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 (NATM), which is also referred to as the Sequential Excavation Method (SEM) or simply conventional tunneling. 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 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, a large ditch is dug into the ground with room for the tunnel. After placing the tunnel, the ditch is covered 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 tunneling mobilizes a large mining operation around a machine that drills the entire diameter of the tunnel concurrently. This approach can be 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.
Continuous Monitoring and Design with NATM and AGRU
NATM is not perfect, needing much coordination, cooperation, and communication to compete with the other construction methods. And even at peak performance, NATM typically performs at a slower rate than the other methods as the team adapts to changing conditions. For NATM to compete successfully, the operation must reduce interruptions in excavation and support work by closely working with all elements of the operation to bring in the right supplies for the changing conditions.
Choosing the right supplier can significantly reduce delays by ensuring that enough waterproofing membrane, water stop profiles, geotextile, and other products are available on-demand as the need arises. AGRU America offers a range of waterproofing membranes in the form of the AGRUFLEX and EASYFIX systems, which takes advantage of engineered thermoplastics—flexibility, ductility, and corrosion-resistance—to deliver protection against water and aggressive soils.
Whether your next tunnel project uses the New Austrian Tunneling Method, Cut-and-Cover, or TBM methods, AGRU has an extensive portfolio of tunnel lining systems.
This article was submitted by AGRU America.
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- “New Austrian Tunneling Method (NATM).” Accessed June 20, 2018. http://www.railsystem.net/natm/.
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