Les propriétés rhéologiques de la craie et leur influence sur le percement de galeries

¹ Ingénieur Géologue E.N.S.G., attaché de Recherches au C.N.R.S., Institut Dolomieu, Grenoble.
² Ingénieur Civil des Mines, Electricité de France Direction de l'Equipement, Division Géologie Géotechnique.

Résumé

Research including laboratory and in-situ tests down to depth of 220 m on the behaviour of chalk was carried out for an underground pumping plant scheme in the Champagne region of France, but it has now been abandoned. Chalk is a soft, light, highly porous material and is saturated with water below the water table. Its particularly fine structure was described with the aid of an electron microscope. Elastic modulus measurements produced the first evidence of remarkably close agreement between laboratory and in-situ data. Chalk shows no scale effect, which indicates a total absence of cracks. The formation is continuous even at a scale of 1 decametre. Modules and strength are affected by moderate axial anisotropy ; strength of the rock is greater in a direction perpendicular to its stratification. Creep and the behaviour of chalk under tri-axial load were also investigated, and the latter was found to result in a considerable reduction in volume. Reworking by impact or vibration converts this brittle rock into a soil near its liquidity limit. The exceptional continuity and homogeneity of chalk and initial stresses very close to a hydrostatic distribution are good conditions for studies of the stability of underground works in elastic-plastic media. The exploration shaft showed the same rock-bursting below a depth of 120 m as in the Mont Blanc tunnel. though this 'danger signal' does not mean that the works are impracticable. It should be possible to avoid any drilling difficulties, especially by drilling vent shafts ahead of the work face to induce decompression. And, as in the Mont Blanc tunnel, the lining should not be designed to take the total load.