CHANGES IN THE ENGINEERING GEOLOGICAL PROPERTIES OF GNEISSIC ROCKS DUE TO WEATHERING

Textural variations and changes in physical properties of gneissic rocks due to weathering were investigated on both field samples of different weathering grades, and fresh rock cores subjected to various artificial weathering environments. Chemical weathering indices, such as weathering potential index and the weathering product index, were also measured. Gneissic rocks generally show structural complexities related to the metamorphic processes, and the presence of these features has substantial effects on developing differentiated weathering zones. The effects of various weathering agents, such as temperature, water, acid and salt, were investigated by soaking in acidic solutions, and by adopting salt crystallization and freezing-thawing tests on fresh rock samples. Physical properties, including porosity, water absorption, uniaxial compressive strength, tensile strength and P-wave velocity were measured. Physical properties did not show much changes from various artificial weathering tests, but soaking in acid showed remarkable change in the physical properties of specimens. From the XRF analysis, Fe₂0₃ and L.O.I increased with increasing weathering grade, while FeO and K₂O decreased. Decomposition of feldspars and resulting clay minerals, such as kaolinite and illite, were observed in highly weathered samples. It seemed to be possible to reference weathering grades for gneissic rocks from good correlations of WPI-WP, and I_mob-WP(PI).