Grapevine root system strength in an area susceptible to shallow landslides for slope stability assessment

During the last few years, several rainfall-induced shallow landslides affected the Oltrepò Pavese (northern Italy) hilly slopes cultivated with vineyards, causing in many cases the destruction of the cultivated areas. A study of the contribution of the grapevines, and in particular of their roots, is thus fundamental for slope stability analyses and consequently for the slope preservation. In this work, the preliminary results of some experimental investigations on grapevine root system contribution to soil reinforcement are presented. The density of grapevine roots shows great variations between the different sites, with highest values in the first 0.6 m. Roots are usually present till the contact between soil and weathered bedrock. The trends obtained showed that the number of grapevine roots seemed lower in slopes affected by shallow landslides. Despite the differences in the soil features, type of bedrock, grapevine age and vineyard practices, a unique relationship between root diameter and root tensile strength can be identified. The root reinforcement was evaluated applying the Fiber Bundle Model and through back analysis on slopes previously affected by shallow stability failures. Additional root reinforcement reached values till 18.3 kPa in slopes with cultivated vineyards. The root reinforcement in correspondence of the sliding surface reaches values till 8 kPa for vineyards and between 0.9 and 1.5 kPa for abandoned vineyards.