Laboratory study examining permeability evolution along an isotropic unloading stress path

A small experimental study sponsored by the British Geological Survey (aligned to the Geosphere Containment Project) and the Mont Terri Consortium (as part of the HA experimental programme) was undertaken by the Transport Properties Research Laboratory (TPRL). The objective of the work was to provide information on the evolution of intrinsic permeability and the stress-dependent specific storage of an Opalinus Clay specimen subject to an isotropic unloading stress history. Isotropic effective stress was reduced in a step wise manner on a specimen of Opalinus Clay subject to a constant hydraulic gradient. Permeability values were found to be in the range 3.8 to 9.5 x 10-21 m2 at average effective stresses of 3.25 to 0.40 MPa respectively, with permeability significantly increasing when effective stress declined to below around 1.0 MPa. However, this study clearly shows that on the scale of this experiment, the specimen continues to exhibit very low permeability. Storage values for the stress regime imposed during this study were found to range from 3.8 to 46.7 x10-5 m-1, exhibiting a general trend of increasing storage with decreasing effective stress. The relationship between void ratio and logarithm of effective stress is reasonably linear for the stress regime of the test indicating the specimen exhibits soil-like behaviour. Evidence suggests that the locus in void ratio-effective stress space corresponds to the rebound-reconsolidation line of theoretical soil mechanics. The general trend of increasing material compressibility and decreasing bulk modulus are symptomatic of an increase in specimen volume due to swelling of the clay during stress unloading. Values of Young’s modulus based on analysis of the unloading data are generally lower than those derived from numerical modelling of the data. However, both determinants exhibit a general decreasing trend as average effective stress declines. The combination of low modulus values and the inability of the numerical model to fit all the data simultaneously, strongly suggests that the specimen is exhibiting a form of visco-plastic deformation during the unloading process.