Abstract
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.
Highlights
Understanding and quantifying the mechanical effect of vegetation on steep slopes began approximately 50 years ago with direct shear tests performed on soil blocks containing roots (Wu 1976, 2013; Stokes et al 2014)
Implications for engineering application A key finding of this study is that where roots are clustered on slopes, there exists a threshold shear strength distribution beyond which increasing the strength of the rooted zone will not provide any further benefit to stability as the critical failure mechanism will already have moved to bypass the stronger zones and fail through the weaker unreinforced zones
The family of fibre break models predicted much higher root cohesion than the pull-out models proposed in this study
Summary
Understanding and quantifying the mechanical effect of vegetation on steep slopes began approximately 50 years ago with direct shear tests performed on soil blocks containing roots (Wu 1976, 2013; Stokes et al 2014). Michalowski and Čermák 2003; Zaimoglu and Yetimoglu 2012; Wood et al 2016) This approach is convenient where the dimensions and spacing of the reinforcement are small and behaviour can be homogenised statistically; otherwise, tests are difficult to perform, time consuming and expensive. As for the latter approach, the soil-root interaction properties can be estimated from axial root properties which can be determined from axial tension or pull-out tests of the roots (e.g. Van Beek et al 2005; Docker and Hubble 2008; Fan and Su 2008; Mickovski et al 2009; Sonnenberg et al 2010; Loades et al 2010; Comino et al 2010; Schwarz et al 2011; Boldrin et al 2017; Liang et al 2017a). The additional resistance within the soil due to the presence of roots may be introduced into stability calculations either as boundary forces (Greenwood et al 2004; Greenwood 2006) or used to evaluate c′r for use in the MohrCoulomb failure envelope equation (Waldron 1977; Wu et al 1979; Pollen and Simon 2005)
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