NATM Tunneling Method Overview

New Austrian tunneling method?(NATM), also known as the?sequential excavation method?(SEM) or?sprayed concrete lining method?(SCL), is a method of modern?tunnel?design and construction employing sophisticated monitoring to optimize various wall reinforcement techniques based on the type of rock encountered as tunneling progresses. This technique first gained attention in the 1960s based on the work of?Ladislaus von Rabcewicz,?Leopold Müller, and?Franz Pacher?between 1957 and 1965 in Austria. The name NATM was intended to distinguish it from earlier methods, with its economic advantage of employing inherent geological strength available in the surrounding?rock?mass to stabilize the tunnel wherever possible rather than reinforcing the entire tunnel.

NATM/SEM is generally thought to have helped revolutionize the modern tunneling industry. Many modern tunnels have used this excavation technique.

The works built by the Sequential Excavation Method are very attractive from the economic point of view and reasonable in karst condition.

The NATM integrates the principles of the behavior of rock masses under load and monitoring the performance of underground construction during construction. The NATM has often been referred to as a "design as you go" approach, by providing an optimized support based on observed ground conditions. More correctly it can be described as a "design as you monitor" approach, based on observed convergence and divergence in the lining and mapping of prevailing rock conditions. It is not a set of specific excavation and support techniques.

NATM has seven elements:

· Exploitation of the strength of native rock mass?– Relies on the inherent strength of the surrounding rock mass being conserved as the main component of tunnel support. Primary support is directed to enable the rock to support itself.

· Shot Crete?protection?– Loosening and excessive rock?deformation?must be minimized. This is achieved by applying a thin layer of shot Crete immediately after face advance.

· Measurement and monitoring?– Potential deformations of the excavation must be carefully monitored. NATM requires installation of sophisticated measurement instrumentation. It is embedded in lining, ground, and?boreholes. In the event of observed movements, additional supports are installed only when needed, with a resultant overall economy to the total cost of the project.

· Flexible support?– The primary lining is thin and reflects recent?strata?conditions. Active rather than passive support is used and the tunnel is strengthened by a flexible combination of?rock bolts, wire mesh and steel ribs, not by a thicker concrete lining.

· Closing of the?invert?– Especially crucial in soft ground, the quick closing of the invert (the bottom portion of the tunnel) which creates a load-bearing ring is important, and has the advantage of engaging the inherent strength of the rock mass surrounding the tunnel.

· Contractual arrangements?– Since the NATM is based on monitoring measurements, changes in support and construction method are possible, but only if the contractual system enables them.

·?Rock mass classification, ranging from very hard to very soft, determines the minimum support measures required and avoids economic waste that comes from needlessly strong support measures. Support system designs exist for each of the main rock classes. These serve as the guidelines for tunnel reinforcement.

Based on the computation of the optimal?cross section, only a thin shot Crete protection is necessary. It is applied immediately behind the excavated tunnel face to create a natural load-bearing ring and minimize the rock's deformation.?Geotechnical?instruments are installed to measure the later deformation of?excavation. Monitoring of the stress distribution within the rock is possible. This monitoring makes the method very flexible, even if teams encounter unexpected changes in the?geotechnical?rock consistency, e.g. by?crevices?or?pit water.?Reinforcement?is done by?wired?concrete that can be combined with steel ribs or lug?bolts, not with thicker shot Crete. The measured rock properties suggest the appropriate?tools?for tunnel strengthening, where support requirements can traditionally be estimated using the RMR or Q System.?Since the turn of the 21st century, NATM has been used for soft ground excavations and making tunnels in porous?sediments. NATM enables immediate adjustments in the construction details, but requires a flexible contractual system to support such changes.

Key features of the NATM design philosophy are:

·?The?strength?of the ground around a tunnel is deliberately mobilized to the maximum extent possible.

· Mobilization of ground strength is achieved by allowing controlled deformation of the ground.

· Initial primary support is installed having?load-deformation characteristics appropriate to the ground conditions, and installation is timed with respect to ground deformations.

·?Instrumentation?is installed to monitor deformations in the initial support system, as well as to form the basis of varying the initial support design and the sequence of?excavation.

When NATM is seen as a construction method, the key features are:

· The tunnel is sequentially excavated and supported, and the excavation sequences can be varied to address the specific rock conditions being encountered efficiently.

· The initial ground support is provided by shot Crete in combination with fiber or welded-wire fabric reinforcement, steel arches (usually lattice girders), and sometimes ground reinforcement (e.g. soil nails,?spilling).

· The permanent support is typically a cast-in-place concrete lining placed over a waterproofing membrane.

·?There is a quick closure of the invert, that is, the bottom part of the tunnel, to create a structural ring that takes advantage of the rock or soil arc naturally created on the top part of the tunnel section