GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 3.2 613 Geological, geomorphological and geognostic surveys of the landslide. The investigated area is located in the south-western side of Asti Reliefs (elevation 130-320 m a.s.l.) in the central Piedmont hilly region (Asti Province), in San Martino Alfieri village. The stratigraphy of southern Asti area consists of a succession of incoherent sediments, related to depositional environments changing with time from marine to deltaic and fluvial (Carraro 1996; Forno et al. , 2015). The sedimentary succession is defined, in the lower bands of the hilly reliefs, by deep marine clay deposits (Lugagnano Clay) and in the higher bands by littoral sandy deposits (Asti Sand) referred to the Pliocene age (respectively to Piacenzian and Zanclean). Above these sediments a widespread silty and subordinately gravelly fluvial cover of middle and upper Pleistocene occurs. The fluvial deposits appear as a terraced fluvial succession that develops at different heights at the top of the hill ridges in the Asti Reliefs frequently involved by surficial landslides (mudflows) (Barbero et al. , 2014a; Barbero et. al. , 2014b; Carraro and Valpreda, 1991). The whole sedimentary sequence is deformed by the Asti Syncline consisting of a wide E-W regional scale fold developed in the central hilly area. The hilly ridge involved in the landslide corresponds to a vineyard area, gently sloping towards SSW. This landslide, with a rotational slip mechanism, develops in a slope, with a convex morphology due to several landslides occurred over time, shaped in the marine succession formed by gray-blue clay massive deposits, with horizontal-parallel bedding (strike E-W) dipping N, about 20°, corresponding to the Lugagnano Clay (Lower Pliocene, Piacenzian) (Fig. 1). The Lugagnano Clay is some hundred meters thick and consists of gray-blue clay and massive silt deposits and sandy marl rich in microfossils, with horizontal-parallel bedding, outcropping in the lower parts of the hilly reliefs into the Tanaro River incision and secondary rivers. The sedimentological features and the paleontological content suggest a deep marine environment (Boni et al. , 1969). Geological, morphological and geotechnical surveys allowed to recognize the morphological features and geomechanical characteristics linked to the oldest landslide and those connected to its triggering (Barbero and Tissoni, in press) (Fig. 1). The sedimentological features of displaced material suggest that the landslide involved the soil developed on the clay substrate and its cover, that show similar features. Geognostic and geotechnical surveys consisting of standard n. 5 cone penetration tests (S.C.P.T.) (P in Fig. 1) and n. 2 rotary drilling borings (S in Fig. 1) were performed in the eighties of last century while, in the last year, n. 2 S.C.P.T. (Pn in Fig. 1) have been carried out on the triggered landslide body. The penetration tests were performed longitudinally to the development of the landslide while, the borings, for logistical reasons, were made on the road. Borings revealed the depth of the bedrock (Lugagnano Clay) at 9 m b.g.l. The sedimentological features and the stratigraphic interpretation of the subsoil under the road, explored by means of borings, are as follows: - 0.00 - 3.50 m: reworked soil (colluvial cover and upper part of the old landslide body) - 3.50 - 7.00 m: less plastic silty clay and yellowish brown to yellow grey clay (old landslide body) - 7.00 - 9.00 m: yellowish clay marl (transitional facies) - 9.00 - 15.00 m: compact grey marl (bedrock) The recent landslide body involves the reworked soil of the oldest one. The geotechnical proprieties of soils were obtained by the execution of analysis for soils classification, according to MIT and Atterberg Limits. The grain size distribution is therefore in a very narrow range: the size fraction below the sieve A.S.T.M. (American Standard Test Sieve Series) nr. 200 (<0.075 mm) is around 90% (60% silt and 30% clay). Soils plasticity features, according to Casagrande Plasticity Chart, correspond to Liquid Limit values between 42 – 53% and Plasticity Index values between 11 - 16%. The soils can be classified as “silty clay of low plasticity”.