Material property changes in granitic rock during long-term immersion in hot water

To study the effects of temperature and time on hydro-mechanical properties of crystalline rocks, a laboratory weathering test was carried out, in which twenty-five cylindrical specimens of biotite granite were immersed in 90 degrees C water for 2000 days. During the test, various measurements were made, in parallel, to investigate the long term changes in the specimens' physical properties such as permeability, porosity, longitudinal wave velocity and so on. Growth of microcracks and synthesis of clay minerals were also investigated to supply fundamental data for the analysis. The experimental results indicate that a discontinuous increase in permeability takes place around 1030 days of immersion, although only a slight increase occurs up to 670 days. This occurs because microcracks become dense because of chemical and physical weathering, and thus become fully interconnected to form large-scale flow paths. Halloysite and smectite were synthesized as weathering products of plagioclase in the laboratory weathering tests. A stably produced clay mineral changes from halloysite to smectite at around 1030 days. This can be explained in terms of the selection of stably produced clay minerals dependent on the chemical condition of the solution. It can be said that chemical weathering, i.e., hydration forming clay minerals, is a key factor when considering the safety and integrity of structures constructed in underground rock masses and subjected to circulation of subsurface waters for a long period of time.