**4. Conclusions**

The paper presents a study focusing on the effect of soil tillage due to agricultural activity on slope stability conditions in a hilly environment. To this aim, a back analysis has been performed using the PG\_TRIGRS model on the study area of Collazzone, central Italy. The code has already been used for the prediction of shallow landslides in the Umbria

Region, but it has not been tested on areas where landslides, triggered by a specific rainfall event, have been directly observed. The proposed probabilistic approach which assumes input data as random variables defined through their probability density functions, can provide a higher level of reliability than deterministic approaches, which assume the input data without uncertainty.

For this preliminary study, we decided to perform the stability analyses under saturated conditions and we have not considered the randomness of the initial water table depth *dw* in order to have only three random variables in input (*c* , *ϕ*, and *ks*). This choice allowed us to reduce the computational times and to better identify the effect of soil tillage on the soil's mechanical properties. However, future upgrades of the method will include the piezometric variability, the unsaturated conditions, and the soil thickness analysis. The study area selected for this purpose is of particular scientific interest because it is characterized by intense agricultural activity that greatly increases its susceptibility to landslides. The latter appears to be related to agricultural managemen<sup>t</sup> systems which modify the physical and mechanical properties of the soil. During the year the ground is tilled and denuded in the rainiest period, while in the summer and spring months the slopes appear covered by vegetation. The continuous change in soil conditions alters the mechanical characteristics of the soil in a non-negligible way. The use of unsustainable agricultural machinery should be limited to avoid loss of cohesion in the cover soil.

From the results obtained in this study, with reference to the area analyzed, soil tillage due to agricultural activity caused a reduction in cohesion of between 20 and 30%; this estimation agrees with the results obtained in studies aimed at evaluating the effects produced by specific tillage on soil's mechanical properties [44]. With reference to the results, the slope stability model is able to predict surface landslides but it classifies, as a precaution, the areas not affected by landslides. In any case, in order to safeguard the Collazzone area from shallow sliding phenomena, it is necessary to practice planned and appropriately selected agricultural techniques.

The method presented in this work is quantitative and reproducible, thus can be applied in other areas with similar environmental contexts, also for comparison with in situ and laboratory tests to refine and optimize the evaluation of the effect of soil tillage on slope stability.

**Author Contributions:** Conceptualization, E.V. and D.S.; methodology, E.V., S.L.G., F.A., F.F. and D.S.; analysis, E.V. and S.L.G.; data curation, F.A., F.F. and E.V.; writing—original draft preparation, E.V. and S.L.G.; writing—review and editing, S.L.G., F.A., F.F. and D.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** Rainfall provided by the administration of the Umbria region. The authors would like to thank two anonymous reviewers for their comments and suggestion, which were useful in improving the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
