Effects of weathering on characteristics of rocks in a subtropical climate: weathering morphology, in situ, laboratory and mineralogical characterization

This paper presents the results of a comprehensive study on the morphology of weathering profiles developed on metamorphic and sedimentary rocks under subtropical climate, focusing on its influence on geotechnical parameters of weathering materials. Several cut and natural slopes were selected for more detailed descriptions of their physical, mineralogic, geologic, structural and geotechnical parameters, focusing on weathered rock materials, including characteristics of matrix (e.g., fabric, mineralogy and degree of weathering) and rock mass (e.g., rock:soil ratio, discontinuity characteristics, JRC—joint roughness coefficient, and JCS—joint compressive strength). Samples from the upper portions of the profiles and down into the transition zone between rock and soil were collected to determine their physical and geotechnical parameters, to prepare and describe thin sections and to perform X-ray diffraction (XRD) analyses and point load tests. Results indicate that weathering profiles are thin, contacts between material layers are sharp and there is a strong structural control on weathering, mainly by stress-relief joints. Soil-like material is present only in portions of the rock mass near the ground surface or along structural discontinuities (foliation and fractures). Thin-section description can provide important indexes and information that can be related to chemical and physical weathering of those rocks and so be used to differentiate materials present along profiles. Also, physical parameters have proved to be useful for the distinction of different weathered materials. Finally, XRD and thin-section description have allowed the identification of important mineralogical changes due to weathering.