Pauline Defossez

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

The 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.

Analyzing key factors of roots and soil contributing to tree anchorage of Pinus species

Key messageRoot anchorage strength and stiffness can be represented by small number of root and soil parameters. Root morphology represents the majority of these factors.AbstractTree anchorage is a primary function for plant survival which may reach its limit under extreme conditions such as windstorms. To better understand the processes and influential factors underlying tree anchorage, we analyzed the mechanical effects of root morphology and the material properties of roots and soil on the tree-overturning process with the recently developed finite element model RootAnchor. The root system was represented by a simplified 3D root pattern derived from an ensemble average of seven measured root systems of 19-year-old Pinus pinaster grown in sandy spodosol. Soil properties were measured by direct shear tests. Taguchi orthogonal arrays were used to examine the sensitivity of the geometric and material factors of roots and soil to tree anchorage. Tree anchorage was characterized by anchorage strength TMc and anchorage stiffness K0. Using a small number of numerical experiments, the sensitivity analysis prioritized only two key factors contributing to tree anchorage among the 34 factors considered. The results showed root morphological traits that played a dominant role in the material properties of roots and soil in tree anchorage. Taproot depth, the dimensions of the Zone of Rapid Taper (ZRT) and basal diameter of the windward shallow roots were the key factors contributing to TMc (variations > 8%). The dimensions of the taproot, root and soil stiffness, and the basal diameter of the leeward shallow roots were the most active factors for K0 (variations > 10%). These results provide insight into simplified tree anchorage expressions for the prediction of wind-induced uprooting.

Determination of the soil compaction in real time on field with plate penetrometer and data filtering system

Soil compaction due to traffic is the process whereby soil bulk density is increased. The compaction level depends on the weight of the vehicle, and the soil susceptibility to compaction, which is characterized by its mechanical properties (that is, the stress-strain relationships). The plate penetrometer can be used to quantify the levels of soil compaction in-situ and supply a larger number of information on the control of this phenomenon, in real time. This paper evaluates the use of this tool with data filtering system assistance on semi confined compression assay, for predicting soil compaction in-situ, through parameters pre-compaction stress (spc) and soil compression Index (Cc). The tests were carried out on Experimental Agricultural Engineering Nucleus (NEEA in Portuguese) at West Parana State University - UNIOESTE - Brazil. The sample collection was carried at points along the tire track, 24 h after soil compaction induced by the rear tractor wheel with one, five and ten passages, on the same track. The information system is able to read all data derived from the compression test and carry out a filtering on the data by means of pre-selected parameters, plotting the compression curves and regression equations. The analysis, carried out “in situ”, avoids laboratorial procedures and gives the farmer more agility in the decision process on the management of the agricultural machinery system.