1 NATM (New Austrian Tunneling Method ) in Tunneling Def of Tunneling: Underground passage dug though hill, under road or river etc. for passage of road, train etc. Various Approaches for Tunnel construction: 1) Drill & Blast System: Under this the mode can be broadly divided into: a) Conventional Tunneling system: In this approach used from earlier times the excavated rock mass is supported with/without use of steel ribs made up generally of ISMB/ISHB with the different spacing depending upon the rock mass type along with lagging generally made of cement concrete M15 these days. The space between the steel support/lagging and excavated section is filled with lower grade of cement concrete generally M10 backfill. Sometimes shotcrete with use of rock bolts is also resorted. However the final lining where required is also given of plain cement concrete generally M20 grade. b) NA TM: This method has been developed basically in Austria so its name make use of providing flexible primary lining in shape of shotcrete , wire mesh, rock bolts ,lattice girder. In case of weaker rock mass the use of pipe forepole/pipe roofing is also resrted for crown support which in turn lead to less overbreak as well as ensure safety during the execution. The main aspect of the approach is dynamic design based on rock mass classification as well as the in situ deformation observed. Hence more economical use of the tunnel support system along with the rational approach of execution.
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NATM (New Austrian Tunneling Method ) in Tunneling
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NATM (New Austrian Tunneling Method ) in Tunneling
Def of Tunneling: Underground passage dug though hill, under road or river
etc. for passage of road, train etc.
Various Approaches for Tunnel construction:
1) Drill & Blast System: Under this the mode can be broadly divided
into:
a) Conventional Tunneling system: In this approach used from
earlier t imes the excavated rock mass is supported with/without
use of steel ribs made up generally of ISMB/ISHB with the
different spacing depending upon the rock mass type along with
lagging generally made of cement concrete M15 these days.
The space between the steel support/lagging and excavated
section is filled with lower grade of cement concrete generally
M10 backfill. Sometimes shotcrete with use of rock bolts is
also resorted. However the final lining where required is also
given of plain cement concrete generally M20 grade.
b) NATM: This method has been developed basically in Austria
so its name make use of providing flexible primary lining in
shape of shotcrete , wire mesh, rock bolts ,lattice girder. In case
of weaker rock mass the use of pipe forepole/pipe roofing is
also resrted for crown support which in turn lead to less
overbreak as well as ensure safety during the execution. The
main aspect of the approach is dynamic design based on rock
mass classification as well as the in s itu deformation observed.
Hence more economical use of the tunnel support system along
with the rational approach of execution.
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2) Tunnel Boring Machine : This method is totally mechanized but
capital intensive. It is very much suitable where there is reasonablly
kind of rock mass to be encountered during construction and not
suitable for highly varying geology. Here lining can be done with
precast member just behind the tunneling.
History of NATM:
The term New Austrian Tunneling Method Popular ly Known as
NATM got its name from Salzburg (Austria). It was first used by Mr
Rabcewicz in 1962. It got world wise recognition in1964. This method has
been evolved as a result of experience gained in Austrian Alpine tunneling
condition. The first use of NATM in soft ground tunneling is done in
Frankfurt metro in 1969. The basic aim of NATM is for getting stable and
economic tunnel support systems. This method has been very useful in
complex diversif ied geological condition where forecasting of the rock mass
is difficult due to rapidly changing geology.
Broad Principles of NATM :
NATM broadly based on the following principles :
• Mobilization of the strength of rock mass - The method relies on
the inherent strength of the 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 mass
deformation should be minimised by applying a layer 25-50mm of
sealing shotcrete immediately after opening of the face.
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• Measurements - Every deformation of the excavation must be
measured. NATM requires installation of sophisticated measurement
instrumentation. It is embedded in lining, ground such as load
cells,extensometers and relectors.
• Primary Lining - The primary lining is thin.It is active support and
the tunnel is strengthened not by a thicker concrete lining but by a
flexible combination of rock bolts, wire mesh and Lattice girders.
• Closing of invert – Early as far as possible closing the invert so as to
complete the arch action and creating a load-bear ing ring is important.
It is crucial in soft ground tunnels
• Rock mass classification - The participation of expert geologist is
very important as the primary support as well as the further designing
of supports etc during the excavation of rock requires the
classification of the rock mass.
• Dynamic Design – The deigning is dynamic during the tunnel
construction.Every face opening classification of rock is done and the
support are selected accordingly. Also the des ign is further reinforced
based on the deformation as noiced during the monitoring.
Classification of Rock Mass type
Rock mass encountered during excavation cannot be said to be
favourable or unfavourable only on the basis of the type of the rock.. Several
other factors also play part in the rock mass behaviour. The excavation in the
rock is depenent on the rock class based on several factors such as –
compressive strength of rock, water condition, number of cleavages,
condition of cleavages , dip and stike of the rock etc.There are various
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approaches of classification of the rock mass and most predominantly are
RQD,RMR and Q factor of the rock mass.
Rock Quality Designation index (RQD)
The Rock Quality Designation index (RQD) was developed by Deere (Deere
et al 1967) to provide a quantitative estimate of rock mass quality from drill
core logs. RQD is defined as the percentage of intact core pieces longer than
100 mm (4 inches) in the total length of core. The core should be at least
NW size (54.7 mm or 2.15 inches in diameter) and should be drilled with a
double-tube core barrel.
RMR Value:
RMR value dends upon the following factors:
1. Uniaxial compressive strength of rock material.
2. Rock Quality Designation (RQD).
3. Spacing of discontinuities.
4. Condition of discontinuities.
5. Groundwater conditions.
6. Orientation of discontinuities.
Based on this the rock mass classification as per RMR is as under:
RMR
Value
100-81 80-61 60-41 41-20 <20
Rock Class I II III IV V
Description Very
Good
Good Fair Poor Very poor
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Q Factor:It depends on the following:
i) Block size
ii) Inter block shear
iii) Active stress
iv) Reduction for joint water flow
v) Presence of weakness zones
Q factor varies from 0.01 to 1000 i.e. from exceptionally poor rock to