Metodologie sperimentali per l’esecuzione di provedi taglio diretto su terre rinforzate con radiciExperimental methodologies for the direct shear testson soils reinforced by roots

Root apparatus profoundly modify the hydrological and geotechnical characteristics of the soil, making it less inclined to collapse when under shear stress. During the last years many authors carried out different researches in the attempt to scientifically establish the actual role of vegetation on the slopes and at which extent. In this sense, models have been developed capable of foreseeing potential risks. These models are based on assumptions which are empirical or have many experimental variables, tweaked with simulations, both in situ and in the laboratory, using plants or artificial fibres of different nature and shear apparatus built on purpose. The experimental tests proposed in this paper have been carried out following the direct shear test, according to the standard defined by Casagrande, and they examine in a small scale the behaviour of rooted soil samples. For the tests we used Castanea sativa M. and Robinia pseudoacacia L. roots. The test conditions are deliberately and necessarily simplified, carried out on recreated test pieces, using two different methods developed during the experiments, using a variable number of roots. The interesting aspect of this kind of test, is the possibility to underline the behaviour of the soil until its failure (but not roots failure) occurs. At the same time we carried out tensile tests on roots of the same species. Besides the laboratory tests, in field tests were carried out with the Bore-hole Shear Test (BST), with the hypothesis of the same experimental conditions of the laboratory. Results were compared to the most recent models for the calculation of the additional cohesion and also the relation between roots activation strength and the resistance to failure was analysed. The results obtained with the proposed methods are coherent with the expected results, and they show an increase of the apparent cohesion and not significant variations of the angle of internal friction. Analogously, the data in situ show that the soil shear strength is positively modified by the increase of the RAR, which is in inverse proportion to the depth. The use of the Bore-hole shear test deserves to be deepened, in order to know the actual efficacy of this tool in perceiving the soil root density.