Geotechnical characterization of the soils along the alignment of Montedoglio-Lake Trasimeno water pipe (Central Italy).

The Lake Trasimeno is one of the major Italian lakes and its economic and environmental significance is quite great. In the last years, the anthropic pressure effects became, though enhanced, only incidental to the consequences of a dry climatic phase that led to a drastic reduction in the lake water levels which are strictly influenced by rainfall rather than by groundwater flows. After the last water crisis (2002-2003 period) a water pipeline from Montedoglio dam to lake Trasimeno (Tuscany-Umbria border) was planned with the purpose to regulate the lake levels and to satisfy the agricultural water demand of Chiana valley. This note gives the results of the geotechnical and mineralogical characterization of some Pleistocene-Holocene fluvial-lacustrine soils found on the test trenches excavated along the 3th lot of the water pipeline (21 km long). About fourteen samples were analyzed along the alignment in order to verify the suitability of the soils as materials of covering and to correlate the geotechnical and mineralogical parameters with other clayely soils of central Italy (Umbria region). Ten samples belong to groups A4 and A6 of the CNR UNI 10006 (silts and not very compressible clays), two of them belong to A2-4 (gravels and silty sands), and only one (C4) belongs to group A7-5 (highly compressible clays of medium plasticity). According to the modified Skempton diagram, all the samples align with the kaolinite line and, unlike other soils of fluvial-lacustrine origin from the Chiana valley, they show a medium-low swelling potential. An analysis of the Proctor curves shows how the samples generally have a maximum dry density ranging between 2.07 and 1.95 g/cm3, while the optimum moisture content varies between 8.6 and 11.4%. The C4 sample differs from the rest, showing a much broader curve, with moisture content values from 12% to 20% and a maximum dry density value of 1.76 g/cm3. In conclusion, the soils found along the alignment can be considered suitable for covering the water pipe, except for those from the area where sample C4 was taken. Consequently, if the soils in question are used, since both the organic carbon content and the sulfates content are suitable, it is recommended their lime stabilization in order to reduce the problems connected with swelling.