Measurement of stress–strain relationship of beech roots and calculation of the reinforcement effect of tree roots in soil–wheel systems

Abstract

Evidence from forestry shows that part of the forest floor bearing capacity is delivered by tree roots. The beneficial effect varies and diminishes with increasing number of vehicle passes. PLAXIS is a commercially available finite element code that can calculate stresses, strains and failure states of soil mechanical problems. It can cope with unsaturated reinforced soil. Tree roots have a stiffness and a failure strength. They show elastic as well as plastic behaviour. They also show fatigue phenomena in repeated loading. Rooting intensity and root patterns of forest trees are complicated, but some information is available. This paper summarizes published data on mechanical properties of tree roots, and presents new measuring results of these properties. The paper further presents results of finite element calculations with PLAXIS for forest soil loaded by a tyre. This includes situations with and without reinforcement by tree roots. The reinforcement effects are, a.o., decrease of wheel rut depth and rolling resistance, decrease of damage to soil structure by the wheel load, and as a negative effect, 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 pattern; adhesive and frictional properties of the soil–root interface; tyre load and contact surface. The presented calculation results, which are based on realistic input data, show the sensitivity of the reinforcement effect to the listed variables.