Site investigation for the effects of vegetation on ground stability

The procedure for geotechnical site investigation is well established but little attention is currently given to investigating the potential of vegetation to assist with ground stability. This paper describes how routine investigation procedures may be adapted to consider the effects of the vegetation. It is recommended that the major part of the vegetation investigation is carried out, at relatively low cost, during the preliminary (desk) study phase of the investigation when there is maximum flexibility to take account of findings in the proposed design and construction. The techniques available for investigation of the effects of vegetation are reviewed and references provided for further consideration. As for general geotechnical investigation work, it is important that a balance of effort is maintained in the vegetation investigation between (a) site characterisation (defining and identifying the existing and proposed vegetation to suit the site and ground conditions), (b) testing (in-situ and laboratory testing of the vegetation and root systems to provide design parameters) and (c) modelling (to analyse the vegetation effects)

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

Vegetation has been previously proposed as a method for protecting artificial and natural slopes against shallow landslides (e.g. as may be triggered by an earthquake); however, previous research has concentrated on individual root soil interaction during shear deformation rather than the global slope behaviour due to the extreme expense and difficulty involved in conducting full-scale field tests. Geotechnical centrifuge modelling offers an opportunity to investigate in detail the engineering performance of vegetated slopes, but its application has been restricted due to the lack of availability of suitable root analogues that can repeatably replicate appropriate mechanical properties (stiffness and strength) and realistic 3-D geometry. This study employed 3-D printing to develop a representative and repeatable 1:10 scale model of a tree root cluster representing roots up to 1.5 m deep at prototype scale) that can be used within a geotechnical centrifuge to investigate the response of a vegetated slope subject to earthquake ground motion. The printed Acrylonitrile Butadiene Styrene (ABS) plastic root model was identified to be highly representative of the geometry and mechanical behaviour (stiffness and strength) of real woody root systems. A programme of large direct shear tests was also performed to evaluate the additional strength provided by the root analogues within soil that is slipping and investigate the influence of various characteristics (including root area ratio, soil confining effective stress and root morphology) on this reinforcing effect. Our results show that root reinforcement is not only a function of root mechanical properties, but also depends on factors including surrounding effective confining stress (resulting in depth dependency even for the same RAR), depth of the slip plane and root morphology. When subject to shear loading in soil, the tap root appeared to structurally transfer load within the root system, including to smaller and deeper roots which subsequently broke or were pulled out. Finally, the root analogues were added to model slopes subjected toearthquake ground motion in the centrifuge, where it was revealed that vegetation can substantially reduce earthquake-induced slope deformation in the soil conditions tested (76% reduction on crest permanent settlement during slippage). Both the realistic 3-D geometry and highly simplified root morphologies, as characterised mechanically by the shear tests, were tested in the centrifuge which, despite exhibiting very different levels of additional strength in the shear tests, resulted in very similar responses of the slopes. This suggests that once a certain minimum level of reinforcement has been reached which will alter the deformation mechanism within the slope, further increases of root contribution (e.g. due to differences in root morphology) do not have a large further effect on improving slope stability.<br/

The influence of vegetation on soil strength

Research is being carried out at Technion in Israel to study the influence of plant roots on the stability of slopes. The present paper describes the part of the investigation concerned with the determination of the additional shear strength contributed to soil by roots. Specifically, results of the following studies are presented: tension tests on roots, pull-out tests of roots from the soil, and direct shear tests on soil and root-reinforced soil. The quantitative results obtained in these investigations provide data which may be used in calculations of slope stability, although this should be done with caution, as pointed out in the paper. Des recherches sont effectuées en ce moment chez Technion en Israël pour étudier l'influence des racines de plantes sur la stabilité des pentes. Cet exposé décrit la partie de cette investigation concernant le calcul du gain de résistance au cisaillement donné au sol par les racines. Plus spécifiquement, nous présentons les résultats des études suivantes: essais de traction sur les racines, essais d'extraction des racines hors du sol, essais de cisaillement direct sur le sol et sur un sol renforcé par des racines. Les résultats quantitatifs émanant de ces investigations ont produit des données qui pourront être utilisées dans le calculs de stabilité de pente mais, comme nous le soulignons dans cet exposé, ceci devra être fait avec prudence. </jats:p

The stability of soil slopes stabilised with vegetation

This is the third and final paper describing a research project, carried out at the Technion in Israel, to study the influence of plant roots on the stability of slopes. The two previous papers described earlier stages of the research, in which root and soil properties were established, and the reliability of the numerical scheme, based on the use of the finite difference code FLAC, was checked. The present paper uses the previous results as a basis for analysing the stability of root-reinforced chalky slopes under various conditions. It is demonstrated that the stability analysis of root-reinforced slopes must consider the roots as individual elements, and take account of their properties, as well as their interaction with the surrounding soil. Use of the ‘equivalent layer ’ approach, in which the reinforced soil is replaced by an equivalent soil with strength properties obtained from laboratory shear tests, is likely to be significantly non-conservative. The influence of root inclination is studied, and it is shown that vertical roots do not contribute significantly to slope stability, whereas a considerable contribution is provided by roots that are perpendicular to the slope face. The numerical scheme may be used for the analysis of more complex soil-root interaction problems. Cet exposé est le troisième et le dernier d'une série décrivant un projet de recherche réalisé au Technion en Israël, afin d'étudier l'influence des racines de plantes sur la stabilité des pentes. Les deux précédents exposés décrivent les premières phases de la recherche, phases au cours desquelles les propriétés des racines et du sol ont é té é tablies et la fiabilité de la formule numérique, basée sur l'utilisation du code de différence finie FLAC, aété vérifiée. Cet exposé utilise les précédents résultats comme base pour analyser la stabilité, dans diverses conditions, des pentes crayeuses renforcées par des racines. Nous démontrons que l'analyse de stabilité des pentes renforceées par des racines doit considérer les racines en tant qu'éléments individuels et tenir compte de leurs proprieétés ainsi que de leur interaction avec le sol environnant. L'utilisation de la méthode ‘couche é quivalente’ dans laquelle le sol renforcé est remplacé par un sol é quivalent, dont les propriétés de résistance ont é té déterminées à partir d'essais de cisaillement en laboratoire, sera probablement non conventionnelle. Nous é tudions l'influence de l'inclinaison des racines et nous montrons que les racines verticales ne contribuent pas de manière significative à la stabilité des pentes alors que les racines perpendiculaires à la face de la pente ont une action considérable. La formule numérique pourra être utilisé pour l'analyse de problèmes d'interaction sol-racines plus complexes. </jats:p

Numerical simulation of direct shear of rootreinforced soil

This paper is the second in a series of three, describing a research project being carried out at Technion in Israel to study the influence of plant roots on the stability of slopes. In the present paper, the numerical simulation of large, direct shear tests performed on soil samples reinforced with roots, is described. Simulation was performed using the finite difference code, FLAC, and two different soil models were used—the hyperbolic model, and a plastic, strain-hardening model. Soil parameters were obtained from triaxial tests, and root properties from tension and pull-out tests described in the previous paper. The method of preparing the numerical scheme in order to obtain equivalence with the experimental set-up is discussed. Good agreement was obtained between the analyses and the results of the laboratory tests for both soil models, providing confidence in the use of the scheme for slope stability analysis. Cet exposéest le second d'une série de trois, décrivant un projet de recherche menéà Technion en Israël pour é tudier l'influence des racines végétales sur la stabilitédes pentes. Dans cet exposé, nous décrivons la simulation numérique d'essais de grands cisaillements directs, essais réalisés sur des é chantillons de sol renforcés de racines. La simulation a étéréalisée en utilisant le code de différence finie, FLAC; deux modèles de sol différents ont é téutilisés le modèle hyperbolique et un modèle plastique, à durcissement-déformation. Nous avons dérivéles paramètres du sol des tests triaxiaux et les propriétés des racines des tests de tension et d'arrachage décrits dans l'étude précédente. Nous examinons la méthode de préparation du système numérique afin d'obtenir une é quivalence avec les installations expérimentales. Nous obtenons une bonne corrélation entre les analyses et les résultats des essais en laboratoires pour les deux modèles de sol, ce qui nous montre la fiabilitéde ce système pour l'analyse de la stabilitédes pentes. </jats:p

A Note on Convex Approximation in Lp

AbstractA convex function f given on [−1, 1] can be approximated in Lp, 1 < p < ∞, by convex polynomials Pn of degree at most n with the accuracy o(n−2/p). This follows from the estimate ∥f−Pn∥p ≤ c · n−2/p·ωφ2(f, n−1)1/q, where 1 ≤ p ≤ ∞, p−1 + q-−1 = 1, φ(x) = (1 − x2)1/2, and ωφ2(f, t) is the Ditzian-Totik modulus of smoothness in the uniform metric