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 weatheredbedrock layer varies greatly, attaining several meters under certainclimatic and lithologic conditions. The hydrology of catchments onbedrock is strongly controlled by the hydrologic functioning of theseweathered bedrock layers, and the understanding and modeling of thisfunctioning requires a knowledge of their hydraulic properties, suchas their water retention curves. There are few references availableon the water retention curve of weathered bedrock. To fill this gap,we measured the water retention characteristic of highly weatheredPlomelin leucogranodiorite in replicate core samples. The study ofweathered granite samples showed that their mean water retentioncurves differed significantly from those estimated for soil samplesof the same grain size distribution using a pedotransfer functionand for other granites with the same weathering index. Even if onecan generate hypotheses to explain the observed differences, thethree approaches we used to compare our results cannot replace theexperimental approach to generate the water retention curve of amaterial such as weathered granite. In addition to enriching thebody of work on water retention curves of weathered bedrocks, theresults of this work suggest that there is a need for developinga database of retention properties of weathered bedrocks in parallelwith the development of a model based on factors more appropriatethan the weathering index.