Digital image correlation-based analysis of hygroscopic expansion in Herrnholz granite

Groundwater-induced deformation of natural rock slopes is typically interpreted to be related to changes in pore pressure. Rock, however, is a porous medium that has been shown to expand on contact with water. Here, we present insights into surface deformation of free-standing 115 x 65 x 35 mm granite prisms with water continuously infiltrating from the upper surface over 16 - 24 hours under ambient laboratory environmental conditions. Digital image correlation analysis indicates a mean extensional strain due to wetting of approximately (4 - 4.7) x 10-4, equivalent to an internal pressure of 40 - 47 MPa, assuming linear-elastic material behaviour. Nanoscale adsorption theory indicates the contribution from adsorption of water onto free surfaces may be 18.4 MPa, while capillary condensation generates pressure up to 30.9 MPa. It is likely a majority of strains is the result of water entering micro pores with radius less than 5 nm, which account for 1.16% of the porosity. Our relatively simple experimental setup may serve as a useful tool for analyzing moisture-induced deformation in a range of geological materials.