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

Evidence from forestry shows that part of the forest floor bearing capacity is delivered by tree roots. The beneficial effect varies and diminishes wi th 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 w ell as plastic behaviour. They also show fatigue phenomena in repeated load ing. Rooting intensity and root patterns of forest trees are complicated, b ut 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 soi l 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 w heel load, and as a negative effect, damage to the tree root system. The ma gnitude of these effects depends on a number of parameters: stiffness and s trength 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 pa ttern; adhesive and frictional properties of the soil-root interface; tyre load and contact surface. The presented calculation results, which are base d on realistic input data, show the sensitivity of the reinforcement effect to the listed variables. (C) 2000 Elsevier Science B.V. All rights reserve d.