VALUTAZIONE DEL RINFORZO RADICALE DEL SUOLO OPERATO DALLE RADICI DELLE PRINCIPALI SPECIE FORESTALI DELLA LOMBARDIA

Forests can prevent and/or mitigate hydrogeomorphic hazards in mountainous landscapes. Their effect is particularly relevant in the case of shallow landslides phenomena, where plants decrease the water content of the soil and increase its mechanical strength. Although such an effect is well known from ages, its quantification is a relatively new challenge. The present dissertation faces the effect on hillslope stability of several forest species typical of Lombardy’s mountain forests. Such effects are estimate in terms of additional root cohesion by the classical Wu (1976) and Waldron (1977) model (W&W) and by Fiber Bundle Model approach (FBM). Samples of European beech (Fagus sylvatica L.), Norway spruce (Picea abies (L.) Karst.), European larch (Larix decidua Mill.), sweet chestnut (Castanea sativa Mill.) and European hop-hornbeam (Ostrya carpinifolia Scop.), Sycamore Maple (Acer pseudoplatanus L.), Grey Alder or Speckled Alder (Alnus incana (L.) Moench), European Ash (Fraxinus excelsior L.), Manna Ash (Fraxinus ornus L.), Black Locust (Robinia pseudoacacia L.), were taken in different locations of Lombardy (Northern Italy) to estimate root tensile strength, Root Area Ratio the root cohesion distribution within the soil. Besides the increasing of knowledge concerning the ability of the mentioned species to reinforce soils, that are fundamental to estimate the stability of forested hillslopes in the Alps, the dissertation reaches some more general results. Referring to root tensile strength, the obtained results confirm that such property is related to root size by a power law relationship. Such relationship, however, is not invariant with species but shows dependence with site’s characteristics, although the observed variability is very high and no specific factor has been resulted as a key factor. Considering roots with a diameter greater than 1 mm, European beech roots seem to be more resistant than Norway spruce, Sweet chestnut and European Ash. Referring to Root Area Ratio, the results herein presented showed a great variability within the same species at the same location and among different locations. Despite the number of rooted profiles investigated, such variability hindered the possibility to identify a specie-specific or site-specific behaviour. Concerning root cohesion modelling, the original W&W model seems to overestimate root reinforcement because it assumes that all the roots crossing the shear surface break at the same time. FBM approach, instead, accounting for the progressive breaking of roots represents a promising perspective in root cohesion evaluation. Root cohesion values present the same species and site variability according to RAR values. The values herein obtained, moreover, are consistent with the results of direct shear tests and back-analysis data and, finally, they are significant for slope stabilisation at depth where shallow landslides generally occur.