Mechanical properties of tree roots for soil reinforcement models

Abstract

Evidence from forestry has shown that part of the forest floor bearing capacity is delivered by tree roots. The beneficial effect however varies and diminishes with increasing number of vehicle passes. Roots potential for reinforcing the soil is known to depend among others on root mechanical properties, distribution, morphology, etc. Rooting intensity and root patterns of forest trees are complicated, but some information is available. The objectives of this study are therefore as follows: (1) addressing the occurrence of field traffic on forest soils, (2) identifying root mechanical properties that play a role in soil reinforcement, (3) measuring root stress-strain relationships, root failure stress and strain and root behaviour under repeated loading and (4) simulating root reinforcement effect using a FEM (Finite Element Method) code capable of accounting for root properties in reinforcement simulations.The repeated loading experiments included repeated loading of tree roots to different loading levels and loading with different loading rates or elongation rates. These studies revealed that tree roots possess stiffness and failure strengths. They show elastic as well as plastic behaviour. They also show fatigue phenomena in repeated loading. Available FEM codes were studied with respect to their capability in dealing with soil reinforcement by roots. PLAXIS which is a commercially available FEM code was used due to its ability to calculate stresses, strains and failure states of soil mechanical problems. It can also cope with unsaturated reinforced soil. The finite element calculations conducted with PLAXIS are intended for soils loaded by forestry vehicles. These involved situations with and without reinforcement by tree roots. The reinforcement effects are, among others, decrease of wheel rut depth and rolling resistance, decrease of damage to soil structure by the wheel load and as a negative effect, physiological damage to the tree root system. The magnitude of these effects depends on a number of parameters: stiffness and strength of the tree roots, soil mechanical properties like cohesion, angle of internal friction, compression index, preconsolidation stress, depth of a hard sublayer (if present), distance between vehicle and tree, rooting patterns, adhesive and frictional properties of the soil-root interface, wheel load and contact surface. The presented simulation results, which are based on realistic input data, show the sensitivity of the reinforcement effect to the listed variables.</p