基于聚类分析的边坡稳定性研究

在边坡稳定性分析中,滑面搜索方法对于得到边坡的稳定性系数至关重要。现今两种常用的边坡稳定性分析方法,极限平衡法以及强度折减法,可以得到极限应力状态下边坡的潜在滑面。而为了评价边坡在当前应力场下的稳定性,提出一种基于聚类分析概念的滑面搜索方法。这种方法综合考虑了边坡各点的位移以及点安全系数,采用K均值聚类法将具有相近性质的点进行分类,把边坡体分为潜在危险区以及稳定区。潜在危险区的边界即为边坡在当前应力状态下的潜在滑面,并运用矢量和方法计算得到其稳定性系数。采用上述方法对3个算例进行验算,并与极限平衡法以及强度折减法进行对比。结果表明：（1）基于聚类分析的边坡稳定性评价方法具有较好的合理性和可靠性;（2）当边坡处于临界状态附近,通过聚类分析得到的潜在滑面与其他两种方法得到的滑面基本一致;当边坡处于较稳定状态时,当前应力状态下的潜在滑面要比强度折减法得到的滑动带范围要更深更广,但求出的稳定性系数也较高;当存在软弱夹层时,矢量和计算出的滑面更为陡峻,稳定性系数比其他两种方法的结果要小。     

多层边坡破坏机制数值模拟研究

通过大量的多层边坡算例来分析FLAC3D强度折减法在不同强度、坡比和层厚的情况下所得安全系数与滑动面位置的变化规律,并与极限平衡法进行对比,探究强度折减法和极限平衡法所得结果产生差异的原因,揭示多层边坡的破坏机制。数值模拟计算结果表明:①对于上层土体强度较软的边坡,当上下层强度相差到一定程度,表现为上层破坏,强度折减法与极限平衡法的安全系数相对差值最大,达5%~7%;上层土体厚度h增加时,安全系数逐渐减小;上层土体位于地表以下时,滑动面通过坡趾,安全系数保持不变。②对于下层土体强度较软的边坡,当上下层强度相差到一定程度,强度折减法所得安全系数基本保持不变,但极限平衡法仍保持增长趋势,最大相对差值可达12%;上层土体厚度h增加时,安全系数也相应增大,h位于9~12 m,边坡表现为深层破坏;而h=12 m时,极限平衡法的滑动面却通过坡趾,但安全系数相差很小。当h的变化范围在3~5 m,两种方法的安全系数相对差值最大,达6%~10%。破坏区分布分析表明,边坡呈拉伸-剪切复合破坏,对于下层土体强度较软的边坡,复合破坏模式更显著,与单一剪切破坏模式相比,最大有10%左右的相对差值。     

边坡局部和整体稳定性评价方法的辩证统一

边坡稳定性评价方法有局部和整体两类,两者之间存在差异有其历史的局限性和存在的合理性。以极限平衡法和有限元强度折减法为例,分析了典型整体与局部稳定性评价方法在滑面搜索、平衡条件、安全系数的定义、材料参数和失稳等5个方面存在的异同。利用PFC算例揭示了真实边坡带有强烈空间和时间特征的渐进破坏特性,发现边坡失稳是自然地形的重塑、再造和稳定的过程,也是一个动势能转换和耗散的过程。针对边坡稳定问题及其特点,提出了基于广义软化的边坡稳定矢量和法的理论构架和技术路线,并进行了程序实现,克服了传统强度折减脱离实际材料劣化特征的不足,安全系数求解仅与应力状态相关,假设条件少,可以实现局部与整体稳定评价的辩证统一,可以反映边坡失稳的渐进特性和参数演化的时空效应。与传统极限平衡法(LEM)和强度折减法(SRM)在滑面和安全系数方面的综合对比发现,局部稳定揭示的滑面更稳定,获取的安全系数是全周期的,文中方法所得安全系数比整体方法的结果稍高,差异是整体方法的假设条件造成的。     

River bank erosion control by soil nailing

A study has been done for analysing soil nailed cuts with circular type wedge failure by friction circle method. Various parameters such as nail length, nail diameter, nail inclination, wall inclination and angle of internal friction of soil have been considered to determine the factor of safety of nailed open cuts. The study shows that for cohesionless soil nailed cut, factor of safety increase with increase of parameters like angle of internal friction of soil, length of nail (L) versus height of cut (H) ratio, cohesion of soil and nail inclination (upto 15°) with horizontal. The study revealed that nails grouted with cement perform better than driven nails. A case study further confirms the analytical findings. Received 7 October     

物质点强度折减法及其在边坡中的应用

物质点法适用于模拟连续介质大变形,如边坡失稳全过程。在物质点法中应用强度折减法,用于边坡稳定性评价。与极限平衡法相比,二者安全系数计算值、滑动面位置结果基本一致;与有限元强度折减法相比,物质点法失稳评价标准的物理意义明确。利用物质点法大变形计算优势,评价边坡失稳后的破坏后果,通过算例说明其评价不同安全系数下的滑坡堆积形态及滑移距离的能力,尤其是评价滑坡对临近建筑物的影响程度的能力。物质点强度折减法可用于边坡稳定性评价及边坡破坏后果评价。     

基于点稳定系数法的斜坡稳定性分析

基于莫尔-库伦强度理论构架,界定了点稳定系数的概念,并推导其计算公式。利用Geostudio软件建立了均质斜坡模型及计算其应力分布,并在此基础上结合MATLAB软件计算斜坡模型中各点的点稳定系数,勾绘出斜坡体内不同稳定度区域,探析了斜坡稳定性,并与传统极限平衡法进行了对比。对比结果表明:对直立斜坡,两种方法的计算结果均为不稳定,但点稳定性系数法勾绘出坡脚及坡脚底部存在两处不稳定区域;对60°斜坡,点稳定系数法的计算结果表明坡脚处存在潜在不稳定区域,而极限平衡法的计算结果表明坡体处于稳定状态;对45°斜坡,两种方法的计算结果均为稳定,计算结果一致。进一步分析得到结论:点稳定系数法不需要假设或指定某一形状滑面进行斜坡稳定性评价,且可考虑应力集中对坡体稳定性的影响;极限平衡法以稳定系数表达计算结果,而点稳定系数法以不稳定区域表达计算结果。在分析了应力和岩土体力学参数因素对点稳定系数法计算结果的敏感性后发现:相对于极限平衡法,岩土体力学参数对点稳定系数法影响更为敏感,存在黏聚力界限点和内摩擦角界限点,且对均质斜坡破坏形式(局部滑动变形破坏或整体压缩变形破坏)起着非常重要的作用。     

倾斜基岩上的边坡破坏模式和稳定性分析

应用离心模型试验对工程中被颇为关注的带有与边坡走向一致的倾斜基岩面,且在该基岩面存在软弱夹层的边坡的稳定性和破坏模式进行了比较详细地研究,并用极限平衡分析方法对试验结果进行了计算分析。模型试验结果表明,该类边坡失稳时,紧贴岩面的软弱夹层就成为滑动破坏面,因而边坡整体沿基岩面向下滑动,且侧向水平位移各处基本一致,表现出典型的平移滑动破坏模式。将稳定安全系数的实测结果与按平移滑动破坏模式的极限平衡分析方法的计算结果相比较后发现,两者相当地吻合,证实了按平移滑动破坏模式所作的极限平衡分析,能良好地预测边坡平移滑动破坏情形下的稳定安全系数。     

应用离散元强度折减对复杂边坡进行稳定性分析

介绍了离散元（DEM）及强度折减法基本原理,利用3DEC软件结合强度折减法对某水电站高边坡进行稳定性分析。对岩体强度参数进行折减,以关键点位移与时间关系曲线发散时的折减系数作为边坡安全系数,根据位移矢量图确定滑面和破坏形态。通过与Dijkstra极限平衡有限元法及Sarma法的计算结果对比,验证该方法的可靠性。在二维计算基础上建立并简化三维模型,模拟边坡三维应力场。结合工程关注区域的应力及位移趋势,采用3DEC中的辅助节理截取典型坡段单宽模型,在已有的三维应力场的基础上对其进行强度折减,弥补了二维强度折减未考虑三维应力场对边坡稳定性影响的不足,把握了工程重点及力学特性,为类似工程提供了一种合理高效的稳定性分析手段。     

基于主动学习径向基函数的边坡系统可靠度分析

相较于极限平衡法,强度折减法在计算边坡稳定性系数上有许多优势,但更大的计算量在一定程度上限制了其在边坡可靠度分析中的应用。为了有效地减少可靠度分析中数值模型的计算次数,以减轻使用强度折减法所带来的计算压力,引入了基于主动学习径向基函数（ARBF）代理模型的高效分析方法：利用主动学习函数在极限状态面附近搜索训练样本更新代理模型,加快模型训练的收敛速度;采用线性核径向基插值函数简化模型参数优化过程,建立简洁、稳定的代理模型。此外,为了充分发挥主动学习代理模型的优势,提出针对土质边坡特性的初始采样策略。当得到稳定的代理模型后,结合蒙特卡罗模拟计算边坡的系统失稳概率。作为对比,基于两个典型边坡算例,测试了两种经典的可靠度方法：主动学习克里金模型（AK）和二次响应面法（QRSM）,论证了引入的主动学习径向基函数代理模型在计算效率上的高效性和计算模型上的稳定性。     

Numerical modelling of soil nails in loose fill slope under surcharge loading

A plane–strain numerical model has been developed to mimic a nailed loose fill slope under surcharge loading. The model has been used to back-analyse a field test that was conducted to examine the behaviour of soil nails in loose fill slopes under surcharge loading. Incremental elasto-plastic analyses coupled with pore water diffusion have been performed to study the internal deformation, water content redistribution in the soil, and the performance of the soil nails during and after the application of surcharge loading. The model parameters describing the mechanical and hydraulic properties of the nailed slope were obtained from field or laboratory tests. Different modelling techniques and boundary conditions for mimicking soil–nail interaction in loose fill material have been examined. Comparisons between numerical predictions and field measurements demonstrate that a new interfacial model, denoted as the embedded bond–slip interface model, is more suitable for mimicking the interfacial behaviour. Despite the simplicity of the numerical model, the predicted responses are in close agreement with the field test results, in particular the mobilisation and distribution of nail forces in response to surcharge loading. Both the numerical and the field test results suggest that soil nails are capable of increasing the overall stability of a loose fill slope for the loading conditions considered in this study. The increase in confining stress along the soil nails near the surcharge area is central to the overall stabilising mechanism. On the contrary, the nail forces mobilised near the nail heads are much smaller, indicating that the beneficial effect of having a structural grillage system at the slope face is limited for the range of surcharge pressures considered in this study.     

Soil Nailing Application in Erosion Control – An Experimental Study

A study has been done for soil nailed cut considering circular type wedge failure by friction circle method. Effect of variation of design parameters such as nail length, nail diameter, nail inclination, wall inclination, angle of internal friction of soil, etc. have been incorporated in the study to determine the factor of safety of nailed open cuts. It is seen that at sites which are susceptible to rainfall induced erosion, the erosion may be checked to a greater degree by soil nailing. This methodology can be adopted at embankment sites, natural slopes, highways, etc. including free board of riverbank area of natural rivers. The study shows that grouted nails perform better than driven nails. A case study further confirms the analytical findings.     

Investigation on failure mechanism of nailed soil slopes under surcharge loading: testing and analysis

This paper presents the laboratory test results of nailed soil slopes to study the behaviour of reinforced slopes under surcharge load. Tests were conducted on nailed soil slopes of three different slope angles (β = 30°, 45°and 60°) and nail inclinations (i = 0°, 15° and 30°). Each slope is reinforced with a number of hollow aluminium nails placed in three rows. The failure response, load-settlement, nail forces at the centre of nails and the distribution of nail forces along the length are examined during the tests. The numerical simulation of the model tests is performed by a 3-D finite element program. The numerical results are found in agreement to the corresponding experimental results. It is observed that the nails inclined at 0° to the horizontal give the maximum improvement in the load carrying capacity of the slopes followed closely by the 15° nail inclination. The maximum nail forces are observed in the topmost row of nails and decreases with the depth. The distribution of the nail forces along the nail are also found to be varying with respect to the failure surface of the slopes.     

基于随机响应面法的边坡系统可靠度分析

提出了一套基于随机响应面法的边坡系统可靠度分析方法。该方法首先从大量潜在滑动面中筛选出代表性滑动面。针对每条代表性滑动面,采用Hermite多项式展开建立其安全系数与土体参数间的非线性显式函数关系（即随机响应面）。然后,采用直接蒙特卡洛模拟计算边坡系统失效概率。在蒙特卡罗模拟中,采用所有代表性滑动面的随机响应面计算每一组样本所对应的边坡最小安全系数。最后,以两个典型多层边坡系统可靠度问题为例验证了该方法的有效性。结果表明：文中提出的边坡系统可靠度分析方法能够有效地识别边坡代表性滑动面,具有较高的计算精度和效率,并且确定代表性滑动面时无需计算滑动面间的相关系数。同时该方法可以有效地计算低失效概率水平的边坡系统可靠度,为含相关非正态参数的边坡系统可靠度问题提供了一条有效的分析途径。此外,多层边坡可能同时存在多条潜在滑动面,基于单一滑动面（如临界确定性滑动面）或者部分代表性滑动面进行边坡系统可靠度分析均会低估边坡失效概率。     

四川梓潼金龙村边坡地震稳定性分析

金龙村边坡位于四川省梓潼县境内,汶川县东,距离汶川地震中心约150 km。通过现场调查,分析了地形、地貌条件和地震等因素对该边坡稳定性的影响。发现滑动面位于土层与基岩的接触面,采用极限平衡法和数值分析法对边坡的稳定性进行了计算,2种计算得到的稳定性结果较为接近。采用动力时程分析法计算得到了边坡位移与加速度变化的关系。     

Multiple response surfaces for slope reliability analysis

This paper develops a multiple response surfaces approach to approximate the limit state function for slope failure by second‐order polynomial functions, to incorporate the variation of the most probable slip surfaces, and to evaluate the slope failure probability p_f. The proposed methodology was illustrated through a cohesive soil slope example. It is shown that the p_f values estimated from multiple response surfaces agree well with those p_f values that have been obtained by searching a large number of potential slip surfaces in each Monte Carlo simulation (MCS) sample. The variation of number of the most probable slip surfaces is studied at different scale of fluctuation (λ) values. It is found that when full correlation assumed for each of random fields (i.e., spatial variability is ignored), the number of the most probable slip surfaces is equal to the number of random fields (in this study, it is 3). When the spatial variability grows significantly, the number of the most probable slip surfaces or number of multiple response surfaces firstly increases evidently to a higher value and then varies slightly. In addition, the contribution of a specific most probable slip surface varies dramatically at different spatial variability level, and therefore, the variation of the most probable slip surfaces should be accounted for in the reliability analysis. The multiple response surfaces approach developed in this paper provides a limit equilibrium method and MCS‐based means to incorporate such a variation of the most probable slip surfaces in slope reliability analysis. Copyright © 2014 John Wiley & Sons, Ltd.     

Stability assessment of Jinlong village landslide,Sichuan

The Jinlong village landslide is located in Zitong County, Sichuan and approximately 150 km east of Wenchuan. The landslide has exhibited creep deformation for nearly 30 years. Field investigations indicated that the slope deformation was caused by the combined effects of unfavorable topographic and geological conditions and earthquakes. The sliding surface is along a contact between silty clay and mudstone. The Wenchuan earthquake accelerated the creep, causing bulging of the ground surface. Shear test results of the silty clay near the sliding surface indicated that the residual strength is consistent with the stability state of the landslide. The factors of safety were calculated using the limit equilibrium method (LEM) and the method of fast Lagrangian analysis of continua in two dimensions. The results using the LEM indicate that the stability is clearly affected by seismic shaking, and when the peak ground acceleration is 0.08g, the slope reaches the limit state. The shear failure surface given by numerical simulation develops on the contact between the clay and silty mudstone, which confirmed the assumed sliding surfaces’ location using the LEM.     

Evaluation of factor of safety for vegetated and barren soil slopes with limit equilibrium computations

Limit equilibrium method (LEM) is the most conventional and popular technique in slope stability analysis. Location of the unique failure surface and the determination of corresponding critical factor of safety (critical-FOS) of the soil slope requires a lot of trial and research. With this amount of trial and research results, we evaluate the field of factor of safety (FOS-field), which represents the critical-FOS at a point within the domain of all possible failure surfaces. Such a field can be very important for the precise judgment of the most critical-FOS of the slope and its perturbation. This paper also presents the evaluation of the FOS-field of vegetated slopes, thus providing an analytical way to examine the effect of vegetation on the soil slope stability. We evaluated the stability of vegetated and barren soil slopes under dry and fully saturated conditions. With dry and fully saturated conditions, the behaviour of slopes in most favourable and worst conditions can be simulated. Evaluation of FOS-field for various slope geometries and conditions show that the more unstable the slope becomes, the more difficult it will be to pinpoint the location of the critical failure surface with the unique least FOS. We verified the LEM codes with finite element method (FEM) and spectral element method (SEM) codes considering a sample problem from Smith and Griffiths’ book (Smith and Griffiths 2003).     

填筑和开挖边坡的稳定性分析

在有限元应力应变分析的基础上,采用以剪应力定义的安全系数和模式搜索的方法确定边坡的安全系数及最危险滑动面。在平面应变条件下,对土层均质的填筑和开挖边坡的稳定性进行了较为全面的研究,并同天然边坡的安全系数和破坏面进行比较,计算结果表明,这3类边坡滑动面的形状基本一致,天然边坡安全系数要高于填筑和开挖边坡;同时比较了采用有限元稳定评价方法和采用极限平衡方法计算结果的差别;最后分析研究了计算参数变化对填筑和开挖边坡稳定性的影响。     

The effect of pressure-grouted soil nails on the stability of weathered soil slopes

Pressure-grouted soil nails have been increasingly used for stabilizing slopes. The pullout resistance of a soil nail is the main factor for reinforcing the slope stability. In this study, a two-dimensional axisymmetric finite element model is developed to simulate the pullout behavior of a pressure-grouted soil nail. This model is verified with field pullout tests result of a pressure-grouted soil nails by comparing with gravity-grouted soil nails. Based on the analysis, a three-dimensional finite element model is proposed for stability analysis of a slope reinforced with pressure-grouted soil nails using the shear strength reduction method. A series of numerical slope stability analyses for a slope composed of weathered soil are performed to investigate the effects of grouting pressure on the slope stability and the behavior of the soil nails. Special attention is given to the installation effect of a pressure-grouted soil nails. It is found from the result of this study that the pressure-grouted soil nails increase the safety factor by fifty percent in a slope by increasing the stiffness of the nailed slope system. Numerical analysis results confirm the fact that the pullout resistance of a soil nail is the main factor for stabilizing slopes rather than the shear resistance of the soil nail.     

Three-dimensional FE analysis of a nailed soil wall curved in plan

A nailed soil wall curved in plan was modelled in three-dimensions by the finite element method for construction, service and ultimate loading conditions. The behaviour of the nailed soil wall, the soil–nail interaction, the role of the reinforcement, and the overall and internal failure mechanisms were investigated. © 1997 by John Wiley & Sons, Ltd.