Modelling the seismic performance of rooted slopes from individual root–soil interaction to global slope behaviour

Abstract

Modelling the seismic performance of rooted slopes from individual root-soil interaction to global slope behaviour T. LIANG Ã , J. A. KNAPPETT Ã and N. DUCKETT † Many natural and man-made slopes are planted with vegetation, and it is known that this can increase the stability of slopes under static conditions. There is anecdotal evidence that vegetated slopes also perform better than fallow slopes during earthquakes. However, the study of the dynamic behaviour of slopes planted with species having dichotomous ('woody') roots is relatively rare owing to the extreme expense and difficulty involved in conducting full-scale dynamic testing on shrubs and trees. In this paper, dynamic centrifuge testing and supporting numerical modelling have been conducted to study this problem. In the centrifuge modelling, ABS plastic rods are used to simulate repeatably the mechanical properties of real roots. The numerical modelling work consisted of two parts. First, a computationally-efficient beam-on-non-linear-Winkler-foundation (BNWF) model using existing p-y formulations from piling engineering was employed to produce a macro-element describing the individual root and soil interaction both pre-and post-failure. By adding contributions from the different root analogues of different diameters, smeared continuum properties were derived that could be included in a fully dynamic, plane-strain continuum, finite-element model in a straightforward way. The BNWF approach was validated against large direct shear tests having stress conditions simulating those in the centrifuge at different potential slip plane depths. The conversion to smeared properties for global time-history analysis of the slope was validated by comparing the continuum finite-element results with the centrifuge test data in terms of both the dynamic response and permanent deformations at the crest, and these demonstrated good agreement. Owing to the simplicity of the BNWF approach and its ability to consider variable root geometries and properties, along with variation of soil properties with depth, it is suggested that the validated approach described will be useful in linking individual root-soil interaction characteristics (root strength and stiffness, diameter variation, root spacing and so on) to global slope behaviour.KEYWORDS: centrifuge modelling; earthquakes; finite-element modelling; numerical modelling; slopes; vegetation INTRODUCTION Vegetation (grasses, shrubs and trees), as an effective and environmentally friendly approach to improving slope stability, improves slope stability mainly through direct mechanical reinforcement of soil and by modifying groundwater conditions by means of evapotranspiration. The net effect of both of these mechanisms is an increase in shear strength within a defined zone around the roots, although only the mechanical effect is present at all times; the hydrological effects potentially disappear following heavy rain. In terms of the direct mechanical effect, many studies have been performed to quantify the increase in soil ...

Small-scale modelling of plant root systems using 3D printing, with applications to investigate the role of vegetation on earthquake-induced landslides

“…The load displacement data also suggested that deep roots were mobilised and broken in sequence rather than simultaneously (see the numerous sudden small drops on the 75-mm curve, Fig. 9)-this has also been observed previously for straight (vertical) root analogues in large DSA tests (Liang et al 2015).…”

“…Here, RAR was defined using the following equation Landslides where A r is the total root cross-sectional area and A is the area of CRZ (CRZ and ZRT are the same for the 3D root cluster) rather than the total cross-sectional area of the DSA. This was selected based on the previous observation of Liang et al (2015) that root influence on shearing properties is confined to an area local to the roots. Test series B utilised a simpler group of straight (vertical) root analogues at the same confining stress (8 kPa), such that the effect of root morphology could be investigated through a comparison with test series A.…”

“…3), while model TL03 was reinforced with straight ABS rods having the same RAR and spatial distribution at the level of the middle of the 3D root cluster to investigate root morphology, similar to the series B DSA tests. Further detail about the design of model TL03 can be found in Liang et al (2015). Model TL04 was a fallow reference case of identical slope geometry.…”

“…Additionally, the analogues used were typically made of either rubber or wood as materials with low and high stiffness and strength, respectively. While these materials bracket the stiffness and strength of typical roots, neither of their properties is ideal (Liang et al 2015;Liang and Knappett 2017).…”

CHD pile performance: part II – numerical modelling

“…The model parameters have subsequently been validated against centrifuge test data (i.e. full-scale stresses) on the dynamic response of sandy slopes (Al-Defae et al, 2013;Liang et al, 2015) and buildings on shallow foundations during earthquake shaking. The use of the correlations in this paper allows for an additional validation to be made against 1g test data (to explore the stress dependency of stiffness) and for a static loading problem.…”