Effect of Ambient Humidity on the Elasticity and Deformation of Unweathered Granite

Moisture variation has been noted as a driver of strains in both natural and anthropogenic bedrock environments. Similarly, increasing water content has been shown to reduce elastic stiffness in a variety of rock materials, though evidence for granites is limited. This study presents strains of an axially stressed (0.1 MPa) Herrnholz granite cylinder with ambient relative humidity alternating between 20% and 90% in a stepwise manner. At each ambient humidity level, the Young's modulus and Poisson's ratio were determined by performing a series of load and unload cycles at 1-12 hr intervals. We observed that when the ambient humidity increased from 20% to 90%, Young's modulus declined by 13% on average, and nearly returned to the initial dry value when the ambient humidity was reduced to 20%. Poisson's ratio increased by 130% in response to the same humidity change, and approximately 60% of it was reversed when dried. These changes occurred linearly with a volumetric strain of up to 8 x 10(-4) of the tested sample, equivalent to 24.5 MPa internal stress. This stress change is attributed to the generation of nanoscale adsorption stress, which includes surface adhesion pressure along pore walls and capillary pressures within characteristic pore spaces. Using a modified adsorption model, the two contributions were estimated to be 8.5 and 16 MPa, respectively. The agreement between laboratory-derived and modeled stress change validates the proposed mechanism of adsorption-induced strains and elastic property variations in unweathered granite. Plain Language Summary The public is well aware that moisture variations caused by rainfall events or freeze-thaw processes have a negative impact on the stability of rock-formed infrastructures, such as stone constructions, water supplies, bridges, and natural rock slopes. However, the underlying mechanisms, such as how liquid water or vapor affects the properties and behaviors of rock, are not well understood. Granite, in particular, is generally known as a hard, low-porosity, and clay-deficient material; however, its softening and expansion under moist conditions is underappreciated. In a stepwise manner, we measured progressive deformation, and changes in Young's modulus and Poisson's ratio of a Herrnholz granite cylinder with ambient humidity alternating between 20% and 90%. Results suggest softening (Young's modulus) and volumetric expansion of unweathered granite in wet conditions, and vice versa in dry conditions.