New Austrian Tunnelling method
Encyclopedia
The New Austrian Tunnelling method (NATM) was developed between 1957 and 1965 in Austria
. It was given its name in Salzburg
in 1962 to distinguish it from old Austrian tunnel
ling approach. The main contributors to the development of NATM were Ladislaus von Rabcewicz
, Leopold Müller
and Franz Pacher
. The main idea is to use the geological stress
of the surrounding rock
mass to stabilize the tunnel itself.
Many have argued that the New Austrian Tunnelling method was not new or Austrian having been previously used else where in Europe and isn't a tunnelling method as much as a philosophy. This aside NATM has no doubt done much to revolutionise tunnelling and bring it into the 21st century
There are seven features on which NATM is based:
Based on the computation of the optimal cross section
, just a thin shotcrete protection is necessary. It is applied immediately behind the Tunnel boring machine
, to create a natural load-bearing ring and therefore to minimize the rock's deformation. Additionally, geotechnical instruments are installed to measure the later deformation of excavation
. Therefore a monitoring of the stress distribution within the rock is possible.
This monitoring makes the method very flexible, even at surprising changes of the geomechanical
rock consistency during the tunneling work, e.g. by crevices or pit water. Such (usual) problems are not solved by thicker shotcrete, but the reinforcement
is done by wire
d concrete which can be combined with steel ribs or lug bolts
.
The measured rock properties lead to the appropriate tool
s for tunnel strengthening. Therefore in the last decade NATM was also applied to soft ground excavations and to tunnels in porous sediment
s. The flexible NATM technique enables immediate adjustments in the construction details, but this requires a flexible contract
ual system, too.
. Recently the scene has been complicated by new terms and alternative names for certain aspects of NATM. This is partly caused by an increased use of the method in the USA, particularly in soft ground shallow tunnels (see External links).
Besides the official name New Austrian Tunnelling Method other designations are used, e.g. Sequential Excavation Method (SEM) or Sprayed Concrete Lining (SCL) are often used in shallower tunnels. In Japan
sometimes other names were used, e.g. Centre Dividing Wall NATM, or Cross Diaphragm
Method (both abbreviated as CDM), and even Upper Half Vertical Subdivision method (UHVS).
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.
However, many engineers use "NATM" whenever shotcrete
is proposed for initial ground support of an open-face tunnel. Especially with reference to soft ground, the term NATM can be misleading. As noted by Emit Brown, NATM can refer to both a design
philosophy and a construction
method.
When NATM is seen as a construction method, the key features are:
Some experts note that many of these construction methods were used in the US and elsewhere in soft-ground applications, before NATM was described in the literature.
In an article of 2002 Romero states the major difference between the viewpoint
s of design and of construction: The deformation of the soil (rem.: at soft-ground tunnels) is not easily ‘controlled’. Therefore it can be concluded that the excavation and support planned for sequentially excavated, shotcrete-lined tunnels .. utilizes NATM construction methods but not necessarily NATM design methods. These details are less essential at tunnels in solid or fair rock.
Austria
Austria , officially the Republic of Austria , is a landlocked country of roughly 8.4 million people in Central Europe. It is bordered by the Czech Republic and Germany to the north, Slovakia and Hungary to the east, Slovenia and Italy to the south, and Switzerland and Liechtenstein to the...
. It was given its name in Salzburg
Salzburg
-Population development:In 1935, the population significantly increased when Salzburg absorbed adjacent municipalities. After World War II, numerous refugees found a new home in the city. New residential space was created for American soldiers of the postwar Occupation, and could be used for...
in 1962 to distinguish it from old Austrian tunnel
Tunnel
A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers...
ling approach. The main contributors to the development of NATM were Ladislaus von Rabcewicz
Ladislaus von Rabcewicz
Ladislaus von Rabcewicz was an Austrian engineer, university professor and one of the main contributors to the New Austrian Tunneling method....
, Leopold Müller
Leopold Müller
Leopold Müller was one of the pioneers of rock mechanics and one of the main contributors to the development of the New Austrian Tunnelling method....
and Franz Pacher
Franz Pacher
Franz Pacher is an Austrian engineer, pioneer of modern tunneling, and one of the main contributors to the development of the New Austrian Tunneling method.-References:...
. The main idea is to use the geological stress
Stress (physics)
In continuum mechanics, stress is a measure of the internal forces acting within a deformable body. Quantitatively, it is a measure of the average force per unit area of a surface within the body on which internal forces act. These internal forces are a reaction to external forces applied on the body...
of the surrounding rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
mass to stabilize the tunnel itself.
Many have argued that the New Austrian Tunnelling method was not new or Austrian having been previously used else where in Europe and isn't a tunnelling method as much as a philosophy. This aside NATM has no doubt done much to revolutionise tunnelling and bring it into the 21st century
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 is not a set of specific excavation and support techniques and has often been referred to as a "design as you go" approach to tunnelling providing an optimized support based on observed ground conditions but more correctly it is a "design as you monitor" approach based on observed convergence and divergence in the lining as well as mapping of prevailing rock conditions.There are seven features on which NATM is based:
- Mobilization of the strength of rock mass - The method 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.
- ShotcreteShotcreteShotcrete is concrete conveyed through a hose and pneumatically projected at high velocity onto a surface, as a construction technique....
protection - Loosening and excessive rock deformation must be minimised. This is achieved by applying a thin layer of shotcrete immediately after face advance. - Measurements - Every deformation of the excavation must be measured. NATM requires installation of sophisticated measurement instrumentation. It is embedded in lining, ground, and boreholeBoreholeA borehole is the generalized term for any narrow shaft bored in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water or other liquid or gases , as part of a geotechnical investigation, environmental site...
s. - Flexible support - The primary lining is thin and reflects recent strataStratumIn geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
conditions. Active rather than passive support is used and the tunnel is strengthened not by a thicker concrete lining but by a flexible combination of rock boltRock boltA rock bolt is a long anchor bolt, for stabilizing rock excavations, which may be tunnels or rock cuts. It transfers load from the unstable exterior, to the confined interior of the rock mass....
s, wire mesh and steel ribs. - Closing of invert - Quickly closing the invert and creating a load-bearing ring is important. It is crucial in soft ground tunnels where no section of the tunnel should be left open even temporarily.
- Contractual arrangements - Since the NATM is based on monitoring measurements, changes in support and construction method are possible. This is possible only if the contractual system enables those changes.
- Rock mass classificationRock mass classificationRock 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...
determines support measures - There are several main rock classes for tunnels and corresponding support systems for each. These serve as the guidelines for tunnel reinforcement.
Based on the computation of the optimal cross section
Cross section (geometry)
In geometry, a cross-section is the intersection of a figure in 2-dimensional space with a line, or of a body in 3-dimensional space with a plane, etc...
, just a thin shotcrete protection is necessary. It is applied immediately behind the Tunnel boring machine
Tunnel boring machine
A tunnel boring machine also known as a "mole", is a machine used to excavate tunnels with a circular cross section through a variety of soil and rock strata. They can bore through anything from hard rock to sand. Tunnel diameters can range from a metre to almost 16 metres to date...
, to create a natural load-bearing ring and therefore to minimize the rock's deformation. Additionally, geotechnical instruments are installed to measure the later deformation of excavation
Earthworks (engineering)
Earthworks are engineering works created through the moving or processing of quantities of soil or unformed rock.- Civil engineering use :Typical earthworks include roads, railway beds, causeways, dams, levees, canals, and berms...
. Therefore a monitoring of the stress distribution within the rock is possible.
This monitoring makes the method very flexible, even at surprising changes of the geomechanical
Geomechanics
Geomechanics is the geologic study of the behavior of soil and rock. The two main disciplines of geomechanics are soil mechanics and rock mechanics. The former deals with the behaviour of soil from a small scale to a landslide scale...
rock consistency during the tunneling work, e.g. by crevices or pit water. Such (usual) problems are not solved by thicker shotcrete, but the reinforcement
Reinforcement
Reinforcement is a term in operant conditioning and behavior analysis for the process of increasing the rate or probability of a behavior in the form of a "response" by the delivery or emergence of a stimulus Reinforcement is a term in operant conditioning and behavior analysis for the process of...
is done by wire
Wire
A wire is a single, usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various...
d concrete which can be combined with steel ribs or lug bolts
Screw
A screw, or bolt, is a type of fastener characterized by a helical ridge, known as an external thread or just thread, wrapped around a cylinder. Some screw threads are designed to mate with a complementary thread, known as an internal thread, often in the form of a nut or an object that has the...
.
The measured rock properties lead to the appropriate tool
Tool
A tool is a device that can be used to produce an item or achieve a task, but that is not consumed in the process. Informally the word is also used to describe a procedure or process with a specific purpose. Tools that are used in particular fields or activities may have different designations such...
s for tunnel strengthening. Therefore in the last decade NATM was also applied to soft ground excavations and to tunnels in porous sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
s. The flexible NATM technique enables immediate adjustments in the construction details, but this requires a flexible contract
Contract
A contract is an agreement entered into by two parties or more with the intention of creating a legal obligation, which may have elements in writing. Contracts can be made orally. The remedy for breach of contract can be "damages" or compensation of money. In equity, the remedy can be specific...
ual system, too.
Philosophy and controversial 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, however most city tunnels are built at shallow depth and need not control the release of the in situ stress, seeking instead to minimise settlement. This has led to a confusion in terminology in that tunnelling engineers use "NATM" to mean different things: some define it as a special technique, but others as a sort of philosophyPhilosophy
Philosophy is the study of general and fundamental problems, such as those connected with existence, knowledge, values, reason, mind, and language. Philosophy is distinguished from other ways of addressing such problems by its critical, generally systematic approach and its reliance on rational...
. Recently the scene has been complicated by new terms and alternative names for certain aspects of NATM. This is partly caused by an increased use of the method in the USA, particularly in soft ground shallow tunnels (see External links).
Besides the official name New Austrian Tunnelling Method other designations are used, e.g. Sequential Excavation Method (SEM) or Sprayed Concrete Lining (SCL) are often used in shallower tunnels. In Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
sometimes other names were used, e.g. Centre Dividing Wall NATM, or Cross Diaphragm
Diaphragm (mechanics)
In mechanics, a diaphragm is a sheet of a semi-flexible material anchored at its periphery and most often round in shape. It serves either as a barrier between two chambers, moving slightly up into one chamber or down into the other depending on differences in pressure, or as a device that...
Method (both abbreviated as CDM), and even Upper Half Vertical Subdivision method (UHVS).
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.
However, many engineers use "NATM" whenever shotcrete
Shotcrete
Shotcrete is concrete conveyed through a hose and pneumatically projected at high velocity onto a surface, as a construction technique....
is proposed for initial ground support of an open-face tunnel. Especially with reference to soft ground, the term NATM can be misleading. As noted by Emit Brown, NATM can refer to both a design
Design
Design as a noun informally refers to a plan or convention for the construction of an object or a system while “to design” refers to making this plan...
philosophy and a construction
Construction
In the fields of architecture and civil engineering, construction is a process that consists of the building or assembling of infrastructure. Far from being a single activity, large scale construction is a feat of human multitasking...
method.
Key features
According to E.Brown (Weblink 2), the key features of the design philosophy refer to:- The strengthStrength of materialsIn materials science, the strength of a material is its ability to withstand an applied stress without failure. The applied stress may be tensile, compressive, or shear. Strength of materials is a subject which deals with loads, deformations and the forces acting on a material. A load applied to a...
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 loadStructural loadStructural loads or actions are forces, deformations or accelerations applied to a structure or its components.Loads cause stresses, deformations and displacements in structures. Assessment of their effects is carried out by the methods of structural analysis...
-deformation characteristics appropriate to the ground conditions, and installation is timed with respect to ground deformations. - InstrumentationInstrumentationInstrumentation is defined as the art and science of measurement and control of process variables within a production, or manufacturing area....
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 excavationEarthworks (engineering)Earthworks are engineering works created through the moving or processing of quantities of soil or unformed rock.- Civil engineering use :Typical earthworks include roads, railway beds, causeways, dams, levees, canals, and berms...
.
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.
- 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 usually (but not always) a cast-in-place concrete lining.
Some experts note that many of these construction methods were used in the US and elsewhere in soft-ground applications, before NATM was described in the literature.
In an article of 2002 Romero states the major difference between the viewpoint
Perspective (cognitive)
Perspective in theory of cognition is the choice of a context or a reference from which to sense, categorize, measure or codify experience, cohesively forming a coherent belief, typically for comparing with another...
s of design and of construction: The deformation of the soil (rem.: at soft-ground tunnels) is not easily ‘controlled’. Therefore it can be concluded that the excavation and support planned for sequentially excavated, shotcrete-lined tunnels .. utilizes NATM construction methods but not necessarily NATM design methods. These details are less essential at tunnels in solid or fair rock.
Further reading
- Johann Golser, The New Austrian Tunneling Method (NATM), Theoretical Background & Practical Experiences. 2nd ShotcreteShotcreteShotcrete is concrete conveyed through a hose and pneumatically projected at high velocity onto a surface, as a construction technique....
conference, Easton (USA), 4-8 Oct 1976.