Effect of the root biotechnical characteristics of Alnus subcordata, Paulownia fortunei and Populus deltoides on the soil mechanics

The effect of root reinforcement depends on the biotechnical characteristics of the root system including the tensile strength of individual roots, the root density and the distribution of the root system in the soil. This research was conducted in the Hyrcanian forest in Iran, where shallow landslides are frequent due to road construction. The effect of the root biotechnical characteristics of Alnus subcordata, Paulownia fortunei and Populus deltoides each one in 2-, 10- and 15-year-old plantations was assessed. The profile trenching method was used in this study to obtain the root area ratio of individual species by counting the number of roots and measuring the root diameter. For each species, single root specimens were sampled and tested for tensile tests in the laboratory using the standard Instron apparatus. The natural moisture content (two weeks after rainfall), Atterberg limits, shear strength of the soil were determined for plantations, stable and unstable sites. Results of this study indicated that plastic index and internal friction angle decreased with increasing root diameter rate. Moreover, internal friction angle and cohesion increased with increasing root area ratio and root diameter and density index. Tensile strength of roots decreased with increasing stand age. The highest and the lowest tensile strengths among species were observed for Paulownia fortunei and Populus deltoides, respectively. The findings of this research contribute to expanding the knowledge of root biotechnical properties of some tree species and to the choice of the most appropriate species for improving plastic index and shear strength in landslide prone areas.

Quantification of root reinforcement in bio-slope stabilization: laboratory and field studies

Bio-slope stabilization is an environmentally friendly and sustainable technique for rehabilitation and maintenance of slope infrastructure. The processes in which these plants stabilize the slope, particularly root reinforcement, and evapotranspiration, mainly involved the unsaturated zone of the slope. Plants are also subjected to continuous change during the life time of bio-engineered slopes. This paper reports on recent studies on influence of roots on saturated and unsaturated soil properties, and suction effects on root reinforcement. These experimental results were then used to estimate the change in in-situ root reinforcement of biostabilized slope in the field. A new methodology based on the mini-rhizotron technique was proposed which linked the root-area ratio with root-reinforced strength and the factor of safety of the slope through laboratory-based relationships. This newly proposed method can be used to evaluate timebased performance of vegetated slope infrastructures in practice.