On the evaluation of internal stability of gap-graded soil: a status quo review

Internal instability is a phenomenon of fine particle redistribution in granular materials under the seepage action and consequent change in the soil’s internal structure and hydraulic and mechanical properties. It is one of the primary causes of failures of sand-gravel foundations and embankment dams. The criteria establishment is considered the key to solving the erosion problems, so the existing internal stability criteria need a review and classification to study the recent development trends in soil seepage and erosion. Therefore, this paper aims at reviewing the internal stability factors of gap-graded soil with a focus on the internal erosion mechanism and internal stability evaluation based on geometric and hydraulic criteria. Firstly, the paper compared the effect of several commonly used geometric criteria for gap-graded soil evaluation, such as particle size, fine content, void ratio, and fractal dimension. Furthermore, it provided a hydraulic criteria overview and analyzed the effects of the hydraulic gradient, hydraulic shear stress, confining pressure, and pore velocity on internal erosion. The geometric–hydraulic coupling methods were introduced, with a detailed elaboration of the erosion resistance index method based on accumulated dissipated energy. The capabilities and limitations of these criteria were discussed throughout the paper. It was found that combined Kezdi’s criterion and Kenney and Lau’s criterion is more reliable to evaluate internal stability of soil. The gap-graded soil with fine particle content higher than 35% is not necessarily internally stable. Finally, the energy-based method (erosion resistance index method) can effectively reproduce the total amount of erosion mass and the final spatial distribution of fine particles and identifies erosion. The review's outcome can be used as a basis to evaluate the internal erosion risk for gap-graded soils. The evaluation methods discussed here can help identify the zones of relatively high erosion potential.

     

Reliability assessment of granular filters in embankment dams

Empirical criteria have been used successfully to design filters of most embankment large dam projects throughout the world. However, these empirical rules are only applicable to a particular range of soils tested in laboratory and do not take into account the variability of the base material and filter particle sizes. In addition, it is widely accepted that the safety of fill dams is mainly dependent on the reliability of their filter performance. The work herein presented consists in a new general method for assessing the probability of fulfilling any empirical filter design criteria accounting for base and filter heterogeneity by means of first‐order reliability methods (FORM), so that reliability indexes and probabilities of fulfilling any particular criteria are obtained. This method will allow engineers to estimate the safety of existing filters in terms of probability of fulfilling their design criteria and might also be used as a decision tool on sampling needs and material size tolerances during construction. In addition, sensitivity analysis makes possible to analyse how reliabilities are influenced by different sources of input data. Finally, in case of a portfolio risk assessment, this method will allow engineers to compare the safety of several existing dams in order to prioritize safety investments and it is expected to be a very useful tool to evaluate probabilities of failure due to internal erosion. Copyright © 2006 John Wiley & Sons, Ltd.     

Discrete numerical modeling of particle transport in granular filters

Granular filters are an essential component in earth dams to protect the dam core from seepage erosion. This paper uses the particle flow method (PFM) to study the mechanism of particle transport in a base soil–filter system. The distributions of the eroded base-soil particles in different filters are traced and analyzed. The eroded mass and intruding depth of the eroded particles into the filters are obtained under different times and hydraulic gradients. The simulation results show that the eroded mass and intruding depth of the base-soil particles into the filter are related to the representative particle size ratio of the base soil to the filter, hydraulic gradient and erosion time. The numerical predictions are also compared with the empirical filter design criterion. The results show that the particle flow model provides an effective approach for studying the filtration micro-property and the erosion mechanism in a base soil–filter system, which is useful for filter design.     

Quantitative analysis of piping erosion micro-mechanisms with coupled CFD and DEM method

Piping, as one of the critical patterns of internal erosion, has been reported as a major cause for failures of embankment dams and levees. The fundamental mechanism of piping was traditionally investigated through experimental trials and simplified theoretical methods in macroscale. Nevertheless, the initiation and progressive evolution of piping is a microscale phenomenon in its essence. The current understanding of the micro-mechanism of piping erosion is limited due to a lack of quantitative analysis and visualized evidence. And in fact, seepage flows can affect the soil fabrics and the development of contact forces between particles. But how these fabrics and contact forces evolve under a critical hydraulic gradient is still not fully understood. In this paper, the detailed process of piping erosion is investigated by using a coupled computational fluid dynamics and discrete element method (CFD–DEM) approach. The treatment of soil–flow interactions in CFD–DEM is explained by exchanging the momentum between the two phases. During the simulation, the piping erosion process is initiated by incrementally ascending differential water head across the soil samples. The three main stages of piping erosion (initial movement, continuation of erosion and total heave) can be identified from monitoring the particle velocity and positions. In addition, the evolution of contact force, hydraulic force, coordination number and void fraction is inspected to provide insight into the micro-mechanism of piping erosion. Two cases are simulated, one with a uniform particle size and a relatively uniform porosity distribution and the other with specific particle size and porosity distributions. An interesting finding from this study is that piping does not always initiate from the free surface and the evolution of piping depends heavily on the particle size and porosity distribution.     

Numerical modelling of fluid-induced soil erosion in granular filters using a coupled bonded particle lattice Boltzmann method

This paper presents a three-dimensional coupled bonded particle and lattice Boltzmann method (BPLBM) with an immersed moving boundary scheme for the fluid-solid interaction. It is then applied to investigate the erosion process of soil particles in granular filters placed within earth dams. The microscopic migration of soil particles can be clearly visualised as the movement of particles can be directly recorded. Three granular filters with different representative size ratios are simulated and the numerical results are seen to match the empirical criteria. In addition, the effect of the representative size ratio of granular filters, hydraulic loading and erosion time are discussed.     

A reliability based approach for evaluating the slope stability of embankment dams

Embankment dams are important and costly civil engineering structures that provide an essential infrastructure for the management of water. One of the critical aspects of dam design is the analysis of stability and safety of the earth structure under various operating and environmental conditions. Traditionally, a deterministic approach is used for such analysis. However, the determination of variables such as soil strength parameters, pore pressure and other pertinent properties involves uncertainties, which cannot be handled in the traditional deterministic methods. It is, therefore, highly desirable to develop a reliability based analytical/numerical methodology for stability analysis of dams taking into account these uncertainties. Reliability and probability theories are developed in this paper for assessing the reliability index and the corresponding probability of failure of multi-layered embankment dams and slopes. Two definitions were used to calculate the reliability index (i.e. the normal distribution and the log–normal distribution). The computer program was developed and validated by the Congress Street open cut failure case. The developed approach was used to study the stability of the King Talal embankment dam. The results are discussed and conclusions drawn.     

Experiments on internal erosion in sandy gravel foundations containing a suspended cutoff wall under complex stress states

Internal erosion is one of the most common failure modes of embankment dams or foundations, and the simplest and most effective preventive measure is to build a cutoff wall. The soil at the bottom of the cutoff wall is usually under complex stress states. The deeper the cutoff wall, the higher is the stress. In this study, the effects of stress conditions on the evolution of internal erosion were investigated in sandy gravel foundations containing a suspended cutoff wall using a newly developed stress-controlled erosion apparatus. Three series of erosion tests were conducted on gap-graded soil under different confining stresses, different deviatoric stresses, and different confining and deviatoric stresses. The results of these tests are as follows: (1) The discharge and permeability decrease with an increase in the confining stress, but the critical hydraulic gradient increases. (2) In the second series of erosion tests, the specimen is compressed under low deviatoric stress; the specimen undergoes shear expansion under high deviatoric stress. (3) In the third series of erosion tests, the confining and deviatoric stresses synchronously change, and therefore, their combined effect on the evolution of internal erosion is complicated. Under low stress, the soil is compressed in the early stage of the experiment, and its structure may change during internal erosion. When the stress level is high, the specimen also undergoes shear expansion, and the degree of expansion is controlled by both confining and deviatoric stresses.     

Experimental investigation of microstructural changes in soils eroded by suffusion using X-ray tomography

Internal erosion is a complex phenomenon which represents one of the main risks to the safety of earthen hydraulic structures such as embankment dams, dikes or levees. Its occurrence may cause instability and failure of these structures with consequences that can be dramatic. The specific mode of erosion by suffusion is the one characterized by seepage flow-induced erosion, and the subsequent migration of the finest soil particles through the surrounding soil matrix mostly constituted of large grains. Such a phenomenon can lead to a modification of the initial microstructure and, hence, to a change in the physical, hydraulic and mechanical properties of the soil. A direct comparison of the mechanical behaviour of soil before and after erosion is often used to investigate the impact of internal erosion on soil strength (shear strength at peak and critical state) using triaxial tests. However, the obtained results are somehow contradictory, as for instance in Chang’s study (Chang and Zhang in Geotech Test J 34(6):579–589, 2011), where it is concluded that the drained strength of eroded soil decreases compared to non-eroded soil, while both Xiao and Shwiyhat (Geotech Test J 35(6):890–900, 2012) and Ke and Takahashi (Geotech Test J 37(2):347–364, 2014) have come to the opposite conclusion. A plausible explanation of these contradictions might be attributed to the rather heterogeneous nature of the suffusion process and to the way the coarse and fine grains are rearranged afterwards leading to a heterogeneous soil structure, a point that, for now, is not taken into account, nor even mentioned, in the existing analyses. In the present study, X-ray computed tomography (X-ray CT) is used to follow the microstructure evolution of a granular soil during a suffusion test, and, therefore, to capture the induced microstructural changes. The images obtained from X-ray CT reveal indeed that fine particles erosion is obviously not homogeneous, highlighting the existence of preferential flow paths that lead to a heterogeneous sample in terms of fine particles, void ratio and inter-granular void ratio distribution.

     

Evaluation of Combined Base Soil Erodibility and Granular Filter Efficiency

Filters managed in zoned dams are designed according to criteria based on the grain size distribution of both filter and eroded soil. However, the constriction size distribution of the filter is the key parameter which governs the filter retention process of flowing eroded particles. To assess the filter efficiency regarding eroded particles, several filters and base soils are tested in a vertical cell with a configuration coupling erosion and filtration processes. For setting the boundary condition of eroded particles at the filter inlet, hole erosion test (HET) was performed on the base soil. The investigation of the evolution of filter behavior shows that the void ratio and the grain shape are of a great influence on filter efficiency. A new approach of filter clogging was proposed by evaluating a damage index which is affected by various parameters such as the ratio D₁₅/d₈₅ and the size of eroded particles. An approach linking the geometrical parameters (damage index) to the hydraulic conductivity leads to an estimation of the filter performance which provides a more quantifiable and realistic criterion. The results indicate that even existing criteria were not met; the tested filters remain efficient as regards to experimental data. An analytical approach based on constrictions size distribution was used and pore reduction was matched with experimental results.

     

Influence of relative density of the granular base soil on filter performance

Perpendicular contact erosion due to poorly designed filters is a frequent hazard for water-retaining structures serving as lifeblood to the community. This phenomenon occurs when the fine particles of a base soil at the contact interface with a coarser material are detached and transported through pores formed by the coarse particles. Therefore, most filter design criteria focus on the gradation of coarse particles or the gradation of pore constrictions. Meanwhile, the parameters of the base soil, such as relative density, are often overlooked. On the one hand, some experts neglect the impact of relative density because perpendicular contact erosion occurs at the interface, where fine particles expose themselves to larger pores. On the other hand, it is a general belief that the more compacted a base soil is, the less susceptible it will be to erosion as the seepage is reduced. This paper discusses this dilemma from a mutual perspective which assesses the influence of relative density from experimental, numerical, and analytical standpoints. The experimental study reveals that there is an optimal relative density which will release the least eroded mass. The influence is crucial as it can change the status of stability to unstable. The physical essence of the phenomenon is expressed by a numerical study at the micro-scale, which investigates the redistribution of flow lines and stress resulting from a particle detachment. The discovery at the micro-scale is confirmed by an analytical evaluation at the macro-scale, which assesses the redistribution of pore constrictions.

     

Internal erosion in dike-on-foundation modeled by a coupled hydromechanical approach

One of the major causes of instability in geotechnical structures such as dikes or earth dams is internal erosion, an insidious process that occurs over a long period of time. Research on this topic is still fairly new and much more needs to be understood in order to solve the problems posed by this phenomenon. This paper proposes a hydromechanical model based on porous continuous medium theory to assess how internal erosion impacts the safety of earthen structures. The saturated soil is considered as a mixture of four interacting constituents: soil skeleton, erodible fines, fluidized fine particles, and fluid. The detachment and transport of the fine particles are described by a mass exchange model between the solid and the fluid phases. An elastoplastic constitutive model for sand-silt mixtures has been developed to monitor the effect of the evolution of both porosity and fines content induced by internal erosion upon the behavior of the soil skeleton. The model has been numerically solved with the finite element method. It has then been applied to the specific case study of a dike foundation subjected to internal erosion induced by the presence of a karstic cavity beneath the alluvium layer. The numerical results show the onset of erosion, the time-space evolution of the eroded zone, and the hydromechanical response of the soil constituting the dike, all of which highlights the effects of the cavity location, the erosion rate, and the fines content.     

高速公路沉降计算中e-lg p法修正探讨

利用已有的高速公路土工试验资料，通过大量的非线性有限元计算得出一个与路堤填筑高度及路基土体压缩系数相关的沉降综合修正式，使得单向 e-lgp 法的计算结果修正后能够考虑土体侧向变形。实例验证表明：经此式修正后的沉降值误差较小，不但比规范法更合理，而且与实测值以及有限元法结果很接近，具有一定的工程参考价值和借鉴意义。     

Numerical modelling of internal erosion during hydrate dissociation based on multiphase mixture theory

It has been reported that sand production, which is a simultaneous production of soil particles along with gas and water into a production well, forced to terminate the operation during the world's first offshore methane production test from hydrate-bearing sediments in the Eastern Nankai Tough. The sand production is induced by internal erosion, which is the detachment and migration of soil particles from soil skeleton due to seepage flow. The inflow of the eroded soil particles into the production well leads to damage of the production devices. In the present study, a numerical model to predict the chemo-thermo-mechanically coupled behavior including internal erosion during hydrate dissociation has been formulated based on the multiphase mixture theory. In the proposed model, the internal erosion is expressed as mass transition of soil particles from soil skeleton to the fluidized soil particles. Since the internal erosion is considered to depend on the soil particle size, mass of soil particles are divided into several groups that have different representative particle diameters, and the constitutive equations for the onset condition and the mass transition rate of the internal erosion are formulated for each group. Also, transportation of soil particles in the liquid phase is formulated for each particle size group in the proposed model. Finally, a simulation of the methane gas production from the hydrate-bearing sediment by depressurization method is presented, and the internal erosion and the dissociation behavior are discussed.     

Development of an empirical model for predicting peak breach flow of landslide dams considering material composition

The existing empirical models do not consider the influence of material composition of landslide deposits on the peak breach flow due to the uncertainty in the material composition and the randomness of its distribution. In this study, based on the statistical analyses and case comparison, the factors influencing the peak breach flow were comprehensively investigated. The highlight is the material composition-based classification of landslide deposits of 86 landslide cases with detailed grain-size distribution information. In order to consider the geometric morphology of landslide dams and the potential energy of dammed lakes, as well as the material composition of landslide deposits in an empirical model, a multiple regression method was applied on a database, which comprises of 44 documented landslide dam breach cases. A new empirical model for predicting the peak breach flow of landslide dams was developed. Furthermore, for the same 44 documented landslide dam failures, the predicted peak breach flow obtained by using the existing empirical models for embankment and landslide dams and that obtained by using the newly developed model were compared. The comparison of the root mean square error (E_rms) and the multiple coefficient of determination (R²) for each empirical model verifies the accuracy and rationality of the new empirical model. Furthermore, for fair validation, several landslide dam breach cases that occurred in recent years in China and have reliable measured data were also used in another comparison. The results show that the new empirical model can reasonably predict the peak breach flow, and exhibits the best performance among all the existing empirical models for embankment and landslide dam breaching.

     

The Effect of Impervious Clay Core Shape on the Stability of Embankment Dams

One of the most important dangers that treat earth dams which can lead to interior failure over a prolonged period is the hydraulic fracturing factor. In the case of zoned dams, due to differences in stiffness of the core and its abutment zone, differential settlements occur between them. This factor is responsible for the arching phenomenon. Differential settlements between core and shell cause cracks within the core initially sub-surface, Those cracks may develop the first impounding causing internal erosion on the dam core. In this research, using a computer modeling of Ghavoshan rockfill dam (located the west part of Iran) as a case study computed by SIGMA/W program, the role of the dam core shape on those factors is demonstrated. It is found that an inclined core shape is preferred in a condition that is especially important settlements of construction during for dam body. The result of finite element analysis indicates desired conditions from the point of view of stress, deformation and resistance against hydraulic fracturing for the same width of dam designs. Moreover, this can be higher priority for embankment dam designs.     

The scaling law in the hole erosion test with a constant pressure drop

A process called ‘piping’, which often occurs in the soil at dams, levees, and dykes, involves the formation and development of a continuous tunnel between upstream and downstream ends. The hole erosion test is commonly used to quantify the critical stress and the rate of piping erosion progression. The aim of this study is to draw up a model for interpreting the results of this test. A characteristic internal erosion time is defined and expressed as a function of the initial hydraulic gradient and the coefficient of surface erosion. It is established here that the product of the coefficient of erosion and the flow velocity is a significant dimensionless number: when this number is small, the kinetics of erosion are low, and the particle concentration does not have any effect on the flow. This finding applies to most of the available test results. Theoretical and experimental evidence is presented showing that the evolution of the pipe radius during erosion with a constant pressure drop obeys a scaling exponential law. Copyright © 2008 John Wiley & Sons, Ltd.     

海堤土工织物滤层的探讨

研究了波动水流对海堤织物滤层的水力要素及过滤机理。粒径小于0.6 mm颗粒间存在粘着力;同时,由于滤层和堤体的渗透流速不一致,则堤体界面的土颗粒将受水流剪应力的作用。在分析了使土颗粒发生变形的水流剪应力、变形土体周壁的屈服应力和波浪压力等后,根据平衡原理,建立了织物滤层的保土性准则。     

A simple approach to evaluating leakage through thin impervious element of high embankment dams

Internal erosion is the most common reason which induces failure of embankment dams besides overtopping. Relatively large leakage is frequently concentrated at defects of impervious element, and this will lead to eventual failure. The amount of leakage depends not only on integrity of impervious element, but also on dam height, shape of valley, shape of impervious element and water level in reservoir. The integrity of impervious element, which represents the relative level of seepage safety, is not easy to be determined quantitatively. A simple method for generalization of steady seepage state of embankment dams with thin impervious element is proposed in this paper. The apparent overall value of permeability coefficient for impervious element can be obtained by this method with reasonable accuracy and efficiency. A defect parameter of impervious element is defined as an index to characterize seepage safety of embankment dams. It equals the ratio of the apparent overall value of permeability coefficient to the measured value in laboratory for intact materials. Subsequently, seepage safety of three dams is evaluated and the evolution of defect level of impervious element of dams is investigated. It is proved that the newly proposed method in this paper is feasible in the evaluation of relative seepage safety level of embankment dams with thin impervious element.     

基于分形理论和模型试验的沟道物源动储量评价模型

汶川地震后,大量松散固体物源堆积在沟道中,使沟道泥石流发生的概率激增。准确的计算泥石流沟道物源的动储量一直是泥石流物源统计的难点。文章以七盘沟下游主沟段沟道物源为研究对象,在实地勘查、资料收集的基础上,以室内模型试验为研究手段,引入分形理论将复杂的土体粒度成分用分维值定量描述,研究不同沟道堆积体在不同降雨作用下的侵蚀规律,建立以降雨强度和分维度为双影响因子的动储量评价模型。研究表明:粗粒土不易起动,但在充足的水动力条件下,侵蚀作用会成倍放大;上细下粗土发生泥石流时侵蚀变化和总的侵蚀规模较小,这种粒序分布形式有益于沟道的稳定;上粗下细土与粗粒土的侵蚀现象类似,但发生大规模泥石流的降雨阈值低于粗粒土;沟道物源中,侵蚀作用效应的排序为:溯源侵蚀>下切侵蚀>侧缘侵蚀>潜蚀;文章所拟合的公式适用于宽缓型沟道泥石流,对于窄陡型沟道泥石流存在一定的局限性。     

A critical review on granular dam filter behaviour – from particle sizes to constriction-based design criteria

Over emphasis on tests results involving base soils with increasing coefficient of uniformity (C_u ) makes the reliability of an empirical particle-based criterion for effective filters questionable. Extra measures such as regrading of base soil are then necessary in order to capture the effectiveness of filter constriction sizes in trapping eroding fines. Recent studies which focused on the role of the sizes of filter voids on overall filtration behaviour have deployed constriction size approaches that were shown to be equally acceptable methods for distinguishing between effective and ineffective filters. This paper examines the evolution of this approach, starting from past empirical and mathematical investigations and proceeding to the eventual development of geometric-probabilistic methods with special reference to embankment dams. Models based on dominant and controlling constriction sizes are introduced, and subsequently the constriction-based retention criteria for granular filters are presented. The proposed retention criteria were verified based on extensive experimental data taken from small- and large-scale filtration tests carried out by various authors. The integration of filter compaction, porosity and C_u , together with the incorporation of analytical principles, provides alternative and rigorous design approaches that remove most limitations of the conventional particle-based criteria, thus making the models essentially more comprehensive and quantifiable.