Weathering of the Oporto granite: geotechnical and physical properties

Abstract

This paper characterizes the weathering products of the two-mica Oporto granite in terms of its mineralogical, chemical, geotechnical and physical properties. This information is used to identify a physical property that can be used as an index of the degree of weathering and to estimate other properties. In outcrop, the Oporto granite is always weathered and weathering profiles frequently exceed 20 m. Within these profiles, granitic saprolites are composed of quartz, feldspars and some mica with kaolinite and gibbsite as final weathering products. The average percentage of secondary minerals determined by bulk X-ray diffraction (XRD) is low in both weathered rock (2.4%) and saprolite (8.7%). Geotechnically, the saprolites exhibit no or low plasticity (plasticity index, PI≤12%). The skeleton character of the saprolites is reflected in the significant proportion of fines (29.5%<75 μm) and low percentages in the <2 μm fraction (6.2%). According to the Unified Soil Classification System (ASTM D 2487-93), most saprolites are SM-silty sands, with some SW-SM-well graded sand with silt, SC-clayey sand, CL-lean clay and ML-silt. Total porosity, free porosity, dry bulk density, ultrasonic velocity and uniaxial compressive strength tests of fresh and weathered rock vary with the degree of granite weathering. This permits correlation between physical properties and the identification of free porosity as the property most strongly influenced by weathering. It is proposed that this could be used as a weathering index, and as the property, which correlates with the highest number of other physical properties; it might be used for their estimation.