Soil structure changes during compaction of a cohesive soil

Abstract

Orientations of particles, pores and other constituents during compaction of an artificially made clayey soil were studied in order to investigate how soil structure, and in turn, engineering parameters such as dry unit weight, porosity, void ratio and compaction characteristics, change during compaction of a cohesive soil at different moisture contents on both the dry and wet sides of the optimum moisture content. The results show that the orientation pattern at the very dry stage of the compaction curve is nearly random. The overall degree of preferred orientation increases as the moisture content increases until the optimum moisture content is reached. Edge–edge contacts between the particles and/or domains at the very dry stage of the compaction curve first become mainly edge–face and then become face–face contacts near the optimum moisture content. Around the optimum moisture content, the overall degree of preferred orientation becomes the highest. From this point on, however, the overall degree of preferred orientation starts to decrease again indicating that further moisture content increase does not cause additional overall preferred orientation. The structure beyond the optimum moisture content is mainly characterised by long strings of differently oriented packets in the form of curved trajectories and mainly face-to-face contacts between the particles and/or domains in the packets. Though there is a high degree of preferred fabric orientation in each individual packet, the overall degree of preferred orientation decreases because the packets are themselves oriented in different directions. The results agree with previous studies showing that the soil fabric dry of optimum is randomly oriented but becomes increasingly oriented towards the optimum. However, as the moisture content increases on the wet side of optimum, the overall degree of preferred orientation decreases as opposed to the generally accepted view.