Effects of compaction state on structural strength of a clayey soil as determined by micropenetrometer tests

In this investigation, the structural strength of a clayey soil compacted at three different dry densities (1.7, 1.6 and 1.5 Mg/m3) and three different compaction water contents (20%, 16% and 12%) was studied by using a micropenetrometer. The penetration curves at various water contents during the drying and wetting cycle were analyzed. The maximum unit penetration strength and the penetration stiffness were used to represent the structural strength. Experimental results show that the structural strength increases with decreasing water content. At a given water content, the larger dry density, the larger structural strength and the smaller hysteresis loop induced by the drying-wetting cycle. At a higher dry density, the structural strength increases and the drying-wetting cycle induced hysteresis loop becomes smaller. Specimens compacted on the wet side of optimum water content exhibit a dispersed structure and those compacted on dry side of optimum water content have an aggregated structure. For soils with a dispersed structure, the structural strength increases with decreasing water content and the growth rate is gradually accelerated. However, for soils with an aggregated structure, when dried to water content lower than 8%, the structural strength increases by a small degree or even decreases. Moreover, the hysteresis loop of the soil specimens with dispersed structure is much smaller than those with an aggregated structure.