Suitability of pre-compression stress as the real critical stress of unsaturated agricultural soils

This study was conducted to examine if the pre-compression stress σpc is the major critical stress influencing soil physical quality indices, i.e. air permeability Kg and air-filled porosity (AFP). Topoils of five soil series were collected from the Isfahan Province in central Iran. Treatments consisted of: five soil types (sandy loam to clay), four matric suctions (10, 20, 50 and 80 kPa), three values for the maximum axial stress σpc (200, 400 and 600 kPa), and three loading types with 10 loading cycles. The loading types were a confined compression test (CCT), a semi-confined compression test (SCCT) and a kneading compression test (KCT). Soil type and matric suction, loading type, maximum applied stress and number of loading cycles significantly affected log [Kg], void ratio e, and AFP. The Significance of the loading cycle's number implies that though the σpc of the soil was not exceeded, the soil physical properties were considerably changed. A gradual transition from elastic-to-plastic deformation was observed on the stress–strain curves obtained from CCT. The stress−strain curves of the SCCT were sharp at σpc due to the semi-confined condition, which allows lateral deformation during loading. Cyclic loading was not always accompanied by significant irreversible strain but this could result in up to 10 times the decrease in Kg. For the fine-textured soils, CCT resulted in a significantly greater reduction in Kg, e and AFP when compared with KCT. The opposite trend was observed for the coarse-textured soils. The KCT homogenised the pore system and resulted in more decrease of Kg when compared with CCT at matric suctions of 10 and 20 kPa. However, at the matric suctions of 50 and 80 kPa, KCT created an open microstructure, which led to relatively higher values of Kg. The results show that the σpc might not be a real critical stress from a view of soil physical quality indices (i.e. Kg and AFP), especially at low matric suctions. Thus, the characterisation of soil compaction may not be completely accounted by a bulk property such as void ratio, but additional information about pore characteristics are needed to describe the effect of compaction on soil physical quality.