Loess soil layers have been deposited due to the accumulation of aeolian sediments. Due to the aeolian formation origin, the special metastable and open structure loess strata have been formed which are usually unsaturated in nature. These features make the behavior of loess sensitive to wetting and loading. Since naturally occurred loess is widespread, it has been recently considered as a promising resource for constructing earth-fill structures. Due to different microstructures of intact and compacted loess, different responses to wetting and loading would be expected. However, the interlink between different hydro-mechanical behavior and microstructures of intact and recompacted loess have not been revealed yet.

This research aims at providing insights into the microstructura...[ Read more ]

Loess soil layers have been deposited due to the accumulation of aeolian sediments. Due to the aeolian formation origin, the special metastable and open structure loess strata have been formed which are usually unsaturated in nature. These features make the behavior of loess sensitive to wetting and loading. Since naturally occurred loess is widespread, it has been recently considered as a promising resource for constructing earth-fill structures. Due to different microstructures of intact and compacted loess, different responses to wetting and loading would be expected. However, the interlink between different hydro-mechanical behavior and microstructures of intact and recompacted loess have not been revealed yet.

This research aims at providing insights into the microstructural effects on the behavior of unsaturated loess. Special attention is given to the water retention curve (WRC), volume changes, dilatancy and shear strength, and dynamic properties. A comprehensive experimental program was conducted including a series of laboratory element tests and microstructural investigations. Tests were conducted by utilizing a modified pressure plate device, an environmental chamber, a newly developed high suction-controlled direct shear box, a resonant column device and an oedometer cell. Furthermore, microstructural studies were carried out on both intact and recompacted loess through scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) tests.

Results of WRC tests showed that hysteresis was significant in the suction range of zero to 140 MPa. However, it was more pronounced for intact loess at suction around the air-entry value (AEV) as a consequence of constricted large pores and the ink-bottle effects supported by microstructural observations. A conceptual model was proposed to link a WRC to the corresponding pore size density (PSD) function and hence capture the key features of the WRC. Compared with recompacted loess, intact loess shrunk less (by 50%) and yielded at a higher suction. This is due to different microstructures associated with intact and recompacted loess.

Recompression index was reduced by one order of magnitude as suction increased from zero to 230 MPa, implying the enhancement of material rigidity due to aggregate stiffening. The peak shear strength continuously increased with suction even at high range (8 to 230 MPa) but at a reduced rate, which cannot be predicted by existing single effective stress concept or two independent stress state variables approach. This is due to the observed linear increase in dilatancy with suction, which is likely caused by the progressive aggregation of silt-clay particles as suction increased. Moreover, recompacted loess showed higher dilatancy than that of intact loess under identical conditions because of more non-uniform distribution of voids in the latter than the former.

Differences in the elastic shear strain threshold are negligible for specimens with different microstructures at the initial state. However, the elastic shear strain threshold significantly decreased (decrease in elastic range) and both damping ratio and shear modulus degradation attained higher rates as suction increased up to 40 MPa. This could be due to suction-induced silt-clay aggregation demonstrated by SEM observation. Therefore, desiccated fine-grained soils at high suction behave similar to coarse-grained sand or gravel.