Loess Area Of China Research Articles

Influence of physical properties and shear rate on static liquefaction of saturated loess

Flow sliding instability of saturated loess slopes is a common geological hazard in loess areas of China. Previous studies have found that the occurrence of flow-slip loess landslides is closely related to static liquefaction and is controlled by physical characteristics and load conditions. In this work, we comprehensively study these influences of physical properties (i.e. initial pore structure, gradation, dry density) and shear rate on the static liquefaction of saturated loess through a series of consolidated undrained triaxial tests, and the effect mechanism of these related factors on the static liquefaction of saturated loess are also discussed. The results present that: (1) The peak deviator stress and the maximum pore pressure of the undisturbed loess are much greater than these of the remodeled one under each level of confining pressure (except 450 kPa). Further, the calculated liquefaction potential index (LPI) of undisturbed loess is much greater, indicating that undisturbed loess is more prone to static liquefaction due to the initial pore structure. (2) The lower the relative clay/silt content ratio of the saturated remodeled loess, the stronger the potential liquefaction ability. With the increase of the relative content of clay from 0.125 to 0.698, the stress-strain curve gradually transitions from strain softening to hardening. (3) The remodeled loess show the steady-state strength tends to continuously increase with the increase of dry density from 1.38 g/cm3 to 1.56 g/cm3, while the LPI increases first and then decreases, and the largest value appears when the dry density reaches to 1.44 g/cm3. The reason is that the value of 1.44 g/cm3 is the normal consolidated condition, which essentially reflects the potential liquefaction change during the transformation from under consolidated to over consolidated state.(4) The effect of shear rate on the stress-strain curve of remolded loess is not significant, but the peak strength and ultimate pore pressure show a trend of increasing and then decreasing with the increase of shear rate. There exists a “critical shear rate “of 0.1 mm/min reflecting the liquefaction of loess is more likely to occur when reaches to this critical value. (5) Based on the comparison of static liquefaction tests, the influencing factors of static liquefaction of saturated loess are: initial pore structure> gradation > dry density > shear rate. This study can provide a systematic evaluation for understanding the influencing factors of static liquefaction capacity of saturated loess (especially remolded one), and also has a reference for explaining the loess flow-sliding failure mechanism under disturbance conditions.

Research on Bearing Difference between Single-Pile Composite Foundation Field Test and Group-Pile Composite Foundation of High-Rise Buildings

The reliability of high-rise buildings’ rigid-pile composite foundations primarily relies on the load test results of single-pile composite foundations. Field load tests were conducted for a high-rise building in the loess area of China, the results conformed to the design requirements. Nevertheless, the buildings experienced significant settlement after construction, which was quite different from the test results. Investigating bearing disparities between the single-pile and group-pile composite foundations in high-rise structures was necessary. Firstly, conducting an indoor interface shear test to propose a pile–soil hyperbolic contact model and integrate it into the numerical model. Subsequently, a finite element model was employed that accounts for pile–soil interactions in order to analyze discrepancies in bearing. The results show that the settlement difference rises by 56.4% as the load escalates by 100 kPa from the initial level and increases by 28.9% as the load ascends by 100 kPa to the final level. The settlement difference changes with an increasing load in a hardened curve pattern. Furthermore, differences in pile bearing characteristics and pile–soil interaction were analyzed. Negative friction occurs within 1/4 of the pile length within group-pile composite foundations, while in single-pile composite foundations, the range contracts to 1/10.

Field practice and ground settlement behaviors of a land creation case in loess area of China

Field practice and ground settlement behaviors of a land creation case in loess area of China

Reply to Comment on “Multi-fractal characteristics of reconstructed landform and its relationship with soil erosion at a large opencast coal-mine in the loess area of China” by Shi et al. (2021)

We responded to the comment by Chen and Huang (2022). We found that the formula of R factor originally quoted was wrong. We replaced it with a new formula suitable for the Loess Plateau, and corrected the errors in the article.

Failure mechanism investigation on loess–mudstone landslides based on the Hilbert–Huang transform method using a large-scale shaking table test

Loess–mudstone landslides occur easily because of earthquakes or rainfall; they account for a large number of landslides in the loess area of China. The Buzi landslide was one of the two largest landslides triggered by the 2013 Minxian–Zhangxian Ms. 6.6 earthquake and resulted in two deaths. Field investigations showed that it was a loess–mudstone landslide with a short sliding distance (approximately 20 m). By simplifying the original site conditions of the landslide, a model slope was built to perform a large-scale shaking table test. The dynamic response of the model was investigated in the time–frequency–energy space using the Hilbert–Huang transform method, and the failure mechanism was clarified from an energy perspective. The Hilbert spectrum indicates that the distribution characteristics of the instantaneous frequency energy are completely different between the loess layer and mudstone layer. Under 0.06 g, large energy is mainly concentrated in a frequency range 10–40 Hz in the loess layer, whereas the energy is mainly concentrated in the 5–10 Hz range in the mudstone layer. This implies that complex reflection and refraction waves occurred in the loess layer. With the increase in intensity, the energy in the loess layer gradually concentrates in the 5–10 Hz range, suggesting that the absorption capacity of the loess layer for high-frequency energy from the relative movement of soil particles increased. The marginal spectra changed from single peak to multipeak with increase in elevation under 0.06 g and changed from multipeak to single peak (with a predominant frequency of approximately 5.6 Hz) with increase in intensity, demonstrating that elevation (>1/2H) can amplify the high-frequency cumulative energy under 0.06 g, showing that input seismic energy gradually takes dominance. Large cumulative energy (10–40 Hz) and instantaneous energy are mainly distributed at the slope shoulder and loess layer. This causes the formation of cracks and a sliding surface in the loess layer. The failure phenomenon of the model slope is close to that of the Buzi landslide, in which sliding occurred at the middle–upper part of the slope within the loess layer.

Comment on “Multi-fractal characteristics of reconstructed landform and its relationship with soil erosion at a large opencast coal-mine in the loess area of China” by Shi et al. (2021)

We were intrigued by the soil erosion modeling conducted by Shi et al. (2021) on post-mining areas in China. However, we discovered that the authors employed an incorrect empirical equation to calculate the rainfall erosivity factor, which resulted in a severe underestimation of the true rate of soil erosion. We presented our findings and discussed a possible source for the true equation.

Combining gradual and abrupt analysis to detect variation of vegetation greenness on the loess areas of China

Combining gradual and abrupt analysis to detect variation of vegetation greenness on the loess areas of China

Multi-fractal characteristics of reconstructed landform and its relationship with soil erosion at a large opencast coal-mine in the loess area of China

Post-mining dumps are typical artificial landforms, with original natural features of soil, topography and vegetation radically altered, and soil and water conservation capacity declined accordingly. In this study, the geomorphic characteristics and soil erosion of reconstructed landforms and their relationships in opencast coal mines are studied. The elevation relief features were described using multi-fractal methods, soil erosion rates were evaluated utilizing the revised universal soil loss equation (RUSLE), and the correlations between geomorphic multi-fractal parameters and soil erosion rate were analyzed within a study area of three soil dumps in the Shanxi Pingshuo Antaibao opencast coal-mine of China. The results show that reconstructed landform in opencast coal mines have multi-fractal characteristics. Generalized dimensions (D0, D1, ΔD and D1/D0) and singularity spectra (α0, Δα and Δf) quantify reconstructed geomorphic features. Soil erosion in the sub-areas is slight, indicating that reclamation measures have alleviated soil erosion exacerbated by mining. Geomorphic multi-fractal parameters reflect the differences of soil erosion rate, the increasing trend of D0, D1 and α0 are consistent with that of average annual soil loss rate. It is feasible to apply these parameters to reflect soil erosion of reconstructed landforms. This study provides a new perspective for portraying the reconstructed geomorphic features and soil erosion on opencast coal-mine dumps.

Spatial distribution characteristics of reconstructed soil bulk density of opencast coal-mine in the loess area of China

Controlling and adjusting the reconstructed soil bulk density (RSBD) in an artificially disturbed ecosystem is one of the main challenges in land reclamation. Exploring the spatial distribution of RSBD is the best starting point to analyze and improve reclamation technology. This study quantifies the spatial distribution pattern and variation of RSBD on reclaimed land in the loess area of China using geostatistical spatial interpolation and multiple regression analysis. Topography (platform, slope, and slope aspects), vegetation cover, soil particle size distribution, and soil erosion were found to affect the spatial variability of the RSBD. The boundary between the sunny slope and shady slope was considered to be the dividing line for the RSBD, which was generally distributed in stripes and patches. The RSBD was higher on the sunny slope than on the shady slope at 0–10 cm, while the opposite was observed at 20–40 cm. However, the RSBD at the depths of 40–60 cm increased from west to east, and the direction of the strong spatial variability also gradually changed from “ Southwest ↔ Northeast ” to “ West ↔ East ”. The unary quadratic regression equations accurately represent the variation laws of RSBD at different soil depths under different topography and vegetation cover (R2 > 0.90). Measures that utilize engineering and vegetation to improve the RSBD should be adjusted according to the specific landscape and topography. This study provides a theoretical basis for the improvement of the RSBD and the optimization of land reclamation technology used in opencast coal-mine dumps in the loess area.

Influences Analysis of Dynamic Response for Slope Supported by Frame with Anchors under Different Anchor Intervals

This article based on the practical engineering in the northwest loess area in China, considered the interaction and collaborative work of frame-anchor-soil, with the help of the finite element software ADINA, established a 3D finite element model of the frame prestressed anchor supporting slope system under earthquake action. In the model, the soil was simulated by elastic-plastic model, and a bilinear elastic model was used to simulate the anchor, friction-element was used to describe the interaction between soil and frame (beams, columns and retaining plate), the frame is simulated by a bilinear elastic-plastic model. By inputting bidirectional seismic waves, the dynamic response of the supporting system under different horizontal and vertical spacing of anchors was compared and analyzed. Finally, obtained the influence law of different anchor spacing on the slope seismic response.

Spatial distribution of soil bulk density and its relationship with slope and vegetation allocation model in rehabilitation of dumping site in loess open-pit mine area.

Studies of soil bulk density (BD) spatial variations of land reclaimed after mining have become a focus of land reclamation and ecological restoration research. However, there have been few studies on the relationship among the reconstructed BD, terrain conditions, and vegetation growth. We examined the southern dumping site of the Pingshuo Antaibao open-pit coal mine located in a loess area in China. Field sampling data, digital elevation models (DEMs), and high-definition images were obtained, and indoor testing, geostatistics, and inverse distance weighting (IDW) were applied. This paper aims to analyze the spatial distribution law of the reconstructed BD and focus on its relationship with slope and vegetation allocation models. We demonstrated that (1) BD increased with soil depth and varied moderately within each layer. (2) The BD variation amplitude of the top 0-20-cm soil layer in both the east-west and south-north directions was small and more similar in the east-west direction than in the south-north direction, which was affected by herbaceous root systems. In the next four layers from 20 to 60 cm, the variation in BD in the east-west direction was far larger than that in the south-north direction, which was affected by vegetation classification. (3) On the whole, BD decreased with increasing slope, but when the slope was between 0° and 21°, BD exhibited a specific change law. (4) From the perspective of vegetation classification, the orders of magnitude of BD in the 0-20-cm and 20-60-cm layers differed. Overall, BD in areas vegetated with Korshinsk Peashrub was the lowest, and BD was moderate in areas with mixed vegetation, while BD was the highest in areas without vegetation or only vegetated with Black Locust. The mixed grass-bush-tree or bush-tree mode attained the best effect in regulating BD. These results can improve the basic principles of land reclamation in mining areas and provide a basis for further optimizing land reclamation technology in practice.

Do reconstructed landforms have multifractal characteristics? A case study at an opencast coalmine dump in the loess area of China

Opencast coal mining causes large changes in landform. The geomorphological features of the reconstructed landform have not been fully understood in the loess area. To quantify the characteristics of the reconstructed landform, multifractal analysis and Principal Component Analysis (PCA) were applied to classify three soil dumps, located at the Antaibao opencast coalmine in China. Most parts of the reconstructed soil dumps had multifractal characteristics. Multifractal parameters reflected heterogeneity of the reconstructed landform from surface relief, regularity and variance in elevation, and the results of PCA further provided a good explanation of the differences and similarities among the three soil dumps. Sub-areas of the South Dump (S1, S2, S3, and S4) were clustered together and this grouping was explained by relatively high D(0), ɑmax and D(q)min, whereas sub-areas of the remaining dumps were sparsely distributed. Long-term restoration and soil erosion may cause the homogenization of the reconstructed landform. Moreover, multifractal parameters, D(q)min, D(0), D(1), D(2), ɑmin, and f(ɑmin) were significantly correlated with Normalized Difference Vegetation Index (NDVI), suggesting susceptibility of the restored vegetation to landform development. Therefore, to improve revegetation practice, micro-topographic measures and silviculture technology are both needed in post-mining areas. The micro-geomorphological patterns could not only improve the efficiency of water use, but also provide a basis for the evolution of geomorphological research in the loess area of China.

Effect of Shield-Tunnel Excavation on Adjacent-Pile Foundation Based on Modified Cam-Clay Model

To ensure the safety of shield tunneling near pile foundation and reduce the disturbance of this phenomenon in an urban metro in the loess area of China, a three-dimensional finite-element analysis model of shield-tunneling construction is established based on ABAQUS numerical simulation software and modified Cam-clay model. The deformation and ground settlement of adjacent-pile foundation during shield tunneling are analyzed. The law of descent is calculated and analyzed. Results show that the influence of tunnel shield on adjacent-pile foundation is primarily manifested in the X-direction displacement of the buried depth of the tunnel. During the shield-excavation process, the X-direction displacement of the pile increases gradually with decreased distance between the excavation face and the pile. When the distance between the pile axis and the excavation face of the tunnel is approximately[-0.5D, +0.5D], the displacement is the largest; when D continues to increase, the X-direction displacement is the largest. The displacement also continues to increase and eventually stabilizes. By obtaining the deformation curve of ground settlement, we find that the ground deformation is the largest above the tunnel axis and gradually decreases to both sides, whereas the pile cap is affected by uneven settlement. In the actual excavation of the tunnel project, strengthening the monitoring of the uneven settlement of the cap is necessary to take timely control measures.

Micro-topographic assessment of rill morphology highlights the shortcomings of current protective measures in loess landscapes

Due to urban expansion and the rapid development of infrastructure in the loess area of China, artificial earth-fill embankments and excavated slopes are increasingly widespread in recent years. Erosion is typical in such loess slopes; however, quantitative statistical analyses of various counter-measures that affects rill erosion are still lacking. Here, we quantified rill morphology and rill erosion development in two newly constructed slopes with different engineering protection measures. We used high-resolution digital surface models (DSMs) acquired using an Unmanned Aerial Vehicle (UAV) to analyze the case areas during two-time periods. Our results from centimeter accuracy differential DSMs demonstrated that rapid rill erosion is prevalent in the study area, expressed as rill density varying between 2.03 km−2 and 8.81 km−2 at different slope surfaces (viz., erosion protected slopes [EPS], landslide protected slopes [LPS], and unprotected slopes [US]). The slope gradient responsible for rill erosion of the EPS, LPS and US are obviously different, and such information is essential for planning preventive measures in each slope type. At the EPS, the severity of erosion is maximum at the top of the ridges, whereas the gap between reinforced concrete lattice and loess deposits are of serious concern at the LPS. The current engineering measures employed in the study area are thus found ineffective for protection against rill erosion. We therefore propose an improved design by implementing an intercepting drain to the existing design for preventing further erosion.

Study of shaking table test of seismic subsidence loess landslides induced by the coupling effect of earthquakes and rainfall

Light rain or moderate rain is the most common meteorological event in the rainy season in the loess area of China, so the probability of landslide hazards induced by the coupling effect of earthquakes and rainfall under the condition of light rain or moderate rain is relatively higher than that under heavy rain. To study the dynamic response characteristics and instability mechanism of loess slopes by the coupling effect of earthquakes and rainfall under the conditions of moderate rain and light rain, a low-angle slope model test of a large-scale shaking table after 10 mm of rainfall was carried out. By gradually increasing the dynamic loading, the evolution of the macroscopic deformation and the instability failure mode of the slope model are observed; the temporal and spatial trends of the amplification effect, acceleration spectrum, pore pressure and soil pressure are analyzed; and the failure mechanism of the slope is determined. The results showed that the amplification effect increased along the slope surface upward, and a strong amplification effect appeared at the front of the top of the slope. Because of the stronger dynamic stress action on the upper part of the slope, the immersed soil in the upper part of the slope experienced seismic subsidence deformation, the saturation in the seismic subsidence soil increased, and the water content temporarily increased locally. With the further increase in the loading intensity, a large number of tension cracks were generated in the seismic subsidence area, and water infiltrated down along the cracks and the wetting range expanded under dynamic action. The range of seismic subsidence and cracks further extended to the deep part of the slope. Under the reciprocating action of the subsequent ground motion, the swing amplitude of the soil mass in the seismic subsidence area, which is divided by a large number of cracks in the upper part of the slope, increased further, resulting in the further reduction in the residual strength of the seismic subsidence soil mass located at the crack tip due to the pull and shear action. Finally, under the combined action of gravity and dynamic force, the upper soil mass in the seismic subsidence area dragged the lower soil mass in the seismic subsidence area downward because the sliding force is greater than the residual strength of the soil mass, which induced a seismic subsidence-type loess landslide. Under the coupling effect of earthquakes and rainfall, the instability mode and mechanism of this landslide are significantly different from those of liquefaction-type landslides.

The comparative analysis of two injection modes and recharge capacity estimation for a radial well in the loess area of China

This study is focused on the theoretical substantiation and practical application of the radial well recharge. Two semi-empirical equations were derived for the estimation of the recharge capacity and clogging coefficient of the recharge well, respectively. Two injection modes were applied to analyze the relations between the recharge capacity, water level, and flow rate. Two methods were used to calculate and verify the recharge capacity and clogging coefficient, with a brief analysis of their variation laws. The recharge capacity of the radial well was found to increase with the injection flow, with the following gradual saturation during two injection stages. The recharge capacity increased with the recharge flow, while the clogging coefficient of the second stage was higher than that of the first stage, and exhibited a different variation pattern. The results obtained strongly indicate that the pumping and recharge are hydrological processes, which differ from each other by their physical essence. The available calculation methods based on the pumping theory were found to overestimate the recharge well flow by more than six times. The refined semi-empirical equation provides a more realistic estimation of the recharge capacity of radial wells.

Small-scale catastrophic landslides in loess areas of China: an example of the March 15, 2019, Zaoling landslide in Shanxi Province

In March 15, 2019, a small-scale loess landslide occurred at Zaoling, Shanxi Province, China, destroying several buildings and killing 20 people. Based on a digital orthophoto map (DOM) and digital elevation model (DEM) derived from Dajiang Pro4 UAV aerial images, coupled with field investigations, this paper describes the geometric features of this slope failure and analyzes its possible inducing factors. Results show that this landslide has a circular chair shape with a sliding direction of 355°. The total projected area of the landslide is 11,800 m2, in which the source is 9880 m2, and the main scarp is 1995 m2. The volume of the source material and loose deposit debris are about 10 × 104 m3 and 12 × 104 m3, respectively. Our research suggests that the inducing factors of this event may include the low strength and structure of the loess under a wet condition, special landforms of the slope, and human activities such as water discharge from a nearby bath center building and coal mining.

Evaluation of gully head retreat and fill rates based on high-resolution satellite images in the loess region of China

The acceleration of large gully erosion in the loess environments of China is a matter of concern because of the changing land use and urban expansion. This paper evaluates the gully head retreat and landfill rates in the Huoxianggou watershed in the loess region of China for the period between 1969 and 2017. High-resolution satellite imageries and ground truths allowed us to estimate the linear and volumetric change rates of gully head. The digital elevation model (DEM) derived from unmanned aerial vehicle (UAV) was used to calculate the geometric parameters of the gully bottom, and the linear and volumetric gully parameters were obtained using the 1969, 2004 and 2017 satellite images of the study area. We estimated the average annual linear retreat rate (Rl) for the study area between 1969 and 2003; Rl was found to be 1.41 ± 0.98 m y−1, the average annual volumetric retreat rate (RV) was 63.79 ± 96.62 m3 y−1, the average annual linear fill rate (Fl) was 3.87 ± 4.09 m y−1, and the average annual volumetric fill rate (FV) was 1002.46 ± 1633.87 m3 y−1. Between 2004 and 2017, Rl was 0.82 ± 0.80 m y−1, RV was 1.80 ± 1.37 m3 y−1, Fl was 3.86 ± 4.99 m y−1, and FV was 708.62 ± 1787.21 m3 y−1. We also estimated the retreat and fill rate of gullies within various land use classes. When the land use type near the gully head was road, the volumetric retreat rate of the gully was threefold that of the farmland, and the rate of the gully head landfill was fourfold that of the farmland. Moreover, the volumetric change rate (CV) and linear change rate (Cl) of all gully heads under the influence of a road, including gully retreat and landfill, had a power exponential relationship (CV = 13.29 C 2.14 , R2 = 0.71). The gullies near the roads in the thick loess area were cut at an abnormally rapid rate, which has not received sufficient attention previously. Therefore, the effect of road construction on gully erosion in the loess area of China should be reassessed.

Using multi-fractal and joint multi-fractal methods to characterize spatial variability of reconstructed soil properties in an opencast coal-mine dump in the Loess area of China

Opencast coal mining activities have significant effects on the reconstructed soil properties, including its physical, chemical and biological properties. To assess the effects of opencast mining and associated dumping activities on reconstructed soil properties, the spatial variability of soil properties, i.e., soil particle size distribution (PSD), organic matter (OM) and total nitrogen (TN), in an inner dump after dumping and before reclamation in Antaibao opencast coal-mine of Shanxi Pingshuo was analyzed using multi-fractal and joint multi-fractal methods. Soil samples at 78 sampling sites at the depths of 0–20 cm and 20–40 cm were collected using an auger. The singularity spectra of soil properties in the study area were analyzed, and three multi-fractal parameters (i.e., D1, D2 and ∆α) were calculated. The singularity spectra and multi-fractal parameters can quantitatively characterize the spatial variability of soil PSD, OM and TN. The variability of soil PSD, OM and TN at the depth of 20–40 cm was higher than that at the depth of 0–20 cm. At same soil depth, the sand and clay contents showed relatively high spatial variability, and the silt content, OM and TN exhibited relatively low spatial variability. The spatial variability in sand content, OM and TN between 0 and 20 cm and 20–40 cm depths had a relatively high correlation, while the clay and silt contents at the depths of 0–20 cm and 20–40 cm showed a relatively low correlation. Multi-fractal and joint multi-fractal methods exhibited more advantages in characterizing spatial variability of reconstructed soil properties. This study provided a theoretical basis for the soil sampling optimization of in the opencast coal-mine dump in the Loess area of China.

Effects of Pipe Roof Support and Grouting Pre-Reinforcement on the Track Settlement

Based on the first shallow tunnel passing below an active railway station in the loess area in China, studies on the tunnel deformation and track settlement during tunneling are performed by using FLAC3D. It is found that, without adopting other reinforcement measures, the maximum track settlement has far exceeded engineering requirements. To reduce deformations induced by the tunneling, the combined presupport technique of the pipe roof and grouting reinforcement is presented and optimal construction parameters are provided. It is concluded that the installation of the pipe roof support plays an important role in controlling tunnel crown settlement and track settlement. The optimal pipe diameter is 159 mm, and the optimal arrangement area of the pipe roof is 150°. Grouting could improve soil strength and reduce deformations. The optimal thickness of the grouting reinforcement ring is 2 m. When the optimal parameters of the combined presupport technique are adopted, calculation results show that the maximum track settlement would reach 13.8 mm, which realized the settlement control goal of a maximum value of 15 mm. At last, the combined presupport technique proposed has been well validated in the “Railway Station” of Metro Line 4.