Haijun Hu

The 49,000-km2 Badain Jaran Desert lies in the centre of Alxa Plateau in the western Inner Mongolian Region [1;2]. The southern part of this desert is characterised by the unique association of lakes with the tallest megadunes of Earth (general height varying between 150 and 350 m). The mean precipitation rate of this region is below 100 mm yr-1 and the evapotranspiration one is ~2600 mm yr-1. Around 140 lakes have been reported, mainly located in the interdunal region and they represent a mean surface of ~23 km2. In order to protect the water resource of this desert, scientific research such as the sources of groundwater and groundwater recharge has been carried out. One of the most interesting resulting hypotheses is the existence of a convective circulation of the groundwater [3;4;1;5]. Indeed, the ascending current of groundwater can 1) supply the lakes and 2) may play role in the cementation of the megadunes, process that is considered as the starting point for their development. Interestingly, at the surface of the megadunes, a dry layer is present and its depth varies between 20 and 50 cm. But below this dry layer, the sand is moistened [6].Space-based thermal images from MODIS of this region display at first approximation a correlation between the topography and the surface temperature evolution. In order to understand the relationship between the surface temperature, topography and soil moisture, a fully coupled hydro-thermal method was adopted to simulate the interaction between the atmosphere and the first metre below the surface. The analysis process includes the determination of material parameters, initial and boundary conditions, the calculations of net solar radiation, actual evaporation and sensible heat. Our methodology relies on the measured temperature distribution by MODIS and the calculation shows the temperature evolution along with the elevation. The factors including sunshine direction (i.e. sunny or shadowed slope) and evaporation on the surface temperature distribution at Badain Jaran will be discussed.[1] Dong et al. (2004), Geomorphology, doi: 10.1016/j.geomorph.2003.07.023; [2] Dong et al. (2009), Geomorphology, doi: 10.1016/j.geomorph.2008.10.015; [3] Chen et al. (2004), Nature, doi: 10.1038/432459a; [4] Chen et al. (2012), Geochemistry International, doi: 10.1134/s0016702912030044 ; [5] Gates et al. (2008), Applied Geochemistry, doi: 10.1016/j.apgeochem.2008.07.019; [6]  Chen et al. (2006), Chinese Science Bulletin, doi; 10.1007/s11434-006-2196-8

Loess soils undergo collapse when the material is loaded and wetted, which leads to engineering problems and disasters. To understand better the collapse mechanisms that govern loess and loess-like deposits under complex stress paths, a distinct element model has been established. A contact model considering water content effect introduced for structural loess was used to cement contacted particles together to simulate unsaturated structural loess under biaxial compression conditions. The numerical structural loess was tested for different wetting and loading paths, i.e., loading-quick wetting (LW^Q), loading-gradual wetting (LW^G), and wetting-loading (WL) paths. The simulated macroscopic mechanical behavior agrees approximately quantitatively in terms of the deviatoric deformation and qualitatively regarding the volumetric deformation when compared with available experimental results. The tests under different mean stresses indicate the existence of initial, maximum, and terminating collapse stress states in a biaxial wetting test. The influence of the wetting procedure, i.e., gradual wetting or quick wetting, has a considerable effect on the mechanical performance for numerical samples. The axial strain of a sample during a loading-wetting (LW) path is larger than that of a sample in a WL path. The wetting-induced volumetric deformation corresponds with the failure of bonded contacts. The deviator fabrics (fabric anisotropy) of total, bonded, frictional, and broken bond contacts were also investigated.

Natural loess and natural loess with filling at ground fissures in Jingyang, Shanxi Province, China, were experimentally studied to investigate the effect of crack filling on the mechanical behavior of loess and the crack development by conventional triaxial compression test (CTC), reduced triaxial compression test (RTC) and reduced triaxial extension test (RTE) under constant water content condition. The tests were carried out at the consolidation pressure near in-situ stress. Different failure types were observed in the tests. In the CTC test, each soil demonstrates strain-hardening behaviour and shear failure plane is the interface between loess and filling for the natural loess with filling sample. In RTC test, each soil behaves strain-softening. Shear failure planes are conjugate planes for the loess sample and shear failure plane is the interface between loess and filling for the natural loess with filling sample. In RTE test, rupture is observed and the failure surface does not cross the filling for natural loess with filling, that is, the failure surface lies in loess totally. The shear strength of loess with filling is almost the same as that of loess.