1. International workshop on micropiles – Venice, 30 may – 2 June 2002
TWENTY YEARS OF EXPERIENCE IN THE USE OF THE UMBRELLA-ARCH METHOD OF SUPPORT FOR TUNNELLING
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G. Carrieri, Geodata S.r.l., Torino, Italy
R. Fiorotto, Casagrande, Italy
P. Grasso, Geodata S.r.l., Torino, Italy
S. Pelizza, Politecnico di Torino, Italy
Presented by: G. Carrieri

2. Conventional tunnelling methods in soft ground
When tunnelling through soft ground, external, additional support (ground reinforcement or ground improvement) at some stage of the excavation is always necessary
The main problem with conventional methods when tunnelling through difficult geotechnical conditions is control of deformation: without support or treatment the ground weakens and tends to sink into the opening (fall of ground from the upper part of the tunnel face, tunnel face extrusion, tunnel face failure), a phenomenon called “decompression”.
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The elimination of this decompression may require a “cavity pre-confinement action” (defined as any active action that favours the formation of an arch effect in the ground ahead the tunnel face) that can be achieved through reinforcement or/and protective intervention ahead the tunnel excavation.

3. Conventional tunnelling methods in soft ground
Reinforcement (or ground improvement) acts directly on the ground’s mechanical characteristics improving its natural strength and deformation properties by means of appropriate ground improvement techniques.
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Protective intervention channels’ stresses around the tunnel periphery thus performing a protective function ensuring that the natural strength and deformation properties of the core are maintained (the main effect is to contain the reduction of σ3 - minor principal stress). The umbrella-arch technique can be included in this definition.

4. Conventional tunnelling methods in soft ground
TYPE OF INTERVENTION
LOCATION
ACTION
Around the cavity
Cavity boundary
Tunnel core
Protective
Reinforcement
GROUTING  
JET-GROUTING    
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FREEZING   
DEWATERING   
FIBREGLASS ELEMENTS   
PILOT TUNNEL   
PRE-CUTTING  
PRE-TUNNEL  
UMBRELLA-ARCH  

5. Conventional tunnelling methods in soft ground
Mechanical pre-cutting
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6. Conventional tunnelling methods in soft ground
Pretunnel
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7. Conventional tunnelling methods in soft ground
Fibreglass elements
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8. Conventional tunnelling methods in soft ground
Umbrella arch
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9. Conventional tunnelling methods in soft ground
Umbrella arch
Umbrella arch
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Bolts
Micropiles
Reinforcement
of face
Temporary
Invert

10. Conventional tunnelling methods in soft ground
Umbrella arch
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11. Conventional tunnelling methods in soft ground
Umbrella arch
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12. Fields of application of the umbrella-arch – Tunnel portals
The portal zone frequently represents the most problematic area during excavation of a tunnel. Many problems are concentred in this area, especially those deriving from the geological nature of the ground, lack of confinement in the immediate overburden and rock mass discontinuities. In general, the following factors influence the portal problems:
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morphology of the site;
geo-mechanical characteristics of the terrain;
stability conditions of the ground;
presence of water;
presence of external constraints (building, infrastructures, etc.)

13. Fields of application of the umbrella-arch – Tunnel portals
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14. Fields of application of the umbrella-arch – Tunnel portals
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15. Fields of application of the umbrella-arch – Shallow tunnels
The construction of shallow tunnels in soft ground requires complete control of ground deformations:
to avoid damage to infrastructures and buildings on the surface,
to ensure the safety of the workmen in various stages of the excavation,
to control the increase of bending moments in the temporary lining that may result from loosening of the ground, asymmetric loading (due to the geomorphology, proximity to another cavity, or surface loads), and heterogeneity of the ground,
to avoid the initiation or recurrence of the phenomenon of slope instability.
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16. Fields of application of the umbrella-arch – Shallow tunnels
Shallow tunnels: Egnatia Odos S3 tunnel (Greece)
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17. Fields of application of the umbrella-arch – Shallow tunnels
Shallow tunnels: Egnatia Odos S3 tunnel (Greece)
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18. Fields of application of the umbrella-arch – Shallow tunnels
Shallow tunnels: Maiko tunnel (Japan)
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19. Fields of application of the umbrella-arch – Shallow tunnels
Shallow tunnels: Maiko tunnel (Japan)
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20. Fields of application of the umbrella-arch – Deep tunnels
When tunnelling through soft ground in deep overburden the magnitude of stress concentrations at the tunnel face always exceeds the strength of the rock (soil) mass.
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Tunnelling through ground belonging to behaviour categories “e” and “f” requires pre-confinement actions. The selection between reinforcement or protective interventions depends on the stress-strain behaviour of the ground.

21. Fields of application of the umbrella-arch – Deep tunnels
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22. Fields of application of the umbrella-arch – Deep tunnels
Ferrocarril
Caracas – Tui medio
(Venezuela)
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Tunnel Pitahaya

23. Fields of application of the umbrella-arch – Deep tunnels
Fault crossing in deep tunnel conditions: Daj Khad shear zone - Headrace tunnel of Nathpa Jhakri Hydroelectric Project (India).
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24. Fields of application of the umbrella-arch – Deep tunnels
Fault crossing in deep tunnel conditions: Daj Khad shear zone - Headrace tunnel of Nathpa Jhakri Hydroelectric Project (India).
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25. Fields of application of the umbrella-arch – Deep tunnels
Fault crossing in deep tunnel conditions: Daj Khad shear zone - Headrace tunnel of Nathpa Jhakri Hydroelectric Project (India).
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26. Fields of application of the intervention ahead the tunnel excavation
TYPE OF INTERVENTION
FIELD OF APPLICATION
Cohesive terrain
Sandy
gravely terrain
Terrain with boulders
Fractured rock
Complex Formations
GROUTING 1 
JET-GROUTING 2 3
FREEZING
DEWATERING 4
FIBREGLASS ELEMENTS 5 6
PILOT TUNNEL
PRE-CUTTING
PRE-TUNNEL
UMBRELLA-ARCH 7
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27. Fields of application of the umbrella-arch
Terrain with boulders
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28. Fields of application of the umbrella-arch
Terrain with boulders
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29. Fields of application of the umbrella-arch
Sandy gravely terrain
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30. Fields of application of the umbrella-arch
Terrain below water table
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31. Fields of application of the umbrella-arch
Terrain below water table
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32. Umbrella arch design Conventional approach
The steel pipe is considered to be a continuous beam on multiple supports
Assuming that the value of "s" (in the case of a stable face) is equal to 1/2 d, the maximum bending moment, (Mmax) is:
Mmax = (1/12)* qu* (3/2 a+0.5)2
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Considering that this design is for a very brief duration, the (allowable) working stress (Sadm) for the steel pipe can be close to its yield stress (1.5>Fs>1.1). The characteristic moment of resistance of the pipe is: Wmin = Mmax/Sadm.

33. Umbrella arch design Numerical modelling
Usually the steel pipe is modelled by beam elements and the grouted area around the steel pipe is modelled by shell elements, the rigidity of the umbrella-arch in the longitudinal direction (tunnel axis direction) is completely different from its rigidity in the circumferential direction
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34. Conclusion
The umbrella-­arch method has been used with demonstrated success in many types of difficult ground formations both in Italy and abroad
its flexibility and the fact that it can be used in different kinds of ground;
the treatment can be varied according to the ground formation,
it is safe because the steel pipes begin to work as a support immediately, even without grouting;
it is simple and quick and allows full face excavation to be carried out in soft soils;
it also has a structural function.
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