Test speed and other factors affecting the measurements of tree root properties used in soil reinforcement models

Measured values of the mechanical properties of tree roots are found to be affected by a number of factors. Shear properties of tree roots are found to be partly influenced by size of the testing equipment, level of soil compaction, deformation of the root material and estimated width of the shear zone in the soil, etc. Among others, tensile root properties are found to be affected by factors such as season, age, root types or species, root preparation before experiment, clamping procedure of the root, and rate of elongation used. The aims of this paper were (1) finding an appropriate clamping procedure for root measurements; and (2) studying the effect of elongation rate on stress–strain relationships of beech (Fagus sylvatica L.) roots. It was found that problems associated with root clamping (i.e., in root experiments) such as breakage of the root sample close to the jaws of the clamping devices and slippage may be eliminated by introducing wooden blocks of appropriate hardness within the jaws of the clamps and exerting a suitable clamping force, respectively. An appropriate wood hardness and suitable clamping force were found by conducting a number of initial trial experiments. Tensile experiments conducted with beech roots pairs showed that elongation rate does influence the stress–strain relationships of roots. In general, stress values of beech roots were found to increase between 8 and 20% for an increase in the rate of elongation from 10 to 400 mm/min. Similar findings have been reported in textile fibres.