Pre-compression stress concept and physical qualities of soils from Central Iran

The concept of pre-compression stress is important to the compressive behavior of unsaturated agricultural soils under quick compression since additional soil compaction might occur only when applied stress exceeds the pre-compression stress. This concept is based on non-significant elastic or significant plastic strain of the soil. This study was conducted in order to examine if the precompression stress can be linked to soil physical qualities, such as air permeability (Kg). Topoils of five different soil series were collected from Isfahan province in central Iran. Attempts were given to have a range of soil texture and organic matter content. Five soil types, four pF values (2, 2.3, 2.7 and 2.9), three maximum or pre-compression stress values (200, 400 and 600 kPa) and three loading types with two sub-types of loading (cyclic loading with 10 cycles and staircase loading) were used in the experiment. Loading types consisted of confined compression test (CCT), semi-confined compression test (SCCT) and kneading compression test (KCT). For cyclic loading, after reaching a pre-set maximum stress and measuring Kg, the loading with the same maximum stress was repeated 9 more times. In staircase loading, the maximum stress was changed between consecutive loading cycles. The effect of loading on void ratio and Kg was affected mainly by soil type and wetness. When reloading of the soils was done several times with the same maximum stress, soil strain was increased more for SCCT and KCT in comparison with CCT. In staircase loading, all the soil types showed a similar response in which, the void ratio and Kg were decreased with increasing compression stress. Likewise, the void ratio and Kg at a given compression stress decreased with an increase in soil moisture. Repeated loading on the soils with high water content (pF values of 2 and 2.3) could distort the contractile skin of water menisci around the soil particles. It was found that the repeated loading effect on Kg is more important at lower pF values or higher water contents. This process may not accompany with significant plastic strain since for some cases more than ten times decrease in Kg resulted from the repeated loading. KCT created more homogenization of pore system and resulted in more decrease in Kg when compared with CCT at higher water contents (pF values of 2 and 2.3). On the other hand, for drier conditions (pF values of 2.7 and 2.9), kneading compression formed a more open microstructure i.e. anisotropic arrangement of soil particles, which led to higher values of Kg when compared with CCT though the strains were almost the same. Results from the cyclic loading showed that the pre-compression stress might not be a real critical stress from the view soil qualities (e.g. Kg) especially for the soils with unstable structure and high water contents. This is due to the fact that the compressive pore water pressure deteriorates and homogenizes pore system continuities. Moreover, the particles of the unstable soils do not have permanent and stable bonds e.g. by organic compounds or cementation in order to resist the cyclic loading. Therefore, changes of soil physical properties during compaction process imply the fact that bulk properties like bulk density (or void ratio) cannot completely describe the soil physical quality state but additional information about pore continuity properties is necessary.