Recent research has outlined that mobilized in situ strength at the reactivation of displacements along a preexisting sliding surface may be slightly greater than laboratory residual strength. Although geotechnical literature explains this difference in various ways—for instance, strength gain due to soil aging—little work has been devoted to this topic. Bromhead ring shear tests were carried out on remolded samples of the Cormons flysch, a typical geological formation in northeastern Friuli-Venezia Giulia (Italy). The specimens came from the shallow weathered crust of the Cormons flysch, near the areas of Montona and Rosazzo. This experimental study aimed at highlighting the effect of a limited number of parameters on strength gain at the reactivation of displacements along aged sliding surfaces. The results of this research confirm that aging may cause a rising of reactivated shear strength above the residual value, depending on time of aging and vertical stress. The longest aging period—about a month—s...

The 1963 Vajont landslide is a reference example of large rockslides involving clay interbeds emplaced in sedimentary rock masses in correspondence with the basal rupture zone (thinly stratified cherty limestone of the Fonzaso Formation dated to Middle–Upper Jurassic). The basal shear zone of the 1963 Vajont landslide was made up of a chaotic assemblage of displaced rock masses, limestone angular gravel, and spread clay lenses. The mineralogical investigations showed that the clays are characterized by complex assemblages of illite/smectite mixed layers (36–96%) admixed with variable amounts of calcite (4–64%) and quartz (0–6%). The clay layers show highly variable plasticity properties and shear strength characteristics. The samples with a large prevalence of clay mineral content (CM) (CM > 79%) are characterized by low values of the residual friction angle (6.7–14.9°), whereas clay materials characterized by a higher content of granular minerals (calcite and quartz) clearly show greater friction angle values (19.5–26.7°). The high permeability of the limestone angular gravel, which caused a rapid reservoir-induced inflow (1960–1963), together with the low friction angle of the clay layers were responsible for the overall shear strength reduction in correspondence with the basal rupture zone, thus favoring the huge sliding on 9 October 1963.

In this work, we describe soil moisture profiles related to typical colluvial slopes that were involved in rainfall-induced shallow failures occurring in alpine and pre-alpine areas of the Friuli Venezia Giulia Region (NE Italy). The trend of the volumetric water content (θ_w) showed a general increase from the ground surface to the bottom soil layer, with two or three marked moisture peaks. The saturation degree (S) varied from 65–70% (topsoil horizon) to nearly saturated basal colluvium (S = 95–100%). Soil moisture data demonstrates that, for a very humid climate, colluvial covers are often close to the saturation condition for most of the year. The calculated suction profiles indicated that maximum values ranging from 40 to 55 kPa often occur in the slope surficial soil (depth < 0.2–0.5 m). This negative pore-water pressure greatly decreases after a heavy rainfall event because of the infiltration process. Complete saturation of colluvial cover in the alpine and pre-alpine regions generally requires rainfall exceeding 150–200 mm for a 24-h storm duration. This results in a recurrence time of Tr ≅ 5–10 years for critical rainfall episodes involving colluvial slopes in the Friuli Venezia Giulia Region. The case histories analyzed demonstrate the importance of performing a detailed lithostratigraphic analysis of the colluvial deposit in order to properly define the suction measurement points, which there should be more of than the three-point determinations usually reported in the literature (for example, z = 0.5, 1.0 and 1.5 m).

Heterogeneous rock masses that include rhythmic alternations of marl, shale, marly limestone, sandstone, siltstone, and argillite, such as Flysch, are particularly prone to generating colluvial deposits on gentle slopes, which are often subject to failures triggered by heavy rainfall. Flysch-derived colluvial soils are made up of highly heterogeneous sediments ranging from clayey loam to rock fragments, and they have been studied more rarely than homogeneous soils. In this work, we present a geotechnical and hydraulic characterisation performed both in situ and in the laboratory on flysch-derived colluvial soils that were involved in a channelised landslide in the pre-alpine area of the Friuli Venezia Giulia region (NE Italy). The investigated soils were characterised by the average values of the grain size composition of about 25% gravel, 20% sand, 30% silt, and 25% clay. The loamy matrix presented low-to-medium values of the liquid and plastic limits, as well as of the plasticity index (LL = 40%, PL = 23%, and PI = 17%, respectively). The values of the peak friction angle for natural intact samples were 33° < ϕ’_p < 38°, whereas the residual friction angle fell to 23–24° at great depths and high vertical stresses, for a prevailing silty–clayey matrix. Variable head permeability tests were performed both in situ and in the laboratory, showing that the values of the vertical and horizontal permeability were very close and in the range 1 × 10⁻⁴–1 × 10⁻⁶ m/s. The soil permeability measured in the field was generally higher than the hydraulic conductivity calculated on laboratory samples. The proposed geotechnical and hydrological characterisation of flysch-derived colluvial soils can be of fundamental importance before the use of more thorough analyses/models aimed at forecasting the possible occurrence of slope failures and evaluating the related landslide hazard. The reported geotechnical and hydraulic parameters of flysch-derived colluvial materials can represent a useful reference for rainfall infiltration modelling and slope stability analyses of colluvial covers that are subject to intense and/or prolonged precipitation. However, when facing engineering problems involving colluvial soils, particularly those coming from flysch rock masses, the intrinsic variability in their grain size composition, consistency, and plasticity characteristics is a key feature and attention should be paid to the proper assumption of the corresponding geotechnical and hydraulic parameters.