Abstract: Altered granite develops in the dam site of Daigusi Reservoir in Gansu Province. The alteration characteristics of altered granite directly affect the engineering characteristics of rock mass and the safety of permanent and temporary hydraulic (construction) buildings. The engineering characteristics of altered granite in the dam site area of Daigusi Reservoir were examined in macro-and-microscopic scales via field survey, sampling test, and geophysical prospecting. Results suggest that the altered granite is mainly caused by magmatic and vein hydrothermal metasomatism, and the alteration type is mostly argillization. Altered granite concentrates in the riverbed and the low elevation parts of the near-river right-side rock in 70-180 meters downstream of Daigusi Hydropower Station; the depth of alteration is concentrated in the range of 20-100 meters below ground surface. The altered granite is featured with remarkably reduced strength, increased porosity and water absorption, as well as sound integrity.

Key words: granite, alteration, engineering geological characteristics, geological survey, Daigusi Reservoir