twenty years of experience in the use of the umbrella-arch method of support for tunnelling

TWENTY YEARS OF EXPERIENCE IN THE USE OF THE UMBRELLA-ARCH METHOD

OF SUPPORT FOR TUNNELLING

TWENTY YEARS OF EXPERIENCE IN THE USE OF THE UMBRELLA-ARCH METHOD

OF SUPPORT FOR TUNNELLING

International workshop on micropiles – Venice, 30 may – 2 June 2002

G. Carrieri, Geodata S.r.l., Torino, ItalyR. Fiorotto, Casagrande, ItalyP. Grasso, Geodata S.r.l., Torino, ItalyS. Pelizza, Politecnico di Torino, Italy

Presented by: G. Carrieri

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”.

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.


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.

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.


TYPE OF INTERVENTION

LOCATION ACTIONAround

the cavity

Cavity boundar

y

Tunnel core

Protective

Reinforcement

GROUTING

JET-GROUTING

FREEZING

DEWATERING FIBREGLASS ELEMENTS

PILOT TUNNEL

PRE-CUTTING

PRE-TUNNEL

UMBRELLA-ARCH

Mechanical pre-cutting



Pretunnel


Fibreglass elements


Umbrella arch

Umbrella arch

Temporary Invert

Reinforcement of face

Umbrella arch

Micropiles

Bolts



Umbrella arch

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:

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.)

Fields of application of the umbrella-arch – Tunnel portals

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.

Shallow tunnels: Egnatia Odos S3 tunnel (Greece)


Shallow tunnels: Maiko tunnel (Japan)


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. 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.


Ferrocarril

Caracas – Tui medio

(Venezuela)

Tunnel Pitahaya

Fault crossing in deep tunnel conditions: Daj Khad shear zone - Headrace tunnel of Nathpa Jhakri Hydroelectric Project (India).


Fields of application of the intervention ahead the tunnel excavation

TYPE OF INTERVENTION FIELD OF APPLICATION

Cohesive terrain

Sandygravely terrain

Terrain with

boulders

Fractured rock

Complex Formatio

ns

GROUTING 1

JET-GROUTING 2 3

FREEZING

DEWATERING 4

FIBREGLASS ELEMENTS 5 6

PILOT TUNNEL

PRE-CUTTING

PRE-TUNNEL

UMBRELLA-ARCH 7

Fields of application of the umbrella-arch

Terrain with boulders


Sandy gravely terrain

Fields of application of the umbrella-arch Terrain below water table


Terrain below water table

Umbrella arch design

The steel pipe is considered to be a continuous beam on multiple supports

Conventional approach

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 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.

Umbrella arch design

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

Numerical modelling

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.

twenty years of experience in the use of the umbrella-arch method of support for tunnelling

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