DEFORMATION AND PORE PRESSURE IN DEHYDRATING GYPSUM UNDER TRANSIENTLYDRAINED CONDITIONS

The development of pore-fluid pressure in dehydrating rocks is a compe tition between the reaction, which produces water, and the change in p ermeability caused by the reduction in solid volume. Our study explore s the development of pore-fluid pressure and the mechanical behavior o f dehydrating rocks under conditions where the relative drainage is tr ansient, i.e. drainage evolves as the reaction progresses. The dehydra tion system studied was gypsum to bassanite plus water. Hydrostatic pr essure and axial compression experiments were performed at 23 degrees to 150 degrees C, confining pressures (P-c) of 0.1 to 200 MPa, pore pr essures (P-p) of 10 to 100 MPa, and strain rates of 7x10(-7) to 6X10(- 5) 1/s. The volume of water expelled from the sample during dehydratio n, the differential stress, the reaction ratio, and the porosity distr ibution were determined as a function of time. The expulsion of water with time is divided into three stages broadly defined as: (I) the for mation of a connected pore network; (II) the maximum water expulsion r ate; and (III) the completion of the reaction. At high effective press ures (P-e = P-c - P-p), gypsum deforms by plastic flow and cataclasis at temperatures below that necessary for dehydration. Above the dehydr ation temperature, samples show weakening and embrittlement, indicativ e of low effective pressures, when deformed in stage I. In stages II a nd III, the sample strength increases gradually with time, eventually exceeding the pre-dehydration strength of gypsum. These results sugges t that high pore pressures (low effective pressures) are only transien t and occur in stage I. The exceptional strengthening in stages II and III occurs because the new phase, bassanite, is stronger than gypsum. The evolution of pore pressure with time in a transiently drained deh ydrating system has been modeled by incorporating fluid release and po rosity change rates into a hydraulic diffusion equation. The numerical simulations show that initially, pore pressure increases following an undrained path and a pore pressure in excess of hydrostatic is possib le. Later, the increase in porosity and permeability makes drainage mo re efficient and the pore pressure decreases rapidly. The magnitude an d duration of the excess pore pressure are sensitively dependent on th e dehydration kinetics and the hydro-mechanical properties. The pore p ressure peak predicted from the numerical simulations correlates with the experimental results that weakening and embrittlement occur only i n stage I. Our results suggest that even under natural conditions wher e rocks have a finite drainage, high excess pore pressures may occur.