The Engineering Geological Characteristics and Alteration Classification of Altered Granite in East Quwu Mountain, Gansu, China

With its excellent physical and mechanical properties, granite is often the first choice for the foundation material for dams in water conservancy engineering. However, alteration can profoundly change the mineral composition, structure, and mechanical behavior of deep granite, posing critical challenges to project safety. The Quwu Mountain area in Baiyin, Gansu Province, a proposed pumped storage reservoir, exposes extensive Silurian granite. Engineering investigation shows that different levels of clay and hydrothermal alteration have taken place in the granite rock mass, and the level of alteration exhibits a distinct vertical zonation as revealed by borehole core logging. In this study, we quantitatively characterize the porosity, compressive strength, wave velocity, and shear parameters of altered granite of different degrees through mineralogical analysis, laboratory tests, and in situ testing. In order to guide the construction in this area, we establish a classification system that distinguishes weak, moderate, and strong alteration degree, based on macroscopic features, RQD, and clay mineral content. Results of this paper show that alteration is dominated by potassium feldspathization and kaolinitization, leading to increased porosity (4–10%) and structural loosening. Strongly altered granite exhibits severe mechanical degradation, moderately altered granite retains medium strength, and weakly altered granite approaches the properties of fresh rock. This research can provide technical support for engineering safety design and risk prevention in the Quwushan reservoir area, but its applicability to other regions requires further validation.