Mineralogical and petrophysical properties evolution of a weathered limestone in southern Belgium

The weathering of limestone results in the partial dissolution of calcium carbonate, leaving a soft and porous material called alterite. Properties of the weathered rock are strongly different from its parent rock due to changes in composition and departure of soluble materials. The resulting increase in porosity modifies the hydrological proprieties of the rock such as permeability, hydraulic conductivity and reservoir capacity. Furthermore, the loss of material weakens the structure of the rock resulting in a decrease in mechanical resistance. This study aims at quantifying the evolution of the mineralogical, petrographic and mechanical properties of the Petit Granit, which is a Carboniferous limestone from the area of Soignies (southern Belgium). Rock samples representing various stages of weathering were collected within the same stratum. Their density, porosity, permeability, content in carbonate and mechanical resistance were analyzed. X-ray diffractions and thin sections analysis were conducted to document the mineralogical and petrophysical changes due to weathering. The results clearly show the influence of weathering on rock properties: a wide gain in porosity implying a decrease in density, a huge increase in permeability and a drop of the mechanical strength to zero. Finally, we show how these properties change with the weathering intensity.