Measuring the Biot coefficient for a fluid-saturated crystalline rock

Biot coefficient a relates the increment of fluid content Δζ to an increment of bulk volume strain Δε_v during the application of an increment of mean stress DP under drained conditions i.e. no change in cavity fluid pressure Dp = 0. The Biot coefficient a is measured as (i) the ratio of increment of fluid content to bulk volumetric strain a = Δζ/Δε_v; (ii) the ratio of increment of mean stress to cavity fluid pressure a = DP/Dp under the conditions of zero volumetric strain; and (iii) a = 1-(K/Ks′), which requires the measurement of two other elastic coefficients, K = drained bulk modulus and Ks′ = unjacketed bulk modulus. Measurement of a using methods (i) and (ii) is challenging for crystalline rock. Hence, method (iii) is often promoted in the literature. The objective of this paper is to use methods (i), (ii), and (iii) to determine a at a Terzaghi effective mean pressure P′ = P - p = 15.0 MPa for Westerly blue granite, a typical crystalline rock. The specimen is saturated using the back pressure saturation approach, where a constant Skempton B-value at a specific Terzaghi effective mean pressure ensures full saturation. The experiments demonstrate that the three methods yield similar results for a at a given ′ nepetro.org/ARMAUSRMS/proceedings-pdf/ARM.