Aeolian loess is widely spread all over the world and covers approximately 10% of the earth surface. Because of the aeolian origin, loess has a metastable and high porosity structure, which is sensitive to hydraulic drying/wetting and loading. This sensitivity may induce significant volumetric deformation and collapse, which influences water retention characteristics. Natural loess deposited as deep as 10 m or more are subjected to seasonal variations of water content (suction). Given that soil is a stress-dependent material, it is, therefore, necessary to understand stress effects on water retention characteristics of loess with volumetric measurements. However, very limited studies have been reported on water retention curve (WRC) of loess. Volumetric changes due to drying/wetting and...[ Read more ]

Aeolian loess is widely spread all over the world and covers approximately 10% of the earth surface. Because of the aeolian origin, loess has a metastable and high porosity structure, which is sensitive to hydraulic drying/wetting and loading. This sensitivity may induce significant volumetric deformation and collapse, which influences water retention characteristics. Natural loess deposited as deep as 10 m or more are subjected to seasonal variations of water content (suction). Given that soil is a stress-dependent material, it is, therefore, necessary to understand stress effects on water retention characteristics of loess with volumetric measurements. However, very limited studies have been reported on water retention curve (WRC) of loess. Volumetric changes due to drying/wetting and effects of stress state on water retention characteristics have been rarely reported in the literature. To better understand intact loess behaviour, re-compacted loess is used as a reference material. The principal objectives of this study are, therefore, to measure the WRC of loess over a wide range of suctions and to investigate the effects of net stress on WRC and wetting/drying induced volumetric behaviour of intact and re-compacted loess.

Three series of tests were carried out on intact and re-compacted specimens to achieve the above objectives. In test series-I, WRC tests of intact and re-compacted loess under zero stress were carried out for a wide range of suctions (zero to 290 MPa). Two different hydraulic paths WD (wetting followed by drying) and DW (drying followed by wetting) were followed to cover suction range, 0-0.4 MPa and 0.88-290 MPa, respectively and volumetric measurements were made as well along the drying and wetting paths. Suction was imposed by axis translation, osmotic and vapour equilibrium techniques to cover different ranges of suction, 0-0.4 MPa, 0.88-4 MPa and 7-290 MPa, respectively. On the other hand, stress effects on WRC and wetting/drying induced volumetric behaviour of intact and re-compacted loess were investigated in test series II and III, respectively in which tests were conducted at different net stresses, i.e., zero, 50 and 110 kPa, respectively, at low suction ranges (0-0.4 MPa). In that case, net stress was applied by using a modified one-dimensional stress-controllable volumetric pressure plate while suction was controlled by the axis translation technique.

Experimental results revealed that hysteretic behaviour is different for intact and re-compacted loess for the studied suction range (i.e., zero-290 MPa). Intact loess shows more pronounced hysteresis around suction value of 10 kPa compared to re-compacted loess due to larger proportion of constricted pores (macro-pores larger than 100 μm) of intact loess. Stress has a significant effect on water retention curve (WRC) of intact and re-compacted loess. Only wetting WRC of intact loess is influenced by an increase in net vertical stress while drying WRC is insensitive to stress effects. On the contrary, stress effect is more pronounced on drying WRC of re-compacted loess. At high suction range (i.e., 0.88-290 MPa), re-compacted loess shows 50% higher drying induced shrinkage (or volumetric strain) than that of intact loess. This difference in volumetric behaviour of intact and re-compacted loess is because the former soil has a higher yield stress and a more stabilized structure than those of the latter. Under a constant net stress, re-compacted loess shows significantly higher wetting induced collapse compared to that of intact loess.