Loess compaction at different water contents: Effects on hydraulic conductivity, compression behavior, microstructure, and water distribution

In this study, compacted loess samples with varying compaction water content but identical dry density were prepared to investigate the evolution of their hydraulic conductivity and compression behavior. Additionally, environmental scanning electron microscopy (ESEM) and nuclear magnetic resonance (NMR) analyses were conducted to gain microstructural insights into loess behavior at the laboratory scale. The results indicate that the maximum saturated hydraulic conductivity is observed at the lowest compaction water content, particularly in the early stage of permeability tests. In particular, for loess compacted at water contents below the optimum (as determined by the modified Proctor compaction test), the hydraulic conductivity decreases throughout the permeability tests. Conversely, when the water content exceeds the optimum level, the hydraulic conductivity shows an increasing trend. In terms of compression behavior, when the as-compacted samples are loaded in oedometer conditions, an increase in material compressibility is observed with increasing compaction water content. Again, a different phenomenological behavior was observed when the compaction water content exceeded the optimum, i.e. an abrupt increase in loess compressibility. ESEM tests provide microstructural confirmation of this evidence, as the surface morphology of the compacted loess changes significantly with increasing compaction water content. The microstructural evolution was also quantified in terms of area ratio using image processing software. Finally, NMR was used to quantify the intra- and inter-aggregate water at different compaction water contents, once again highlighting a threshold for the presence or absence of inter-aggregate water similar to the optimum water content.