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The negative skin frictional caused by loess collapse will decrease the bearing capacity of single pile, which is essential to the design of pile foundations in loess areas. In this study, a method for estimating the subsidence of soil layer at any depth is firstly proposed based on the total self-weight collapse value. Secondly, a new load transfer constitutive model for pile-soil interface is developed, which considers the nonlinear stress-strain relationship and the ultimate shear strength of soil. Then, a load transfer calculation model for pile foundation is established, which can calculate the pile axial force, the pile skin frictional, neutral point position and the settlement of a single pile. The calculation results are compared with the test data that obtained from a pile foundation on-site immersion test and the effectiveness of the calculation method is verified well. This calculation method may be useful for designing pile foundations in collapsible loess regions. [ABSTRACT FROM AUTHOR]

The quantification of the moistening deformation for wetting-sensitive unsaturated loess is of direct interest in regions implementing geotechnical engineering. Many constitutive models are available for describing the stress–strain behavior of unsaturated soils. However, few constitutive models directly describe moistening deformation. In this study, two types of triaxial tests were carried out on undisturbed loess. A triaxial compression test was conducted to determine the shear strength boundary of a specimen, and a triaxial moistening test with different stress ratios was used to study the moistening deformation law. The volumetric strain caused by moistening was used as the hardening parameter. A humidity variable was defined as the moisture level, and this parameter could reflect the moistening degree. The experimental rule and plastic theory were combined to establish the constitutive model. The model could directly calculate the deformation amount caused by moistening under different stress states. The model is applicable to the loess in the Guanzhong area in the Chinese Loess Plateau. [ABSTRACT FROM AUTHOR]

• Electrical resistivity tomography was useful to monitor infiltration process in loess. • Day-time temperature fluctuation affected electrical resistivity in loess. • Matrix flow in loess stagnated at near-surface under constant influx. • Preferential flow transferred to matrix flow for downward progression in deeper soil. • Flood irrigation recharged groundwater via hidden cracks over intact surface. The effect of preferential flow (PF) in loess is of direct interest as flood irrigation is commonly found in loess regions where precipitation is scarce. This study was to assess the applicability of time-lapse electrical resistivity tomography (ERT) data on monitoring infiltration in loess and to analyze the effects of preexisting preferential path on infiltration process at a localized scale. An in situ single-ring infiltrometer (SRI) test was conducted above a preexisting near-surface crack under a constant influx (10 cm water head for 8 h), and the test was monitored by the means of 2-D time-lapse ERT and validated by an exploratory trench. The ERT results observed some increasing uncertainties in the electrical resistivity in part due to daily temperature variation; however, they showed that the infiltration process under the presence of preferential path can be effectively monitored by time-lapse ERT with a combination of unit electrode spacing (UES). The test results showed a high relative reduction of electrical resistivity (>80%) due to matrix flow (MF) and PF within 0–0.5 h of the test, and followed by a transformation of preferential flow to a more matrix-dominated unsaturated flow, i.e. preferential-to-matrix flow (PMF), with continuous downward progression into the deeper subsurface (greater than 1.2 m) while MF stagnated at the near-surface (about 0.4 m). The results suggested that the presence of even a near-surface preferential path significantly accelerate the infiltration into the deeper loess by the means of PMF, as compared to observed slow infiltration of pure matrix flow in previous studies. [ABSTRACT FROM AUTHOR]

Proton nuclear magnetic resonance (NMR) is widely applied to characterize the microscopic properties of hydrogen‐containing porous media. The transverse relaxation time cutoff (T2c) value is the crucial parameter for the quantitative analysis of NMR data. Currently, there is no universal method for the determination of the T2c in clayey soils. This study aimed to develop a laboratory method for determining the T2c of remoulded loess by the freezing point of loosely bound water. Malan loess, a kind of typical clayey silt, was used as test material. Based on the soil freezing characteristic, NMR measurements were performed on remoulded loess with different macro‐parameter controls during the cooling process to obtain the T2 spectrum at each target temperature. By analysing the variation of unfrozen water content with temperature reduction, the freezing point of loosely bound water and the T2c value within the freezing‐point range was determined. The freezing point of loosely bound water in remoulded loess is about −3 to −5°C and that of firmly bound water is less than −5°C. Accordingly, the T2c value of remoulded loess is determined to be 1.5–1.8 ms. The assessment of heating and cooling process and different methods for determining the T2c shows that the laboratory method by the freezing point is effective and reliable, and the T2c determined by statistical methods is worthy of further study and improvement. The saturated permeability of remoulded loess is evaluated according to the determined T2c, and two NMR‐based permeability equations can well reflect pore water distribution in remoulded loess, but to a certain extent, both equations ignore soil microstructure, pore connectivity and chemical effects of pore solution. The laboratory method by the freezing point and the determined T2c value of remoulded loess fill the gap of NMR measurement in loess analysis and are of great significance for low‐plastic clays and clay types. [ABSTRACT FROM AUTHOR]

From 2011 to 2013, a mega project, known as the Gully Land Consolidation Project (GLCP), was implemented in the hilly gully region atop China's Loess Plateau. However, the GLCP involved large-scale slope excavation and gully backfilling that changed the local geological environment, which led to serious geohazards, such as erosion, soil salinization, and dam failure. In this study, various geohazards caused by the GLCP in the Gutun watershed (GTW) were investigated by combined remote sensing analysis, geophysical exploration, and field surveys, and the relationships between the hazards were also explored. According to the achieved results, increased soil erosion with an average doubling from 2018 to 2020 is widely distributed in the GTW. Furthermore, 195 areas containing clear evidence of salt precipitates were observed in some of the newly created arable lands, especially downstream of the dam. This was mainly attributed to the high water table, evaporation, and soluble salt concentration of the loess. Fifty-nine newly built silt dams, primarily located in the branch channels and at the gully mouth of the Gutun channel, broke in 2020. The osmotic damage and softening caused by the combined effect of the incomplete compaction of the dam body and concentrated heavy rainfall were the main reasons of the dam breaks. The different types of disasters in the GTW after the implementation of the GLCP show a strong spatial relationship that follows the surface water flow path and forms a disaster chain consisting of slope erosion, silt dam breaks, and the soil salinization of near-dam farmlands downstream. This disaster chain amplifies disaster risks and losses. These findings can guide the improvement of the GLCP and inform geohazard mitigation strategies in areas impacted by the GLCP. [ABSTRACT FROM AUTHOR]

[Display omitted] • Mathematical morphology was effective to distinguish the loess evolutionary stages. • A clearer evolutionary pattern showed the process from loess tablelands to hills. • Residual tablelands were the most vulnerable areas where tablelands disappear. The geomorphic evolution of loess tablelands (LTs), loess ridges (LRs) and loess hills (LHs) affects the availability of local farmland. The present study distinguished the evolutionary stages of different loess geomorphic units using mathematical morphology and analyzed the evolutionary process from LT to LR to LH from geomorphic characteristics and the physical meaning of the geomorphic parameters. Remote sensing images were used to interpret the LT-LR-LH landforms in the Heshui County watershed. The interpreted results indicated a gradual decrease in LT and increase in LR and LH in the Heshui County watershed from southwest to northeast. Geomorphic parameters were used to assess the evolutionary stage and establish the classification criteria. The results showed that geomorphic parameters were useful to distinguish the evolutionary stages of different loess geomorphic units, and circularity (C) played a key role in the setting of classification criteria. An evolutionary pattern of LT-LR-LH landforms was quantified and visualized. Generally, the evolution of loess tablelands followed a trend from LT to LR to LH with the further subdivision of LT-LR-LH into 9 stages of loess geomorphic units. This study provides new insight into the evolution of LT-LR-LH loess landforms and revealed the location where LTs disappeared. These classification criteria and evolutionary patterns may be extended to other loess areas to assess the extent of landform evolution. [ABSTRACT FROM AUTHOR]

A mega project, Mountain Excavation and City Construction (MECC), was launched in the hilly and gully region of the Chinese Loess Plateau in 2012, in order to address the shortage of available land and create new flat land for urban construction. However, large-scale land creation and urban expansion significantly alters the local geological environment, leading to severe ground deformation. This study investigated the topographic changes, ground deformation, and their interactions due to the MECC project in the Yan'an New District (YND). First, new surface elevations were generated using ZiYuan-3 (ZY-3) stereo images acquired after the construction in order to map the local topographic changes and the fill thickness associated with the MECC project. Then, the interferometric synthetic aperture radar (InSAR) time series and 32 Sentinel-1A images were used to assess the spatial patterns of the ground deformation in the YND during the postconstruction period (2017–2018). By combining the InSAR-derived results and topographic change features, the relationship between the ground deformation and large-scale land creation was further analyzed. The results indicated that the MECC project in the YND has created over 22 km2 of flat land, including 10.8 km2 of filled area, with a maximum fill thickness of ~110 m. Significant uneven ground deformation was detected in the land-creation area, with a maximum subsidence rate of approximately 121 mm/year, which was consistent with the field survey. The strong correlation between the observed subsidence patterns and the land creation project suggested that this recorded uneven subsidence was primarily related to the spatial distribution of the filling works, along with the changes in the thickness and geotechnical properties of the filled loess; moreover, rapid urbanization, such as road construction, can accelerate the subsidence process. These findings can guide improvements in urban planning and the mitigation of geohazards in regions experiencing large-scale land construction. [ABSTRACT FROM AUTHOR]

Landslide disasters are associated with severe losses on the Loess Plateau of China. Although early warning systems and susceptibility mapping have mitigated this issue to some extent, most methods are qualitative or semi-quantitative in the site-specific range. In this paper, a quantitative spatial distribution model is presented for site-specific loess landslide hazard assessment. Coupled with multi-temporal remote sensing images and high-precision UAV cloud point data, a total of 98 loess landslides that have occurred since 2004 on the Heifangtai terrace were collected to establish a landslide volume-date and retreating distance database. Eleven loess landslides are selected to construct a numerical model for parameter back analysis, and the accuracy of the simulation results is quantitatively evaluated by the centroid distance and overlapping area. Different volumes and receding distance rates of landslides are fitted to determine the relationship between cracks and potential volume, and different volumes and parameters are combined to simulate the spatial distribution of potential loess landslides. The results of this study reveal that landslide volumes mainly range between 1 × 103 and 5 × 105 m3, and the historical occurrence probability reaches 0.551. The optimal parameters are estimated by the maximum likelihood method to obtain a uniform distribution parameter value probability model, and the results show that the error of the estimated length within a range of 0.05 from the optimal parameter does not exceed 15%. In the selected slope slide case, farmland near the toe of the slope primarily includes exposed hazards with probabilities greater than 0.7. This work provides a useful reference for local disaster reduction and a theoretical methodology for hazard assessments. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Three groundwater domes are found in the center and eastern part of the Heitai terrace. • The hydrological processes are characterized by multi-temporal 2D ERT and sensors. • Landslide deposits block the seepage and result in local groundwater level rising. • Landslide type and magnitude are affected by groundwater seepage and movement. Abstract In the Heifangtai terrace, NW China, flooding irrigation is common for the cropland production and has increased the groundwater table by 20–30 m over the past five decades. It is believed that excessive irrigation has induced a large number of loess landslides in this area. However, these loess landslides responses to hydrological changes have still not been well understood. Therefore, 43 boreholes and 51 2D Electrical resistivity tomography (ERT) profiles with a total length of 40 km have been investigated to analyze the hydrological characteristics for the whole terrace and characterize the effects of hydrological changes on the occurrence of different types of loess landslides. Our main findings are as follows: (1) Three groundwater domes are found in the center and eastern part of the Heitai terrace and the groundwater table is gradually rising in the whole terrace based on the ERT results. (2) The rising rate of groundwater table at the center of the terrace is twice greater than that at the margin of the terrace and it takes approximately 4 months for irrigation water to penetrate from the top surface to the saturated layer. (3) Landslide deposits could block drainage channels of groundwater, increase the localized hydraulic gradient and result in rising of local groundwater table at the edge of the terrace. (4) The resultant hydraulic gradient impels the groundwater from the center to seep out on the eastern and southern margins and new landslides are prone to occur in the main scarp of landslides with rising groundwater table. The type and magnitude of landslides are affected by groundwater seepage and movement. In order to fundamentally control the occurrence of landslides, effective drainage system and the restrained amount of irrigation water are needed. [ABSTRACT FROM AUTHOR]

Agricultural irrigation has increased the groundwater level in the Heitai terrace (part of the Heifangtai terrace) by 20 m over nearly five decades, which causes 3–5 landslides each year at the edge of the terrace. The Heitai terrace is of great interest in the study of loess-related landslides; but there is no unanimous agreement on the types of either the landslides in this study site or the loess-related slope failures in general. On the basis of aerial images (res. 5 cm), Digital Elevation Model (res. 10 cm), and field investigations, we analyzed the distribution and failure mode of the landslides in Heitai. The geological structure and characteristics of 69 landslides (vol. 5 × 10 3 –6 × 10 6 m 3 ) are studied. The preliminary results of groundwater recharge in the terrace and formation of the apparent spring lines on the slope surface are analyzed to better understand the failure modes. We divided the landslides in Heitai into two groups based on the location of the failure surface, i.e. loess landslide and loess-bedrock landslide, of which the development is governed by the angle between the principle direction of slope deformation and the dip of bedrock bedding. We further analyzed the failure mode of each type observed in Heitai, defined as follows: loess-bedrock planar slide, loess-bedrock irregular slide, loess flowslide, loess slide, and loess flow. The proposed types of loess-related landslide are to be incorporated in the Varnes classification (Varnes, 1978) in consideration of the engineering properties of loess, and to provide backward compatibility for Heitai and potentially other regions in the Loess Plateau of China. [ABSTRACT FROM AUTHOR]

[Display omitted] • Farming percolation process was reproduced by field tests and numerical modelling. • Vadose zone could be divided into transient flow zone and steady flow zone. • Vertical percolation rate at 0.5 m/day is double that of the horizontal direction. • Propagation of the wetting front is hampered due to air outflow in transient zone. • Farming water could pass through the thick unsaturated loess layer within 100 days. Loess slopes along the Yellow River in China are prone to failure in response to intensive irrigation. Existing studies found that the wetting front depth subjected to intensive farming water could barely reach 6 m in loess terrace. Long-term field monitoring revealed that irrigation water takes a short time to recharge the groundwater. Yet the dynamic process and mechanism of irrigation water percolation through thick unsaturated loess remains unrevealed. To address this problem, the Heifangtai terrace was selected as the study area and a series of unique full-scale controlled field irrigation experiments were carried out combined with numerical modelling. The variations in water content, pore-water pressure, and electrical resistivity over time were monitored. From the field testing, the unsaturated stratified loess layer could be divided into two vadose zones based on the water percolation characteristics, namely, transient flow zone (0–6.5 m) and steady flow zone (>6.5 m). Variation in water content presented a hysteresis effect in the transient flow zone compared with the steady flow zone. In the whole seepage process, the vertical percolation rate of 0.5 m/day was double that of the horizontal percolation rate. Furthermore, the irrigation water percolating process was modelled using a finite element code, PANDAS, which was validated by the field testing. Numerical simulation results demonstrated that the movement of wetting front was hampered by the air outflow within transient flow zone and that irrigation water reached the deep groundwater table through the thick unsaturated stratified loess within 100 days, which was consistent with the field monitoring data. This research aids in a better understanding of the irrigation water percolation process in unsaturated stratified loess in the loess region. [ABSTRACT FROM AUTHOR]

The Heifangtai terrace located in Yongjing County of Gansu Province in northwest China is characterized by a large number of landslides with frequent failure events related to agricultural irrigation activities. Thus, it is a task of great importance to identify and monitor the active loess slopes and sort out the impact factors. In this study, we employed the time series InSAR method to map the active slopes in the Heifangtai terrace with two TerraSAR-X data stacks covering the year of 2016 and two Sentinel-1 data stacks from 2016 to 2018. The detected active spots are mainly distributed along the edges of the terrace. Time series displacements derived from TerraSAR-X and Sentinel-1 observations were validated by comparisons against GPS and crack gauge measurements. Good agreement between InSAR and GPS was achieved at slow moving locations, while unwrapping errors induced by sudden accelerations make InSAR underestimate the pre-failure displacements with respect to GPS measurements. The trends of InSAR-measured displacement and crack gauge measurements also match well with correlation higher than 0.75. InSAR-measured displacements at the Jiaojia landslide group were highly correlated with the bury of groundwater record, which suggested that the rising of groundwater level may seriously decrease the stability of loess slopes. • Unstable slopes in the Heifangtai terrace are mapped with TerraSAR-X and Sentinel-1 InSAR analyses from 2016 to 2018. • InSAR results are validated by comparisons with GPS and crack gauge measurements. • Displacement patterns of typical landslides within the study area are analyzed. • Intensive irrigation on the terrace significantly affects the stability of loess slopes. [ABSTRACT FROM AUTHOR]