Finite element analyses of slope stability problems using non-associated plasticity

In recent years, finite element analyses have increasingly been utilized for slope stability problems. In comparison to limit equilibrium methods, numerical analyses do not require any definition of the failure mechanism a priori and enable the determination of the safety level more accurately. The paper compares the performances of strength reduction finite element analysis(SRFEA) with finite element limit analysis(FELA), whereby the focus is related to non-associated plasticity. Displacement-based finite element analyses using a strength reduction technique suffer from numerical instabilities when using non-associated plasticity, especially when dealing with high friction angles but moderate dilatancy angles. The FELA on the other hand provides rigorous upper and lower bounds of the factor of safety(FoS) but is restricted to associated flow rules. Suggestions to overcome this problem, proposed by Davis(1968), lead to conservative FoSs; therefore, an enhanced procedure has been investigated. When using the modified approach, both the SRFEA and the FELA provide very similar results. Further studies highlight the advantages of using an adaptive mesh refinement to determine FoSs. Additionally, it is shown that the initial stress field does not affect the FoS when using a Mohr-Coulomb failure criterion.

Finite element analysis of slope stability by expanding the mobilized principal stress Mohr's circles-Development, encoding and validation

In recent years,finite element analysis is increasingly being proposed in slope stability problems as a competitive method to traditional limit equilibrium methods(LEMs)which are known for their inherent deficiencies.However,the application of finite element method(FEM)to slope stability as a strength reduction method(SRM)or as finite element limit analysis(FELA)is not always a success for the drawbacks that characterize both methods.To increase the performance of finite element analysis in this problem,a new approach is proposed in this paper.It consists in gradually expanding the mobilized stress Mohr’s circles until the soil failure occurs according to a prescribed non-convergence criterion.The present approach called stress deviator increasing method(SDIM)is considered rigorous for three main reasons.Firstly,it preserves the definition of the factor of safety(FOS)as the ratio of soil shear strength to the mobilized shear stress.Secondly,it maintains the progressive development of shear stress resulting from the increase in the principal stress deviator on the same plane,on which the shear strength takes place.Thirdly,by introducing the concept of equivalent stress loading,the resulting trial stresses are checked against the violation of the actual yield criterion formed with the real strength parameters rather than those reduced by a trial factor.The new numerical procedure was encoded in a Fortran computer code called S^(4)DINA and verified by several examples.Comparisons with other numerical methods such as the SRM,gravity increasing method(GIM)or even FELA by assessing both the FOS and contours of equivalent plastic strains showed promising results.

Multi-factor sensitivity analysis on the stability of submarine hydrate-bearing slope

There are many factors affecting the instability of the submarine hydrate-bearing slope (SHBS),and the interaction with hydrate is very complicated.In this paper,the mechanical mechanism of the static liquefaction and instability of submarine slope caused by the dissociation of natural gas hydrate (NGH) resulting in the rapid increase of pore pressure of gas hydrate-bearing sediments (GHBS) and the decrease of effective stress are analyzed based on the time series and type of SHBS.Then,taking the typical submarine slope in the northern South China Sea as an example,four important factors affecting the stability of SHBS are selected,such as the degree of hydrate dissociation,the depth of hydrate burial,the thickness of hydrate,and the depth of seawater.According to the principle of orthogonal method,25 orthogonal test schemes with 4 factors and 5 levels are designed and the safety factors of submarine slope stability of each scheme are calculated by using the strength reduction finite element method.By means of the orthogonal design range analysis and the variance analysis,sensitivity of influential factors on stability of SHBS are obtained.The results show that the degree of hydrate dissociation is the most sensitive,followed by hydrate burial depth,the thickness of hydrate and the depth of seawater.Finally,the concept of gas hydrate critical burial depth is put forward according to the influence law of gas hydrate burial depth,and the numerical simulation for specific submarine slope is carried out,which indicates the existence of critical burial depth.

Stability analysis of submarine slopes in the area of the test production of gas hydrate in the South China Sea

In this paper, the mechanical properties of gas hydrate-bearing sediments (GHBS) were summarized and the instability mechanism of submarine hydrate-bearing slope (SHBS) was analyzed under the background of the test production of gas hydrate in the northern part of the South China Sea. The strength reduction finite element method (SRFEM) was introduced to the stability analysis of submarine slopes for the safety of the test production. Two schemes were designed to determine the physical and mechanical parameters of four target wells. Through the division of the hydrate dissociation region and the design of four working conditions, the range and degree of hydrate dissociation at different stages during the test production were simulated. Based on the software ABAQUS, 37 FEM models of SHBS were set up to analyze and assess the stability of the submarine slopes in the area of the test production. Necessary information such as safety factors, deformation, and displacement were obtained at different stages and under different working conditions. According to the calculation results, the submarine slope area is stable before the test production, and the safety factors almost remains the same during and after the test production. All these indicate that the test production has no obvious influence on the area of the test production and the submarine slopes in the area are stable during and after the test production.

基于残余位移增量判据的土石围堰边坡稳定性分析

针对有限元强度折减法在复杂水文地质环境下土石围堰边坡稳定性评价中效果欠佳的问题,引入残余位移增量判据,基于有限元强度折减法,考虑深厚覆盖层土石围堰不同水位下的渗流特征,通过计算坡面节点平均残余位移增量,获取边坡稳定系数,进一步通过位移增量分布图,探讨了围堰边坡潜在滑面位置及其形成机制。结果表明,在复杂水文地质环境下的土石围堰边坡稳定性评价中,残余位移增量判据相较经典失稳判据适用性更强。研究结果能够为工程实践提供指导。

基于强度折减法的河漫滩软土浅埋偏压连拱隧道洞口边坡稳定性研究

以南京市乌龙山大跨浅埋偏压连拱隧道为背景,基于有限元强度折减法分析了隧道施工过程中洞口边坡的安全系数和塑性区分布特征,探明了不同坡角和隧道埋深下边坡稳定性情况与破坏模式发展规律,并提出相应施工建议。结果表明:边坡角度越大、隧道埋深越小时,隧道开挖过程中上覆边坡的安全系数越低。及时施作隧道衬砌能大幅度提高坡体稳定性,但当隧洞顶部埋深超过洞高的1.5倍后,衬砌加固效果相对有限;当边坡角度为60°时,坡体潜在滑动面将沿隧道浅埋侧拱顶附近产生,当隧道洞顶埋深达1.5倍洞高时,边坡破坏面不再经过隧道,而是分布于拱顶上方;针对隧道洞口段安全性较差问题,建议根据不同隧道埋深和边坡角度采用不同范围的坡表注浆加固方案。当隧道埋深>1.5倍洞高时,无须坡表注浆;当隧道埋深<1.5倍洞高且边坡角度<60°时,应在隧道浅埋侧施工前对坡脚至中隔墙区域进行注浆,且加固深度不应小于隧道拱腰埋深;当边坡角度达60°时,深埋和浅埋侧隧道施工前均需进行坡表注浆。

基于强度折减黄土路基高边坡稳定性分析与设计优化

为开展黄土路基高边坡的治理工作,保障交通安全,文章基于有限元强度折减法,开展边坡稳定性分析研究。首先,基于数值模拟,构建路基高边坡有限元模型,标定力学参数。其次,利用有限元强度折减法,从等效塑性应变贯通、收敛计算、边坡位移突变等方面开展边坡稳定性计算分析。最后,依托荣乌高速黄土路基高边坡工程治理实例,开展路基边坡合理断面和边坡防护研究。结果表明:采用简化Bishop计算的安全系数相对偏于安全,提出的稳定性计算方法更加准确且符合实际。

Quasi-Monte Carlo Simulation-Based SFEM for Slope Reliability Analysis

Considering the stochastic spatial variation of geotechnical parameters over the slope, a Stochastic Finite Element Method (SFEM) is established based on the combination of the Shear Strength Reduction (SSR) concept and quasi-Monte Carlo simulation. The shear strength reduction FEM is superior to the slice method based on the limit equilibrium theory in many ways, so it will be more powerful to assess the reliability of global slope stability when combined with probability theory. To illustrate the performance of the proposed method, it is applied to an example of simple slope. The results of simulation show that the proposed method is effective to perform the reliability analysis of global slope stability without presupposing a potential slip surface.

基于有限元强度折减法加固边坡稳定性分析

本文围绕某街道的加固边坡在实地考察结果的基础上,采用重力式挡墙的加固方式进行加固,利用有限元软件MIDAS/GTS NX建立三维模型,根据强度折减法理论对其进行稳定性分析,检验支护方案对边坡的滑面强度储备是否满足要求,利用其结果对加固支护方案进行深入研究.结果表明,采用重力式挡墙方案进行加固后,边坡安全系数由1.15提高到1.41,边坡的稳定性有了大幅度提高,边坡加固挡墙后改变了坡脚的应力应变状态,能够有效防止坡脚的局部破坏,提升边坡的稳定性.

基于强度折减法的边坡三维有限元稳定分析

为研究地震工况下边坡的稳定响应,基于强度折减法,利用三维有限元模型对边坡进行静力分析和动力时程分析,对比不同工况条件下边坡失稳特征和边坡稳定安全系数。结果表明:高达98.6 m的水库软岩边坡开挖,通过喷砼、系统锚杆和局部预应力锚索等支护后,受地震工况影响,边坡整体位移量和特征断面位移量增加显著,最大位移增加量达到7.6倍;边坡塑性区深度增加明显,最大塑性区深度增加了1.5倍;坡体主应力值的变化量较小,且均为受压状态;在静力、动力工况条件下,边坡总体稳定,边坡安全系数降低约25%;静力、动力工况所对应有限元强度折减法安全系数均满足规范要求。

基于极限平衡理论与有限元强度折减法的某涉水高边坡稳定性分析

该文以深圳某大型涉水高边坡为研究对象,将极限平衡法和有限元强度折减法原理相结合,从边坡稳定性角度出发,应用理正及有限元软件分别对开挖边坡整体抗滑稳定性和局部面层塑性破坏进行计算分析。研究结果表明,原始边坡安全系数大于规范允许值,边坡防护结果符合稳定性要求,但边坡浅层面在地应力卸荷作用下仍会产生破坏问题,因此,采用锚杆框格梁支护结构来减少边坡面层卸荷崩解破坏对工程安全造成的影响,支护后边坡稳定系数为1.47,满足规范允许值要求。利用圆弧滑动极限平衡法对开挖边坡进行多断面整体稳定分析的同时,采用有限元软件对边坡典型断面的护坡结构进行设计,能够更为全面、准确、快速地分析大型边坡工程的整体抗滑稳定性,并满足局部稳定安全要求,可为类似工程提供借鉴。

基于极限平衡法和有限元法的边坡稳定性综合分析

为了研究软岩边坡弱层影响下的边坡稳定性,建立含有弱层的边坡模型,采用极限平衡法与有限元法模拟受弱层影响下的边坡稳定性,探讨多种评判指标,综合描述边坡稳定状态。结果表明,通过有限元法计算,保留弱层时,边坡稳定性系数小于1,边坡模型被剪切应变带贯通,在弱层中底部剪出,上部呈现滑移趋势,边坡破坏模式为滑移剪切式;清除部分弱层后,边坡稳定性系数大于1.3,剪切应变带未扩展贯通整个边坡模型;采用极限平衡法计算边坡稳定性,滑弧基本与有限元法的剪切应变带一致,从而验证计算模型的准确性。

基于ABAQUS的省道某公路边坡稳定性及参数敏感性分析

文章以某省道公路为例,将有限元方法和强度折减方法相结合,首先研究了降水强度对边坡安全系数的影响,之后分析了不同坡高、坡角、内摩擦角及黏聚力下边坡安全系数的变化情况。研究表明:降水强度越大,安全系数下降越明显;边坡安全系数随高度的减小而升高,随内摩擦角的减小而降低;在相同的内摩擦角下,安全系数随坡角的减小而逐渐升高。

有限元强度折减法边坡失稳判据的统一性

目前有限元强度折减法边坡稳定性分析中广泛使用的边坡失稳判据主要有3类,即有限元数值迭代不收敛判据、特征部位位移突变判据、广义塑性应变或等效塑性应变贯通判据。对于上述3类判据的选用,一直以来存在较大的分歧。而从本质上看,3类边坡失稳判据具有一致性和统一性。之所以使用不同判据时计算结果表现出差异,是由于人为误判和有限元数值计算误差造成的。在具有足够计算精度的情况下,3类判据所得结果一致。

基于Drucker-Prager准则的边坡安全系数定义及其转换

探讨了基于D-P(Drucker-Prager)准则的边坡稳定安全系数定义形式,提出了各D-P准则之间的安全系数转换关系,并据此建立了基于D-P准则的边坡稳定安全系数与传统莫尔–库仑准则条件下安全系数的关系表达式。目前,ANSYS有限元软件采用的岩土材料屈服准则为莫尔–库仑六边形外接圆D-P准则,在利用有限元强度折减法计算边坡稳定安全系数时,可以先求出外接圆D-P准则条件下的安全系数,然后利用所提出的安全系数转换公式就可直接计算出各D-P准则条件下的安全系数。对于平面应变条件下的强度问题,平面应变莫尔–库仑匹配D-P准则(分关联和非关联两种情况)与莫尔–库仑准则等效,因此,通过转换就可以在ANSYS程序中实现莫尔–库仑准则,而不需要进行二次开发。这样就解决了基于D-P准则的有限元强度折减安全系数与传统工程中采用的安全系数(基于莫尔–库仑准则)间的接轨问题。大量算例结果表明:在平面应变条件下采用平面应变莫尔–库仑匹配D-P准则求得的安全系数与传统极限平衡条分法中用Spencer法求得的安全系数非常接近,且误差在1%～2%,已经具有相当高的计算精度,也同时证明所提出的方法是可行的。

三类边坡失稳判据的统一性

将热力学理论中材料破坏的能量机制引入边坡工程中,推导了强度折减过程中边坡的能量方程,利用FLAC^(3D)二次开发了能量的计算程序,并应用于某经典边坡算例中。通过与Spencer法的计算结果对比发现:强度折减法中边坡的稳定性与能量变化关系紧密,由此提出了4种新的失稳判据——能量突变失稳判据,且4种能量突变失稳判据理论上是统一的。通过对比发现:动能突变判据与数值计算收敛性判据、重力势能降突变判据与特征点垂直位移突变判据、耗散能增量突变判据与塑性区贯通判据分别存在对应关系,证明了常见的3类失稳判据也是统一的。通过算例试算发现,3类判据判定结果不相等主要是由收敛标准和网格精度等人为因素造成的,本质上是因为数值计算是一种近似解。工程应用中,可通过3类失稳判据判定结果的一致性来自评安全系数的计算精度,一致性越好,精度越高。提高计算精度的根本办法是严格收敛标准和精细网格划分,但应同时兼顾数值计算的时长,从而实现计算效率最大化。

强度折减有限元法中边坡失稳判据的研究

为了研究强度折减有限元法中各种边坡失稳判据的适用性,在典型算例分析计算的基础上,提出了"失稳判据的选择应和所采用的数值计算的迭代方法、应变模式假定等问题相匹配"的观点.研究结果表明,当采用数值计算不收敛判据时,应采用小应变模式假定,并利用弧长控制方法;当采用坡面位移突变判据时,应采用大应变模式假定;塑性应变区贯通判据可作为上述两种判据的参考.大量算例的计算结果表明,强度折减有限元法计算获得的边坡稳定性系数和极限平衡法计算结果的相对差小于3%.

考虑抗剪强度参数空间变异性的边坡稳定性分析

提出基于商业软件ABAQUS的边坡稳定分析的随机有限元法。采用Karhunen-Loeve级数展开方法建立土体抗剪强度参数的随机场模型,在强度折减有限元法的基础上模拟边坡失稳过程,并采用塑性屈服区贯通以及计算不收敛判据作为边坡失稳判据,利用python脚本文件分析最危险滑面分布规律。运用该方法研究土体内摩擦角和黏聚力的空间变异性对边坡稳定性的影响。研究结果表明:所提的边坡稳定分析的随机有限元法操作简单易于实现,能够从安全系数以及最危险滑面2个方面分析土体抗剪强度参数的空间变异性对边坡稳定性的影响;内摩擦角随机场中相关长度的改变对边坡的稳定性影响不明显;而黏聚力的空间变异性对边坡稳定性影响则相对更大,黏聚力随机场中水平与竖直相关长度的比值不同,边坡稳定性的变化规律不同。此外,内摩擦角与黏聚力的相关系数越大,失效概率越高,最危险滑面分布越广。

基于能量突变的强度折减法边坡失稳判据

为了建立有限元强度折减法(SSR-FEM)中边坡失稳的能量突变判据,基于能量原理,推导强度折减过程中边坡能量反应的计算理论,二次开发折减过程中能量反应的FLAC3D计算程序。获得二维边坡能量反应随折减系数的变化规律,与常见3类失稳判据判定的安全系数对比。并利用三维边坡算例进一步验证能量突变判据的适用性。研究结果表明:当边坡达到临界失稳状态时,重力势能降、动能增量和耗散能增量均突然增大,而弹性应变能增量突然减小,表明能量突变可作为一类边坡失稳判据。能量突变判据具有物理意义明确、整体性强和易于判断等优点,尤其适用于三维边坡。

非饱和土边坡稳定分析方法探讨

在对非饱和土边坡进行稳定分析时,应该全面地考虑基质吸力对边坡稳定的贡献。首先,分别探讨了基于Fredlund非饱和土强度表达式和Bishop非饱和土有效应力强度公式将强度折减有限元法推广到非饱和土边坡稳定分析中的具体方法;然后,开发了可以考虑基质吸力两种处理方法的强度折减有限元计算程序;最后,给出了一个非饱和土边坡稳定分析的对比算例,说明了二者的不同特点。