The Low Compaction Grading Technique on Steep Reclaimed Slopes: Soil Characterization and Static Slope Stability

Since the Surface Mining and Control Reclamation Act of 1977, US coal mining companies have been required by law to restore the approximate ground contours that existed prior to mining. To ensure mass stability and limit erosion, the reclaimed materials have traditionally been placed with significant compaction energy. The Forest Reclamation Approach (FRA) is a relatively new approach that has been successfully used to facilitate the fast establishment of native healthy forests. The FRA method specifies the use of low compaction energy in the top 1.2–1.5 m of the contour, which may be in conflict with general considerations for mechanical slope stability. Although successful for reforestation, the stability of FRA slopes has not been fully investigated and a rational stability method has not been identified. Further, a mechanics-based analysis is limited due to the significant amount of oversize particles which makes the sampling and measurement of soil strength properties difficult. To investigate the stability of steep FRA slopes (steeper than 20°), three reclaimed coal mining sites in the Appalachian region of East Tennessee were investigated. The stability was evaluated by several methods to identify the predominant failure modes. The infinite slope method, coupled with the estimation of the shear strength from field observations, was shown to provide a rational means to evaluate the stability of FRA slopes. The analysis results suggest that the low compaction of the surface materials may not compromise the long-term stability for the sites and material properties investigated.     

挡墙加固非饱和土边坡整体稳定性分析

在对挡墙加固边坡进行设计和评价时应综合考虑基质吸力和挡墙对边坡整体稳定性的影响。应用可以考虑基质吸力的弹塑性强度折减有限元程序,分析了挡墙加固非饱和土边坡的整体稳定性,揭示了基质吸力对加固边坡整体稳定性的影响程度,同时亦分析了整体潜在滑动面位置的变化情况,并进行了关于吸力摩擦角的敏感性分析。     

Spectral element analysis to evaluate the stability of long and steep slopes

This paper evaluates the stability of long and steep slopes examining the effect of the critical length/depth ratio, L/H, and critical slope angle, β _cr, by comparing the results of infinite slope equation with the spectral element method. In addition, the influence of uphill and downhill boundaries on stability of long and steep slopes is also evaluated in theoretical domains. As an example, this paper presents a stability analysis of long and steep vegetated and barren slopes in saturated and seismic conditions, and also evaluates the effectiveness of the infinite slope equation for those slopes. In the vegetated slopes, the root zone may not extend to the whole depth of slope, which may lead to overestimating the factor of safety by infinite slope equation. This paper examines the applicability of the infinite slope equation for infinitely long, steep, and shallow slope model by comparing the results of the spectral element method.     

非饱和土基质吸力对边坡稳定的影响

降雨及浸水往往引起天然土坡及人工土坡的滑坡,其中,土中基质吸力的消失是一个重要原因。通过试验和计算分析,揭示了基质吸力对边坡稳定所起的重要作用。非饱和土边坡稳定计算中的关键参数 的确定,尚有待于进一步研究。     

The role of infiltration processes in steep slope stability of pyroclastic granular soils: laboratory and numerical investigation

Rainfall-induced landslides on steep slopes are a common feature in much of Italy’s mountain areas covered by shallow-pyroclastic deposits. Generally, these deposits are unsaturated and have a slope angle higher than 40°–50°; hence their stability is due to the positive effect of matric suction on soil shear strength. During rainfall, rainwater infiltration causes a decrease in suction, which in turn causes changes in soil mechanical and hydraulic properties, leading towards an instability process. However, the response of pyroclastic soil slopes to rainwater infiltration is not fully understood. The aim of this study is to link slope instability to the infiltration process on the basis of advanced geotechnical characterization, in situ monitoring and numerical analysis calibrated through a back-analysis of well-instrumented flume tests.     

抗滑桩加固非饱和土边坡三维稳定性分析

讨论了抗滑桩加固边坡的稳定分析研究状况,在全面地考虑基质吸力和抗滑桩对边坡稳定性的贡献后,利用所发展的可考虑基质吸力的弹塑性强度折减有限元程序,探讨了抗滑桩加固前后的非饱和土边坡整体稳定性,并且与相应不考虑基质吸力条件下的边坡稳定性进行了对比分析。     

非饱和-非稳定渗流条件下的边坡临界滑动场

受季节性降雨或水库运行的影响,岸坡外水位及坡内孔隙水压力场的变化较大,不利于岸坡的稳定性。在水位变化过程中,利用非饱和-非稳定渗流有限元计算得到孔隙水压力场,基于非饱和土的渗流和抗剪强度理论,对水位变化过程中的边坡临界滑动场法进行改进,提出可考虑水位变化与岸坡非饱和-非稳定渗流过程的边坡临界滑动场数值模拟方法。将改进后的水位变化过程中的边坡临界滑动场法分别应用于黏土、粉土岸坡在水位升降过程中的稳定性分析,研究了水位升降速率及基质吸力对岸坡稳定性的影响,并揭示了边坡在水位变化过程中的稳定性变化历程。研究表明,该方法计算结果合理、可靠,更适用于涉水边坡的稳定性计算,且岸坡稳定性变化历程受水位升降速率、基质吸力等多种因素共同影响,只有在考虑非稳定渗流的基础上同时考虑基质吸力的作用才能正确得出水位变化过程中岸坡稳定性变化规律和实质。     

暗穴环境下非饱和黄土边坡稳定性分析

黄土暗穴在水土流失方面作为一种独特的侵蚀方式,往往会威胁到建设在非饱和黄土层之上的公路边坡稳定性。基于考虑基质吸力的强度折减有限元程序,本文通过开展不同位置暗穴、单双暗穴和基质吸力赋存与丧失等条件下边坡稳定性的对比研究,着重对影响边坡稳定性的暗穴、基质吸力等因素进行了研究。研究结果表明,当暗穴接近地表时由于存在一定的减载效应而利于坡体的稳定,而当暗穴接近边坡坡脚时则成为黄土边坡稳定性的不利因素,在边坡底部存在的双暗穴明显削弱了边坡的稳定性,基质吸力使得非饱和黄土边坡安全系数增大,潜在滑动面变深,反之则使安全系数减小,潜在滑动面上移,基质吸力的丧失或减小以及暗穴的削弱作用使得潜在滑动面位置变化较大。     

雨水入渗对非饱和土坡稳定性影响的参数研究

很多国家和地区的斜坡失稳与雨水入渗有密切关系。通过参数分析研究可以深化对这种关系的认识和理解,因而对滑坡灾害的预测和预防有重要意义。针对香港地区一种典型非饱和土斜坡,用有限元法模拟雨水入渗引起的暂态渗流场,然后将计算得到的暂态孔隙水压力分布用于斜坡的极限平衡分析。计算中采用延伸的摩尔－库伦破坏准则以便考虑基质吸力对抗剪强度的贡献,研究了降雨特征、水文地质条件及坡面防渗处理等因素对暂态渗流场和斜坡安全因数的影响。数值模拟结果表明：降雨强度、降雨历时和雨型对暂态渗流场及斜坡稳定性有明显的影响;土体的渗透系数,尤其是渗透系数各向异性的影响特别显著;斜坡中相对隔水层的存在以及斜坡防渗护面的效果等因素的影响均不容忽视。     

非稳定渗流条件下非饱和土边坡稳定分析

大量的滑坡灾害表明, 水的作用是影响边坡失稳的重要原因,合理地评价边坡的稳定性,需考虑边坡基质吸力场的变化与水的渗流作用。通过稳定与非稳定渗流条件下的非饱和土堤算例,探讨了Bishop和Fredlund非饱和土抗剪强度公式应用于强度折减有限元法的差异,并与极限平衡法的结果进行对比。结果表明,Bishop和Fredlund非饱和土抗剪强度公式的本质是相同的,而从非稳定渗流条件下的分析结果来看,强度折减有限元法分析的安全系数与极限平衡法分析临水坡和背水坡安全系数的最小值相当接近,强度折减有限元法更适合分析边坡的整体稳定性。此外,采用Bishop强度公式可以通过用饱和度代替基质吸力参数来体现基质吸力对强度的影响,在分析饱和度较高的非饱和土边坡稳定性时较为方便。     

Stability analysis of laterally confined slope lying on inclined bedding plane

A stability analysis of a laterally confined slope model, lying on an inclined bedding plane, was presented to evaluate the lateral shear resistance by considering the loading paths and failure envelopes. Two slope models were prepared on a bedding plane by compaction, one with and one without lateral confinement. The compacted models are related to the geological conditions at shallow depths where brittle deformation can occur and an excavation can induce horizontal field stress that significantly influences the stability of the slope. Three distinct loading paths, controlled by either tilting the angles or increasing the surcharge loads, were applied to achieve the failure of the slope models. Rankine’s passive earth pressure due to compaction was reduced by the shear strength reduction ratio. The shear strength reduction ratio was estimated through the least-squares fitting method based on the results of model tests at failure when the loading paths intersected the failure envelope. Provided that the effect of lateral confinement in a rock mass can be described by the shear strength reduction ratio, the proposed equations will be beneficial for slope stability analyses of laterally confined slopes on bedding planes. A case study of an undercut pit wall in an open-pit mine was demonstrated by showing that the unknown shear strength reduction ratio can be back-analyzed from the rainfall-induced landslide case. Therefore, the design of other undercut slopes with different geometries and groundwater conditions in the rock mass, which have undergone the same geological process as the back-analyzed case, is possible.

     

生态护坡对边坡岩土体基质吸力的影响研究

对植被蒸腾作用以及阻滞地表径流作用进行了研究,提出生态护坡对边坡岩土体基质吸力的影响主要有3个方面：(1) 植被的蒸腾作用消耗边坡岩土体中的水分,使边坡体基质吸力升高,对边坡稳定有利;(2) 植被阻滞地表径流,增加降雨入渗量,使边坡体含水率增高,基质吸力降低,对边坡稳定不利;(3) 植被的存在究竟使边坡体基质吸力如何变化以及对边坡稳定是否有利,最终决定于降雨量和雨型。在此基础上,更进一步提出在降雨丰沛、暴雨较多的地区进行生态护坡时,应对边坡体采取防排水处理措施。     

降雨条件下土坡饱和-非饱和渗流及稳定性分析

基于非饱和土力学理论,利用有限元法对降雨条件下边坡的饱和-非饱和渗流及稳定性进行了探讨。数值分析表明,雨水入渗使边坡非饱和区土体的基质吸力减小,是导致边坡稳定性降低的主要因素。进而考察了基质吸力的不同取值及裂隙的不同位置对边坡稳定性的影响,结果表明：在降雨初期基质吸力取值越大,得到的边坡安全系数越高,但经过长时间降雨即边坡土体基本达到饱和后,基质吸力的取值不同对边坡安全系数基本上没有影响;裂隙的存在为雨水入渗提供了良好的通道,因而导致边坡稳定性相对于无裂隙状态来说有所降低。降雨初期下游裂隙使边坡稳定性降低较快,但随着降雨历时的增加,即下游土体逐渐变为饱和状态后,边坡的稳定性变化则主要由上游裂隙控制。     

Rain-triggered slope failure of the railway embankment at Malda,India

The common slope stability analysis is incapable of accurately forecasting shallow slides where suction pressures play a critical role. This realization is used for elaborate stability analyses which include soil suction to better predict rainfall-induced slides at railway embankment at Malda where three known cases of slope failures and train derailments occurred after heavy rainfall. The relationship between the soil–water content and the matric suction is established for the embankment soil. It is then used in the coupled analyses of seepage and slope stability to estimate performances of the embankment at different intensity and duration of rainfall. The numerical simulations are performed with the FE code Geo-Studio. The numerical results show significant reduction in the factor of safety of the railway embankment with the increase in the intensity and duration of rainfall. The effectiveness of the proposed mitigation measures including placement of 2 m-wide free draining rockfill across the slopes and drilling 5-m-long sheet pile wall at the toe of the embankment is studied numerically. The study confirms that the proposed mitigation measures effectively increase the factor of safety of the embankment and stabilizing it even in case of a heavy rainfall of 25 mm/h over 12 h.     

Role of apparent cohesion in the stability of Dominician allophane soil slopes

This study examines the effect of loss of apparent cohesion from rainwater infiltration upon the stability of partly saturated, allophanic soil slopes of Dominica (West Indies). The parent material of the Dominican allophanic soils are the andesitic and dacitic volcanic rocks from ten volcanic centres of mainly Pleistocene age. Although simplifying assumptions are made to assess the depth of wetting front and magnitudes of true and apparent cohesion values of the partly saturated allophanic soils, certain inferences of practical significance emerge from the study.

Matric suction contributes to the effective stress of unsaturated soils and increases the shear strength of these soils by imparting them an apparent cohesion strength component. This apparent cohesion strength component of the partly saturated Dominician allophanic soils is considered to be related to the matric suction term (u_a−u_w) by a parabolic relationship.

The partly saturated allophanic soil slopes of Dominica would invariably fail from loss of apparent cohesion upon saturation of the soil mantle by the infiltrating water front only if (a) the average slope angle (β) is greater than or equal to the drained shear strength parameter of the unsaturated soil (Φ_d) (slopes with β ≥ Φ_d are referred to as category 1 slopes in this study) and (b) the drained cohesion parameter (c_d) belonging to the partly saturated soil of the category 1 slope is mainly contributed by matric suction induced apparent cohesion (c_app) and contribution from true cohesion (c′) is absent. However, the possibility of the category 1 slopes (for the case 1 situation) failing in the saturated condition from effective stress reduction due to rise in ground water table is shown to be an unfeasible proposition. In contrast, for the case 1 situation, the partly saturated category 2 slopes (β < Φ_d) are insusceptible to failure from reduction in effective stress due to loss of matric suction but fail in the saturated condition from effective stress reduction due to rise in ground water table. If the allophanic soils were to possess a sufficient magnitude of true cohesion (c′), even the partly saturated category 1 slopes possessing slope angles (β) much in excess of their Φ_d values (β − Φ_d = 15.1°) would remain stable upon total elimination of the matric suction induced cohesion. However, partly saturated category 1 slopes possessing a true cohesion component and insusceptible to failure from reduction in effective stress upon loss of matric suction, invariably fail in the saturated condition from reduction in effective stress due to rise in ground water table.

Using the methodology developed in this study, two previously reported case histories pertaining to rain induced translational landslides in the residual soil areas of Brazil are re-examined and the results obtained in this study are found to be in agreement with the findings of the previous researchers.     

考虑基质吸力作用的土坡稳定性分析

非饱和土基质吸力的有效应力作用,使得土体的抗剪强度得以提高。基于Fredlund的非饱和土抗剪强度理论,在土坡极限平衡分析方法中,同时考虑分条的土条底面与条间的基质吸力分布对土体抗剪强度的贡献,进而构筑考虑基质吸力作用的土坡极限平衡分析方法。构筑基于极限平衡分析方法的目标函数,应用SLP优化方法寻求最危险滑动面。用具体计算实例说明这种分析思路可以评价土坡的稳定性评价的影响。     

基于模糊因素的岩质边坡地震稳定性多模型组合评价

岩质边坡地震稳定性评价是岩土边坡地震失稳防治的基础工作,针对评价过程中评价因素的多源模糊性,借鉴工程可变模糊集理论,将模糊可变评价模型应用于岩质边坡地震稳定性评价中。综合考虑岩质边坡内在结构和外部自然条件,选取岩土体特性、新构造运动特征、坡高、坡角、年均降雨量和场地地震烈度6个指标作为评价指标体系并建立等级标准;将改进熵权算法引入该模型中,利用指标实际数据离散性求权重;通过改变模糊可变评价模型参数对岩质边坡地震稳定性进行线性与非线性组合评价,并将均值作为最终评价结果。将该方法应用于天然边坡与路堑边坡实例中,结果表明,模糊可变评价模型评价结果合理、客观,具有更高的可靠性与稳定性,为岩质边坡地震稳定性评价工作提供了一种新的研究方法与思路     

降雨入渗影响下边坡中的非饱和渗流特性

为了分析降雨入渗影响下非饱和土坡渗流特性,利用自制降雨模拟系统和实时监测系统,对降雨诱发非饱和土坡失稳过程进行全方位、多参量的实时监测,研究不同降雨条件下,不同坡度、不同压实度边坡坡体不同位置雨水入渗率和湿润峰的实变规律.结果表明:降雨入渗条件下,陡坡和高压实度土体不利于雨水入渗,而缓坡和低密实度土体入渗率变化快;实际土体吸力和含水量实时变化规律不同步,提出试验湿润峰概念,含水率（吸力）湿润峰点可按含水率（吸力）实时曲线的过渡区和雨后残余含水率（吸力）的线性交叉点确定;考虑单向吸湿或脱湿路径下土体含水率和吸力具有唯一对应关系,含水率湿润峰点与吸力湿润峰点的绝对值时差即为形成湿润峰所需时间;对比湿润峰实测值与Lumb半经验值散点分布规律,基于Lumb湿润峰深度计算公式提出非线性修正表达式.      

Numerical Modelling of Steep Slopes in Open Rock Quarries

The key question regarding steep rock slopes along rock quarries is their stability because a rock slope failure can have critical results. In this study, the aim is to investigate the areas with potential risk for jointed karstic limestones in a rock quarry. First, to determine rock mass properties, scan-line surveys were performed, and the major orientations of discontinuities were analyzed using stereographic projection. Then, the physicomechanical properties of the slope-forming rock were determined in the laboratory, and geomechanical properties of the rock mass were determined using an empirical failure criterion. Finally, the quarry slope stability was assessed in accordance with numerical modelling. According to the results obtained, the numerical modelling of steep rock slopes can be efficiently evaluated by using finite element method. Beside this, the presence of joints intersecting the main discontinuity sets, the filling materials of discontinuities resulting from weathering of limestone and surface deposits, surcharge load due to mine waste dumped on the slopes and excavation blasting during construction of quarry area play a key role when modelling the steep rock slopes by using finite element method.     

降雨入渗条件下多级黄土高边坡稳定性分析

在西北地区进行基础工程建设,遇到了大量的多级黄土高边坡工程。边坡工程属于永久性工程,难免遇到降雨状况。降雨入渗会引起边坡土体饱和度与土体重度的变化,进而导致边坡体基质吸力与有效应力场的变化,使边坡的稳定性受到较大的影响。本文通过考虑边坡体不同深度处基质吸力随雨水入渗的变化情况,建立了渗流场与应力场的耦合计算模型,对降雨入渗条件下多级黄土高边坡的稳定性进行分析。最后,依托实际边坡工程,利用PLAXIS 3D岩土有限元软件,建立边坡稳定性计算模型,通过设置降雨量随时间的变化函数与渗流边界条件,进行降雨入渗条件下多级黄土高边坡的稳定性数值计算,并与理论分析结果进行了对比分析。根据数值计算结果可得到多级黄土高边坡在降雨入渗条件下的变形、基质吸力、有效应力、潜在滑移面以及安全系数的变化情况,从而对降雨入渗影响下多级黄土高边坡的稳定性做出分析。研究结果可为降雨入渗条件下多级黄土高边坡的稳定性的研究提供一定的指导作用。