Soil-particle and pore orientations during consolidation of cohesive soils

Abstract

Orientations of particles, pores and other constituents during consolidation of an artificially made clayey soil were studied in order to investigate how orientations, and in turn, engineering parameters such as void ratio and compressibility, change during consolidation of a cohesive soil. The results show that the orientation pattern before loading is generally random, even though there may be some degree of preferred orientation caused by the overburden pressure. Loadings smaller than the maximum effective (preconsolidation) stress cause only a small degree of preferred orientation and a small change in the soil structure, which may be the reason why small amounts of compression and void ratio change occur on the recompression curve. When the sample is loaded to stress levels greater than the preconsolidation stress, however, there is an increasing degree of preferred orientation. The higher the loading, the higher the percentage of measurements showing a nearly horizontal preferred orientation, and the lower the average orientation angle with the horizontal. The soil structure is no longer able to sustain the increased loads, and the soil-particles, pores and other constituents are rearranged or reoriented into a new more stable and stronger structure, resulting in much larger void ratio changes and a practically linear curve (the virgin curve) with a steeper slope. This may be the reason why a large amount of compression occurs on the virgin curve. The orientation tends to become perpendicular to the maximum principal stress direction, and an anisotropic structure develops as the loading increases. The effect of side friction on soil structure becomes evident as the loading increases, indicating that smearing the inside of the oedometer ring with high-vacuum silicon grease results in little, if any, reduction of side friction.