduct which has held the record for tunnel length since 1945.