Tree resistance to wind: the effects of soil conditions on tree stability

Wind damage represents more than 50% by volume of forest damage in Europe. Recent evidence suggests that wind damage could double or even quadruple by the end of the century with dramatic consequences for the forest economy and the ecological functioning and survival of European forests. Most trees during storms are uprooted. While a large amount of work has been done over the last decade on understanding the aerial tree response to turbulent wind flow, much less is known about the root-soil interface, and the impact of soil moisture on tree uprooting. This paper investigates at tree scale the effects of soil conditions, such as water saturation during storms, on tree stability. Our analysis is based on (i) the critical bending moment that induces tree uprooting measured from static pulling experiments (ii) the soil mechanical properties as function of climatic conditions measured and modeled from laboratory measurements (iii) new techniques developed for studying the mechanics of tree structure incorporating 3D roots architecture and numerical biomechanics modeling

Quantifying the effect of matric suction on the compressive properties of two agricultural soil using an osmotic oedometer

International audienceThe compaction of cultivated soils by agricultural machines considerably affects both the structure and physical properties of soil, thus having a major impact on crop production and the environment. The soil mechanical strength to compaction is highly variable both in time and space because it depends on soil type (texture), soil structure (porosity) and soil moisture (suction). This paper is devoted to the effect of soil suction on the compression index Cc which is one of the mechanical parameters that describes the soil mechanical strength to compaction. We used an oedometer compression tests with suction control implemented by using the osmotic technique to study the compression index of a loamy soil and a sandy soil. Soil samples were prepared by compacting soil powder passed through 2 mm sieve, to a dry bulk density of 1.1 or 1.45 Mg m-3. The mechanical stress and the suction ranges considered corresponded to field conditions, with vertical stress less than 800 kPa and suction less than 200 kPa. The results show that the compression index Cc changed little with suctions ranging from 10 to 200 kPa for the two soils at different initial densities. By contrast, the variation of Cc is significant when soil suction is close to zero for the loamy soil at an initial dry bulk density of 1.1 Mg m-3. From a practical point of view, this variation in compression index with suction is a useful result for modelling soil strain due to traffic and predicting the compaction of cultivated soils

Analysing the form of the confined uniaxial compression curve of various soils

International audienceThe soil compaction by vehicles is a major factor responsible for physical degradation of cultivated soils. Uniaxial confined compression tests are usually performed to characterise the compaction properties of soil. Two main forms of compression curve have been observed: (i) the bi-linear curve having an elastic rebound curve at low stresses and a linear virgin compression curve at higher stresses; (ii) the S-shaped curve having deviation of the virgin compression curve at high stresses. In the present work, uniaxial confined compression tests were performed on four soils having various textures and different plasticity. Tests were performed on undisturbed and remould samples, at various initial dry bulk densities and water contents. The S-shaped compression curves were observed more frequently when the clay content and/or the initial water content were high. In addition, the S-shaped curves were observed more frequently on remould soils than on undisturbed soils. The difference between the compression of air-filled pores and that of meso-pores storing water subjected to high capillary forces could explain the observed S-shaped curves

Soil compaction by wheeling: change in soil suction due to compression

International audienceSoil compaction due to traffic has been increasingly recognized as a considerable problem facing intensive agriculture. Most of the models used to estimate soil deformation during the passage of machines are based on the concept of total stress: then they have neglected an important stress variable for unsaturated soils, i.e. the matrix suction. The aim of the present work is to evaluate the validity of this hypothesis by studying suction variation during a static compression test. A standard oedometer cell equipped with a tensiometer was used to measure soil suction in situ for different vertical stresses. Measurements were carried out on remoulded soil samples obtained by compacting a loamy soil at different initial water suctions (< 100 kPa). The results showed that the suction remained almost constant until a stress threshold value t beyond which the suction decreased as the stress increased. This stress threshold increased with the initial suction. These results corroborated the hypothesis of a constant suction during deformation usually assumed to model soil compaction during traffic for soils with suction higher than 20 kPa. The results obtained highlighted the effect of soil structure on the stress threshold: t was found to be higher for soil samples with initial aggregates < 2 mm for those with initial aggregates < 0.4 mm. This was interpreted at pore scale by comparing qualitatively the evolution of pore-size distribution and the expected distribution of water in the pores. This interpretation was based on pore-size distribution measurement by mercury intrusion

A study on the air permeability as affected by compression of three French soils

International audienceSoil air permeability is one of the most important parameters which govern the aeration in agricultural soils and thus has a significant effect on the plant growth and crop production. Therefore, it appears important, when analysing the effect of soil compaction due to agricultural machinery, to correlate air permeability with soil capacity parameters such as air-filled porosity, degree of saturation, water content, etc. In the present work, the relationship between air permeability, soil capacity parameters and vertical stress was analysed by performing confined uniaxial compression tests accompanied by air permeability measurements. Three French soils having different textures were studied. Tests were performed on remoulded and undisturbed soils, at various initial dry bulk densities and water contents. For the remoulded soils, the air permeability has been found strongly correlated with the applied vertical stress for sandy loam; by contrast, no obvious correlation could be established for clay. As far as the undisturbed soils are concerned, the air permeability could be correlated with the air-filled porosity for sandy loam and silty-clayey loam but also no evident correlation could be established for clay. Examination of an existing model predicting the air permeability from the air-filled porosity using one tortuosity/connectivity parameter showed that this parameter varies in a small range for sandy soils and in a larger range for clayey soils

A method to predict the soil susceptibility to compaction of surface layers as a function of water content and bulk density

International audienceIdentifying the vulnerability of soils to compaction damage becomes an increasingly important issue in the planning and execution of farming operations. Soil compaction models are efficient tools for predicting soil compaction due to agricultural field traffic. Most of these models require the knowledge of the stress/strain relationship, as well as the mechanical parameters and their variation with different soil physical properties. Because the soil compaction depends on its water content, bulk density and texture, a good understanding of the relation between them is essential for defining suitable farming strategies according to climatic changes. In this work we propose a new pedotransfer function for 10 French representative soils collected from cultivated fields, a vineyard and forests. We investigate the relationship between soil mechanical properties and easily measurable soil properties as well as water content and bulk density. Confined compression tests were performed on remoulded soils of a large range of textures at different initial bulk densities and water contents. The use of remoulded samples allowed us examining a large range of initial conditions with low variability of measurement. A good linear regression was obtained between soil precompression stress, compression index, initial water content, initial bulk density and soil texture. The higher the clay content, the higher the soil capacity to bear higher stresses at higher initial water content without reaching severe compaction state. The initial water content played an important role in clayey and loamy soils. In contrast, for sandy soils, the mechanical parameters were less dependent of initial water content but more related to the initial bulk density. These pedotransfer functions are expected to hold for soils of surface layers with tillage but further measurements on intact samples are needed to tests their validity

Models of soil compaction due to traffic and their evaluation

International audienc

Improving finite element models of roots-soil mechanical interactions

Two finite element modelling approaches have been applied and compared to simulate direct shear tests of soils reinforced by roots. The “frictional behaviour” method used with solid elements predicted shear strength of the root-soil system 4.6-6.9% higher than that predicted with “embedded beam elements”. This difference can be considered as negligible. Embedded beam elements were thus chosen to discretize digitized Maritime pine root systems in a tree anchorage model. Results exhibited contrasted mechanical responses when considering different root material properties

Modélisation du comportement aérodynamique de l'arbre au vent

National audienc

Modelling compaction of cultivated soils due to traffic: effect of soil water content.

International audienc