Quantifying the contribution of grapevine roots to soil mechanical reinforcement in an area susceptible to shallow landslides

Hilly slopes cultivated with vineyards are often affected by rainfall-induced shallow landslides that cause destruction and loss of the cultivations. For this reason, the assessment of mechanical contribution from grapevine roots is fundamental for slope stability analyses and consequently for the slope preservation. In this context, our work aims to quantitatively evaluate the soil reinforcement given by grapevine roots. The selected study area (13.4 km2), located in the region of Oltrepò Pavese in Northern Italy, is characterized by a high shallow landslides density and is constituted by vineyards in steep slopes. The tested soils are Haplic Calcisols, Petric Calcisols and Calcic Gleysols, with silt loamy or silty clay loamy textures, from high to very high carbonate content and low organic carbon and nitrogen contents. The rootstock of the grapevine is a combination of Vitis berlandieri and Vitis riparia with root systems, which reach average depths of up to 1.5 m. The grapevine root density (number of roots and Root Area Ratio) is rather variable and is strongly correlated to soil permeability. In fact, the results show that low permeable soils have small number of roots and occur near recent shallow slides. Despite the differences of soil features, type of bedrock, grapevine plants age, vineyards row orientation and season collection, a unique relationship between root diameter and root tensile strength has been identified. Root reinforcement, related to the grapevine root system and evaluated using Fiber Bundle Model, shows the lowest values in correspondence of sites characterized by the lowest soil permeability, as in the study case Calcic Gleysols. The sites with these soil features, actually, are also those most affected by shallow instability in the past, indicating that their great susceptibility to shallow landslides. The results of this study also highlight the role played by different amounts of grapevine root reinforcement on the slope stability during rainfall conditions, which could lead to triggering, on the study area.