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3D Numerical Investigation of Face Stability in Tunnels With Unsupported Face

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Abstract

The paper studies the stability of unsupported tunnel faces by analyzing the results of a large number of 3D numerical analyses, in various ground conditions and overburden depths. The analyses calculate the average face extrusion (Uh) by averaging the axial displacement over the tunnel face. Limiting face stability occurs when the average face extrusion becomes very large and numerical convergence becomes problematic. Using the results of the numerical analyses, a dimensionless “face stability parameter” is defined, which depends on a suitable combination of ground strength, overburden depth and tunnel size. The face stability parameter correlates well with critical tunnel face parameters, like the safety factor against face instability, the average face extrusion, the radial convergence of the tunnel wall at the excavation face, the volume loss and the deconfinement ratio at the tunnel face. Thus, semi-empirical formulae are proposed for the calculation of these parameters in terms of the face stability parameter which is obtained from basic tunnel and ground parameters. Thus, useful conclusions can be drawn for the safety factor against face instability, the volume loss and the deconfinement ratio at the excavation face.

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adapted from Lunardi, 2008)

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Abbreviations

A:

Tunnel section area (m2)

C:

Soil cohesion (Mohr–Coulomb failure criterion)

D:

Tunnel width (m)

E:

Young modulus of the ground

Ei :

Intact rock Young modulus

Em :

Rockmass Young modulus

GSI:

Geological Strength Index

H:

Overburden depth measured from the tunnel axis up to the ground surface

L:

Length of the tunnel core

mb, s, a:

Parameters of the Hoek–Brown failure criterion

pi :

Fictitious radial internal pressure

po :

Average overburden pressure at the tunnel axis (average of vertical and horizontal geostatic stresses).

SF:

Safety factor of the tunnel face against instability

Uh :

Average face extrusion

UR :

Radial convergence of the tunnel wall

V:

Volume of the core, ahead of the tunnel face

VL:

Volume loss = ΔV / V

Ko :

Horizontal geostatic stress coefficient

ΔV:

Reduction of V, due to tunnel wall convergence

Λf :

Face stability parameter

Λ:

Deconfinement ratio

V:

Poisson ratio of the ground

σci :

Intact rock strength

σcm :

Ground strength (for soils and rockmasses)

φ:

Soil friction angle (Mohr–Coulomb failure criterion)

Ωf :

Face extrusion parameter

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Acknowledgements

The present PhD thesis research of Mr. D. Georgiou was supported by scholarship funding from the Onassis Foundation and the Evgenides Foundation.

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  1. National Technical University of Athens, Zografou, Athens, Greece

    D. Georgiou, A. Kalos & M. Kavvadas

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  1. D. Georgiou
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  2. A. Kalos
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  3. M. Kavvadas
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Correspondence to D. Georgiou.

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Georgiou, D., Kalos, A. & Kavvadas, M. 3D Numerical Investigation of Face Stability in Tunnels With Unsupported Face. Geotech Geol Eng 40, 355–366 (2022). https://doi.org/10.1007/s10706-021-01900-1

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