Effects of internal and external forces on pre-compaction stress and air-premeability of fice soils from Central Iran

Stresses and strains are imposed on soils by natural processes, such as wetting and drying cycles, as well as by anthropogenic actions such as tillage and traction. The combined effects of these stresses on soil behavior, however, have not been studied intensively. In central parts of Iran, soils are low in organic matter with unstable structure and behave as slaking, crusting and hardsetting soils. Previous studies showed that such soils could be compacted even in the absence of external forces. This research was conducted to study the combined effects of loading types, soil matric potentials (pF values of 2.7 and 2.9) and drying and wetting cycles on soil physical and mechanical properties of five different topsoils from Isfahan, Iran. Loading types consisted of confined compression test (CCT), semi-confined compression test (SCCT) and kneading compression test (KCT). Each loading type had three maximum stress levels (200, 400 and 600 kPa) with two sub-types of loading (cyclic loading with 10 cycles and staircase loading). Before starting the compaction process, a drying and wetting cycle was applied on all the soil samples. In cyclic loading of CCT and KCT, after reaching the pre-set stresses and measuring void ratio and air permeability (Kg), the drying and wetting cycle was applied. Then, void ratio and Kg were measured again and loading with the same maximum stress was continued 9 more times. For staircase loading of CCT and KCT, the drying and wetting cycle was imposed after the first (200 kPa) and the second (400 kPa) cycles. The void ratio and Kg were determined before and after the drying and wetting cycles. The results showed that the pre-compaction stress and the sharpness of pre-compaction region were significantly affected by external and internal forces, i.e. the highest effective stress resulting from drying and wetting cycles. Therefore, any change of soil structure might play an important role in pre-compaction of soils. It was interesting that for the silty clay loam soil with unstable structure, drying and wetting cycle decreased the Kg of the sample with pre-compaction stress of 200 kPa and increased it with pre-compaction stresses of 400 and 600 kPa although void ratio changes were negligible. It implies that drying and shrinking acts similar to consolidation under an external stress. Drying is comparable to virgin compression and rewetting is comparable to rebounding. For the coarser soils, an increase in Kg after a cycle of drying and wetting was observed. Since, the ability of such soils in creation of micro-cracks upon drying and wetting is limited, it might be concluded that drying and wetting will cause reformation of menisci around the soil particles in such a way that improve soil physical quality (Kg). During soil compression, particles and water are brought to unstable positions and the drying and wetting cycles could rearrange them even if there is not significant change in soil packing state (void ratio). The results are in agreement with Henionen’s (1977) findings which showed, under natural conditions, soils are approaching a specific packing state depending on their intrinsic properties, however, the rate of these changes are that divides soils into structurally stable (low rate) and unstable (high rate). It can be concluded that compressive behavior of soils, especially with unstable structure, could significantly be affected by internal stresses. The repeated loading as well as drying and wetting cycles would change the virgin compression line (VCL) although the stress was not changed apparently. The changes are more obvious for Kg. It means the effects of external stresses were ameliorated by the drying and wetting cycles.