A simplified method for analysing the mechanics of clod: clod interactions in topsoils with a wide range of clay content

Classical concepts of mechanics applied to continuous materials cannot be used directly for associations of soil clods occurring in ploughed and harrowed topsoils. Therefore, we are proposing a new simplified method for analysing the clod : clod mechanics, for a wide range of clay content. We pressed together axially two adherent remoulded soil balls and recorded a dimensionless strain flattening ratio R-f as a function of the compression stress. The strain-stress relationships of the studied adherent soil balls were (i) modeled using either a common statistical curve-fitting approach or the concept of effective stress, and (ii) compared with the strain-stress relationships obtained for the cylindrical counterparts. The axial stress applied on the two adherent soil balls was heterogeneous, ranging from a minimal value to a maximal one, which were both related to the R-f counterpart value. The relationship between the maximal compression stress and the flattening strain was fitted with a power-law equation, with either a negative or a positive power-law coefficient for either the convex stress-strain curve of the plastic clayey soils or the concave stress-strain curve of the friable granular soils, respectively. The internal soil strength, determined as the intercept of the straight stress-suction line for the flattening test (R-f value of 0.47), was of 2.2 and 1.2 kPa (minimal stress), and of 10 and 5.4 kPa (maximal stress) for the silty and the sandy remoulded topsoils, respectively. The minimal axial compressive stress for two adherent remoulded topsoil balls at a flattening strain ratio of 0.47 increased nonlinearly as a function of the axial compressive stress for its cylinder counterpart at a volumetric axial ratio of 0.2. The drier and the more cohesive is the soil, the more difficult is the geometrical macroscopic change from a spherical shape to a flattened surface area. The results show the merit of applying the flattening test for two adherent soil balls both for determining mechanical parameters relevant to the behaviour of adherent tilled topsoils. (C) 1999 Elsevier Science B.V. All rights reserved.