In situ and laboratory measurements of very low permeability in the Tournemire argillites (Aveyron). Comparison of methodologies and scale effect.
Boisson, JY; Cabrera, J; Bertrand, L; Heitz, JF.
BULLETIN DE LA SOCIETE GEOLOGIQUE DE FRANCE, 169: (4) 595-604.
At the request of the Institut de Protection et de Surete Nucleaire (irsn - Institute of Nuclear Safety and Protection), ANTEA visited the Tournemire site (Aveyron) to carry out an hydraulic characterization of the 200 m-thick Toarcian and Domerian formations accessible by tunnel. Permeability measurements were made using the borehole pulse-test method either in the global hole or perpendicular to more permeable fractured zones. The tests yielded an approximate value for the hydraulic head and an order of magnitude for the permeability at 1 to 10 metre scale (10(-11) to 10(-13) m/s). A borehole was then equipped for a long-duration (6 months) measurement of the hydraulic head in the rock body. Laboratory measurements were made on 4 cm-diameter core samples taken from different boreholes. The tests, carried out under triaxial stress, required preliminary saturation-consolidation of the test samples. Through applying steady-state flow or hydraulic pulse, it was possible to measure a permeability in the order of 10(-14) m/s for the matrix of the clayey material.
The difference between laboratory and in situ values is explained by the presence of fractures in the rock body. Moreover, it seems that the hydraulic conditions of measurement in the field around the hole could have an influence on the final result.