Wenwu Chen

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.

Development of a Robust Stereo-PIV System for 3-D Soil Deformation Measurement

This paper presents a robust stereo-particle image velocimetry (Stereo-PIV) system developed for three-dimensional (3-D) soil deformation measurement in geotechnical engineering. Also known as digital image correlation, PIV is a popular image processing technique to measure two-dimensional (2-D) fluid velocity in fluid dynamics. The Stereo-PIV technique extends 2-D deformation measurement to 3-D based on a binocular vision model, where two cameras with a well posed geometrical setting, are utilized to image the same object. Although commercial Stereo-PIV systems are available in the market, the applications of this technique are still limited by their high cost and special hardware requirements. This study presents a robust Stereo-PIV system, which utilizes two standard complementary metal–oxide–semiconductor (CMOS) cameras and two open software packages to apply this technique to geotechnical engineering testing. In order to apply this technique, three steps have to be performed: the first step is the calibration for the system to get the parameters of the two cameras with respect to the region of interest (ROI); the second step is to obtain a series of paired images of the ROI during deformation and calculate the corresponding 2-D displacement fields from these images; and the last step is to construct a 3-D displacement field based on the camera parameters calculated in the first step and the corresponding 2-D displacement vector pairs from the second step. The developed system is applied in a model test to measure 3-D sand deformation around a laterally loaded pile. The results demonstrate that the system can obtain a reasonable result and can be readily adjusted for more advanced 3-D deformation measurement in geotechnical engineering research.

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.

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.

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.

Hazard assessment of potentially dangerous bodies within a cliff based on the Fuzzy-AHP method: a case study of the Mogao Grottoes, China

The Mogao Grottoes are among the most famous sites on the World Heritage list. Several large-scale preservation projects were implemented in 1962, 1982, and 2008, respectively, to improve their preservation conditions. According to field investigation and assessment in recent years, the cliff is stable on the whole because of the reinforcement projects. Among them, there are still 42 potentially dangerous bodies, which may not be stable in some conditions. For the purpose of building the monitoring and early warning system and the long-term preventive preservation of the Mogao Grottoes, an innovative analytical method based on the Analytic Hierarchy Process (AHP) and Fuzzy-AHP was applied to assess the hazard of potentially dangerous bodies within the cliff. Firstly, the hazard was classified into six groups: very high, high, moderate, low, very low, and no hazard, in this study. Secondly, the AHP method was applied to calculate the impact index of each causative factor, and then the hazard assessment of dangerous bodies was made based on statistical analysis. Finally, the Fuzzy-AHP method was applied to calculate the correlation of each factor and determine the comprehensive hazard class. The results indicate that Fuzzy-AHP seems to be more accurate than AHP in hazard assessment. Meanwhile, there is a very high risk body that can threaten 10 caves, three high risk bodies that can threaten 13 caves, and 15 moderate risk bodies can threaten 69 caves in total, while the remaining bodies are defined as low or very low grades. Overall, the results of this study provide much data and a theoretical model in the construction of a monitoring and early warning system currently. Furthermore, the new numerical simulation method also holds the potential application to assess the hazards of other types of heritage sites.

Geological Line Selection for the Qinghai-Tibet Railway Engineering

With unique geographical position and severe natural condition, Qinghai–Tibet Plateau has also developed various unfavorable geological phenomena, and these geological problems bring many impacts to the Qinghai–Tibet Railway line selection. In this chapter, main engineering geological problems along Qinghai–Tibet Railway and principles of railway line selection under different geological conditions are summarized.

Line Selection in Active Fault Zones and Meizoseismal Areas

This chapter discusses the spatial distribution of the Kunlun Mountain, Bengcuo, and Yambajan–Damxung–Gulu seismotectonic zones and explains the relationship between active faults and earthquakes, which can severely damage railway structures. The line selection and engineering properties of the Qinghai–Tibet Railway in high-intensity earthquake zones and active fault zones are implemented not only as per the geological-engineering line-selection principles for high-earthquake-intensity regions but also considering the railway safety, reliability, and cost.

Railway-Line Selection in Slope Areas

The main mountain areas along the Qinghai–Tibet Railway are Kunlun Mountains, Hoh Xil Mountains, Fenghuo Mountain, Kaixinling, and Tonglha Mountains, along the slopes of which are distributed various unfavorable geological hazards. Route selection and optimization are crucial in minimizing their adverse effects on the construction of the Qinghai–Tibet Railway.