Induration and Biot’s Coefficient of Palaeogene Limestone

In engineering geology and classification of rock masses for civil engineering purposes, the degree of induration for a rock serves as a useful classification parameter. Induration is a measure of how well the grains of a sedimentary rock are cemented together - from loosely cemented/soft rock to very competent/slightly metamorphic rock. The Biot coefficient links to the degree of cementation in the capacity of how it relates the elastic deformations with the change in pore pressure. A hypothesis is that the degree of induration could be correlated to the magnitude of the Biot coefficient. This is tested on 11 Copenhagen Limestone specimens of varying porosity and densities obtained from one borehole with a limestone interval of 30 m. Their induration varies from H2 to H5. Elastic wave propagation measurements are used to establish the Biot coefficient and determination of the mineralogy for H5 specimens aids in performing fluid substitution with Gassmann’s equation. For the soft H2 specimens the shear wave could not be obtained and isoframe modelling is applied. A correlation is found; although, the Biot coefficient for the intermediate indurations H3 and H4 overlap. The dry density is found to adequately correlate with the Biot coefficient.