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In order to determine the mechanical resistance of several forest tree species to rockfall, an inventory of the type of damage sustained in an active rockfall corridor was carried out in the French Alps. The diameter, spatial position and type of damage incurred were measured in 423 trees. Only 5% of trees had sustained damage above a height of 1.3 m and in damaged trees, 66% of broken or uprooted trees were conifers. Larger trees were more likely to be wounded or dead than smaller trees, although the size of the wounds was relatively smaller in larger trees. The species with the least proportion of damage through stem breakage, uprooting or wounding was European beech ( L.). Winching tests were carried out on two conifer species, Norway spruce ( L.) and Silver fir ( Mill.), as well as European beech, in order to verify the hypothesis that beech was highly resistant to rockfall and that conifers were more susceptible to uprooting or stem breakage. Nineteen trees were winched downhill and the force necessary to cause failure was measured. The energy () required to break or uproot a tree was then calculated. Most Silver fir trees failed in the stem and Norway spruce usually failed through uprooting. European beech was either uprooted or broke in the stem and was twice as resistant to failure as Silver fir, and three times more resistant than Norway spruce. was strongly related to stem diameter in European beech only, and was significantly higher in this species compared to Norway spruce. Results suggest that European beech would be a better species to plant with regards to protection against rockfall. Nevertheless, all types of different abiotic stresses on any particular alpine site should be considered by the forest manager, as planting only broadleaf species may compromise the protecting capacity of the forest, e.g., in the case of snow avalanches.

The mechanical behaviour of individual roots and their interaction with soil controls plant anchorage and slope stabilisation, and this is controlled by plant genotype. Tensile tests were performed on roots of tobacco ( ‘Samsun’) plants with lignin biosynthesis pathways affected by down-regulating cinnamyl-alcohol dehydrogenase (CAD) enzyme production. Altering this pathway resulted in root stiffness <50% of the unmodified control, although failure stress was not different. Like most biological tissues, the roots had non-linear mechanical behaviour, were irregular in shape, and heterogeneous. Particle image velocimetry (PIV), applied for the first time to the tensile testing of materials, identified the localised strain fields that developed in roots under tension. PIV uses a cross correlation technique to measure localised displacements on the surface of the root between sequential digital images taken at successive strain intervals during tensile loading. Further analysis of root sections showed that non-linear mechanical behaviour is affected by cellular rupture, with a clear step-wise rupture from cortex to stele in some younger roots. This will affect slip planes that develop under pull-out at the root–soil interface. By assessing localised axial and radial strain along a root section with PIV, we have been able to determine the true stress that controls ultimate failure and the true stress–strain behaviour along the root length. The techniques used have clear potential to enhance our understanding of mechanical interactions at the root–soil interface.

Simple and complex analytical models of root reinforcement and the associated requirements and limitations are reviewed. Simple models include the limiting equilibrium solution and the cable and pile solutions. The complex model is the finite element method (FEM). The simple models were used to analyze published data from laboratory and shear tests and pullout tests on soils reinforced with synthetic materials and root systems. The models can be used for approximations when the model requirements are met. The FEM was used to simulate experiments and provided more detailed information. These results provide insight on the failure mechanisms. This forms the basis for suggestions on models to be used in stability analysis of slopes.

Forest vegetation is known to increase hillslope stability by reinforcing soil shear resistance and by influencing hydrologic conditions of soil. Although the importance of plant root systems for hillslope stability has received considerable attention in recent years, the quantification of such an effect needs more investigation. In this paper, we present a synthesis of the data gathered in the last 5 years for some species in different locations of the Alps and Prealps of Lombardy (Northern Italy) with the aim to increase our knowledge on root tensile strength and on root area ratio distribution within the soil. Concerning root tensile strength we developed tensile strength–diameter relationships for eight species: green alder ( (Chaix)D.C.), beech ( L.), red willow( L.), goat willow ( L.), hazel ( L.), European ash ( L.), Norway spruce ( (L.) Karst.) and European larch ( Mill.). Results show a great variability among the different species and also for the same species. In general, however, root strength (in terms of tension) tends to decrease with diameter according to a power law, as observed by other authors. Comparing the power law fitting curves for the considered species, it can be observed that they fall in a relatively narrow band, with the exception of hazel, which appears the most resistant. Concerning the evaluation of root distribution within the soil we estimated the root area ratio (the ratio between the area occupied by roots in a unit area of soil) according to its depth for five species (beech, Norway spruce, European larch, mixed hazel and ash) in three locations of Lombardy. Results show that there is a great variability of root density for the same species well as for different points at the same locality. The general behaviour of root density, in any case, is to decrease with depth according to a gamma function for all the studied species. The results presented in this paper contribute to expanding the knowledge on root resistance behaviour and on root density distribution within the soil. The studied location have allowed the implementation of soil–root reinforcement models and the evaluation of the vegetation contribution to soil stability.

Vegetation can significantly contribute to stabilise sloping terrain by adding cohesion to soil: this reinforcement depends on the morphological characteristics of the root systems and the tensile strength of single roots. The paper presents the results of research carried out in order to evaluate the biotechnical characteristics of the root system of three typical Mediterranean plant specieswhich can affect slope stability. The species considered in the present study are L. (a perennial herbaceous monocotyledonous), L. and L. (two dicotyledonous shrub species). The plant specimens were collected in the Basilicata region (Southern Italy) by excavation to obtain the whole root systems. Single root specimens for each species were sampled and tested for tensile strength measurement and the complete root systems were analysed to evaluate the root density distribution with depth in terms of root area ratio. The resulting data have been used to calculate the reinforcing effect in terms of increased shear strength of the soil using the model of Wu (1976, Investigation of landslides on Prince ofWales Island. Geotech. Eng. Rep. 5 Civil Eng. Dep. Ohio State Univ. Columbus, Ohio, USA) andWaldron (1977, Soil Sci. Soc. Am. J. 41(3), 843–849), a simple and widespread model based on the reinforced earth theory. The results show that root reinforcement exerted by L. is stronger than the reinforcement exerted by and in the upper layers of the soil, while presents higher reinforcement values in deeper horizons. presents lower values than either of the other species studied.

Vetiver grass (), also known as , is a graminaceous plant native to tropical and subtropical India. The southern cultivar is sterile; it flowers but sets no seeds. It is a densely tufted, perennial grass that is considered sterile outside its natural habitat. It grows 0.5–1.5 m high, stiff stems in large clumps from a much branched root stock. The roots of vetiver grass are fibrous and reported to reach depths up to 3 m thus being able to stabilise the soil and its use for this purpose is promoted by the World Bank. Uprooting tests were carried out on vetiver grass in Spain in order to ascertain the resistance the root system can provide when torrential runoffs and sediments are trying to uproot the plant. Uprooting resistance of each plant was correlated to the shoot and root morphological characteristics. In order to investigate any differences between root morphology of vetiver grass in its native habitat reported in the literature, and the one planted in a sub-humid environment in Spain, excavation techniques were used to show root distribution in the soil. Results show that vetiver grass possesses the root strength to withstand torrential runoff. Planted in rows along the contours, it may act as a barrier to the movement of both water and soil. However, the establishment of the vetiver lags behind the reported rates in its native tropical environment due to adverse climatic conditions in the Mediterranean. This arrested development is the main limitation to the use of vetiver in these environments although its root strength is more than suffcient.

Highway embankments and cutting slopes in the United Kingdom, particularly in the South East of England, are often constructed of or within stiff over-consolidated clays. These clays are prone to softening with time leading to shallow slope failures and costly repairs. Reinforcement by natural vegetation is potentially a cost-effective method of stabilising these types of slopes over the medium–long term. However, there is a lack of information on how natural vegetation reinforces and stabilises clay slopes. To investigate this problem, the potential reinforcement of selected oak ( L.) and hawthorn ( Jacq.) roots was assessed by conducting in situ root pull-out experiments on a London Clay cutting in south-east England. Pull-out tests were carried out using specifically designed clamps and either a hand pull system with a spring balance and manual recording of force for oak roots or a jacking system with electronic data logging of applied force and displacement for hawthorn roots. Oak roots had a mean pull-out resistance of 7 MPa and that of hawthorn roots was 8 MPa. The electronic data logging of applied force (pull-out resistance) and displacement of the hawthorn roots provided additional data on the failure of branched roots which could be correlated with variations in root morphology. The failure of the roots can be categorised into three modes: Type A: single root failure with rapid rise in pull-out resistance until failure occurs; Type B: double peak failure of a forked or branched root and Type C: stepped failure with multiple branches failing successively. The different types of root–soil bonds are described in relation to root anchorage and soil stability.

The roots of trees provide an important contribution towards the stability of hill slopes. Tree roots in the soil act very similarly to steel fibers in reinforced concrete and provide resistance to shear and tensile forces induced in the soil. In addition, the roots also absorb water from the soil, which reduces moisture content, again helping to increase the stability of the slope. As Iran has a long history of landslides, our research deals with the effect of tree roots on slope stability, in particular, the following species which are of economic and environmental interest: tea ( L.), citrus (), lilaki ( Dsf.) and angili ( D.C.) (Mosadegh, 1996). The study was carried out in Roudsar Township in Gilan State of Iran. Of the overall surface area of 1800 ha, 288 ha were considered suitable for the purposes of this study. A large part of the area had slopes of steep gradients on which natural vegetation was present. Other parts of the same area have been cleared and planted with tea and citrus crops. Soil samples were taken from an area of approximately 70 ha for testing in the laboratory. Direct shear tests were carried out on soil samples and the factor of safety (FOS) calculated. Results showed that the FOS was increased in soils with tree roots present. The global slope FOS was then determined using Bishop’s method. We calculated the FOS in order to protect slopes where the gradient exceeds 25%. In this case study, the minimum FOS was assumed to be 1.3, which corresponds to e.g. vegetation with 40–60% crown cover, a soil internal friction angle of 15° and a slope angle of 21°. When soil internal friction angle equals 15° and slope angle is >31°, slope stability cannot be increased by any vegetation species.

The effect of root reinforcement on slope stability has been well researched through empirical studies, but to date few mechanistic studies have examined the influence of tree roots on slope stability. Furthermore, the previous research has lacked consideration of the effect of landslide displacement on root reinforcement. This paper will analyze the influence of root reinforcement on safety factors (Fs) as a function of slope displacement. A model of a root system as a cluster of straight bars inserted from unstable soil into bedrock is used to reliably estimate increases in the shear resistance of the soil. The relationship between root reinforcement and lateral displacement is analyzed under two conditions: ultimate stress and pullout resistance of root fibers. The species used in the present research was the Japanese cedar ( (L.f.) D. Don.), the most common tree species in Japan. The spatial distribution of root size and root inclination was taken from field experiments performed by Japan Sabo Technical Center in 1998. The reinforcement capacity of root fibers is considered as a function of the horizontal displacement of the landslide and the depth of the slip surface. By combining the data obtained from field experiments with a calculation model of inclined roots, this paper analyzes the Ozawa slope safety factor. Thus, root reinforcement and the slope safety factor were calculated for various displacements in the process of landslide movement.

The mechanics of root reinforcement have been described satisfactorily for a single root or several roots passing a potential slip plane and verified by field experiments. Yet, precious little attempts have been made to apply these models to the hillslope scale pertinent to landsliding at which variations in soil and vegetation become important. On natural slopes positive pore pressures occur often at the weathering depth of the soil profile. At this critical depth root reinforcement is crucial to avert slope instability. This is particularly relevant for the abandoned slopes in the European part of the Mediterranean basin where root development has to balance the increasing infiltration capacity during re-vegetation. Detailed investigations related to root reinforcement were made at two abandoned slopes susceptible to landsliding located in the Alcoy basin (SE Spain). On these slopes semi-natural vegetation, consisting of a patchy herbaceous cover and dispersed Aleppo pine trees, has established itself. Soil and vegetation conditions were mapped in detail and large-scale, direct shear tests on the topsoil and pull-out tests performed in order to quantify root reinforcement under different vegetation conditions. These tests showed that root reinforcement was present but limited. Under herbaceous cover, the typical reinforcement was in the order of 0.6 kPa while values up to 18 kPa were observed under dense pine cover. The tests indicate that fine root content and vegetation conditions are important factors that explain the root reinforcement of the topsoil. These findings were confirmed by the simulation of the direct shear tests by means of an advanced root reinforcement model developed in FLAC 2D. Inclusion of the root distribution for the observed vegetation cover mimics root failure realistically but returns over-optimistic estimates of the root reinforcement. When the root reinforcement is applied with this information at the hillslope scale under fully saturated and critical hydrological conditions, root pull-out becomes the dominant root failure mechanism and the slip plane is located at the weathering depth of the soil profile where root reinforcement is negligible. The safety factors increase only slightly when roots are present but the changes in the surface velocity at failure are more substantial. Root reinforcement on these natural slopes therefore appears to be limited to a small range of critical hydrological conditions and its mitigating effect occurs mainly after failure.