Influences of sand cover on erosion processes of loess slopes based on rainfall simulation experiments

Aeolian-fluvial interplay erosion regions are subject to intense soil erosion and are of particular concern in loess areas of northwestern China. Understanding the composition, distribution, and transport processes of eroded sediments in these regions is of considerable scientific significance for controlling soil erosion. In this study, based on laboratory rainfall simulation experiments, we analyzed rainfall-induced erosion processes on sand-covered loess slopes （SS） with different sand cover patterns （including length and thickness） and uncovered loess slopes （LS） to investigate the influences of sand cover on erosion processes of loess slopes in case regions of aeolian-fluvial erosion. The grain-size curves of eroded sediments were fitted using the Weibull function. Compositions of eroded sediments under different sand cover patterns and rainfall intensities were analyzed to explore sediment transport modes of SS. The influences of sand cover amount and pattern on erosion processes of loess slopes were also discussed. The results show that sand cover on loess slopes influences the proportion of loess erosion and that the compositions of eroded sediments vary between SS and LS. Sand cover on loess slopes transforms silt erosion into sand erosion by reducing splash erosion and changing the rainfall-induced erosion processes. The percentage of eroded sand from SS in the early stage of runoff and sediment generation is always higher than that in the late stage. Sand cover on loess slopes aggravates loess erosion, not only by adding sand as additional eroded sediments but also by increasing the amount of eroded loess, compared with the loess slopes without sand cover. The influence of sand cover pattern on runoff yield and the amount of eroded sediments is larger than that of sand cover amount. Furthermore, given the same sand cover pattern, a thicker sand cover could increase sand erosion while a thinner sand cover could aggravate loess erosion. This difference explains the existence of intense erosion on slopes that are thinly covered with sand in regions where aeolian erosion and fluvial erosion interact.

Numerical experimental study on optimum design of anchorage system for Xiashu loess slope

In FLAC ^(3D),cable element or modified pile element can be used to build slope anchoring model.However,the difference between the two structural elements and their influence on the calculation results have not been studied in depth.In order to solve this problem,the Xiashu loess slope anchoring models based on cable element and modified pile element were constructed respectively.A variety of anchoring schemes were designed by orthogonal experiment method,and then they were brought into the model for calculation and the calculation results were analyzed by range analysis and variance analysis.The results show that the modified pile element can bear the bending moment and reflect the strain softening property of the grout.From the perspective of slope safety factor,the anchorage length and anchor bolt spacing are the main factors affecting the stability of the slope,and the anchorage angle is the secondary factor.The grout in cable element is assumed to be an elastic-perfectly plastic material,so the safety factor of the slope can be significantly increased by increasing the length of the anchor bolts.This will bring potential risks to the slope treatment project.Therefore,in the calculation of the slope anchoring model,the modified pile element is more suitable for simulating the anchor bolt.

Effects of Shrub on Runoff and Soil Loss at Loess Slopes Under Simulated Rainfall

Improved understanding of the effect of shrub cover on soil erosion process will provide valuable information for soil and water conservation programs.Laboratory rainfall simulations were conducted to determine the effects of shrubs on runoff and soil erosion and to ascertain the relationship between the rate of soil loss and the runoff hydrodynamic characteristics.In these simulations a 20° slope was subjected to rainfall intensities of 45,87,and 127 mm/h.The average runoff rates ranged from 0.51 to 1.26 mm/min for bare soil plots and 0.15 to 0.96 mm/min for shrub plots.Average soil loss rates varied from 44.19 to 114.61 g/(min·m^2) for bare soil plots and from 5.61 to 84.58 g/(min·m^2) for shrub plots.There was a positive correlation between runoff and soil loss for the bare soil plots,and soil loss increased with increased runoff for shrub plots only when rainfall intensity is 127 mm/h.Runoff and soil erosion processes were strongly influenced by soil surface conditions because of the formation of erosion pits and rills.The unit stream power was the optimal hydrodynamic parameter to characterize the soil erosion mechanisms.The soil loss rate increased linearly with the unit stream power on both shrub and bare soil plots.Critical unit stream power values were 0.004 m/s for bare soil plots and 0.017 m/s for shrub plots.

The mechanism of slope instability due to rainfall-induced structural decay of earthquake-damaged loess

Natural loess slopes are characterized by a strong geological structure,which is an important factor in maintaining slope stability.The magnitude and duration of the earthquake may disturb the soil structure at different levels degrees,locally changing the arrangement between soil particles.The process of rainfall humidification weakens the cementation between soil particles,and the disturbance and humidification change the structural state of the soil,which in turn causes sliding of the slope along with the decay of soil mechanical properties.As slope instability is often the result of a series of post-earthquake ripple effects,it is of great scientific significance to study the mechanism of slope instability due to the structural decay of earthquake-damaged loess exacerbated by rainfall.In this paper,the impact of structural decay of loess on slope stability is simulated by GEOSTUDIO software under three conditions:pre-earthquake rainfall,post-earthquake rainfall and earthquake,taking the landslide in Buzi Village,Min County,Gansu Province as an example.The comparative analysis of the calculation results shows that the structural properties of the slope without earthquake disturbance are influenced by infil-tration amount.When it is fully saturated,the structural properties are similar to those of saturated soil,and the safety factor is reduced by 12.9%.In addition,the earthquake intensity and duration have different degrees of structural damage to the soil.When the structure is fully damaged,it is similar to that of remodelled soil,and the safety factor is reduced by 45.84%.Notably,the process of the earthquake and the following humidification generates the most serious damage to the loess structure,with a reduction in the safety factor of up to 56.15%.The quantitative analysis above obviously illustrates that the post-earthquake rainfall causes the most severe damage to structural loess slopes,and the resulting landslide hazard should not be underestimated.

Stability analysis of a high combination system of soil loess slope reinforced by the nails and stabilization piles

While the soil nails and the corresponding compound technology are widely used as the support techniques for deep foundation pit and normal slopes, few related engineering cases are found for high loess slopes. By utilizing the finite element software of PLAXIS 8.5, the behavior of a high loess slope reinforced by the combination of soil nails and stabilization piles （hereinafter for CSNSP） is studied in this paper. It can be found that the potential slide surface of the slope moves to deeper locations during the process of the multi-staged excavations. The measure of reducing the weight of the top of the slope is a positive factor to the stability of the loess slope, while the rainfhll is a negative factor. The slope can＇t be stable if it＇s reinforced only by stabilization piles or soil nails during the process of the multi-staged excavations. The soil nail contributes greater to the overall system stability when the excavation depth is relatively shallow, while the stabilization pile takes it over when the excavation depth reaches a large value. Compared to the results from the Sweden circular slip surface, the data derived from the method of phi/c reduction is relatively large when the slope is unreinforced or reinforced only by stabilization pile, and the data turns to be small when the slope is strengthened by soil nails or the combination system of soil nails and stabilization piles.

Simulation on slope uncertainty derived from DEMs at different resolution levels:a case study in the Loess Plateau

Slope is one of the crucial terrain variables in spatial analysis and land use planning, especially in the Loess Plateau area of China which is suffering from serious soil erosion. DEM based slope extracting method has been widely accepted and applied in practice. However slope accuracy derived from this method usually does not match with its popularity. A quantitative simulation to slope data uncertainty is important not only theoretically but also necessarily to applications. This paper focuses on how resolution and terrain complexity impact on the accuracy of mean slope extracted from DEMs of different resolutions in the Loess Plateau of China. Six typical geomorphologic areas are selected as test areas, representing different terrain types from smooth to rough. Their DEMs are produced from digitizing contours of 1:10,000 scale topographic maps. Field survey results show that 5 m should be the most suitable grid size for representing slope in the Loess Plateau area. Comparative and math-simulation methodology was employed for data processing and analysis. A linear correlativity between mean slope and DEM resolution was found at all test areas, but their regression coefficients related closely with the terrain complexity of the test areas. If taking stream channel density to represent terrain complexity, mean slope error could be regressed against DEM resolution (X) and stream channel density (S) at 8 resolution levels and expressed as (0.00158+0.031S-0.0325)X-0.0045S2-0.155S+0.1625, with a R2 value of over 0.98. Practical tests also show an effective result of this model in applications. The new development methodology applied in this study should be helpful to similar researches in spatial data uncertainty investigation.

Deriving the slope-mean shielded astronomical solar radiation spectrum and slope-mean possible sunshine duration spectrum over the Loess Plateau

Solar radiation is often shielded by terrain relief, especially in mountainous areas, before reaching the surface of the Earth. The objective of this paper is to study the spatial structures of the shielded astronomical solar radiation(SASR) and the possible sunshine duration(PSD) over the Loess Plateau. To this end, we chose six test areas representing different landforms over the Loess Plateau and the software package of Matlab was used as the main computing platform. In each test area, 5-m-resolution digital elevation model established from 1:10,000 scale topographic maps was used to compute the corresponding slope, SASR and PSD. Then, we defined the concepts of the slope-mean SASR spectrum and the slope-mean PSD spectrum, and proposed a method to extract them from the computed slope, SASR and PSD over rectangular analysis windows. Using this method, we found both spectrums in a year or in a season for each of the four seasons in the six test areas. Each spectrum was found only when the area of the corresponding rectangular analysis window was greater than the corresponding stable area of the spectrum. The values of the two spectrums decreased when the slope increased.Furthermore, the values of the stable areas of the spectrums in a year or in a season were positively correlated with the variable coefficients of the slope or the profile curvature. The values of the stable areas of the two spectrums in a year or in a season may represent the minimum value of test areas for corresponding future research on the spatial structures of the SASR or PSD. All the findings herein suggest that the spatial structures of the PSD and the SASR are caused by the interactions between solar radiation and terrain relief and that the method for extracting either spectrum is effective for detecting their spatial structures. This study may deepen our understanding of the spatial structure of solar radiation and help us further explore the distribution of solar energy in mountainous regions.

Optimal Scale Selection for DEM Based Slope Segmentation in the Loess Plateau

Optimal scale selection is the key step of the slope segmentation. Taking three geomorphological units in different parts of the loess as test areas and 5 m-resolution DEMs as original test date, this paper employed the changed ROC-LV (Lucian, 2010) in judging the optimal scales in the slope segmentation process. The experiment results showed that this method is effective in determining the optimal scale in the slope segmentation. The results also showed that the slope segmentation of the different geomorphological units require different optimal scales because the landform complexity is varied. The three test areas require the same scale which could distinguish the small gully because all the test areas have many gullies of the same size, however, when come to distinguish the basins, since the complexity of the three areas is different, the test areas require different scales.

Uncertainty of Slope Length Derived from Digital Elevation Models of the Loess Plateau, China

Although many studies have investigated slope gradient uncertainty derived from Digital Elevation Models(DEMs), the research concerning slope length uncertainty is far from mature. This discrepancy affects the availability and accuracy of soil erosion as well as hydrological modeling. This study investigates the formation and distribution of existing errors and uncertainties in slope length derivation based on 5-m resolution DEMs of the Loess Plateau in the middle of China. The slope length accuracy in three different landform areas is examined to analyse algorithm effects. The experiments indicate that the accuracy of the flat test area is lower than that of the rougher areas. The value from the specific contributing area(SCA) method is greater than the cumulative slope length(CSL), and the differences between these two methods arise from the shape of the upslope area. The variation of mean slope length derived from various DEM resolutions and landforms. The slope length accuracy decreases with increasing grid size and terrain complexity at the six test sites. A regression model is built to express the relationship of mean slope length with DEM resolution less than 85 m and terrain complexity represented by gully density. The results support the understanding of the slope length accuracy, thereby aiding in the effective evaluation of the modeling effect of surface process.

Characteristics of Soil Respiration of Terraces and Slope Farmland in Loess Hilly and Gully Regions

Soil CO_2 emissions of terraces and slope farmland in loess hilly and gully regions were measured by using Infra Red Gas Analysis(IRGA),and the diurnal variation characteristics of soil respiration rate in different slope positions of terraces and slope farmland were analyzed.The results show that the diurnal variation curves of soil respiration rate of terraces and slope farmland in loess hilly and gully regions had a single peak.The soil respiration rate of terraces reached the peak during 13:00-15:00,while the soil respiration rate of slope farmland reached the peak from 11:00 to13:00,and it was the lowest at next 07:00.The daily average of soil respiration rate in slope farmland was 0.86μmol/(m^2·s),accounting for 93.48% of that of terraces.The daily average of soil respiration rate in different slope positions of terraces and slope farmland is shown as follows:the bottom of the slope>the middle of the slope>the top of the slope.At the top of the slope,the daily averages of soil respiration rate in terraces and slope farmland were the same;at the middle and bottom of the slope,the daily average of soil respiration rate in terraces was larger than that of slope farmland.

黄土抗剪强度参数均值与方差的Bayes估计及其应用

为解决黄土强度参数估计问题,从工程勘察项目中共收集统计了3384组Q1、Q2、Q3黄土强度参数黏聚力c、内摩擦角φ值的测试数据,不考虑二者的相关性,将参数均值和方差都作为随机变量,建立起黄土强度参数c、φ的正态-逆伽马先验分布。基于Bayes理论,利用共轭先验法推导了参数后验分布和后验概率密度函数的期望值求解公式,确定了估计的误差。以陕西泾阳黄土边坡为例,利用所建立的先验分布和边坡土层测试强度指标,求取参数的后验分布,进一步估计了边坡失效概率的概率分布和稳定系数均值的概率分布。结果表明:在95%的置信度下,按稳定系数评价,边坡均处于基本稳定状态,且稳定系数置信区间小;按失效概率评价,边坡接近稳定状态,但失效概率的置信区间较大,黄土强度参数的方差控制着边坡失效概率的置信区间,将方差作为随机变量,考虑方差变异性,能更科学地评估黄土工程的可靠度。

考虑中间主应力影响的非饱和黄土边坡抗滑桩间距研究

理论研究和工程应用中土体破坏准则的合理选用对研究和设计结果影响较大,同时由于中间主应力效应的影响,导致平面应变试验和常规三轴试验的试验结果的差异也被广泛证实。基于土拱破坏理论,采用理论推导的方式将统一强度理论、统一滑移线场理论和土拱破坏理论应用于非饱和黄土边坡抗滑桩间距研究中,并根据试验数据确定了非饱和黄土中间主应力参数b,推导了基于土拱理论的非饱和黄土边坡抗滑桩间距表达式。通过工程实例研究了非饱和黄土边坡抗滑桩间距随中间主应力参数b和基质吸力的变化规律。研究结果表明,考虑中间主应力效应的影响可增大抗滑桩设计间距,从而降低工程造价。研究对优化非饱和黄土地区抗滑桩间距具有重要的参考价值。

水库黄土岸坡塌岸预测模型及参数敏感性研究

塌岸宽度预测是水电工程重要研究问题之一。现有的经验图解法基于塌岸最终稳定坡形进行塌岸宽度预测,难以反映塌岸的过程、特征,且对岸坡岩土体的物理力学性质考虑也较少。基于此,研究提出了基于极限平衡理论的黄土岸坡塌岸宽度预测模型,确定了模型关键参数取值,并对参数敏感性进行了分析。结果显示,基于极限平衡理论进行预测塌岸宽度时应选取合理的水下堆积系数,水下堆积系数随岸坡高度和水深的增加而增加。而岸坡角度对塌岸宽度的影响较小,且主要影响首次塌岸宽度,对后续塌岸宽度的影响不大。随着岸坡坡度的增大,塌岸宽度呈波动上升趋势。同时,塌岸参数敏感性分析结果显示,参数敏感性排序由大到小依次为岸坡高度、水下岸坡内摩擦角、水下岸坡黏聚力、水上岸坡重度、水深与岸高比、水上岸坡黏聚力、岸坡坡度、水上岸坡内摩擦角、水下岸坡重度,这与岸坡稳定性的参数敏感性分析结果基本一致。研究结果对黄土岸坡塌岸宽度预测具有重要意义。

Investigation on the Deformation and Failure Patterns of Loess Cut Slope Based on the Unsaturated Triaxial Test in Yan'an,China

The large-scale implementation of the Gully Stabilization and Land Reclamation(GSLR)project induces various failures of loess slopes due to excavation in Yan'an,China.However,the deformation and failure behavior of these excavated loess slopes have not been fully understood.In this study,field investigation was undertaken for analyzing the distributions and failure features of excavation-induced loess slope failures.It is found that plastic failure mainly occurs in Q_(3) loess layers and brittle failure in Q_(2).To understand the underlying failure mechanism,a series of triaxial shear tests were conducted on intact Q_(3) and Q_(2) loess samples that with different water contents,namely natural water content(natural),dry side of the natural value(drying 5%),and wet side(wetting 5%).The characteristics of stress-strain curves and failure modes of the samples were analyzed.Results show that the stress-strain curves of Q_(2) samples are dominated by strain-softening characteristics,while Q_(3) samples mainly exhibit strain-harden features except in the drying state.Correspondingly,shear failures of Q_(3) specimens are mainly caused by shear crack planes(single,X or V-shaped).For Q_(2) loess,the dominance of tensile cracks is observed on the surface of damaged specimens.These disclose the different failure modes of excavated slopes located in different strata,that is,the arc sliding failure of Q_(3) loess slopes and the stepped tensile failure of Q_(2) loess slopes,and are helpful in the design and management of the ongoing GSLR projects in the Loess Plateau.

地聚物固化黄土力学性能及边坡稳定性研究

研究以中国西北地区近年来黄土路基滑坡为背景.通过击实试验和直剪试验,测得不同地聚物掺量下固化土的试验数据,并通过扫描电镜(SEM)和X射线衍射(XRD)对钢渣-粉煤灰地聚物固化土进行微观结构分析,同时结合pF-Meter仪器测定不同地聚物掺量下固化土的基质吸力与体积含水率.后基于强度折减法,将试验数据代入COMSOL Multiphysics中,通过饱和-非饱和理论的Richards方程,建立固化黄土路基边坡有限元模型.分析降雨前后路基层中地聚物掺入量和坡比对路基边坡的整体稳定性的影响.结果表明:当固化土中地聚物掺入量为25%时,内摩擦角为31.2°,内摩擦角增幅相比于重塑黄土提高了62.5%.随着地聚物掺入量的增加,黏聚力呈现出先升后降的趋势,当地聚物掺入量为20%时达到最高峰,为81.09 kPa,相比于重塑黄土提高了75.86%.路基边坡的稳定性随着地聚物掺入量的增加先上升后下降,随着坡比减小而上升.在特定条件下,边坡稳定性会随着降雨时间的延长而降低,随着降雨强度的增大而减小.

季冻黄土地区框锚边坡预应力锚杆内力计算方法

冻融土体与锚杆之间相互作用机理研究的不足,导致工程实践中未能充分考虑土体冻融对锚杆内力的影响。针对该问题,基于西宁市季冻湿陷性黄土冻融循环三轴剪切试验,建立考虑冻融次数影响的湿陷性黄土黏聚力经验关系式;根据锚固体与周围土层的变形协调关系,推导冻融条件下框架预应力锚杆锚固段轴力和剪应力的解析式,将理论计算结果与现场试验结果进行对比验证,并分析冻融次数对锚固段轴力和剪应力的影响。结果表明:随冻融次数的增加,土体黏聚力呈指数型下降,锚固段轴力和剪应力在前3次冻融循环中急剧增大,冻融次数达到7次后趋于稳定;相较锚固段末端,锚固段始端轴力受冻融次数的影响更大,但锚固段始端至末端的剪应力随冻融次数的变化规律基本相同;现场试验结果验证了计算方法的合理性。

聚丙烯纤维加筋黄土边坡防护原位测试及改进策略

为了给加筋黄土边坡防护技术的大面积推广提供理论依据和参考,对聚丙烯纤维加筋黄土防护边坡的土壤含水率及边坡形态进行了持续6个月的原位测试,结果表明:聚丙烯纤维加筋黄土具有良好的边坡防护效果,加筋黄土防护边坡土壤含水率总体趋于稳定,不同土层土壤含水率平均变异系数比原状边坡降低10.89%,这得益于加筋黄土防护层形态总体维持完整,能够长期抵抗降雨、干湿循环、冻融循环等不利外部环境的持续影响;然而,聚丙烯纤维加筋黄土防护边坡上植被生长缓慢,且防护层与原状土体易分异脱离。提出了对加筋黄土进行保水改良、采用快速植生的多功能层设计和可操作性强的材料物理防脱技术等加筋黄土防护边坡的改进策略。

基于不同失稳判据的黄土边坡稳定性数值计算分析

安全系数是边坡稳定性评价的一项重要指标,依据不同失稳判据计算得到的安全系数存在差异,会直接影响边坡稳定性状态的评价。以万花山治沟造地工程台阶型开挖边坡为对象,运用FLAC3D有限差分软件,采用强度折减法,选择计算不收敛判据、塑性区贯通判据、位移突变判据3种失稳判据,对黄土边坡安全系数进行数值计算。对比分析3种判据所得的分析结果及优缺点表明:3种失稳判据得出的安全系数大小排序为位移突变判据>计算不收敛判据>塑性区贯通判据。计算不收敛判据使用简便,求解快速,但不能展示折减过程坡体塑性区的变化情况;塑性区贯通判据不宜单独作为边坡失稳判据,需要结合不平衡力综合评估,但可以展示坡体剪切和拉伸塑性区;位移突变判据求解过程复杂,但能真实地反映坡体位移变化导致的临界破坏状态。建议类似工程优先选用求解过程快速的计算不收敛判据,然后结合塑性区贯通情况与坡体位移变化状态对边坡稳定性进行综合评价。

黄土高原高速公路边坡草本群落植被特征及其与土壤的关系

研究黄土高原高速公路边坡不同年限喷播混合草种草本群落植被特征和物种多样性,探讨草本群落特征、物种多样性与土壤物理化学性质之间的关系。以空间代替时间方法,研究黄土高原高速公路边坡草本群落分别恢复15 a、12 a、8 a、4 a和2 a草本群落特征、α物种多样性及其与土壤物理化学性质Mantel检验相关性。边坡恢复15 a共调查到8科13属13种,恢复12 a共调查到7科10属10种,恢复8 a共调查到3科5属5种,恢复4 a共调查到3科5属5种,恢复2 a共调查到4科6属6种;Patric物种丰富度恢复15 a>12 a>8 a>2 a>4 a,Shannon-Wiener指数、Simpson指数恢复15 a>12 a>2 a>8 a>4 a,Pielou均匀度指数恢复15 a>2 a>8 a>12 a>4 a;Patric物种丰富度与总孔隙度和毛管孔隙度、有机质、全氮、全磷呈显著正相关关系(P<0.05),Shannon-Wiener指数与有机质、全磷呈显著正相关关系(P<0.05)。边坡不同年限喷洒混合草种后,草本群落多为多年生草本植物组成,但群落物种组成、数量上存在差异;Patric物种丰富度、Shannon-Wiener、Simpson指数随着恢复年限的增长呈上升趋势;土壤总孔隙度、毛管孔隙度、有机质、全氮、全磷等是Patric物种丰富度与Shannon-Wiener指数的关键环境因子。

基于梯度提升树模型的坡耕地土壤水蚀模拟与分析

[目的]针对黄土高原坡耕地土壤侵蚀过程复杂、人为干扰强烈且难以量化的特点,利用机器学习定量解析主要影响因素对坡耕地土壤水蚀的作用与贡献,模拟分析坡耕地土壤水蚀特征并探究其机理,为坡耕地土壤侵蚀的预报提供基础支撑。[方法]基于黄土高原子洲试验站坡耕地小区1959—1969年产流产沙观测数据,精细化表征其影响因子,运用梯度提升树模型对侵蚀量和径流深的变化及其影响因素的贡献进行分析。[结果]数据集中次降雨侵蚀量(0~122.72 t/km^(2))、径流深(0.02~17.20 mm)、降雨历时(2~1410 min)及平均雨强(0.02~4.63 mm)属强变异,变异系数均>1,且多数变量呈右偏态;在相同训练集和测试集划分情况下,对侵蚀量模型预测精度(R^(2)=0.81)略优于径流深模型(R^(2)=0.80),但侵蚀量模型的层数(8层)大于径流深模型(5层),表明侵蚀机理相较径流机理更为复杂;通过梯度提升树模型与SHAP算法对自变量重要性进行排序发现,影响侵蚀模型与径流模型的自变量重要性不同。[结论]受特征提取的限制,在侵蚀量与径流深较小时预测结果不理想,未来研究应当通过引入更多自变量组合方式寻找更多相关变量以提高对侵蚀事件的预测。产流和产沙的主要影响因素存在差异,降水本身特征对产流过程起主要作用,侵蚀产沙过程中主要受到降水与地形相关自变量的共同影响。基于数据驱动,为揭示黄土高原坡耕地侵蚀机理提供参考,并为区域坡耕地土壤侵蚀防治提供科学依据。