Effective stress and pore water dynamics in unsaturated soils: Influence of soil compaction history and soil properties

Abstract

Effective stress is an important strength indicator for understanding and fundamentally predicting soil behaviour due to mechanical disturbance. This study investigated the influence of `soil properties on effective stress in relation to matric potential, and soil sampling direction (vertical/horizontal). Undisturbed soil samples representing 3 different locations were collected at 4 depths and in 2 sampling directions, to determine which soil properties mostly affect the effective stress in response to the magnitude of the stress applied. The soil properties included texture, organic carbon content, dry bulk density, void ratio, pre-compression stress and prevailing matric potential. The results showed that both the effective stress and the concomitant changes in pore water pressure largely depend on the prevailing matric potential in the soil. In addition, changes in soil structure caused by subsurface soil compaction as a result of tillage, organic carbon and clay content influenced the magnitude of the effective stress and pore water pressure in soils during loading. The porewater pressure measured under static loading is sampling direction dependent due to differences in hydraulic conductivity and pore continuity. Porewater pressure increases (become less negative) with increases in the fraction of fine pores in the soil while it may either remain constant or decrease, when the medium and coarse pore fractions in the soil increases. This implies that the smaller the pore connectivity and continuity, the less negative the changes in pore water pressure become. Also, the higher the pre-compression stress (sometimes due to higher organic carbon content), the more consistent the applied stress induced an increase in the calculated effective stress.