Chinese workers building what would be the world's longest tunnel have come up against an unexpected problem in one of the driest areas on Earth - gushing streams of water.

That is according to engineers involved in the project in China's far western Xinjiang region, more than half of which is made up of vast expanses of arid desert.

"High groundwater levels have caused frequent water inrush accidents that seriously affected the construction schedule," said Deng Mingjiang, a professor with the Chinese Academy of Engineering, in a paper published in domestic peer-reviewed journal Tunnel Construction last month.

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Deng's team has been working on an ambitious project to introduce snowmelt from the Altai Mountains into the deserts of northern Xinjiang through deeply buried tunnels more than 500km (310 miles) long.

Kashuang tunnel, the longest of the three mega tunnels in the project, is expected to stretch for 280km (174 miles), more than twice as long as the Delaware Aqueduct - the main water supply tunnel of New York City - which has held the record since 1945.

But from time to time, the tunnel boring machines (TBMs) run into an unusually rich groundwater source that bursts out so forcefully that it could fill a swimming pool in an hour.

Every time the flooding alarm flashed red, the report said, workers would need to evacuate and the giant boring machine would be made to stop and pull back immediately or risk serious damage.

A wet zone would mean the TBMs could only manage to advance 200 metres (660 feet) per month, or about half the normal pace in a dry region like Xinjiang.

The frequent setbacks had threatened the safety of everyone on site, the paper said.

The Xinjiang project aims to drain water from upstream regions of the Irtysh river, which originates in China from glaciers in the Altai Mountains and flows north into the Arctic Ocean through Kazakhstan and Russia.

China's increasing use of Irtysh water has irked its two neighbouring countries.

However, according to a Beijing-based researcher studying the issue, who requested not to be named due to the issue's political sensitivity, the three countries have kept the dispute out of the public eye, focusing instead on some joint infrastructure projects under China's multi-nation Belt and Road Initiative.

These include hydropower plants and irrigation infrastructure for the entire Irtysh river basin, home to about 15 million people across the three nations and with good potential for economic development.

Authorities in Russia and Kazakhstan did not respond to the Post's request for comment.

A tunnelling project usually involves one or two TBMs. Also known as "moles", such machines represent one of the largest robotic devices in existence, with each costing tens of millions of US dollars.

The rotating blades of the machines can cut through nearly all types of hard rock, with onboard sensors adjusting operations automatically.

The Xinjiang water supply project had 20 moles working at different sites at the same time. Even in China, where large-scale infrastructure-building is the norm, such a fleet was a "rare sight to behold", Deng and his colleagues said in their paper.

However, unlike other major infrastructure projects in China, news on the Xinjiang tunnel has not appeared in any state media. Its route remains classified, and no official deadline has been announced.

By the end of June, two years since starting construction, nearly 60 per cent of tunnel works had been completed, according to the engineers.

However, the frequent flooding could cause serious delays. A 10km tunnel for India's northern Dul Hasti hydropower station, for instance, took 12 years to build - in part because of gushing water, according to Deng.

Not only are geological conditions in Xinjiang's deserts extremely complex, nearly half the estimates arrived at via geological surveys in the route-planning stage turned out to be wrong, according to the paper.

But the engineers came up with some new methods to deal with the unprecedented challenge.

Traditional ground penetrating radars can detect the presence of water in advance, but they work only when the tunnel boring machines stop.

Deng's team and their collaborators have developed a new type of seismic detector that can be mounted on the TBM itself and uses vibrations to detect water and other impediments behind the rocks.

So the tunnelling team was usually prepared when a leak occurred, activating a pump to drain the water out of the tunnel, after which another team would seal the leak.

But the mole could not move forward at all until the engineers had identified the precise source and volume of the water and found a way to divert the flow away from the tunnel, the paper from Deng and his team said.

It remains unclear how much water the tunnel would drain out from the Irtysh river.

But the second-largest river in Xinjiang could provide more than 11 billion cubic metres (2.9 trillion gallons) of snowmelt a year, according to official data.

Going by average water consumption levels in China, this amount would meet the needs of more than 20 million people, or the entire population of Xinjiang.

Xinjiang is China's largest provincial region, and more than 60 per cent of it is covered by the Gobi and other deserts. Urumqi, the regional capital, is further from the oceans than any other city worldwide.

But the arid region is becoming warmer and wetter because of global warming, Chinese scientists have noted.

Large-scale infrastructure construction in the region could suffer severe losses if it does not take into account climate change-related natural disasters such as flash floods, they warned in a recent study.

Researchers have also found that many deserts in Xinjiang used to be lakes, some of which existed until the emergence of the ancient Silk Road about 2,000 years ago.

In 2015, a team of Chinese researchers found geophysical evidence suggesting the existence of a massive body of water under Xinjiang's deserts.

According to their estimates, the volume of water under the Tarim basin alone could be as big as all five Great Lakes in North America combined.

This article originally appeared in the South China Morning Post (SCMP), the most authoritative voice reporting on China and Asia for more than a century. For more SCMP stories, please explore the SCMP app or visit the SCMP's Facebook and Twitter pages. Copyright © 2021 South China Morning Post Publishers Ltd. All rights reserved.

Copyright (c) 2021. South China Morning Post Publishers Ltd. All rights reserved.

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