Fissuring and weathering of clays as controlling factors for slope evolution in the Montemesola area (southern Italy)

Weathering in the form of the processes of chemical alteration and physical breakdown may result in the production of thick sequences of weathered materials, whose physical and geotechnical properties have been strongly reduced, and which thus may be highly prone to slope failures and erosion. Even though much of the scientific literature on the topic is devoted to the analysis of crystalline rocks, weathering of sedimentary rocks is also important in many geological settings. Recognition and mapping of materials showing different grades of weathering is crucial for the identification of varying susceptibility to erosion. Different weathering grade horizons can be distinguished, and their spatial distribution on hillslopes generally mapped, by means of: 1) surveying of the weathering profile on natural and excavated slopes; 2) mineralogical and petrographical analyses to determine weathering classes; and 3) characterization of the weathering grade by means of in-situ and laboratory tests. This paper deals with analysis of stiff clays (Argille Subappennine) in the Taranto province (southern Italy), and discusses methodologies and techniques used for a better understanding of the weathering conditions of the clay materials on the Montemesola hillslopes, their susceptibility to erosion, and the control on the evolution of the argillaceous slopes. Argille Subappennine of Pleistocene age are widespread along the easternmost part of the southern Apennines of Italy and in the Apulian foreland. Not much focus has been so far directed toward their study with regard to erosion and instability phenomena; nevertheless, the diffuse presence of Argille Subappennine along the Ionian coast, and the choice of several quarries in these materials as sites for industrial landfills, have recently highlighted the need towars a better understanding of these materials, aimed at assessing any possible environmental degradation. A multi-disciplinary approach involving geology, geomorphology, hydrogeology, and geotechnique was therefore performed; in addition, recently published data on mineralogy and geotechnique of Argille Subappennine have also been considered. The work started from a detailed geological and structural analysis conducted on several natural and excavated slopes in the area: this phase of study allowed to identify the main systems of discontinuity in the rock mass. Fissuring seems characterized by good persistence along the overall exposed outcrops, and is typically marked by different colour which gives an overall yellow aspect to the upper portion of the clay slopes, strongly contrasting with the gray colour of the lower, less-weathered to unweathered, clays. A discrimination between fissures and the other discontinuities (the latter being related to lamination of argillaceous deposits, drying, and tensional release in the outermost portion of the slopes) has also been attempted. At the outcrop scale the fissures are marked by the yellow colour, which is indicative of the preferential ways of infiltration of water into the rock mass. Continuity and persistence of the main systems of fissuring in the clays is furtherly evidenced by morphology of exposed slopes, where erosional landforms (namely, rills, gullies, and badlands) are preferentially oriented according to the main directions of fissuring. The geotechnical analysis and the laboratory tests have been performed through: i) geotechnical characterization of the two portions exposed on the slopes (unweathered and weathered clays); ii) permeability tests; iii) tests of chemical interactions with likely contaminants (to evaluate the suitability of the argillaceous materials to host landfills). In addition, specific analysis addressed to evaluate the dispersivity of contaminants in the