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We all know that tunnels are essential to modern infrastructure, but what is the best way to drill a tunnel? The three defining methods are the Tunnel Boring Machine method, the Cut-and-Cover approach, and the New Austrian Tunneling Method. Each method holds a set of strengths and weaknesses that make it the better approach for certain applications.
In this article, we will discuss one method in particular: the new New Austrian Tunneling Method (NATM) and compare its merits to the others.
What is NATM?
NATM is both a construction method and a design philosophy. The philosophy looks to use the strength of the surrounding soil to the greatest extent possible to strengthen the tunnel structure. In other words, ground conditions drive the tunneling operation. The NATM philosophy also promotes constant monitoring.
The NATM construction method is about flexibility—drilling and designing depending on the results of the ongoing monitoring. The operation occurs sequentially to take most advantage of the ground conditions. Additionally, NATM installs ground support on the go and on an as-needed basis, adding reinforcement to the shotcrete where necessary. The final, permanent support is usually (but not always) a cast-in-place concrete lining placed over a waterproofing membrane.
NATM is best suited for short-range (> 2 km) tunnels in regions with variable soil conditions. Its philosophy and construction method yield a more cost-effective, flexible tunneling operation when compared with the other methods.
Comparing NATM with Cut-and-Cover
The Cut-and-Cover approach functions exactly as the name suggests. First, engineers dig a large ditch into the ground with room for the tunnel. After placing the tunnel, engineers cover the ditch with soil. This method is effective for building stations and other large segments of a tunnel with greater space requirements.
While Cut-and-Cover is very flexible, it significantly disrupts the ground surface making it ill-suited in dense urban environments or ecologically sensitive areas.
Comparing NATM with TBM
The Tunnel Boring Machine method has recently earned a lot of press with Elon Musk and the Boring Company, who look to challenge TBM conventions (decreasing tunnel diameters) and pump R&D into improved power, automation, and efficiency. TBM mobilizes a large mining operation around a machine that that drills the entire diameter of the tunnel concurrently. This approach has typically been very costly (designing for the worst-load case) and time-consuming.
However, once setup, a TBM operation can run indefinitely as the budget permits.
- “When and Why Choose NATM Over TBM and Cut & Cover Tunneling Techniques.” Accessed June 20, 2018. https://www.dr-sauer.com/faqs/when-and-why-choose-natm-over-tbm-and-cut-and-cover-tunnelling-techniques.
- “The New Austrian Tunneling Method (NATM) & shotcrete: growing side by side.” Accessed June 20, 2018. http://bestsupportunderground.com/natm-shotcrete/?lang=en.
- “New Austrian Tunneling Method (NATM).” Accessed June 20, 2018. http://www.railsystem.net/natm/.
- “Rekindled NATM debate – SCL debate opens.” Accessed June 20, 2018. https://www.tunneltalk.com/Discussion-Forum-Aug12-NATM-SCL-SEM-pros-cons-and-misunderstanding.php.