A critical evaluation of predictive models for rooted soil strength with application to predicting the seismic deformation of rooted slopes

This paper presents a comparative study of three different classes of model for estimating the reinforcing effect of plant roots in soil, namely (i) fibre pull-out model, (ii) fibre break models (including Wu and Waldron’s Model (WWM) and the Fibre Bundle Model (FBM)) and (iii) beam bending or p-y models (specifically Beam on a Non-linear Winkler-Foundation (BNWF) models). Firstly, the prediction model of root reinforcement based on pull-out being the dominant mechanism for different potential slip plane depths was proposed. The resulting root reinforcement calculated were then compared with those derived from the other two types of models. The estimated rooted soil strength distributions were then incorporated within a fully dynamic, plane-strain continuum finite element model to assess the consequences of the selection of rooted soil strength model on the global seismic stability of a vegetated slope (assessed via accumulated slip during earthquake shaking). For the particular case considered in this paper (no roots were observed to have broken after shearing), root cohesion predicted by the pull-out model is much closer to that the BNWF model, but is largely over-predicted by the family of fibre break models. In terms of the effects on the stability of vegetated slopes, there exists a threshold value beyond which the position of the critical slip plane would bypass the rooted zones, rather than passing through them. Further increase of root cohesion beyond this value has minimal effect on the global slope behaviour. This implies that significantly over-predicted root cohesion from fibre break models when used to model roots with non-negligible bending stiffness may still provide a reasonable prediction of overall behaviour, so long as the critical failure mechanism is already bypassing the root-reinforced zones. © 2019, The Author(s)

A new mixing technique for randomly distributed fibre-reinforced expansive soil

In the state of Gujarat, India, a significant part of the area is covered by expansive soil deposits. The swelling and shrinkage characteristics of expansive soil as a result of wetting and drying cause problems for safety and/or performance of structures. The concept of reinforcing soil with fibres, resulting in a randomly distributed fibre-reinforced soil (RDFS), has been studied in significant details, especially for reinforcing granular soils. Studies on fibre-reinforced expansive soils as reported in the literature are very limited. One of the major problems in the study of fibre-reinforced expansive soils is how to mix fibres uniformly. Therefore, the main objective of this paper is to study the available mixing methods, assess their suitability and develop a new mixing technique, which will be cost-effective for mixing fibres with expansive soil, especially in developing countries. In the present study, a surface modification method with cement has been proposed, where the waste tyre fibres (WTFs) were coated with cement to improve the bonding between fibres and clay minerals. The effect of this modification on the strength property of reinforced soil has been examined with unconfined compressive strength analysis. The results show that the shear strength between cement coated fibres and soil is significantly higher than the shear strength between fibres and natural soil