The New Australian Tunnelling Method (NATM) has been found in recent studies and trials to be extremely risky, even for highly professional construction companies exercising extreme care when digging. Despite such obvious risks, the China Civil Engineering Construction Corporation (CCECC) apparently intends to use the NATM method in future projects. Though the stated advantage to the method, according to some, is less traffic hold-up for the construction site and its vicinity, it is quite clear that the risks and drawbacks out way its advantages.
These risks include polluted aquifers (often requiring costly clean-up projects), dangers to workers and passengers as a result of collapsed tunnels and stations (also requiring costly remedial construction), and at worst, passenger and worker fatalities. It is clear that the risks of using the NATM method are ultimately costlier and more dangerous than minimized traffic jams justify, which are unavoidable even with the NATM, since trucks are required to remove debris when using such a system.
The NATM method, considered a "deep mining" method, is used for digging underground stations and tunnels for trains and rails in many cases. Using the deep mining method on loose soil, where deep mining (30 meters approximately) is required comes with even greater risk. Such digging can lead to collapse, endangering on-site workers at the time of construction, as well as those standing or driving above the area dug, and naturally, passengers using the tunnels and stations dug with the risky method.
The NATM method has had disastrous consequences in many locations where it has been used. In 1994, a tunnel under construction using NATM at the Heathrow Airport collapsed and continued to fail on several occasions, bringing chaos to the heart of the airport. The Heathrow disaster also led to the suspension of tunnelling on two London Jubilee Line Extension projects, London Bridge and Waterloo, raising fears about the stability of the ground during any delay. Like the Heathrow tunnels, both passed substantially through London clay and involved the use of dry mix shotcrete. Work on both tunnels was halted by London Underground engineers within hours of the first slippage occurring at Heathrow Airport, whose restoration took some two years. Furthermore, the cost of the near lethal incident stood at approximately £ 150 million. Some say it was miraculous that there were no fatalities in the incident.
The same method of construction was used at the Washington Dulles Airport in 1999 in the United States and in Germany in 1994 at the Munich Metro, this time, resulting in fatal consequences. When the crew for the 770-ft-long underground tunnel for a moving walkway connecting the airport's main terminal with a nearby concourse was only about 50 feet from being completed, the tunnel collapsed, trapping a worker inside. Despite more than 24 hours of rescue efforts, workers could not reached the trapped man, whose body was later recovered from the rubble of the collapsed tunnel.
Another collapse using the NATM method occurred on the Taiwan High Speed Rail Project, connecting the cities of Taipei and Kaohsiung in 2002. One of the nine shallow mined tunnels collapsed with little warning, affecting nearly 50 km of the tunnel. Luckily, the crew of the Taiwan project was evacuated shortly before the collapse, thus no injuries or deaths occurred.
According to studies, the dangers seem to arise despite careful execution of the tunnel method, as many complications occur as a result of unexpected water conditions, including heavy rainfall which can infiltrate the soil. Tunnelling in a water bearing soil can result in a long-term interaction between the soil surrounding the tunnel and the tunnel lining. Infiltration of such moisture and external pore-water pressures can therefore be problematic and possibly lead to tunnel collapse. Furthermore, pore-water pressure can potentially accelerate leakage and cause deterioration of the tunnel's lining, thereby weakening it. This can be particularly troublesome to structural and functional components of the tunnel, and can possibly lead to structural failure.
A further, yes not less serious, cause for concern with the NATM deep mining method is that most underground stations and tunnels require digging beneath the water table of construction sites. Doing so has several negative consequences, especially in areas which have unexpected heavy rainfall or which are situated near large bodies of water. Such digging requires the pumping of more than 20 million cubic meterswater, as compared to less than 1 m3 for the "cut and cover" method often used as a safer alternative to NATM.
Reaching the water table has clear risks for the quality of underground water supplies in and around construction sites. Since in cases where soil is loose, relatively deep digging is required to minimize the risk of collapse, underground water tables are oftentimes reached and require pumping. The risk then exists that any nearby sea water may floods the area, making it impossible to pump. According to several studies, a serious risk exist using such a method including pollution of aquafer water from water already polluted (used water), and the mixing of salt water from the sea or ocean with aquifer water. Such a situation can be at worst irreversable and and at best, a rehabilitation project requiring staggering costs to clean polluted aquifer water.
Considering the obvious risks and proven dangers as a result of using the NATM method, it is therefore used only when other methods are not options. This is especially true when viable alternatives such as the "cut and cover" method now come with only a slightly higher price tag than the NATM method. Considering such facts, it remains unclear, therefore, why firms such as the CCECC continue to utilize the dangerous and costly method in their proposed projects when other, safer methods are readily available.
