Influence of Weathering on Mechanical Behaviour of Garnetiferous Quartzo-Feldspathic Biotite Gneiss

In the present study, garnetiferous quartzo-feldspathic biotite gneisses of different weathering grades from Eastern Ghat Mobile belt of southern peninsular India were investigated in the laboratory for their uniaxial compressive strength (UCS), tangent modulus (E), and Poisson’s ratio in saturated condition. To understand the influence of weathering, gneisses were divided and investigated broadly into three different grades, namely fresh rock (FR), moderately weathered (MWR), and highly weathered (HWR). The mineralogical composition of these gneissic rock variants was quantified by X-ray diffraction (XRD) analysis. The XRD patterns depicted the presence of kaolinite and illite minerals in MWR and HWR grades. With the increase of weathering degree, the clay minerals were also increased. For HWR grade, it was observed that the diffraction peak becomes broader which indicates the presence of poorly crystalline mineral. The mode of failure of rock in compression changed from multi-axial splitting to pure shear, with increase of degree of weathering. Furthermore, physical and slake durability characteristics were studied. An exponential trend was observed in slake durability values of HWR rocks when tested for 9 consequent cycles. In comparison to other grades, remarkable failure was observed in HWR gneisses on saturation, which has resulted in drastic reduction of the strength and deformability characteristics.