A method for stability analysis of vegetated hillslopes: an energy approach

Abstract

Stability analyses of vegetated hillslopes are usually carried out by the limit equilibrium (LE) method where shear displacement is not taken into account. Experiments show that soil with roots produces a shear stress - displacement curve with higher peak shear stress at larger shear displacements than fallow soil. If the safety factor is obtained by the LE method, the ability of soil with roots to resist large shear strains due to soil-root interaction may be underestimated. A new approach is proposed that incorporates, within the stability analysis, the ability of soil with roots to withstand strain. It is based on a consideration of the energy consumed during the shearing process of the soil-root system. This is developed using characteristics of the shear stress - displacement curve of a soil-root system obtained from in situ direct shear tests under simulated overburden pressure and pore-water pressure conditions. The method is limited to vegetated hillslopes where the stability analysis can be approximated by a simplified infinite slope model. Shear stress - displacement data for two tree species were obtained from hillslopes where shallow landslides commonly occur in rainstorms under near-saturated conditions. Using these results the energy approach (EA) and LE methods are compared. A procedure is also outlined to predict the safety factor for hillslopes with different plant densities. Extension of the EA method for general two-dimensional slope stability analysis involving nonlinear shear planes is also explained.

Tree-root control of shallow landslides

“…In slope stability, the process of root reinforcement remains hidden because direct observations have not yet been made on steep hillslopes. Field and laboratory experiments (e.g., Zhou et al, 1998;Ekanayake and Phillips, 1999;Roering et al, 2003;Docker and Hubble, 2008) generally explore only a small part of the complex root reinforcement mechanisms.…”

Observations of “coarse” root development in young trees of nine exotic species from a New Zealand plot trial

“…Danjon et al 1999;Mulatya et al 2002;Docker and Hubble 2008), and thus the statistical power of many relationships are generally limited by small sample sizes. Nevertheless, such data, be they from replicated trials or from single tree observations, are necessary to both improve our general understanding of a species' performance and to develop, calibrate, and validate either conceptual or quantitative models that incorporate root information for use in predicting the effects of vegetation on slope stability (Ekanayake & Phillips 1999, 2002Schwarz et al 2010;Phillips et al 2011;Stokes et al 2014) and hence the effectiveness of revegetation policies (Phillips et al 2013b).…”

An exploratory analysis of vegetation strategies to reduce shallow landslide activity on loess hillslopes, Northeast Qinghai-Tibet Plateau, China

“…tests show that the shear strength of soil without roots is lower than the shear strength of root-soil composite systems (Waldron 1977;Waldron and Dakessian 1981;Ziemer 1981;Operstein 2000;Nilaweera and Nutawera 1999;Ekanayake and Phillips 1999;Wu et al 1979). The greater the tensile strength (or shear strength) of a single root, the stronger its ability to reinforce soil shear strength (Gray and Sotir 1996;Zhu et al 2008;Hu et al 2009;Yang et al 2009).…”

“…Hathaway and Penny 1975;Burroughs and Thomas 1977;Schiechtl 1980;Nilaweera and Nutalaya 1999;Greenwood et al 2004;Genet et al 2005;Norris 2005). When subject to shear stress, root tensile strength is manifest through the tensile resistance of roots (Wu et al 1979;Ekanayake and Phillips 1999;Operstein and Frydman 2000). Single root tensile strength decreases via a power relationship as root diameter increases (e.g.…”