Determination of Tree Root Mechanical Properties

After several extreme natural disasters, such as earthquakes, hurricanes, and floods, huge trees are still alive and standing while manmade infrastructure has collapsed and failed. The natural foundation adopted by nature for supporting trees is significantly different from those constructed for supporting manmade infrastructure. Therefore, a new, economical foundation configuration can be developed by studying the tree root system. This study is part of a significant effort to understand the root configuration and load transfer mechanism using three-dimensional finite element modeling to develop innovative foundations for infrastructure systems. Specifically, in this study, the mechanical properties of roots from several trees were measured in the laboratory, and the results are reported. To accomplish this, three common tree species in the Southeastern United States were selected: pine, sweetgum, and white oak. Different-sized tree roots were collected from all interested tree species and were modified to make cylindrical samples with varying diameters and constant heights. The samples were then tested in a laboratory under compressional axial loading. The results show that the varying diameters of the samples have a correlation for the white oak roots: as the diameter increased, the compressional elastic modulus decreased. However, this was not evident for the pine and sweetgum samples. Also, the white oak roots had the largest compressional elastic modulus, followed by the pine and sweetgum roots, respectively. From the stress-strain plots, the sweetgum roots were determined to be the most ductile, followed by the white oak roots; therefore, the pine roots were the most brittle. Also, the stress-strain plots show that the white oak roots had the maximum peak and yielding stress values of the tested tree species, followed by pine and sweetgum, respectively. Finally, the failure mode was important to note while testing the root samples. These failure modes were generalized into two main categories: buckling and shearing. The main failure mode for the white oak and sweetgum roots was buckling, while shearing was the main failure mode for the pine roots. However, there were other important observations noted during testing and discussed in this study.