A coupled hydro-mechanical model for subsurface erosion with analyses of soil piping and void formation

Acta Geotechnica - A coupled hydro-mechanical erosion model is presented that is used for studying soil piping and erosion void formation under practical, in-situ conditions. The continuum model...     

An auto-adaptive moving mesh method for the numerical simulation of piping erosion

A numerical approach for simulating piping erosion is proposed based on an auto-adaptive moving mesh. This approach adapts a flownet mesh (FM) to discretize the domain. It reduces the simulation of two-dimensional transport of moving particles to one-dimensional problem by taking advantages of flow tube concept. Furthermore, it employs a stepwise procedure to decouple the solid-liquid interaction equations for modeling piping erosion. Specifically, the FM is updated and evolved automatically in accordance to simulated erosion since the flownet is refined in high-velocity regions, where the piping erosion progresses. Therefore, the FM is auto-adaptive to the piping erosion paths.     

Simulations of hydro-fracking in rock mass at meso-scale using fully coupled DEM/CFD approach

The paper deals with two-dimensional (2D) numerical modelling of hydro-fracking (hydraulic fracturing) in rocks at the meso-scale. A numerical model was developed to characterize the properties of fluid-driven fractures in rocks by combining the discrete element method (DEM) with computational fluid dynamics (CFD). The mechanical behaviour of the rock matrix was simulated with DEM and the behaviour of the fracturing fluid flow in newly developed and pre-existing fractures with CFD. The changes in the void geometry in the rock matrix were taken into account. The initial 2D hydro-fracking simulation tests were carried out for a rock segment under biaxial compression with one injection slot in order to validate the numerical model. The qualitative effect of several parameters on the propagation of a hydraulic fracture was studied: initial porosity of the rock matrix, dynamic viscosity of the fracking fluid, rock strength and pre-existing fracture. The characteristic features of a fractured rock mass due to a high-pressure injection of fluid were realistically modelled by the proposed coupled approach.

     

Progression of backward erosion piping with sudden and gradual hydraulic loads

Acta Geotechnica - With the utilization of underground space, backward erosion piping (BEP) has been observed in many underground structures. Different from BEP in embankments or river dikes, the...     

A finite element method for localized erosion in porous media with applications to backward piping in levees

This paper presents a computational method able to effectively model both the simultaneous processes typically observed in backward erosion piping, ie, the pipe tip propagation and the conduit cross-section enlargement. The numerical method is based on the novel formulation of a problem of localized erosion along a line propagating in a multidimensional porous medium. In this line, a conduit with evolving transverse size is embedded, which conveys a multiphase flow. The two systems, porous medium and pipe, are bridged by exchange terms of multiphase fluid mass and by a shared fluid pressure field. On the contrary, different fields are considered to describe flows, which are assumed as Darcian in the porous medium and turbulent in the conduit. These two flows drive pipe propagation and enlargement, respectively, as modeled by means of proper erosion kinetic laws. The corresponding numerical formulation is based on the combination between one- and multidimensional finite elements, to model the erosion conduit and the porous medium, respectively. Several simulations are proposed to demonstrate the ability of the proposed approach in reproducing available experimental data of real-scale tests on levees. Our results point out the crucial role played by the combined influence of pipe propagation and enlargement, as well as of three-dimensional (3D) effects. We also assess the mesh independence of the proposed numerical solution, particularly as concerns the calculated pipe propagation history.     

基于多期DEM数据的滑坡变形定量分析

滑坡的定量变形分析是滑坡研究中的难点问题.为揭示黑方台灌区滑坡的变形演化过程,借助1977年、1997年、2001年和2010年4个时期测制的地形图资料,以ArcGIS为平台,建立了基于多期DEM数据的滑坡变形定量分析模型,并对黑方台塬边32处滑坡分阶段进行了变形量与变形速率计算.1977-1997年,滑坡后壁后移侵蚀速率平均为4.47m/a;1997-2001年,后移侵蚀率平均为3.46m/a;2001-2010年,后移侵蚀率平均为1.10m/a.同时建立了灌溉量与滑坡变形量的相关关系式,并对滑坡的变形演化趋势进行了预测,到2015年,滑坡后壁后移距离平均为0.79m,到2020年,滑坡后壁后移距离可减少到0.20m.     

Temporal evolution of backward erosion piping in small-scale experiments

Backward erosion piping (BEP) is a form of internal erosion which can lead to failure of levees and dams. Most research focused on the critical head difference at which piping failure occurs. Two aspects have received less attention, namely (1) the temporal evolution of piping and (2) the local hydraulic conditions in the pipe and at the pipe tip. We present small-scale experiments with local pressure measurements in the pipe during equilibrium and pipe progression for different sands and degrees of hydraulic loading. The experiments confirm a positive relation between progression rate and grain size as well as the degree of hydraulic overloading. Furthermore, the analysis of local hydraulic conditions shows that the rate of BEP progression can be better explained by the bed shear stress and sediment transport in the pipe than by the seepage velocity at the pipe tip. The experiments show how different processes contribute to the piping process and these insights provide a first empirical basis for modeling pipe development using coupled seepage-sediment transport equations.

     

Modeling of fluid–solid interaction in granular media with coupled lattice Boltzmann/discrete element methods: application to piping erosion

In this article, we present a numerical method to deal with fluid–solid interactions and simulate particle–fluid systems as encountered in soils. This method is based on a coupling between two methods, now widely used in mechanics of granular media and fluid dynamics respectively: the discrete element (DE) method and the lattice Boltzmann (LB) method. The DE method is employed to model interactions between particles, whereas the LB method is used to describe an interstitial Newtonian fluid flow. The coupling presented here is a full one in the sense that particle motions act on fluid flow and reciprocally. This article presents in details each of the two methods and the principle of the coupling scheme. Determination of hydrodynamic forces and torques is also detailed, and the treatment of boundaries is explained. The coupled method is finally illustrated on a simple example of piping erosion, which puts in evidence that the combined LB–DE scheme constitutes a promising tool to study coupled problems in geomechanics. Copyright © 2011 John Wiley & Sons, Ltd.     

三相耦合渗流侵蚀管涌机制研究及有限元模拟

管涌的发生、发展过程是土骨架相在渗流作用下侵蚀为可动细颗粒相,并随水相在孔隙通道中运移流失的过程。在该过程中,渗流与侵蚀相互耦合,相互促进,水相、土相、可动细颗粒相互作用,因此,管涌过程是一个多场、多相耦合的高度非线性的动态过程。现有的管涌试验结果表明,只有当水力梯度大于起始水力梯度时,细颗粒相才会随水相从土体中运移流失,土体才会发生管涌侵蚀,且管涌稳定后土体的孔隙率（稳定孔隙率）和水力梯度之间存在对应关系,根据该结果,提出管涌稳定孔隙率的概念,修正传统的渗流侵蚀本构方程,建立多孔介质中三相耦合的修正的渗流侵蚀管涌控制方程。最后,针对特定应力状态下的土体建立稳定孔隙率和水力梯度之间的对应关系。基于Galerkin有限元法编制有限元程序,在轴对称情况下对该土体的管涌过程进行数值模拟。结果表明,修正后的管涌控制方程能更全面地描述管涌发生、发展直至稳定状态的特性。     

均质土坝管涌发展过程的渗流场空间性状研究

堤坝发生管涌破坏时,渗流场具有强烈的空间特性,管涌发展过程的数值模拟对于堤坝工程管涌预防与治理都具有重要的参考价值。提出以一维通道嵌入三维块体的方法,克服了在三维问题有限元模拟方法上存在的困难。在探讨均质土坝管涌发展过程模拟方法的基础上,使用数值方法分析了土坝管涌发展过程的渗流场时空分布特性;揭示了管涌发展过程中渗流力集中状况及其变化规律。研究表明,改善渗流场的形态是防治堤坝工程管涌的关键措施。     

Analysis of the pipe depth development in small-scale backward erosion piping experiments

Acta Geotechnica - Backward erosion piping is an important failure mechanism for water-retaining structures. It results in the formation of shallow pipes at the interface of a sandy or silty...     

定量恢复大别造山带侏罗-白垩纪的隆升和剥蚀

以造山带为物源区的周缘盆地中的沉积物记录着物源区的成分特征,保存着造山带的演化历史.根据地层资料,利用质量平衡方法得到大别造山带周缘的合肥盆地、江汉盆地、安庆-潜山盆地和信阳盆地在侏罗-白垩纪的沉积总量为1.685×105km3.大别造山带周缘盆地的平均沉积速率分别为早侏罗世0.407×106 m3/a,中晚侏罗世0....     

双强度折减法的量化分析

双强度折减法（DRM）采用黏聚力和内摩擦角的非等比例的折减策略,更加符合真实情况,但对于该方法本身的研究仍然缺乏定量的分析与合理的阐述;通过安全系数的定义、折减路径以及临界状态这几个概念,在强度参数坐标系中,对双折减法和传统强度折减法（SRM）进行量化分析与对比,两种方法具有统一性,表现为DRM相当于从虚拟初始强度点开始折减的SRM.边坡失稳的渐进过程本质上为强度衰减的过程,其中c、tan φ遵循正比例函数关系;同时,强度储备的意义反映为初始强度面积S向临界强度面积S'逐步地逼近的过程,从而得到安全系数表达式;强度折减法的研究应围绕强度来进行,对于强度参数的折减只是强度弱化的实现形式,通过安全系数定义的分析以及算例结果数据的回归分析,表明临界状态是边坡的固有属性,一旦边坡的几何尺寸给定,它将是一个与初始强度参数无关的量.SRM和DRM的结果的异同分析对比,实际上是对临界点的分析.     

基于局部单元劈裂的FEM/DEM自适应分析方法

为了模拟岩体中裂纹的萌生、扩展,Munjiza提出了有限元法/离散元法(FEM/DEM)耦合分析方法。因为裂纹是沿单元边界进行扩展的,亦即裂纹扩展具有网格依赖性,为获得较好的裂纹扩展形态,需要划分密集的初始网格。为解决上述难题,基于FEM/DEM耦合分析方法,提出了基于局部单元动态劈裂的FEM/DEM自适应分析方法,以克服裂纹扩展形态对网格的依赖性。该方法在最初建模时无需划分很密的初始网格,随着荷载的施加,对裂纹尖端附近的局部单元进行动态劈裂,为裂纹的后续扩展提供了更多可能的扩展方向,使得裂纹扩展不必沿着初始网格的单元边界扩展,即可以沿着单元内部进行扩展,裂纹扩展形态更为平滑,与实际情况更为接近。同时相对原FEM/DEM耦合分析方法一开始就划分很密的网格而言,新方法可以划分较为稀疏的初始网格,计算成本降低。最后,通过巴西劈裂算例与原FEM/DEM耦合分析方法对比,分析表明,新方法在一定程度上克服了裂纹扩展形态对初始网格的依赖性。     

深部岩石考虑残余强度时三轴试验离散元定量模拟及参数分析

深部岩石在工程中具有高应力、大变形等典型特点,因此,高围压下考虑岩石残余强度的三轴试验对于分析深部岩石力学特性具有重要意义。离散单元法是分析岩石力学特性的重要数值方法,但是长期以来采用离散单元法定量模拟岩石的三轴试验一直存在诸多挑战,即数值模拟与室内试验得到的应力-应变全过程曲线难以定量匹配。采用改进的三维胶结抗弯-扭模型对深部砂岩考虑残余强度时的三轴试验进行了定量模拟,实现了数值模拟与室内试验应力-应变全过程曲线的定量匹配,获得了岩石较大的峰值/残余内摩擦角及非线性强度包线,克服了经典BPM模型存在的3个突出问题。通过参数分析,研究了峰值/残余内摩擦角及黏聚力与离散元微观参数之间的关系,同时这些大量的算例也证明了该模型具有较高的计算效率,可以满足模拟三维室内常规试验的要求。     

Modelling of piping collapses and gully headcut landforms:Evaluating topographic variables from different types of DEM

The geomorphic studies are extremely dependent on the quality and spatial resolution of digital elevation model(DEM)data.The unique terrain characteristics of a particular landscape are derived from DEM,which are responsible for initiation and development of ephemeral gullies.As the topographic features of an area significantly influences on the erosive power of the water flow,it is an important task the extraction of terrain features from DEM to properly research gully erosion.Alongside,topography is highly correlated with other geo-environmental factors i.e.geology,climate,soil types,vegetation density and floristic composition,runoff generation,which ultimately influences on gully occurrences.Therefore,terrain morphometric attributes derived from DEM data are used in spatial prediction of gully erosion susceptibility(GES)mapping.In this study,remote sensing-Geographic information system(GIS)tech-niques coupled with machine learning(ML)methods has been used for GES mapping in the parts of Semnan province,Iran.Current research focuses on the comparison of predicted GES result by using three types of DEM i.e.Advanced Land Observation satellite(ALOS),ALOS World 3D-30 m(AW3D30)and Advanced Space borne Thermal Emission and Reflection Radiometer(ASTER)in different resolutions.For further progress of our research work,here we have used thirteen suitable geo-environmental gully erosion conditioning factors(GECFs)based on the multi-collinearity analysis.ML methods of conditional inference forests(Cforest),Cubist model and Elastic net model have been chosen for modelling GES accordingly.Variable's importance of GECFs was measured through sensitivity analysis and result show that elevation is the most important factor for occurrences of gullies in the three aforementioned ML methods(Cforest=21.4,Cubist=19.65 and Elastic net=17.08),followed by lithology and slope.Validation of the model's result was performed through area under curve(AUC)and other statistical indices.The validation result of AUC has shown that Cforest is the most appropriate model for predicting the GES assessment in three different DEMs(AUC value of Cforest in ALOS DEM is 0.994,AW3D30 DEM is 0.989 and ASTER DEM is 0.982)used in this study,followed by elastic net and cubist model.The output result of GES maps will be used by decision-makers for sustainable development of degraded land in this study area.     

A combined method to model grain crushing with DEM

This paper presents a combined method to model grain crushing effects with discrete element method.This method combines the two most commonly used concepts to model grain crushing in DEM,i.e.the replacement method and the agglomerate method,so that it is both accurate and efficient.The method can be easily implemented.The performance is shown by several DEM simulations of biaxial tests.Particles with different crush-abilities are modeled.DEM simulation results with and without grain crushing are compared and discussed.The change of grain size distribution due to grain crushing is also investigated.     

The effect of sudden critical and supercritical hydraulic loads on backward erosion piping: small-scale experiments

Acta Geotechnica - Backward erosion piping is an important failure mechanism for cohesive water-retaining structures which are founded on a sandy aquifer. This paper studies the effect of the...     

基于稳定性耦合分析法的余王扁边坡稳定性分析

边坡稳定性分析的极限平衡法与有限元法(FEM)的耦合分析法,首先利用有限元法分析获得边坡岩土体的整体应力场,在此基础上利用极限平衡法进行边坡的稳定系数求解。该方法既反映了边坡的稳定和变形之间的关系,又克服了极限平衡法与有限元法的不足,使二者的优点相互补充,获得的稳定系数基于极限平衡理论体系,可以同传统的稳定系数评价体系接轨。以西安市雁塔区余王扁削坡后边坡为例,用稳定性耦合分析法对其进行了稳定性分析,并把分析结果与各种极限平衡法的计算结果进行了比较,证明了耦合分析方法的可靠性和可行性。     

DEM investigations of internal erosion: Grain transport in the light of micromechanics

Internal erosion by suffusion can change dramatically the constitutive behavior of granular materials by modifying the fabric of granular materials. In this study, the effect of an internal fluid flow on granular materials is investigated at the material point scale using the numerical coupling between a discrete element method (DEM) and a pore-scale finite volume (PFV) coupling scheme. The influence of the stress state and the hydraulic loading (direction and intensity) on the occurrence of grain transport in dense widely graded granular samples is thus investigated and interpreted in terms of micromechanics. In particular, it is shown that grain transport is increased when the macroscopic flow direction is aligned with the privileged force chain orientation. The stress-induced microstructure modifications are shown to influence the transport distances by controlling the number of rattlers.