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

Abstract

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.