GENERATION AND MAINTENANCE OF PORE PRESSURE EXCESS IN A DEHYDRATING SYSTEM .1. EXPERIMENTAL AND MICROSTRUCTURAL OBSERVATIONS

The generation and maintenance of excess pore pressures in dehydrating gypsum aggregates were investigated using experiments and microstruct ural analyses. A triaxial deformation apparatus, was equipped with a p ore fluid system connected directly to the dehydrating sample. This sy stem was operated in constant fluid volume mode to monitor pore pressu re increase under undrained conditions, and in constant pore pressure mode to monitor fluid expulsion under drained conditions. X ray diffra ction and backscatter scanning electron microscopy were used to charac terize the spatial relationship among gypsum, the product phase bassan ite, and the pores. In addition, we measured the permeability and pore compressibility of the starting material and explored the influence o f effective and pore pressures, temperature, and axial load on fluid e xpulsion. Three stages of fluid expulsion and microstructural evolutio n during dehydration of an initially low-porosity, low-permeability gy psum aggregate are defined: (1) Initially, fluid released by the react ion is trapped in isolated or discontinuous pore networks and high por e pressures are possible. (2) An interconnected pore network eventuall y develops and fluid readily escapes. (3) Fluid expulsion slows down d rastically as the reaction nears completion. As a result of coupling b etween dehydration and porosity production, both the cumulative volume of fluid expelled and the expulsion rate increase with increasing tem perature, effective pressure, and axial load and with decreasing pore pressure. Our hydrological and microstructural data, combined with pre vious mechanical data, provide a better understanding of the relations hips among changes in fluid volume, porosity, and pore pressure excess , and the deformation behavior bf a dehydrating system where drainage evolves with time.