Mitigation of levee failures using deep mixed columns and geosynthetics

Stability of levees is critical to the safety of human and structures, especially at high water levels. Levees may fail due to the existence of soft soil foundations or seepage of water through the levees or rapid drawdown. Deep mixing technology has been considered one of the good alternatives to solve foundation and seepage problems while geosynthetics can be used to stabilize slopes during rapid drawdown. Studies have shown that deep mixed columns and geosynthetics can increase the stability of highway embankments over soft soils. In those studies, however, no ponding water exists on either side of the embankment, which is not the case for levees. Experimental studies have shown that deep mixed columns under a combination of vertical and horizontal force could fail due to shear or tension/bending or rotation. A finite difference method, incorporated in the FLAC (Fast Lagrangian Analysis of Continua) Slope software, and a limit equilibrium method (specifically Bishop's method), incorporated in the ReSSA software, were adopted in this study to investigate the stability of the levee with ponding water or under rapid drawdown. In this study, deep mixed columns were installed in continuous wall patterns, which were modeled as 2D deep mixed walls. Geosynthetic layers were modeled using cable elements with grout properties between geosynthetic and soil in the numerical analysis. Mohr-Coulomb failure criteria were used for the levee, the soft soil, and the deep mixed walls. The stability of a levee at different stages (end of construction, average service condition, high water surge, and rapid drawdown from the service condition and the highest water level condition) was examined. The study clearly demonstrated that the deep mixed walls can enhance the stability of the levee by providing shear/moment resistance and hindering seepage through the levee and geosynthetics can enhance the riverside slope stability of the levee by providing tensile resistance to the soil.     

Numerical simulation of the developing course of piping

Piping is the main cause of dangerous situations and one of the most serious threats to levees in China. It is unlikely that Darcy’s law is suitable for evaluation of piping zones; therefore, the ordinary seepage computation based method of simply increasing the permeability coefficient of the piping zone is not appropriate. The discrete element method (DEM) may be a promising alternative to the use of Darcy’s law, but the large number of particles hampers its effective application in practice. In this study, a new approach based on combination of pipe flow in the erosion channel and seepage analysis in the remainder of the levee is proposed. Based on the changeable boundary between the two types of computational domains, the element free Galerkin method (EFG) was employed to facilitate the efficiency of coupling iteration. Examples show that this approach can simulate the main characteristics of developing processes in the erosion channel, which implies that this simplified method can be employed practically.     

Foundation Stability of a Levee

Stability of a levee and its foundation soil during flood is governed by seepage forces. In this study, the seepage forces in the flow domain have been determined using conformal mapping technique. Stability of foundation soil has been studied for a levee with and without cut-off wall on permeable soil of infinite depth. In situ effective normal and shear stresses due to self-weight, additional induced stresses due to levee structure and ponded water on upstream sloping face have been determined by assuming soil to be elastic. Besides, components of seepage stress acting at a point on two orthogonal planes, i.e., on vertical and horizontal planes, are considered and the Mohr’s circle is drawn. Mohr–Coulomb failure envelope is drawn for the foundation soil with known effective cohesion ( $c^{\prime}_{1}$ c 1 ′ ) and angle of internal friction ( $\varphi^{\prime}_{1}$ φ 1 ′ ), and then the stability of the foundation soil has been assessed. Probable zone of general shear failure considering seepage flow has been identified. Zones vulnerable to foundation failure for different width of levee and depth of cut-off wall have been computed. The hydraulic unstable zone in downstream side of the levee resembles a log spiral. The provision of cut-off wall restricts the progress of failure zone only in the downstream side; otherwise, the zone of failure will enter the foundation of the levee. The stability of the foundation soil of an existing ring levee along the rivers Ganga, Sone and Punpun, near Patna (India), has been evaluated.     

青草沙水库大堤渗透特性的显著性检验分析

青草沙水库地处长江人海口,堤身及堤基由大量沙土和粉土组成,要求对大堤的渗流特性进行专题研究,掌握大堤渗流规律,确保大堤的安全运行。分别于2007年建设期和2010年运行期进行了渗透试验,并将两次试验的渗透参数进行了对比,利用数理统计中样本显著性检验分析,对大堤运行期的渗透参数进行了检验分析,为大堤的渗透变形研究提供了有效的数据。     

堤内压盖渗流分析的临界破坏点法

根据洞庭湖区堤防地质特征,建立二元结构堤基堤内压盖模型。基于渗流有限元理论和虚单元固定网格法,编制了堤内压盖渗流有限元程序。通过在压盖上方增加大量强透水性空气单元的方式,解决了压盖渗流计算中经常遇到的2次入渗问题。同时,研究了压盖宽度对防渗效果的影响,得到在堤内有、无水塘的情况下压盖宽度与堤内最大渗透坡降的影响曲线,指出影响曲线主要呈线性变化的趋势,并提出了用于确定临界压盖宽度的临界破坏点法,该法以压盖宽度-堤内最大渗透坡降影响曲线的延长线与天然覆盖层的允许渗透坡降线的交点作为临界破坏点,临界破坏点左侧的堤内区域为可能破坏区域,需要压盖加固;右侧为安全区域,不需加固。临界破坏点法的应用,大幅度地减小了有限元计算模型,避免了多次重复的有限元计算,显著地提高了计算效率,与规范、有限差分等传统方法相比优势明显。     

Three-dimensional hydromechanical modeling of internal erosion in dike-on-foundation

Currently, numerical studies at the real scale of an entire engineering structure considering internal erosion are still rare. This paper presents a three-dimensional (3D) numerical simulation of the effects of internal erosion within a linear dike located on a foundation. A two-dimensional (2D) finite element code has been extended to 3D in order to analyze the impact of internal erosion under more realistic hydromechanical conditions. The saturated soil has been 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 have been modeled with 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 the porosity and the fines content induced by internal erosion upon the behavior of the soil skeleton. An unsaturated flow condition has been implemented into this coupled hydromechanical model to describe more accurately the seepage within the dike and the foundation. A stabilized finite element method was used to eliminate spurious numerical oscillations in solving the convection-dominated transport of fluidized particles. This numerical tool was then applied to a specific dike-on-foundation case subjected to internal erosion induced by a leakage located at the bottom of the foundation. Different failure modes were observed and analyzed for different boundary conditions, including the significant influence of the leakage cavity size and the elevation of the water level at the upstream and downstream sides of the dike.     

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.

     

Levee failure mechanisms during the extreme rainfall event: a case study in Southern Taiwan

Extreme weather has recently caused many disasters worldwide. In August 8, 2009, Southern Taiwan suffered from serious floods during Typhoon Morakot. In this extreme rainfall event, the Chiuliao first levee in the Laonong River basin experienced catastrophic failure. Therefore, this study focuses on the levee failure mechanisms based on variations in levee water levels. Specifically, this study investigates four mechanisms based on limit state equilibrium. The first mechanism involves the slope stability under hydrostatic conditions at various water levels. The results of this analysis show that the levee cannot fail under this mechanism. The second mechanism involves the levee slope stability with steady-state seepage. Because the water levels are different on the protected and flood sides, the water recedes much faster on the flood side than the protected side. Based on this analysis, the levee slope might fail when the water level at the protected side is close to the top of levee and the water level at the flood side starts to recede. The third and fourth mechanisms involve the levee foundation failure in terms of sliding and overturning failure. The results of this study indicate that the levee foundation is more prone to sliding failure than overturning failure. Based on these results, this study shows that the levee failed when the water level at the protected side neared the top of levee while the water level at flood side started to recede. At this moment, the levee may fail because of both the slope failure with seepage and sliding failure of the levee foundation.     

深厚覆盖层地基潜蚀研究综述

潜蚀是深厚覆盖层渗透稳定性问题中比较主要和突出的一种表现型式,开展深厚覆盖层潜蚀问题的相关试验及理论研究,对于保障中国已建、待建重大水电工程的安全均具有重要的理论和实际意义。首先,区分了潜蚀与向后侵蚀管涌,指出潜蚀与向后侵蚀管涌的发生机制完全不同,两者不能混淆在一起。与向后侵蚀管涌相比,潜蚀更具隐蔽性,其发生发展机制更加复杂。其次,从潜蚀发生的几何条件、水力条件及潜蚀数学模型等3个方面详细梳理总结了潜蚀研究的相关进展。最后,结合雅鲁藏布江下游水电开发等国家重大工程,提出未来应着重加强对原状覆盖层土体内部稳定性评价,对极端条件、复杂渗流条件、复杂地层条件下潜蚀发生发展机制,潜蚀时间效应及其长期影响的评价和控制,对潜蚀本构关系及其数学模型等方面的试验及理论研究。     

软基海堤结构稳定性研究

以软土地基上的海堤为研究对象,根据软土变形的时效特征和自重应力场与渗流场的耦合特性,建立了软土蠕变模型和渗流作用下的耦合应力场分析模型,将堤身、堤基作为整体进行了变形特性分析。并采用强度折减有限元法分析了海堤在自重、波浪力和渗透力耦合作用下的整体稳定性。分析结果表明：软基海堤结构处于多种荷载效应的综合作用之下,不仅对堤基和堤身断面的位移场和应力场影响较大,而且对其整体稳定性也有很大影响。通过计算值与实测值的对比,证明了所建计算模型和所采用的计算方法的正确和适用,所得结果具有理论意义和实用价值。     

Investigation of slope instability induced by seepage and erosion by a particle method

A novel particle based Bluff Morphology Model (BMM) developed by the authors is extended in this paper to investigate the effect of two dimensional seepage on the stability and collapse of soil slopes and levees. To incorporate the seepage in the model, Darcy’s law is applied to the interactions among neighbouring soil particles and ghost particles are introduced along the enclosed soil boundary so that no fluid crosses the boundary. The contribution of partially saturated soils and matric suction, as well as the change in hydraulic conductivity due to seepage, are predicted well by the present model. The predicted time evolution of slope stability and seepage induced collapse are in reasonable agreement with the experimental results for homogeneous non-cohesive sand and multiple layered cohesive soils. Rapid drawdown over a sand soil is also investigated, and the location and time of the levee collapse occurrence are well captured. A toe erosion model is incorporated in the BMM model, and the location and quantity of erosion from lateral seepage flow is well predicted. The interplay of erosion, seepage and slope instability is examined.     

微生物-膨润土联合矿化防渗模型试验研究

砂土地基发生渗漏时容易引发地基塌陷和工程结构破坏等工程安全问题。利用微生物-膨润土联合矿化的方法开展了大尺寸砂柱的防渗模型试验,研究了砂土的颗粒粒径、浆液的液固比与注浆次数等因素对砂柱的渗透特性、内部侵蚀特性及膨润土与碳酸钙沉淀分布的影响,分析了处理后试样的封堵稳定性和微观结构,进一步评估了微生物-膨润土联合矿化方法的处理效果。结果表明,通过微生物-膨润土联合矿化方法可以有效地提高砂土的防渗效果与封堵稳定性。经过1～3次注浆后,试样的渗透系数最大可以降低4个数量级,并且渗透过程中的侵蚀速率也得到了成倍的降低,最低达到了0.51 g/(s·m2)。针对膨润土与碳酸钙沉淀在砂土封堵中起到的作用,分析了微生物-膨润土联合矿化方法的防渗机制。该研究成果证实了微生物-膨润土联合矿化方法应用于砂土防渗工程的可行性,为微生物注浆技术解决砂土地基渗漏问题提供了重要参考。     

柱下独立基础稳健性设计与分析

岩土工程随机参数统计特征的不确定性,使得岩土工程可靠度设计存在一定风险。岩土工程稳健性设计能够充分考虑随机参数的不确定性结合结构安全性、稳健性和经济性实现最优设计。针对随机参数统计特征的不确定性对柱下独立基础设计的影响,基于可靠度理论和岩土工程稳健性设计方法,考虑岩土参数、混凝土和钢筋材料力学参数统计特征不确定性的影响,以独立基础几何尺寸作为可控设计参数进行设计分析。将独立基础地基承载力、地基变形、基础结构冲切破坏和基础弯曲破坏4种失效模式视为串联系统,进行多失效模式下的结构体系稳健性设计,分析了多失效模式下结构几何参数与结构体系可靠度的关系。结合稳健性和经济性,进行了独立基础多目标优化设计,确定柱下独立基础设计的最优解。     

Reliability-based design and its complementary role to Eurocode 7 design approach

Reliability-based design (RBD) can play a useful complementary role to overcome some limitations in the Eurocode 7 (EC7) design approach, for example in situations with parameters not covered in EC7, different parametric sensitivities across different problems, cross-correlated or spatially correlated parameters, design aiming at a target reliability or failure probability, or when uncertainty in unit weight of soil is modeled. The complementary role played by RBD under these circumstances is illustrated and discussed for a shallow foundation, a reinforced rock slope, a Norwegian clay slope with spatial variability, a laterally loaded pile requiring implicit numerical analysis, and an anchored sheet pile wall. A pragmatic RBD approach involving first-order reliability method (FORM) only and a more rigorous RBD approach involving both first-order and second-order reliability method (SORM) are offered. Both approaches are implementable using either spreadsheet-based FORM and SORM procedures, or using various commercially available FORM/SORM packages.     

燕山水库坝基防渗墙优化设计

因现场资料不足或分析手段落后等原因,当前进行水库坝基防渗墙设计,往往存在过于保守的倾向。以河南省燕山水库为例,基于优化设计的思路,采用目前较为成熟的二维有限单元法,对不同防渗条件下的坝基渗流场分别进行了模拟,并将计算所得比降、流量与允许比降、允许流量做了对比,从定量的角度提出了既安全、又经济的防渗方案:防渗墙厚度取0.8m,深度取打入破碎带5m。     

悬挂式防渗墙结合堤后压渗盖重防渗效果的试验研究

在实验室模型槽中进行了有悬挂式防渗墙以及堤后压渗盖重的渗流模型试验,取得了不同贯入度以及不同堤后压渗盖重宽度防止堤基渗透变形的系列试验成果。通过一系列试验结果表明,悬挂式防渗墙和堤后压渗盖重可有效地提高强透水层堤基抵抗渗透变形的能力,且在控制渗透变形向上游发展的作用上功效显著。研究表明,堤后压渗盖重的防渗效益往往低于较大贯入度的防渗墙,但从经济角度来看,不失为一种有效可靠的防渗措施。     

洞庭湖区堤防垂直防渗模型研究

根据洞庭湖区堤防工程地质、地形特性,建立了多元结构堤基垂直防渗概化模型,并按照优化思想,采用渗流有限元法,对垂直防渗墙在砂卵石、相对不透水堤基中的贯入深度和堤内最大渗透坡降的位置、大小的关系做了系统地分析研 究。再根据有限元计算结果,全面地评价了两种型式防渗墙的防渗效果,提出了临界最优贯入深度的概念。最后,将有限元法应用到大东口电排高喷防渗墙工程中,实际工程的安全运营表明了研究结果的正确性和实用性。     

Numerical investigation of rainfall-induced fines migration and its influences on slope stability

Rainfall-infiltration-induced fines migration within soil slopes may alter the local porosity and hydraulic properties of soils, and is known to be a possible cause of the failure of slopes. To investigate the intrinsic mechanisms, a mathematical formulation capable of capturing the main features of the coupled unsaturated seepage and fines migration process has been presented. Within the formulation, an unsaturated erodible soil is treated as a three-phase multi-species porous medium based on mixture theory; mass conservation equations with mass exchange terms together with the rate equations controlling fines erosion and deposition processes are formulated as the governing equations and are solved by the FEM method. The influences of both the fines detachment and deposition on the stability of slopes under rainfall infiltration have been investigated numerically. The results show that depending on whether the fines move out or get captured at pore constrictions, both desired and undesired consequences may arise out of the fines migration phenomenon. It is suggested that more attention should be paid to those slopes susceptible to internal erosion whose safety analysis cannot be predicted by traditional methods.     

水流冲刷过程中河岸崩塌问题研究

采用土工数值分析方法,对水流冲刷过程中的河岸崩塌问题进行了分析。分析了水流冲刷引起的河岸后退距离及后退过程中坡内的渗流情况与坡体的稳定性;探讨了水流冲刷过程中岸坡崩塌的发生机理;指出了坡内的渗流作用与坡前水流的冲刷作用是岸坡发生崩塌的主要动力因素,而岸坡土体的物质组成和分布、岸坡坡度等则是岸坡发生崩塌的主要内在因素;并提出了相应的工程防护措施。     

Optimization of analytical equations of groundwater seepage into tunnels: A case study of Amirkabir tunnel

In this study, with investigation of validity limits of analytical equations in computing groundwater seepage discharge into tunnels in different values of r/h (tunnel radius/water table height above tunnel), using optimization by regression analysis, we have tried to converge the results of analytical equations for any value of r/h. There are various experimental, analytical and numerical methods by which it is possible to calculate the amount of groundwater seepage into tunnels. Due to their simplicity and practical base theory, analytical methods have been applied more frequently. Studies imply that amount of real seepage into tunnel is significantly less than what is anticipated from analytical equations; on the other hand, the results of seepage calculations using these equations are depended on tunnel geometry and medium conditions. Previous investigations confirmed that in a tunnel for which, r/h is more than 0.4; these results are highly different both from one another and from the real seepage inflow. In this paper, optimization has been successfully performed on analytical equations so that the results of these methods are converged to each other for any value of r/h.