Deep weathering and alteration in granites - a product of coupled processes

Weathering and alternation in granite has a deep inpact on both geotechnical properties of the rock as well as of the rock mass. In a granite rock mass, the discontinuity pattern together with joint cohesion and friction plays a major role in tunnel construction and road cut slopes. These pheneomena and their cou-pling could be exclusively studied at the Königshainer Berge Tunnel Project (Germany) where two tubes provided an unique cross section through zones of intensive weathering and hydrothermal alteration within two-mica granites. Some parts of the granites underwent two stages of hydrothermal alteration before beeing under surface conditions. Therefore it was necessary to distinguish between the effect of hydrothermal altera-tion and weathering on mineralogy as well as on geotechnical rock properties. Only after the endogene al-teration processes had finished, the rock has been exposed to the exogene processes of weathering. The change of mineral content and the increase of pore volume promotes the action of chemical decomposition.