Living root bridges

Abstract

The Khasi tribes of Meghalaya have a tradition of nurturing and guiding aerial roots of Ficus elastica across rivers and deep gorges to grow bridges. Layers of rocks and soil creates a walkway. It is known that bones grow in response to stress. Therefore, it is of interest to investigate if the dead load on the bridge contributes to changes in the material properties of the aerial roots of F. elastica. This study investigates the strength and stiffness of aerial roots of F. elastica, and models the living root bridge for a FE analysis. The ultimate strength and stiffness of aerial roots of F. elastica were investigated by testing 56 free-hanging root samples in uniaxial tension. These results were compared with 41 tensile test results of roots that were prestressed, almost to failure. The ultimate strength and modulus of elasticity for prestressed and free-hanging roots were plotted against aerial root diameter, moisture content and length. The unloading behavior of aerial roots was also investigated. The aerial roots of F. elastica show some elastic-plastic behavior. Ultimate strength and modulus was found to decrease with increasing diameter for both prestressed and free-hanging roots. Strength and modulus was found to be higher on average for prestressed samples than for free-hanging samples. Photogrammetry was used to model the Nohwet root bridge from 92 photos. A 3D model was produced on Solidworks for a FE analysis of the bridge for three external loading conditions. This was used to develop a specification for photogrammetry for future works. The real strength in living root bridges comes from aerial roots grown in successive layers over several decades resulting in a complex root network.