Varicolored clays of the Marecchia Valleys geothecnical and mineralogical characterization of the Maioletto fortress sectio (northean Marchean).

The Appenine chain appears to be constituted by clay rich allochtonous complex. Among these the "Marecchia Valley Sheet" occurs in the Montefeltro area (northern side of the Marchean region). At Maioletto fortess site about 115 m thick section occurs, which is constituted by interbeddded calcareous and arenaceous laterally discontinuous thin layer. The sampling was aimed to get an overall representativeness of the section and of each cromatically different horizons. Samples were analysed for granulometric distribution and Atteberg's limits. The investigated layers can be classified as clay with silt, being the sand-size fraction almost always absent. The clay fraction are compositionally characterized by illite, smectites, chlorite kaolinite and mixed layers in different proportion. Along the analysed vertical profile the recurrent appearance of smectites and illite/smectite minerals can be mainly related to supply of vitric volcanoclastic material, while, illite, chlorite, and kaolinite to terrigenous sedimentation. The composition of the clay minerals seems to greatly influence the geotechnical parameters, such as activity and plastic index, as well as the chromatic characters of the collected samples. As a general rule, the dark grey layers show a medium to high plasticity while the lighter ones a medium to low plasticity. The comparasion of activity and relative amounts of expansible fraction of clay minerals has pointed out the occurence of at least three different groups of samples, characterized by: 1) high values of activity (between 1.26 and 2.01) and expansible fraction (between 60% and 68%); mediu values of activity (between 0.82 and 1.12) and expansible fraction ( between 37.5% and 50%); low values of activity (between 0.53 and 0.74) and expansible fraction (between 17:5% and 30%).