Lina Ma

On the Loess Plateau of China, the number of projects involving the excavation of mountains and the filling of valleys to create new land is rising. The loess excavated from the mountain is directly used as building material. After being filled into the valley and remolded, it serves as the foundation for overlying structures. However, significant differences in mechanical behavior exist between intact and remolded loess, leading to various issues such as differential settlement. Understanding the microstructure of loess is essential for improving our comprehension of its macro-level hydrological and mechanical behavior. Due to the inherent limitation of the conventional methods (e.g., scanning electron microscopy and mercury intrusion porosimetry) for microstructure investigation, an in-depth understanding of the three-dimensional (3D) microstructural differences between intact and remolded loess remains elusive. To address this gap, this study employs advanced X-ray micro-computed tomography (micro-CT) to investigate the 3D microstructure of both intact and remolded loess from two typical man-made new land creation projects. The three-dimensional microstructures are segmented into a series of cubes of varying dimensions to identify the structure’s representative volume element (RVE) and assess the uniformity of both loess types. The pore network of the RVE is quantitatively analyzed to characterize the microstructure. The results reveal significant disparities between the microstructure of intact and remolded loess, particularly in terms of uniformity, pore size distribution, and pore connectivity. Remolded loess exhibits a denser structure with poorer pore connectivity and greater heterogeneity compared to intact loess. These microstructural differences are attributed to the distinct formation processes of the two types of loess.

The evolution of the landslide erosion of bank slopes in reservoir regions is crucial for disaster prevention and mitigation in hydropower projects, and it is also an important topic to investigate the impact of anthropogenic activities on the environment. A dispute exists on the landslide erosion of bank slopes under reservoir impoundment. In this paper, we chose the Xiluodu reservoir as a typical case for analysis. The reservoir is located on the eastern Qinghai–Tibetan Plateau and ranks as the fourth largest hydropower station in the world. Firstly, we identified landslides using the multi-temporal image interpretation method before and after impoundment in the reservoir area, and established a landslide dataset. On this basis, we analyzed the spatiotemporal distribution of these landslides, and derived the landslide erosion rate based on the quantitative relation between landslide volume and area. The results showed that the landslide erosion rate increased sharply during the initial impoundment period and decreased exponentially. We then analyzed reservoir-induced landslides worldwide and found that most reservoir bank slopes tend to stabilize after about five years of impoundment, eventually even becoming more stable than pre-impoundment, regardless of the location, scale, water fluctuations, and geoenvironment. Thus, in the long term, reservoir construction tends to reduce erosion and contribute to bank stability. This study provides a preliminary answer to the controversial issue of the impact of reservoir construction on the natural environment.

A gigantic project named Gully Land Consolidation (GLC) was launched in the hill-gully region of the Chinese Loess Plateau in 2011 to cope with land degradation and create new farmlands for cultivation. However, as a particular kind of remolded loess, the newly created and backfilled farmland may bring new engineering and environmental problems because the soil structure was disturbed and destroyed. In this study, current situations and characteristics of GLC are introduced. Test results show that physical-mechanical properties and microstructural characteristics of backfilled loess of one-year and five-year farmland are significantly affected by the Gully Land Consolidation project. Compared to natural loess, the moisture content, density, and internal friction angle of backfilled loess increase. On the contrary, the porosity, plasticity index, particle size index, and cohesion index decrease. Through SEM tests, it is observed that the particles of backfilled loess are rounded, with large pores filled with crushed fine particles, which results in skeleton strength weakness among particles and pores. The pore size distribution (PSD) of the four types of loess (Q₃ loess, Q₂ loess, one-year farmland, and five-year farmland) was measured using mercury intrusion porosimetry (MIP) tests, showing that the pore size of Q₃ loess is mainly mesopores 4000–20,000 nm in size, accounting for 67.5%. The Q₂, five-year, and one-year farmland loess have mainly small pores 100–4000 nm in size, accounting for 52.5%, 51.7%, and 71.7%, respectively. The microscopic analysis shows that backfill action degrades the macropores and mesopores into small pores and micropores, leading to weak connection strength among soil particles, which further affects the physical-mechanical properties of loess. The disturbance of backfilled loess leads to an obvious decrease in cohesion and a slight increase in internal friction compared to natural loess. The farming effect becomes prominent with increased backfill time, while the loess soil moisture content increases gradually. Both the cohesion and internal friction of the backfilled loess soil decrease to different degrees. This study is helpful to investigate sustainable land use in the Chinese Loess Plateau and similar areas.