Fanyu Zhang

Effect of pore-water chemistry on undrained shear behaviour of saturated loess

Pore-water chemistry can have a fundamental influence on the shear behaviour of soil. To study the effects of salt concentration and desalinization on undrained shear behaviour, a series of ring shear tests was conducted on Chinese loess saturated with various NaCl solutions of differing concentration, under the same initial void ratio and consolidating stress conditions. Results showed that the peak and steady-state shear strengths increase as the NaCl concentration increases only up to a certain level. When this level is exceeded, any further increase of NaCl concentration results in lower shear strengths. After desalinization the peak and steady-state shear strengths recover to those of the original sample, showing that the salinization of loess is reversible. A pore pressure ratio of 0.6 can be used as a criterion to assess whether liquefaction can be induced in saturated loess. These findings may provide a useful explanation for progressive or seasonal activity of some irrigation-induced loess landslides in the Chinese Loess Plateau.

Relationships between landslide types and topographic attributes in a loess catchment, China

Topographic attributes have been identified as the most important factor in controlling the initiation and distribution of shallow landslides triggered by rainfall. As a result, these landslides influence the evolution of local surface topography. In this research, an area of 2.6 km2 loess catchment in the Huachi County was selected as the study area locating in the Chinese Loess Plateau. The landslides inventory and landslide types were mapped using global position system (GPS) and field mapping. The landslide inventory shows that these shallow landslides involve different movement types including slide, creep and fall. Meanwhile, main topographic attributes were generated based on a high resolution digital terrain model (5 m × 5 m), including aspect, slope shape, elevation, slope angle and contributing area. These maps were overlaid with the spatial distributions of total landslides and each type of landslides in a geographic information system (GIS), respectively, to assess their spatial frequency distributions and relative failure potentials related to these selected topographic attributes. The spatial analysis results revealed that there is a close relation between the topographic attributes of the post-landsliding local surface and the types of landslide movement. Meanwhile, the types of landslide movement have some obvious differences in local topographic attributes, which can influence the relative failure potential of different types of landslides. These results have practical significance to mitigate natural hazard and understand geomorphologic process in thick loess area.

Spatial variation in geotechnical properties and topographic attributes on the different types of shallow landslides in a loess catchment, China

Shallow landslides play an important role in influencing topographic attributes and geotechnical properties. The landslides inventory is mapped using global position system (GPS) and field mapping. Three types of landslides, i.e. slide, creep and fall, are shown in the landslide inventory. Based on a post-landsliding digital elevation model (DEM), main topographic attributes were generated in Arcgis. To assess their frequency distributions, these maps are overlaid with the spatial distributions of all landslides and different types of landslides in geographic information system (GIS), respectively. The spatial analyses reveal that the local surface topographic attributes after landslide have a close relation with the landslide movement type, and different types of landslides always result in different local topographic attributes. In addition, 10 exploratory shafts conducted on the different types of landslides and landscapes were excavated, and the undisturbed soil samples are collected up to 7–9 m depth in the Malan layer, then the main geotechnical properties of the samples are measured. These data show that the vertical spatial variation in geotechnical properties is closely related to the different landslide movement types and landscape features. It is important to better understand the geomorphologic process and geohazard associated with shallow landslides in the Chinese Loess Plateau.

Comparison and combination of different models for optimal landslide susceptibility zonation

It is important to compare different methods and apply combined models for landslide susceptibility zonation on a regional scale for land-use planning and hazard mitigation. The purpose of this study is attempt to obtain an optimal landslide susceptibility zonation in a severely landslide affected region where the available data are very limited. Six single models (analytical hierarchy process (AHP), logistic regression (LR), fuzzy logic (FL), weight of evidence integrated logistic regression (WL), artificial neural network (ANN) and support vector machine (SVM)), were applied to obtain the single landslide susceptibility zonations along the middle reaches of the Bailong River from Zhouqu to Wudu, southern Gansu, China, then these single models were compared, after which the three single models that performed better (LR, ANN and SVM) were selected to prepare the combined zonations. Six conditional independent environmental factors were selected as the explanatory variables that contribute to landslide occurrence (elevation, slope, aspect, distance from fault, lithology and settlement density). The mapped landslides in this region were randomly partitioned into two sets: 80% of the landslides were used for the model training and the remaining 20% were used for validation of the models. Receiver operating characteristic and cost curves were plotted as means of evaluating the quality of the susceptibility zonations for the single and combined models. Results show that the single LR, ANN and SVM are models with superior prediction performance and are more suitable for constructing the combined models in this study. Compared with single models, the combined models provided an improved prediction capability and reduced uncertainties.

A study of a flowslide with significant entrainment in loess areas in China

Flowslides are a frequent type of natural disaster in loess areas and may result in the significant loss of properties and/or casualties. The Dagou flowslide is a typical event in a loess area and is accompanied by significant sediment entrainment. To analyze the mechanisms responsible for flowslides and to obtain the parameters for a runout analysis, a field investigation was conducted. Specimens were sampled on site to carry out laboratory tests, including a triaxial test, sieve analysis, and chemical component analysis. The parameters were used in the runout study employing an energy-based runout model. An analytical entrainment model was adopted to calculate the entrainment after considering physical properties and the mechanism of the entrainment process of the loess. Finally, the entrainment model was incorporated into the runout model to simulate the post-failure process of this case. Energy dissipation due to the deformation of slices was considered as it was thought to be important for a slide with a significant deformation. The simulation results were compared with the measurements, including runout distance, total volume, erosion depth, deposition height at different sections, and velocities at specific locations. The results indicate that the energy-based runout model, together with the entrainment model, can capture the kinematic characteristics of the Dagou flowslide. Therefore, it is feasible to use this model to predict the runout characteristics of flowslides in similar areas. Copyright (c) 2017 John Wiley & Sons, Ltd.

Influence of initial dry density and water content on the soil-water characteristic curve and suction stress of a reconstituted loess soil

In the northwest of China, many loess landslides have occurred without clear triggering factors (i.e., rainfall, earthquake, human activities, etc.). To better understand and analyze the hydro-mechanical properties of these slopes and then provide evidence for their stability analysis subjected to matric suction, it is essential to clarify the soil–water characteristic curve (SWCC). In this study, we conducted a set of experimental trials to examine the influences of initial dry density, water content upon the SWCCs of a loess soil taken from a loess landslide area, by using a conventional volumetric pressure plate extractor. Two common SWCC models have been investigated to evaluate which one is better for loess soil. The suction stress characteristic curves (SSCCs) were also estimated and analyzed. We found that behaviors of SWCCs would be different when the matric suction was greater than a certain value. The two SWCC equations have approximately the same performance in describing the SWCC. The rates of desorption decrease and residual water content increases with increasing the initial dry density, while the initial dry density has little, if any, influence on the air-entry value (AEV). The specimen compacted under higher initial water content would exhibit a higher AEV value and residual water content but lower rate of desorption as compared with the lower initial water content. The magnitude of suction stress had an approximately linear relationship with matric suction before the AEV value, the SSCC shapes will be markedly varied with the initial dry density and water content.

Multiple evidences indicate no relationship between prehistoric disasters in Lajia site and outburst flood in upper Yellow River valley, China

All around the world, ancient legends exist about the Great Flood, and the influence of the Great Flood in human evolutionary history is a hotly debated topic. In China, the legend of the prehistoric Great Flood and Yu the Great’s flood control has a long history and is considered to be closely linked to the establishment of the Xia Dynasty. However, there is a lack of solid scientific evidence. Against this issue, some scholars have proposed that an earthquake around 1920 BCE in the Guanting Basin along the upper reaches of the Yellow River led to the creation of a lake by damming in the Jishi Gorge and that the outburst of water from that lake led to the massive flood in the Yellow River region. These studies provided new scientific evidence for the Chinese legend. In this paper, we date skeletal samples embedded in earthquake sand blasting from the Lajia site, analyze remains from natural disasters (such as earthquakes and floods) and also archaeological remains. In addition, we compared the skeletal samples dating with that of sediments from the dammed-up lake. Our results are inconsistent with those of previous scholars. The earthquake at the Lajia site occurred no earlier than 1800 BCE, and the dammed-up lake in Jishi Gorge had disappeared by 3600 BCE. Thus, the formation and outburst of the dammed lake, the sudden death of ancient humans at the Lajia site and the ancient earthquake were independent events occurring at different times. In addition, the massive flood in the upper reaches of the Yellow River did not actually happen. Thus, we argue against and invalidate the hypothesis that the massive flood was related to Yu the Great’s flood control and the establishment of the Xia Dynasty.

Human-induced landslide on a high cut slope： a case of repeated failures due to multi-excavation

Abstract The paper attempts to represent a case of repeated failures on a high cut slope due to multi-excavation. The characteristics of each failure induced by excavation are analyzed through geological investigation, and then a geological model at different failure stages is proposed. The geological analysis shows that the excavation-induced repeated failures are related to the exposure of the weak bedding plane and the toe unloading of the cut slope. Numerical modeling is conducted based on a sequential method, taking into account the main failure stages of cut slope. The simulation results fairly coincide with the practical phenomena observed in field. It is shown that the decrease in normal stress of displaced mass on cut slope will induce the increase in shear stress in bedding planes and that at the toe of the cut slope. The released stress leads to repeated gravitational instabilities of cut slope due to the decrease in normal stress and the increase in shear stress along the bedding planes of mudstone.

Physicochemical and Mechanical Properties of Lime-Treated Loess

This paper studies the changes in physicochemical and mechanical properties of lime-treated loess at different lime contents and curing days. The physical tests were conducted in particle size distribution, Atterberg limits, specific gravity, specific surface area and cation exchange capacity of lime-treated loess, while its chemical tests included electrical conductivity, total dissolved solids, pH, and oxidation reduction potential of extracts from 1:5 soil to water. Finally, the relevant mechanical tests were performed in unconfined compressive strength. The results showed that the physicochemical and mechanical properties of lime-treated loess were altered during hydration and pozzolantic reactions, which caused the formation of aggregation and new cementitious minerals. As a result, the performance of lime-treated loess acquired great improvement compared to that of untreated loess. The improvement can attribute to the first immediate aggregation formation due to flocculation induced by cation exchange, and then the filling between the aggregations or particles due to the formation of new cementitious minerals. In addition, the results indicated that the chemical properties of lime-treated loess are more relatively sensitive to environmental change than their physical and mechanical properties.

Feasibility Analysis of Loess Slope Improved by Lime Pile

Lime has powerful characteristics of hygroscopicity, expansibility and gelation with soil, so a slope can be strengthened by placement of lime piles. In this paper, experiments have been performed to investigate the diffusion characteristic of lime in loess, and the influence of lime to the physical and mechanical properties of loess also have been examined, in order to verify the feasibility of using pile to improve the stability of slope. The results show that it is feasible to improve stability of slope by lime piles. Pile spacing should be set about 8.5 times as many as the diameter of the pile when the pile holes arranged according to the plum flower form from this test. Moreover, lime piles strengthen the loess slope mainly by gathering and cementing the clay grains in loess.