Deep weathering of granitic rocks: A case of tunnelling in NW Spain

This paper highlights a case of extensive development of saprolith from granitic bedrock in NW Spain. The weathering front at the study area extends to more than 250 in in depth. The drilling of a railway tunnel (Madrid-Galicia high-speed line) through the bedrock and saprolith resulted in significant technical issues and economic losses. Cross-sections revealing the 3D complexity of saprolith development are mainly based on fieldwork, boreholes, laboratory tests, and the excavation works. Saprock is always observed overlying unweathered igneous bedrock. Two saprolite facies, one clay-rich and the other clay-poor overlie the saprock: the recognition is based on several-criteria. These deposits gradually thicken from the boundaries between the granitoid and metasedimentary roof pendants. Factors controlling saprolith development are discussed, and the chemical and mineralogical changes and trends that occur are described. Variations in some geotechnical properties and the loss of quality of the rock masses from an engineering perspective are also quantified. (C) 2015 Elsevier B.V. All rights reserved.