Hydration swelling of crystalline rocks

Long-duration deformation experiments at room temperature have shown that crystalline rocks with low porosities, such as granites, diorites, gneisses and metabasites, swell when saturated with water and shrink when dried out. The effect is reversible and reproducible. The swelling cannot be completely hindered by confining pressures up to 15 MPa. The volume strain (from the dry to the saturated state) is of the order of a tenth of the porosity. The volume strain is decreased in brine with increasing molarity and becomes much smaller by saturation with heptane. Swelling and shrinking can also be observed with sorption and desorption of water, that is a change in the relative humidity of the air. It is proposed that the swelling effect is due to surface forces, that is van der Waals attraction and electrostatic double-layer and solvation repulsion. The pore pressure causing the swelling was estimated to be of the order of 15 MPa. This indicates the presence of very flat pores with distances between the surfaces of about 1 nm. The surface forces probably also play an important role in determining the seismic properties of the rocks at low confining pressures.