Water retention and volumetric characteristics of intact and re-compacted loess

A laboratory testing program was conducted to investigate the effects of microstructure on the water retention curve (WRC) and wetting-drying induced volume change in loess. The axis translation and vapor equilibrium techniques were adopted to control suction in the range of 0-400 kPa and 4-140 MPa, respectively. Hysteresis in the WRC of loess was observed for the entire range of suction studied. Compared to re-compacted loess, intact loess exhibits a more pronounced hysteresis in the suction range below 20 kPa, which can be explained by the ink-bottle pore neck effect or constricted pores. The hypothesis is supported by microstructural evidence of mercury intrusion porosimetry and scanning electron microscopy tests. However, re-compacted loess exhibits larger hysteresis than intact loess for suctions above 30 kPa. A conceptual model was introduced, which links WRC to the corresponding pore-size density (PSD) function. Regarding volume change, more noticeable drying-induced shrinkage, but yielding at a lower suction, was observed for re-compacted loess. This is consistent with the compression test results. Stress has a significant effect on change of PSD and constricted macropores leading to a shift in the main wetting curve and a less pronounced hysteresis. Intact loess exhibits a stress-dependent wetting-induced collapse and drying-induced shrinkage.