La Craie

Notes on the stability of tunnels driven through chalk
M . PANET.
Sound and unfissured chalk possesses favourable characteristics (satisfactory behaviour, low permeability,
ease of excavation) for the construction of underground structures. But weathered and
fractured zones lead to considerable difficulties.
In the short term, sound and unfissured chalk behaves in an elasto-fragile manner, and when the
thickness of overlay is considerable one observes classic but not brutal decompression failures which
can be easily stabilized by bolting.
Chalk reveals important deformations in the long term ; laboratory creep tests show deferred
failures under stresses markedly below the resistance measured under rapid loading.
Tunnel linings must therefore be calculated in the light of this rheological behaviour of chalk ;
for in counteracting deferred convergence strains the lining is subjected to increasing stresses with
the passage of time.
La craie, Bull. Liaison Labo. P. et Ch., special V, oct. 1973, p. 159.