What is the core principle of NATM?

To use the inherent strength of the surrounding rock/soil mass as the primary structural component by applying a thin, flexible support system shortly after excavation to control deformation. (Clause 4)

What are the key support elements in NATM?

The primary support elements are shotcrete, rock bolts, and sometimes steel ribs or lattice girders, which work together with the ground to form a composite load-bearing ring. (Clause 7)

Why is monitoring so important in NATM?

Monitoring (e.g., convergence measurements) provides real-time data on ground deformation, allowing engineers to verify the design, ensure stability, and adjust support measures as needed. It is the cornerstone of the observational approach. (Clause 9)

What is a 'round length'?

The length of tunnel excavated in one cycle of drilling, blasting, and mucking. Its length is determined based on the stand-up time and stability of the ground. (Clause 8.3)

IS 15124

: 2002

Guidelines for construction of underground structures using New Austrian Tunneling Method (NATM)

CurrentSpecializedGuidelinesBIMGeotechnical · Tunneling and Underground Structures

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This standard provides guidelines for the design and construction of tunnels and other underground structures using the New Austrian Tunneling Method (NATM). It covers the core principles, including the use of observational data, sequential excavation, and flexible support systems like shotcrete and rock bolts to mobilize the strength of the surrounding ground mass.

Provides guidelines for the design and construction of underground structures, including tunnels, using the New Austrian Tunneling Method (NATM).

Quick Reference Values

Initial shotcrete layer thickness50-75 mm

Minimum 28-day compressive strength for shotcrete30 MPa

Typical rock bolt spacing (moderate rock)1.5 m x 1.5 m

Maximum allowable convergence limit1-2% of tunnel span

Typical round length in fair rock1.5 - 3.0 m

Monitoring station distance from face2-3 m

Practical Notes

The success of NATM is critically dependent on high-quality, real-time monitoring and the ability to adapt the support system based on observed ground behavior.

Contractual frameworks must be flexible (e.g., using rock classes and item rates) to accommodate changes in support requirements encountered during excavation, which is inherent to the NATM philosophy.

Proper and timely installation of the initial shotcrete layer is crucial to limit ground deformation and ensure early mobilization of ground support.