New Austrian tunneling method

Last updated November 17, 2024

The new Austrian tunneling method (NATM), also known as the sequential excavation method (SEM) or sprayed concrete lining method^[1] (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.^[2]

Principles

The NATM integrates the principles of the behaviour 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.
Shotcrete protection – Loosening and excessive rock deformation must be minimised. This is achieved by applying a thin layer of shotcrete 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 shotcrete 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 geomechanical 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 shotcrete.

The measured rock properties suggest the appropriate tools for tunnel strengthening, where support requirements can traditionally be estimated using the RMR or Q System.^[4] 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.

Variant names

NATM was originally developed for use in the Alps, where tunnels are commonly excavated at depth and in high in situ stress conditions. The principles of NATM are fundamental to modern-day tunnelling, and NATM fundamentally involves specifically addressing the specific soil conditions being encountered. Most city tunnels are built at shallow depth and do not need to control the release of in situ stress, as was the case with the original NATM in the Alps. Projects in cities place a higher priority on minimizing settlement and so tend to use different support methods from the original NATM. That has led to a confusion in terminology in that tunnelling engineers use "NATM" to mean different things.

New terms have arisen, and alternative names for certain aspects of NATM have been adopted as its use has spread. That is partly caused by an increased use of the tunneling method in the United States, particularly in soft ground shallow tunnels.

Other designations are seen for this modern tunneling style; Sequential Excavation Method (SEM) or Sprayed Concrete Lining (SCL) are often used in shallower tunnels. In Japan, the terms Centre Dividing Wall NATM or Cross Diaphragm Method (both abbreviated to CDM) and Upper Half Vertical Subdivision method (UHVS) are used.

The Austrian Society of Engineers and Architects defines NATM as "a method where the surrounding rock or soil formations of a tunnel are integrated into an overall ring-like support structure. Thus the supporting formations will themselves be part of this supporting structure."^[5]

Some engineers use NATM whenever they propose shotcrete for initial ground support of an open-face tunnel. The term NATM can be misleading in relation to soft-ground tunnels. As noted by Emit Brown, NATM can refer to both a design philosophy and a construction method.^[6]

Key features

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 efficiently address the specific rock conditions being encountered.
The initial ground support is provided by shotcrete in combination with fibre or welded-wire fabric reinforcement, steel arches (usually lattice girders), and sometimes ground reinforcement (e.g. soil nails, spiling).
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.

Safety

The 1994 Heathrow Airport tunnel collapse led to questions about the safety of the NATM. However, the subsequent trial blamed the collapse on poor workmanship and flaws in construction management, rather than on the NATM.^[7]

Related Research Articles

<span class="mw-page-title-main">Geotechnical engineering</span> Scientific study of earth materials in engineering problems

Geotechnical engineering, also known as geotechnics, is the branch of civil engineering concerned with the engineering behavior of earth materials. It uses the principles of soil mechanics and rock mechanics to solve its engineering problems. It also relies on knowledge of geology, hydrology, geophysics, and other related sciences.

A tunnel is an underground or undersea passageway. It is dug through surrounding soil, earth or rock, or laid under water, and is usually completely enclosed except for the two portals common at each end, though there may be access and ventilation openings at various points along the length. A pipeline differs significantly from a tunnel, though some recent tunnels have used immersed tube construction techniques rather than traditional tunnel boring methods.

Retaining walls are relatively rigid walls used for supporting soil laterally so that it can be retained at different levels on the two sides. Retaining walls are structures designed to restrain soil to a slope that it would not naturally keep to. They are used to bound soils between two different elevations often in areas of inconveniently steep terrain in areas where the landscape needs to be shaped severely and engineered for more specific purposes like hillside farming or roadway overpasses. A retaining wall that retains soil on the backside and water on the frontside is called a seawall or a bulkhead.

A tunnel boring machine (TBM), also known as a "mole" or a "worm", is a machine used to excavate tunnels. Tunnels are excavated through hard rock, wet or dry soil, or sand, each of which requires specialized technology.

Rock mass classification systems are used for various engineering design and stability analysis. These are based on empirical relations between rock mass parameters and engineering applications, such as tunnels, slopes, foundations, and excavatability. The first rock mass classification system in geotechnical engineering was proposed in 1946 for tunnels with steel set support.

<span class="mw-page-title-main">Rock mechanics</span> Study of the mechanical behavior of rocks

Rock mechanics is a theoretical and applied science of the mechanical behavior of rocks and rock masses.

Shaft mining or shaft sinking is the action of excavating a mine shaft from the top down, where there is initially no access to the bottom. Shallow shafts, typically sunk for civil engineering projects, differ greatly in execution method from deep shafts, typically sunk for mining projects.

<span class="mw-page-title-main">Concrete slab</span> Flat, horizontal concrete element of modern buildings

A concrete slab is a common structural element of modern buildings, consisting of a flat, horizontal surface made of cast concrete. Steel-reinforced slabs, typically between 100 and 500 mm thick, are most often used to construct floors and ceilings, while thinner mud slabs may be used for exterior paving.

Drilling and blasting is the controlled use of explosives and other methods, such as gas pressure blasting pyrotechnics, to break rock for excavation. It is practiced most often in mining, quarrying and civil engineering such as dam, tunnel or road construction. The result of rock blasting is often known as a rock cut.

Underground Wine cellars are subterranean structures for the storage and the aging of wine. They are an integral component of the wine industry worldwide. The design and construction of wine caves represents a unique application of underground construction techniques.

A deep foundation is a type of foundation that transfers building loads to the earth farther down from the surface than a shallow foundation does to a subsurface layer or a range of depths. A pile or piling is a vertical structural element of a deep foundation, driven or drilled deep into the ground at the building site.

Soil nailing is a remedial construction measure to treat unstable natural soil slopes or unstable man-made (fill) slopes as a construction technique that allows the safe over-steepening of new or existing soil slopes. The technique involves the insertion of relatively slender reinforcing elements into the slope – often general purpose reinforcing bars (rebar) although proprietary solid or hollow-system bars are also available. Solid bars are usually installed into pre-drilled holes and then grouted into place using a separate grout line, whereas hollow bars may be drilled and grouted simultaneously by the use of a sacrificial drill bit and by pumping grout down the hollow bar as drilling progresses. Kinetic methods of firing relatively short bars into soil slopes have also been developed.

Landslide mitigation refers to several human-made activities on slopes with the goal of lessening the effect of landslides. Landslides can be triggered by many, sometimes concomitant causes. In addition to shallow erosion or reduction of shear strength caused by seasonal rainfall, landslides may be triggered by anthropic activities, such as adding excessive weight above the slope, digging at mid-slope or at the foot of the slope. Often, individual phenomena join to generate instability over time, which often does not allow a reconstruction of the evolution of a particular landslide. Therefore, landslide hazard mitigation measures are not generally classified according to the phenomenon that might cause a landslide. Instead, they are classified by the sort of slope stabilization method used:

<span class="mw-page-title-main">Cellular confinement</span> Confinement system used in construction and geotechnical engineering

Cellular confinement systems (CCS)—also known as geocells—are widely used in construction for erosion control, soil stabilization on flat ground and steep slopes, channel protection, and structural reinforcement for load support and earth retention. Typical cellular confinement systems are geosynthetics made with ultrasonically welded high-density polyethylene (HDPE) strips or novel polymeric alloy (NPA)—and expanded on-site to form a honeycomb-like structure—and filled with sand, soil, rock, gravel or concrete.

The Mt. Lebanon Tunnel is a light rail tunnel in Allegheny County, Pennsylvania, also known as the Dormont/Mt. Lebanon Transit Tunnel, part of the Pittsburgh Light Rail system.

Steel fibre-reinforced shotcrete (SFRS) is shotcrete with steel fibres added. It has higher tensile strength than unreinforced shotcrete and is quicker to apply than weldmesh reinforcement. It has often been used for tunnels.

The rock mass rating (RMR) is a geomechanical classification system for rocks, developed by Z. T. Bieniawski between 1972 and 1973. Since then it has undergone multiple modifications out of which, RMR89 is commonly used. Recently RMR14 has been proposed to improve the RMR performance by incorporating new experiences from tunnel practices. Continuous functions and a software "QuickRMR" for RMR89 and RMR14 have also been proposed by Kundu. RMR combines the most significant geologic parameters of influence and represents them with one overall comprehensive index of rock mass quality, which is used for the design and construction of excavations in rock, such as tunnels, mines, slopes, and foundations.

Laird is an underground light rail transit (LRT) station under construction on Line 5 Eglinton, a new line that is part of the Toronto subway system. It is located in the Leaside neighbourhood in East York at the intersection of Laird Drive and Eglinton Avenue. It is scheduled to open in 2024.

Tunnels are dug in types of materials varying from soft clay to hard rock. The method of tunnel construction depends on such factors as the ground conditions, the ground water conditions, the length and diameter of the tunnel drive, the depth of the tunnel, the logistics of supporting the tunnel excavation, the final use and shape of the tunnel and appropriate risk management. Tunnel construction is a subset of underground construction.

The Analysis of Controlled Deformation in Rocks and Soils, translated from Italian Analisi delle Deformazioni Controllate nelle Rocce e nei Suoli (ADECO-RS), also known as The New Italian Tunneling Method (NITM), is a modern tunnel design and construction approach. ADECO-RS was proposed by Pietro Lunardi in the 1980s on the basis of long in-depth research into the stress-strain behavior of more than 1,000 km of tunnel and more than 9,000 faces. In the past few decades, ADECO-RS has been widely used in Italian railway, highway and large underground construction projects and has been incorporated into Italian tunnel design and construction specifications.

References

↑ Alun Thomas (2019). Sprayed Concrete Lined Tunnels – 2nd ed. Abingdon, UK: Taylor & Francis. p. 288. ISBN 9780367209759.
↑ Levent Özdemir (2006). North American Tunneling. Abingdon, UK: Taylor & Francis. p. 246. ISBN 0-415-40128-3.
↑ Zhang, D., Xiong F., Zhang L., 2016
↑ RMR and Q System
↑ Alan Muir Wood (2002). Tunneling: Management by Design. Abingdon, UK: Taylor & Francis. p. 328. ISBN 978-0-419-23200-1.
↑ Victor Romero (2002). "NATM In Soft-ground: A Contradiction Of Terms?". Jacobs & Associates.
↑ Shani Wallis. "Heathrow failures highlight NATM (abuse?) misunderstandings". Special Edition Focus: UK Case Studies. TunnelTalk. pp. 47–50.