Experimental Determination of Hydrodynamic Properties of Weathered Granite

Abstract

Weathering processes transform hard fresh rock into regolith materials, composed of weathered bedrock and soils. The thickness of the weathered bedrock layer varies greatly, attaining several meters under certain climatic and lithologic conditions. The hydrology of catchments on bedrock is strongly controlled by the hydrologic functioning of these weathered bedrock layers, and the understanding and modeling of this functioning requires a knowledge of their hydraulic properties, such as their water retention curves. There are few references available on the water retention curve of weathered bedrock. To fill this gap, we measured the water retention characteristic of highly weathered Plomelin leucogranodiorite in replicate core samples. The study of weathered granite samples showed that their mean water retention curves differed significantly from those estimated for soil samples of the same grain size distribution using a pedotransfer function and for other granites with the same weathering index. Even if one can generate hypotheses to explain the observed differences, the three approaches we used to compare our results cannot replace the experimental approach to generate the water retention curve of a material such as weathered granite. In addition to enriching the body of work on water retention curves of weathered bedrocks, the results of this work suggest that there is a need for developing a database of retention properties of weathered bedrocks in parallel with the development of a model based on factors more appropriate than the weathering index.