http://www.saetaequina.com/files/1-s2_0-S0950061811006763-main.pdf

A simple review of soil reinforcement by using natural and synthetic fibers

Slope stability models traditionally use simple indicators of root system structure and strength when vegetation is included as a factor. However, additional root system traits should be considered when managing vegetated slopes to avoid shallow substrate mass movement. Traits including root distribution, length, orientation and diameter are recognized as influencing soil fixation, but do not consider the spatial and temporal dimensions of roots within a system. Thick roots act like soil nails on slopes and the spatial position of these thick roots determines the arrangement of the associated thin roots. Thin roots act in tension during failure on slopes and if they traverse the potential shear zone, provide a major contribution in protecting against landslides. We discuss how root traits change depending on ontogeny and climate, how traits are affected by the local soil environment and the types of plastic responses expressed by the plant. How a landslide engineer can use this information when considering slope stability and management strategies is discussed, along with perspectives for future research. This review encompasses many ideas, data and concepts presented at the Second International Conference ‘Ground Bio- and Eco-engineering: The Use of Vegetation to Improve Slope Stability—ICGBE2’ held at Beijing, China, 14–18 July 2008. Several papers from this conference are published in this edition of Plant and Soil.

Desirable plant root traits for protecting natural and engineered slopes against landslides

Environment, Power, and Society

In Mediterranean environments, gully erosion is responsible for large soil losses. It has since long been recognized that slopes under vegetation are much more resistant to soil erosion processes compared to bare soils and improve slope stability. Planting or preserving vegetation in areas vulnerable to erosion is therefore considered to be a very effective soil erosion control measure. Re-vegetation strategies for erosion control rely in most cases on the effects of the above-ground biomass in reducing water erosion rates, whereas the role of the below-ground biomass is often neglected or underestimated. While the above-ground biomass can temporally disappear in semi-arid environments, roots may still be present underground and play an important role in protecting the topsoil from being eroded. In order to evaluate the potential of plant species growing in Mediterranean environments to prevent shallow mass movements on gully or terrace walls, the root reinforcement effect of 25 typical Mediterranean matorral species (i.e. shrubs, grasses herbs, small trees) was assessed, using the simple perpendicular model of Wu et al. (Can Geotech J 16:19–33, 1979). As little information is available on Mediterranean plant root characteristics, root distribution data were collected in SE-Spain and root tensile strength tests were conducted in the laboratory. The power root tensile strength–root diameter relationships depend on plant species. The results show that the shrubs Salsola genistoides Juss. Ex Poir. and Atriplex halimus L. have the strongest roots, followed by the grass Brachypodium retusum (Pers.) Beauv. The shrubs Nerium oleander L. and the grass Avenula bromoides (Gouan) H. Scholz have the weakest roots in tension. Root area ratio for the 0–0.1 m topsoil ranges from 0.08% for the grass Piptatherum miliaceum (L.) Coss to 0.8% for the tree Tamarix canariensis Willd. The rush Juncus acutus L. provides the maximum soil reinforcement to the topsoil by its roots (i.e. 304 kPa). Grasses also increase soil shear strength significantly (up to 244 kPa in the 0–0.1 m topsoil for Brachypodium retusum (Pers.) Beauv.). The shrubs Retama sphaerocarpa (L.) Boiss. and Anthyllis cytisoides L. are increasing soil shear strength to a large extent as well (up to 134 and 160 kPa respectively in the 0–0.10 m topsoil). Whereas grasses and the rush Juncus acutus L. increase soil shear strength in the topsoil (0–0.10 m) to a large extent, the shrubs Anthyllis cytisoides (L.), Retama sphaerocarpa (L.) Boiss., Salsola genistoides Juss. Ex Poir. and Atriplex halimus L. strongly reinforce the soil to a greater depth (0–0.5 m). As other studies reported that Wu’s model overestimates root cohesion values, reported root cohesion values in this study are maximum values. Nevertheless, the calculated cohesion values are used to rank species according to their potential to reinforce the soil.

/pdf/root-tensile-strength-and-root-distribution-of-typical-3jfe6smof3.pdf

Root tensile strength and root distribution of typical Mediterranean plant species and their contribution to soil shear strength

Improvements to existing methods in the literature have been made for predicting cumulative plastic deformation for fine-grained subgrade soils. The soil deviator stress, number of stress applications, soil physical state, and soil type are considered. The improved method incorporates multilevels of deviator stresses and multisoil physical states that result from load-level variations, as well as seasonal and weather changes throughout traffic. Measurements of plastic deformation for a railroad-track subgrade are presented and show a significant influence of soil physical state, soil type, traffic tonnage, and wheel loads on the accumulation of plastic deformation. Comparisons between predicted and experimental results show good applicability of the improved method.

Cumulative plastic deformation for fine-grained subgrade soils

Many embankments and cuttings associated with the transportation infrastructure in the UK are only marginally stable. Engineering techniques such as soil nailing, geosynthetic reinforcement, improved drainage and ground improvement by stabilisation are available to improve stability but the cost can be high. A lower cost solution may be to utilise vegetation, either self seeded or planted. The benefits and drawbacks associated with vegetation have been the subject of some debate. The problems caused by vegetation in relation to building foundations are well documented and confirm that vegetation can have very significant influences on geotechnical parameters. Appropriate properly maintained vegetation can have the same significant influence to help provide additional stability to soil slopes. This paper considers the potential engineering influences of vegetation and how it can be characterised on site within a geotechnical framework for stability assessments. The direct reinforcement available from the roots of trees and shrubs is identified as providing one of the most significant contributions to slope stability. Case studies in the UK, Greece and Italy demonstrate how results from in-situ root pull out tests may be used to estimate the potential reinforcement forces available from the roots. A scheme is presented to designate zones of influence within the soil according to the size and nature of the vegetation.

/pdf/assessing-the-contribution-of-vegetation-to-slope-stability-xzoo0uxith.pdf

Assessing the contribution of vegetation to slope stability

Vegetation has generally been recognised for its aesthetic landscaping qualities in the urban environment, especially along transportation corridors and for use as noise barriers. The detrimental effects of vegetation are also recognised. Trees and shrubs draw out moisture from the ground through evapotranspiration processes, which leads to the seasonal shrinkage and swelling of clay soils. In adverse climatic conditions, e.g. prolonged hot and dry summers, moisture reduction in clay soils may cause substantial damage to buildings and property. This paper reports on recent projects and studies in the UK and Europe, including the ECO-SLOPES Project (http://construction.ntu.ac.uk) which investigated and defined the positive roles of vegetation in improving the stability of sloping ground. In urban areas, bioengineering techniques have been applied to combat the problems of soil erosion and the shallow landslides that result in the instability of earthworks on the UK's transport network. The engineering influences of vegetation including moisture and pore water pressure changes, and root reinforcement effects are assessed and techniques for monitoring these influences discussed. The inclusion of the vegetation effects are demonstrated in routine limit equilibrium stability analysis. It is concluded that the roots of appropriately planted and maintained vegetation are likely to provide a 10% increase in the factor of safety of potential shallow slip surfaces. A computer based slope decision support system is presented to assist engineers to assess the likelihood of a 'slope' being suitable for bioengineering techniques. The slope decision support system is freely available on the ECO-SLOPES website for users to try and feedback on its applicability. Resume: La vegetation toujours a ete reconnue pour ses qualites d'amenagement esthetiques dans l'environnement urbain, surtout le long des couloirs de transport et comme les barrieres de bruit. Il y aussi a eu la reconnaissance repandue des effets nuisibles de vegetation. Les arbres et les arbrisseaux rallongent l'humidite du sol par les procedes de evapotranspiration, qui mene au recul et l'accroissement saisonniers de sols d'argile. Dans les conditions climatiques defavorables, par ex. a prolonge des etes chauds et secs, la reduction d'humidite dans les sols d'argile peut causer des dommages substantiels aux bâtiments et a la propriete. Ce papier fait un rapport sur des projets et des etudes recents dans le Royaume-Uni et Europe, y compris les ECO-SLOPES Projette (http://construction.ntu.ac.uk) qui a examine et a defini les roles positifs de vegetation dans ameliorer la stabilite de sol en pente. Dans les secteurs urbains, les techniques de genie biologique ont ete appliquees combattre les problemes d'erosion de sol et les glissements de terrain peu profonds qui a pour resultat l'instabilite de remparts sur le reseau de transport de Royaume-Uni. Les influences d'ingenierie de vegetation y compris l'humidite et les changements de pression d'eau de pore, et les effets de renforcement fondamentaux sont evalues de le et les techniques pour controler ces influences ont discute. L'inclusion des effets de vegetation est demontree dans l'analyse de stabilite d'equilibre de limite de routine. Il est conclu que les racines de vegetation avec a-propos plantee et maintenue vont en toute probabilite fournir une 10% augmentation dans le facteur de surete de surfaces d'erreur peu profondes potentielles. Un ordinateur a base le systeme de soutien de decision de pente est presente pour aider des ingenieurs pour evaluer la probabilite d'une pente est convenable pour les techniques de genie biologique. Le systeme de soutien de decision de pente est librement disponible sur le site web de ECO-SLOPES pour les utilisateurs pour essayer de le et les reactions sur sa validite d'application.

Assessing the role of vegetation on soil slopes in urban areas

Repeated and sustained loading tests on undrained samples of normally consolidated lacustrine clay are used to prove the existence of a relationship between pore water pressures and axial strains.The behavior of the clay is studied by investigating the pore pressure and strain responses under repeated loading, and comparing them with the responses from the more usual sustained loading. Samples under repeated loading fail at a stress below the compressive strength of the material as obtained from a standard strength test.It has been shown that, for a repeated stress level below a critical value a nonfailure equilibrium state is reached, closed stress–strain hysteresis loops occur and the soil behavior is essentially elastic. For a repeated stress above the critical value, the effective stress failure envelope is reached and each loading cycle leads to further nonrecoverable deformations and ultimate failure.The pore pressures and axial strains produced by loading may be divided into recoverable and nonreco...

Pore Pressures and Strains After Repeated Loading of Saturated Clay

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).

/pdf/site-investigation-for-the-effects-of-vegetation-on-ground-6zb0ex7piv.pdf

Site investigation for the effects of vegetation on ground stability

Geosynthetics: protecting the environment

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