HYDRATION SWELLING OF WATER-ABSORBING ROCKS - A CONSTITUTIVE MODEL

Water-absorbing rocks are formed from minerals that can hold water in their crystal structure or between grain boundaries. Such water absorp tion is often accompanied by a change in the crystal dimension that ma nifests itself as a swelling of the rock. Swelling is particularly pro nounced in rocks containing phyllosilicates because of the ease with w hich these minerals hydrate; it is thus of geological and geotechnical relevance in shales, clay-rich soils and zeolitized tuffs. The model of hydration swelling that we present here is based on extended versio ns of the equations of poroelasticity and Darcy's transport law, which we derive using a nonequilibrium thermodynamics approach. Our equatio ns account for the hydration reaction under the assumption that the re action rate is fast in comparison with the rate at which hydraulic sta te changes are communicated through the rock, i.e. that local physico- chemical equilibrium persists. Using a finite-element scheme for solvi ng numerically the governing equations of our model we simulate the cr eep of shales during a routine swelling test and calculate the stress and strain distributions around wellbores drilled in shale formations that undergo swelling. We show that swelling effects promote tensile f ailure of the wellbore wall.