ARS | Publication request: Modeling root-reinforcement with a Fiber-Bundle Model and Monte Carlo simulation

Submitted to: Ecological Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 14, 2009
Publication Date: January 1, 2010
Citation: Thomas, R.E., Bankhead, N.L. 2010. Modeling root-reinforcement with a Fiber-Bundle Model and Monte Carlo simulation. Ecological Engineering. 36(1):47-61.

Interpretive Summary: This paper uses a root-reinforcement model to quantify the effect of plant and tree roots on the strength of a soil. First, different ways of dividing the load acting on a soil between the roots in the soil were investigated. Then model runs were carried out to simulate plants with different types of root networks, for example plants that have one main vertical root versus those that have many roots growing parallel to the soil surface. Plants growing on slopes and floodplains were modeled. The results suggest that the constant multiplier value that has commonly been used to approximate the angles of the roots when a soil block fails (a value of 1.2) is too large in most cases and is only found where frictional forces between soil particles are high. In addition it was found that to correctly model the way that roots break in a soil when a soil block fails, the load must be divided equally between the roots. A third finding was that the amount of reinforcement provided by a certain number of roots is affected by the root architecture of the plant, and whether the plant is growing on a slope or a flat surface, such as a floodplain. The amount of reinforcement provided by the roots changes under different circumstances because some roots buckle (are compressed) when the soil starts to fail, and others become taut (are tensioned). The amount of strength provided to the soil by a given plant therefore changes with plant root architecture and the position of the plant compared to the failing soil mass. The research presented in this paper is useful to those trying to quantify streambank and slope stability and to practitioners designing planting schemes to strengthen streambanks and slopes using vegetation.

Technical Abstract: This paper uses sensitivity analysis and a Fiber-Bundle Model (FBM) to examine assumptions underpinning root-reinforcement models. First, different methods for apportioning load between intact roots were investigated. Second, a Monte Carlo approach was used to simulate plants with heartroot, plateroot and taproot/herringbone networks growing on slopes and floodplains. Results suggest that: 1. The commonly-adopted value (1.2) for a term accounting for initial root orientation, shear distortion angle and soil friction angle is too large and is only attained for friction angles >35°; 2. To obtain the correct dynamics, equal load apportionment must be used in FBMs; 3. Root architecture has a significant impact on loading curve shape and the peak load supported by a root bundle; and 4. Plants with different root architectures are suitable for stabilizing different features. For example, 500 Eastern Sycamore roots of differing network types provided median reinforcement of 4.86-15.08 kPa on a slope and 9.49-14.82 kPa on a floodplain. These latter variations, and the duration and displacement over which reinforcement is provided, are controlled by the proportions of compressed and tensioned roots as soil shearing initiates. Root-reinforcement may vary dramatically dependent upon the location of a plant relative to a failing soil mass.