Hydraulic and Hydromechanical Laboratory Testing of Large Crystalline Rock Cores

In this paper, fracture stiffness in rock samples is determined by means of hydromechanical laboratory testing. The aim is three-fold: to develop a procedure for sampling, to update testing equipment and to relate fracture stiffness to the geological history (e.g., stress history and fracture infillings). The hydraulic properties of twenty rock cores (diameter 190 mm, c. 100 mm high) from the Äspö Hard Rock Laboratory were tested in a permeameter cell under different isotropic pressures up to 2.5 MPa. The flow rate through individual fracture samples was recorded. Four of the samples were re-tested in the permeameter cell using an updated hydromechanical procedure with deformation measurement across the fracture. Four load cycles of gradually increasing cell pressure were applied, resulting in a clearly observed hysteresis effect in the first and second cycles. Hydraulic aperture changes calculated using the cubic law were compared with their mechanical equivalents. The aperture changes followed similar trends, although these differed between the samples. Fracture stiffness was determined from the tests, and the stiffness to hydraulic aperture relationship was found to follow previously published patterns linked to the storativity of fractures. Differences in stiffness are explained in the context of the geological history of individual samples, particularly their stress history. The paper presents a conceptualisation of the stiffness behaviour, which includes flow properties, geometric properties and the geological stress history of the tested samples.