Changes in the Physical, Mineralogical and Geomechanical Properties of a Granitic Rock from Weathering Zones in a Tropical Climate

Abstract

The changes in the composition and structure of the rock matrix caused by weathering lead to great alterations in its mechanical and physical properties. When considering this, geologists and engineers should pay special attention to materials from transition zones, namely, between the soil and rock over the weathering profiles. This research aimed to perform a wide characterization of the physical, mineralogical and geomechanical properties of a syenogranite rock in various degrees of weathering developed in a Brazilian tropical climate. Laboratory tests were performed to characterize the rock matrix properties at 5° of weathering. The mineralogical composition, the types and number of microstructures, the main physical and chemical changes, and the micropetrographic and microfracturing indexes of the matrix were obtained from analysing petrographic thin sections. The physical and geomechanical characterization included the determination of physical properties, index properties, strength and strain. The thin sections showed that the tropical climate had a great influence on the mineralogical changes, with the occurrence of mainly chemical processes from the medium to the highest degree of weathering. Among the properties evaluated, porosity was the most sensitive to the weathering, reaching 51 times higher in the highly weathered rock. The microfracturing index did not change much, showing a 42.0% increase from the low value of the fresh rock to the slightly weathered rock. Regarding the mechanical properties, the uniaxial compressive strength was the most sensitive to weathering, as observed by a loss of approximately 99.0% of its initial strength when in highly weathered rock conditions. In agreement with previous studies published on granites worldwide, the larger variations in the syenogranite rock properties were also presented from the medium degree of weathering. An experimental study on the use of Digital Image Correlation produced satisfactory results for determining the axial strain and elasticity modulus for more weathered syenogranite materials. New correlations and a quali-quantitative procedure for its classification have been proposed and tested based on the concepts of applicability, representativeness and significance. From 10 proposed correlations, 50% were considered with high or total applicability, and its use is highly recommended for estimating the parameters for syenogranite. Alternatively, from 31 correlations found in the literature, only 16% have shown high or total applicability for syenogranite.