A Heuristic Method to Evaluate the Effect of Soil Tillage on Slope Stability: A Pilot Case in Central Italy

Among the various predisposing factors of rainfall-induced shallow landslides, land use
is constantly evolving, being linked to human activities. Between different land uses, improper
agricultural practices can have a negative impact on slope stability. Indeed, unsustainable soil tillage
can modify the mechanical properties of the soils, leading to a possible increase of the instability
phenomena. However, the effects of soil tillage on slope stability are poorly investigated. To address
this topic, the PG_TRIGRS model (a probabilistic, geostatistic-based extension of TRIGRS) was
applied to a cultivated, landslide-prone area in central Italy, thoroughly studied and periodically
monitored through systematic image analysis and field surveys. A heuristic approach was adopted
to quantitatively evaluate the effect of soil tillage on the mechanical properties of the soil: after a first
run of the model with unbiased parameters, the slope stability analysis was carried out assuming
several percentages of reduction of the effective soil cohesion to mimic an increasing impact of soil
tillage on the strength conditions. Then, a comparison between observed landslides and the spatial
distribution of the probability of failure derived from the application of PG_TRIGRS was carried
out. A back analysis with contingency matrix and skill scores was adopted to search for the best
compromise between correct and incorrect model outcomes. The results show that soil tillage caused
a 20 to 30% reduction in soil cohesion in the analyzed area.