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

The 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. Soil mechanical resistance is highly variable both in time and space because it depends on soil texture, soil porosity and soil water state (matric suction). This paper is devoted to the effect of soil matric suction on the compression index Cc, which is one of the mechanical parameters that describes the soil mechanical resistance to compression. We performed oedometer compression tests to study the compression index of a loamy soil and a sandy clay, with matric suction control, by using the osmotic technique. Soil samples were prepared by compacting soil passed through a 2 mm sieve at dry bulk densities of 1.1 and 1.45 Mg m−3. The mechanical stress and matric suction ranges considered corresponded to field condition, with vertical stress less than 800 kPa and matric suction less than 200 kPa. The results show that the compression index Cc was constant or changed little with matric suctions for the two soils at different initial densities. For the loamy soil, the average compression index Cc equals 0.73 for an initial bulk density of 1.1 Mg m−3 and 0.41 for 1.45 Mg m−3 for matric suction higher than 20 kPa. For the sandy clay, the average compression index Cc is 0.35. From a practical point of view, our study shows that the mathematical determination of mechanical parameters should be standardized due to the S-shaped compression curves that may significantly impact the compression index estimation. The observation of constant compression index with matric suction is a useful result for modelling soil strain due to traffic and predicting the compaction of cultivated soils.