Minero-petrographic, geochemical, and physical alteration of himachal gneiss an impact on uniaxial strength due to weathering

The present study provides multidisciplinary research on the alteration of petrographic, mineralogical, geochemical characteristics, and physical properties of Himachal gneiss due to weathering. Three different weathering grades fresh, slightly, and moderately weathered gneiss are collected from Kullu, Himachal Pradesh, India. The petrographic and mineralogical analyses are done by thin section and XRD analyses. The microstructural and geochemical studies of gneiss samples were performed using scanning electron microscopy (SEM) and X-ray fluorescence (XRF) respectively. The petrographic images revealed a large amount of alteration in the plagioclase due to weathering. The pervasive sericitization of plagioclase is observed in moderately weathered gneiss samples. The quartz grains are intact and only minor fracturing in some quartz grains is observed in moderately weathered gneiss samples. The SEM images indicate that the inter-granular and intra-granular micro-cracks increase with the weathering grades. The XRF results are used to obtain the different chemical weathering indices and their suitability for Himachal gneiss. Among twenty-five chemical weathering indices, the CIA, CIW, alkaline ratio, PIA, LOI, and WI show a suitable trend and can be used for assessing the weathering of Himachal gneiss. In addition, alterations in the physical properties of gneiss are observed due to weathering. The bulk, dry, saturated densities and slake durability index decrease while the porosity increases with the progressive weathering of gneiss. The progressive weathering significantly reduces the gneiss's uniaxial compressive strength (UCS). For all three weathering grades of gneiss, the maximum and minimum values of UCS were observed at 90˚ and 30˚ foliations, respectively.