An Integrated approach to elaborate 3-D geological and geotechnical models: a case study from the Daunia Sub-Apennine (Apulia, Southern Italy)

Abstract

In the Daunia Sub-Apennine (Apulia, southern Italy) slope instability processes due to rainfalls and earthquakes are widespread and cause significant damage to buildings and other structures, and, in some cases, loss of life. A detailed slope stability assessment requires information on the predisposing and triggering factors, and a good knowledge of the geological and environmental conditions as well. As concerns seismic- induced landslides, conventional methods used for slope stability analysis can be divided in: i) force-based pseudo-static methods, ii) displacement-based methods, and iii) stress-strain methods. Detailed representations of geological and geotechnical units as well as static and dynamic geotechnical characterization of materials have to be considered for the correct choice of the method for slope stability analysis, since these are fundamental for slope behaviour prediction and modelling. The purpose of this paper was to present a methodological approach for elaborating detailed 3-D geological and geotechnical models for areas very heterogeneous in terms of geological and soil properties. In the southern portion of the eastern Daunia Sub-Apennine, the outermost formations of the chain domain crop out. These are represented by the Cretaceous-Miocene and Pliocene wedge-top basin units, followed upwards, in the easternmost portion, by the Plio-Pleistocene foredeep units and by Quaternary filling deposits. A high susceptibility to slope failures for the area is testified by the large number of slope movements consisting in mud flows, roto-translational and composite landslides, and soil slips. Field variability of the slope movements from site to site for mechanisms, velocity, depth of rupture surfaces and volume of materials involved is due to the presence of structurally complex formations, characterised by very poor mechanical properties and high variability of their lithological and structural features. The construction of geological and geotechnical models able to represent realistic information is conditioned by the efficacy of the methods used for assessing the spatial lithofacies distribution and parametrization. The case study of Deliceto is here presented, where the 3-D geological model was built based on the results of stratigraphic correlations between core logs and 2-D geological sections. In-situ surveys were performed by means of classical geological and geomorphological methods, and continuous coring boreholes. Silty-clayey sandstone materials (Conglomerates and Sandstones of Castello Schiavo) crop out along the slopes and rest on clayey silts and marls of turbidite origin (Flysch di Faeto). The Flysch di Faeto Fm. is a structurally complex geological unit which consists of three main lithofacies: 1) silty clays; 2) silty marly clays 3) marls and shales. A series of geotechnical laboratory tests, carried out in accordance with international standards for the static anddynamic characterization of materials, made it possible to obtain a detailed 3-D geotechnical model. In particular, resonant column (RC), cyclic torsional shear (CTS) and standard and cyclic triaxial (TXC) tests were performed on the silty clayey geotechnical unit of the Flysch di Faeto Fm., because it is highly susceptible to geotechnical fatigue resulting from cyclic stresses. The results of the laboratory tests confirmed a variable post-cyclic degradation in the range 40-80% and 12-36%, respectively for the secant shear modulus (G) and the undrained cohesion (cu) associated with an increase between 2.92% and 19.90% for the damping ratio (D), demonstrating the heterogeneity of the material in terms of geological and geotechnical characteristics.