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Tunnel heading stability

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Benoit Jones

on 23 November 2015

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Transcript of Tunnel heading stability

Stability of tunnel headings
Theorems of plastic collapse
Upper bound
Lower bound
If a structure is loaded to this value it
must
collapse
'Kinematically admissible mechanisms'
If a structure is loaded to this value it
cannot
collapse
'Statically admissible stress fields'
True collapse load
Rigorous proofs exist which show that the upper and lower bounds bracket the true collapse load.

To estimate the
true collapse load
, we can:
refine upper and lower bound calculation methods until the bracket is small
use limit equilibrium
use numerical models
use centrifuge tests
use field data from real collapses
"If you take any compatible mechanism of slip surfaces and consider an increment of movement and if you show that the work done by the stresses in the soil equals the work done by the external loads, the structure must collapse (i.e. the external loads are an upper bound to the true collapse loads)." Atkinson (2007)
"If you can determine a set of stresses in the ground that are in equilibrium with the external loads and do not exceed the strength of the soil, the structure cannot collapse (i.e. the external loads are a lower bound to the true collapse loads)." Atkinson (2007)
Undrained
Drained
Davis, E. H., Gunn, M. J., Mair, R. J. & Seneviratne, H. N. (1980). The stability of shallow tunnels and underground openings in cohesive material. Geotechnique, Vol. 30, No. 4, pp. 397-419.
Leca, E. & Dormieux, L. (1990). Upper and lower bound solutions for the face stability of shallow circular tunnels in frictional material. Geotechnique, Vol. 40, No. 4, pp. 581-605.
This is for c'-phi' soils above the water table only
Undrained
Drained
Centrifuge modelling
Numerical modelling
Vermeer, P. A., Ruse, N. & Marcher, T. (2002). Tunnel heading stability in drained ground. Felsbau 20, No.6, pp.8-18.
An example:
An example:
Walter, H., Coccia, C. J., Wallen, R. B., Ko, H.-Y. & McCartney, J. S. (2010). Modeling of tunnel lining deformation due to face instabiity. Proc. Int. Conf. Phys. Mod. Geot. Engrg.
University of Cambridge 10m beam centrifuge
Design charts
Undrained
Drained
Undrained is when k < 1e6 to 1e7 m/s according to Anagnostou & Kovari
Use Mair's design charts based on centrifuge tests
Drained is when k > 1e6 to 1e7 m/s and advance rate < 0.1 to 1 m/hr, according to Anagnostou & Kovari
Use Anagnostou & Kovari's method based on limit equilibrium to calculate support pressure
N.B.: If in doubt, calculate both undrained and drained stability
Davis, E. H., Gunn, M. J., Mair, R. J. & Seneviratne, H. N. (1980). The stability of shallow tunnels and underground openings in cohesive material. Geotechnique, Vol. 30, No. 4, pp. 397-419.
Leca, E. & Dormieux, L. (1990). Upper and lower bound solutions for the face stability of shallow circular tunnels in frictional material. Geotechnique, Vol. 40, No. 4, pp. 581-605.
This is for c'-phi' soils above the water table only.
[limit equilibrium is both kinematically and statically admissible and combines considerations of both compatibility and equilibrium]
[not such a problem for undrained, but for drained case there is a big gap between bounds]
Note: A&K's method can include the effects of seepage
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