Stress and Strain Resulting from Water Adsorption on an Induced Tensile Fracture in Fine Grained Granite

Abstract:

Increases in the water content of intact rock, either through exposure to high ambient humidity, or the addition of liquid water, have been shown to simultaneously increase strain and decrease elastic moduli. The magnitude of volumetric extensional strains in response to wetting of intact granite can reach 10^-4, while the bulk elastic modulus can increase by as much as 30% with respect to those measured in an oven-dried condition. These magnitudes are not trivial when considered alongside a) observations of reversible or non-reversible strains in critical geomorphological or geotechnical settings (e.g. landslides, underground excavations, or wells created for geothermal energy production), or b) reductions in elastic moduli observed in association with (for example) damage induced by fracturing, or melting of permafrost.

In this study, we expand on investigations into the wetting response of intact rock, by inducing a partial tensile fracture in a controlled three-point-bending test undertaken on a fine grained granite. After removing the specimen from the loading frame, we instrument a free-standing sub-sample containing the induced fracture, and observe the effect of unidirectional capillary-driven water uptake in two orientations (from the crack tip, and the crack mouth) using a combination of time lapse digital image correlation and transmitted acoustic energy. Observations encompassed a full wetting-drying cycle in each orientation, and lasted a total of over 40 days.

In both configurations, we observed progressive wetting of the crack surface for a period of over 100 hours. This wetting generated significant localised strains at the crack (with magnitudes again up to 10^-4), as well as distributed strains in the adjacent intact rock as the wetting front progressed. Strains induced on the crack itself indicate internal stresses in the order of several megapascal were generated during wetting, and were almost completely recovered on drying. These stresses were of a similar magnitude to those induced by crack closure on unloading of the three-point-bending test, and along with a significant loss in acoustic transmission, our observations indicate wetting of the sample generates a complete loss of normal stress on portions of the crack.