The 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
A tunnel is an underground passageway, dug through surrounding soil, earth or rock, and enclosed except for the entrance and exit, commonly at each end. A pipeline is not a tunnel, though some recent tunnels have used immersed tube cons ...
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
Franz Pacher (28 April 1919 in Prostřední Suchá, present-day Czech Republic – 3 March 2018 in Salzburg, Austria) was an Austrian civil engineer and a pioneer of modern tunneling. He is one of three men who are considered to be the chief devel ...
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
Rock most often refers to:
* Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids
* Rock music, a genre of popular music
Rock or Rocks may also refer to:
Places United Kingdom
* Rock, Caerphilly, a location in Wales ...
mass to stabilize the tunnel wherever possible rather than reinforcing the entire tunnel.
NATM/SEM is generally thought to have helped revolutionise 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 conditions.
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
Shotcrete, gunite (), or sprayed concrete is concrete or mortar conveyed through a hose and pneumatically projected at high velocity onto a surface, as a construction technique, first used in 1907 invented by Carl Akeley. It is typically ...
protection – Loosening and excessive rock
deformation
Deformation can refer to:
* Deformation (engineering), changes in an object's shape or form due to the application of a force or forces.
** Deformation (physics), such changes considered and analyzed as displacements of continuum bodies.
* Defor ...
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
borehole
A borehole is a narrow shaft bored in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water ( drilled water well and tube well), other liquids (such as petrol ...
s. 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
In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...
conditions. Active rather than passive support is used and the tunnel is strengthened by a flexible combination of
rock bolt
A rock bolt is a long anchor bolt, for stabilizing rock excavations, which may be used in tunnels or rock cuts. It transfers load from the unstable exterior to the confined (and much stronger) interior of the rock mass.
Rock bolts were first use ...
s, 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 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 excavatabili ...
, 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
Cross section may refer to:
* Cross section (geometry)
** Cross-sectional views in architecture & engineering 3D
*Cross section (geology)
* Cross section (electronics)
* Radar cross section, measure of detectability
* Cross section (physics)
**Abs ...
, 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
Geotechnical engineering is the branch of civil engineering concerned with the engineering behavior of earth materials. It uses the principles of soil mechanics and rock mechanics for the solution of its respective engineering problems. It al ...
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
Pit water, mine water or mining water is water that collects in a mine and which has to be brought to the surface by water management methods in order to enable the mine to continue working.
Origin
Although all water that enters pit workings or ...
.
Reinforcement
In behavioral psychology, reinforcement is a consequence applied that will strengthen an organism's future behavior whenever that behavior is preceded by a specific antecedent stimulus. This strengthening effect may be measured as a higher freq ...
is done by
wire
Overhead power cabling. The conductor consists of seven strands of steel (centre, high tensile strength), surrounded by four outer layers of aluminium (high conductivity). Sample diameter 40 mm
A wire is a flexible strand of metal.
Wire is c ...
d concrete that can be combined with steel ribs or lug
bolts, not with thicker shotcrete.
The measured rock properties suggest the appropriate
tool
A tool is an object that can extend an individual's ability to modify features of the surrounding environment or help them accomplish a particular task. Although many animals use simple tools, only human beings, whose use of stone tools dates ba ...
s 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
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
s. 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."
Some engineers use NATM whenever they propose
shotcrete
Shotcrete, gunite (), or sprayed concrete is concrete or mortar conveyed through a hose and pneumatically projected at high velocity onto a surface, as a construction technique, first used in 1907 invented by Carl Akeley. It is typically ...
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
A design is a plan or specification for the construction of an object or system or for the implementation of an activity or process or the result of that plan or specification in the form of a prototype, product, or process. The verb ''to design'' ...
philosophy'' and a ''
construction
Construction is a general term meaning the art and science to form objects, systems, or organizations,"Construction" def. 1.a. 1.b. and 1.c. ''Oxford English Dictionary'' Second Edition on CD-ROM (v. 4.0) Oxford University Press 2009 and com ...
method''.
Key features
Key features of the NATM design philosophy are:
* The
strength
Strength may refer to:
Physical strength
*Physical strength, as in people or animals
* Hysterical strength, extreme strength occurring when people are in life-and-death situations
*Superhuman strength, great physical strength far above human c ...
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
Instrumentation a collective term for measuring instruments that are used for indicating, measuring and recording physical quantities. The term has its origins in the art and science of scientific instrument-making.
Instrumentation can refer to ...
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 shotcrete in combination with fibre or welded-wire fabric reinforcement, steel arches (usually lattice girders), and sometimes ground reinforcement (e.g. soil nails,
spiling
Spiling is a traditional technique used in temperate regions of the world for the prevention of erosion to river and stream banks.
Willow spiling is currently used in the United Kingdom; live willow rods are woven between live willow uprights an ...
).
* 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
Heathrow Airport (), called ''London Airport'' until 1966 and now known as London Heathrow , is a major international airport in London, England. It is the largest of the six international airports in the London airport system (the others be ...
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.
Tunneltalk: Heathrow failures highlight NATM (abuse?) misunderstandings
/ref>
See also
* Geotechnical engineering
Geotechnical engineering is the branch of civil engineering concerned with the engineering behavior of earth materials. It uses the principles of soil mechanics and rock mechanics for the solution of its respective engineering problems. It als ...
* Rock mass classification 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 excavatabili ...
* Tunnels
A tunnel is an underground passageway, dug through surrounding soil, earth or rock, and enclosed except for the entrance and exit, commonly at each end. A Pipeline transport, pipeline is not a tunnel, though some recent tunnels have used ...
* Analysis of controlled deformation in rocks and soils
References
Further reading
* Johann Golser, ''The New Austrian Tunneling Method (NATM), Theoretical Background & Practical Experiences.'' 2nd Shotcrete
Shotcrete, gunite (), or sprayed concrete is concrete or mortar conveyed through a hose and pneumatically projected at high velocity onto a surface, as a construction technique, first used in 1907 invented by Carl Akeley. It is typically ...
conference, Easton, Pennsylvania
Easton is a city in, and the county seat of, Northampton County, Pennsylvania, United States. The city's population was 28,127 as of the 2020 United States census, 2020 census. Easton is located at the confluence of the Lehigh River, a river tha ...
(USA), 4-8 Oct 1976.
{{Authority control
Rock mass classification
Tunnel construction