Findings of Case Histories on the Long-Term Performance of Seepage Barriers in Dams

In a vast majority of cases, seepage barriers increase the reliability of dams. However, it is important to recognize that seepage barriers often drastically increase hydraulic gradients around the boundaries of the barrier, and through any windows or defects in the barrier. The result is increased water pressures and hydraulic gradients behind and around the barrier. These increased pressures and gradients have potential to provide the catalyst for initiation of several modes of internal erosion that were either unlikely or less likely without the seepage barrier. As a consequence, seepage barriers give rise to the potential for additional mechanisms of internal erosion and piping in the dam and the foundation. A compendium of 30 case histories of dams that have had seepage barriers in place for over 10 years has been assembled, and observations and insights garnered from these case studies were compiled. Finite-element seepage and deformation analyses have been performed to provide better understanding of the performance of seepage barriers and the mechanisms that affect their performance. This paper presents a summary of our findings from the case histories and the analyses performed.     

Assessing Embankment Dam Filters That Do Not Satisfy Design Criteria

A method is presented for assessing the particle size distribution of the filters in a dam compared to the soil the filter is protecting, to determine whether the filters are sufficiently fine to give no, excessive, or continuing erosion. The method is based on the analysis of the results of laboratory tests and the characteristics of dams that have experienced piping incidents. The method can be used to determine whether filters that are coarser than required by modern filter design criteria will eventually seal or experience continuing erosion leading to possible failure of the dam in the event piping initiates.     

Uncertainty of Predictions of Embankment Dam Breach Parameters

Risk assessment studies considering the failure of embankment dams often require the prediction of basic geometric and temporal parameters of a breach, or the estimation of peak breach outflows. Many of the relations most commonly used to make these predictions were developed from statistical analyses of data collected from historic dam failures. The prediction uncertainties of these methods are widely recognized to be very large, but have never been specifically quantified. This paper presents an analysis of the uncertainty of many of these breach parameter and peak flow prediction methods. Application of the methods and the uncertainty analysis are illustrated through a case study of a risk assessment recently performed by the Bureau of Reclamation for a large embankment dam in North Dakota.     

Embankment Dam Breach Parameters and Their Uncertainties

Potential flood hazards that would be created by breached embankment dams need to be evaluated to select spillway design floods and to prepare emergency action plans. The breaches are often modeled simply, usually in the shape of a trapezoid that is defined by its final height, base width or average width, and side slopes, along with the time needed for the opening to form completely. Data collected from 74 embankment dam failures were used to develop mathematical expressions for the expected values of the final width and side slope of a trapezoidal breach along with its formation time. Information is provided that allows variances of the predicted quantities to be calculated as well. The findings of the statistical analysis were then applied in a Monte Carlo simulation to estimate the degree of uncertainty of predicted peak flows and water levels downstream from breached embankment dams.     

Evaluation of Seepage from an Embankment Dam Retaining Fly Ash

Observations, data, and analyses used to investigate the cause of fly ash-laden seepage from the right abutment of an earthen dam are presented herein. The investigation shows that the sediment-laden seepage occurred through permeable/jointed bedrock in the right abutment that was exposed by a landslide prior to construction of the dam. When the level of the impounded fly ash reached the level of the prior landslide, the fly ash-laden seepage migrated through the jointed bedrock of the abutment and exited on the downstream right abutment. The joint bedrock was exposed to the fly ash reservoir because the landslide removed the clayey colluvium and/or residual soil overlying the jointed bedrock that formed a natural impervious barrier to seepage. This sediment-laden seepage initially was a great concern because of the potential for erosion and piping in earth dams. However, the rapid investigation into and subsequent monitoring of the seepage revealed that accumulation of fly ash and other coarser particles created a filter cake that reduced the seepage and eventually sealed the joints and fractures in the sandstone abutment. No fly ash-laden seepage has been observed on the downstream abutment since April 2004 after first appearing on February 16, 2004. This filter cake development and self-healing process averted additional seepage and illustrates the beneficial effects of fly ash-laden seepage in controlling reservoir leakage.     

Breaching Parameters for Earth and Rockfill Dams

Dam risk analysis is at the heart of dam failure prevention and mitigation. In order to assess dam risk, it is essential to conduct a quantitative analysis of the process of a dam breach, which can be described by such parameters as breach geometry, breaching duration, and peak outflow rate. The main objective of this paper is to develop robust empirical formulas with physical meaning for predicting dam breaching parameters based on past dam failure data. A database of 182 earth and rockfill dam failure cases has been compiled; among these cases nearly one-half are for large dams higher than 15 m. A multiparameter nonlinear regression model is recommended to develop empirical relationships between five breaching parameters (breach depth, breach top width, average breach width, peak outflow rate, and failure time) and five selected dam and reservoir control variables (dam height, reservoir shape coefficient, dam type, failure mode, and dam erodibility). The relative importance of each control variable is evaluated. The dam erodibility is found to be the most important factor, influencing all five breaching parameters. The reservoir shape coefficient and the failure mode also play an important role in the prediction models. Two case studies are presented to show the application of the empirical models developed in this paper.     

Overtopping Breaching of Noncohesive Homogeneous Embankments

Homogeneous small-amplitude embankments were constructed in flumes from a range of uniform noncohesive materials and breached by overtopping flows under constant reservoir level conditions. Embankment erosion evolves from primarily vertical to predominantly lateral in nature. The breach channel initially erodes the downstream face of the embankment with an invert slope parallel to the face, the breach invert slope then progressively flattening to a terminal value by rotating about a fixed pivot point along the base of the embankment, the location of this pivot point being a function of the size of the embankment material. The breach channel is of a curved (hourglass) shape in plan. Below the water line, breach cross-section width B variation with elevation y above the breach invert is nondimensionally described by B? = 2k?y?0.5, where for the breach cross section at the embankment crest k? = ?2.82[ln(Hb?)]+0.351, and Hb is the centerline breach crest elevation. Breach discharge Qb can be nondimensionally expressed as a function of the head hb on the breach-crest centerline and the breach crest length in plan Lb using Qb? = 0.242?Lb?(hb?)1.5. All expressions presented are applicable to full-width breach sections (double the half-breach section tested). The present findings enable prediction of the development with time of breach cross section, breach longitudinal profile, eroded volumes, and breach flows. The findings can be utilized for predictions of erosion and flooding occurring as the result of embankment failure, although in an engineering sense the quantitative findings of the present work await confirmation for larger embankments.     

Risk Indexing Tool to Assist in Prioritizing Improvements to Embankment Dam Inventories

A risk indexing tool is proposed to assist in the prioritization of maintenance, repair, and evaluation tasks on embankment dams that are generally less than 33 m (100 ft) high and for which there is little or no instrumentation, limited or no information concerning as-built conditions, and little or no information on the performance history. Under such circumstances, there may not be enough information available to perform anything other than an “indexing” type of analysis to assist in prioritization. The risk indexing tool is based upon identifying potential deficiencies in the physical condition of the dam and rating the overall importance of these deficiencies to the safety of the structure. It is meant as an index of risk (indication) and not as a direct measure of risk. Checklists are presented for onsite inspections to determine current physical condition. Condition is defined in terms of a condition function that is based upon a condition indexing scale. Four potential failure modes are considered: (1) overtopping; (2) external erosion; (3) piping; and (4) mass movement (slope instability). Absolute probabilities are specifically not factored into the analysis. Conditional probabilities are estimated for each failure mode using a Bayesian updating procedure based on dam attributes. A simple failure criticality analysis is performed wherein specific changes in physical condition of the dam are considered to contribute to the probability of failure for each mode. From this failure criticality analysis and the conditional probabilities of failure, the relative importance of the various changes in physical condition is determined. The physical condition and the relative importance are then combined for each observable deficiency to form a risk index. These risk indices are used to prioritize expenditures for improvements on the premise that actions to address the most significant physical deficiencies are preferred.     

Numerical Modeling of Breach Erosion of River Embankments

The process of breach erosion of river embankments depends on the interaction among flow, sediment transport, and the corresponding morphological changes. Levees often consist of noncohesive material with a wide range of grain sizes. The dam material is mainly eroded due to the transport capacity of the overtopping water. Both bed load and suspended load are of importance. For breach formation, the lateral erosion due to slope instabilities has a significant impact. A depth averaged, two-dimensional numerical model was developed to account for these processes. The sensitivity of the discharge through the breach related to different processes and material parameters was investigated and compared to experimental and field data. The results show that the most sensitive parameter of an erosion-based dike-breach simulation is the breach side-slope angle which determines the lateral erosion. The application of the described Model 2dMb to different embankment failures at the Elbe River illustrates its capability in simulating overtopping breaching.     

Time lag between reduction of sediment supply and coastal erosion

The disruption of sediment supply from river to coast by dam development has been a topic of global concern. In Japan, among the top 30 dams in terms of height, 14 dams were constructed prior to 1970 and another 6 dams before 1980. However, the coastline erosion did not surface up as a grievous problem until the 1980s. According to the River Bureau of Japan, the overall erosion rate along the coastline of Japan was 0.72×106 m2/yr prior to 1980, but sharply increased to 1.6×106 m2/yr since 1980. Therefore, there was a time lag between the disruption of sediment supply by dam and beach erosion. This paper presents a case study on what may have delayed the response of beach to the effect of dam construction.     

Prioritization of Ten Embankment Dams According to Physical Deficiencies

Ten embankment dams managed by the Massachusetts Department of Environmental Management Office of Dam Safety have been prioritized according to their physical deficiencies. Based upon visual inspection reports by several independent engineers engaged by the Department of Environmental Management and using a computer program that automates the risk indexing procedure, the 10 dams have been assigned a risk index (or priority ranking) number that identifies potential deficiencies in the physical condition of each structure and provides an evaluation of the overall importance of these deficiencies. Comparisons of the computed total risk indices for all dams in an inventory provides a basis for prioritizing future maintenance and repair actions for that inventory. Examination of the risk indices for each physical condition at a single dam can also give an indication as to which deficiencies may be the most significant. This paper will assist dam safety engineers in evaluating the potential benefits of the application of risk indexing to dams within their inventory. A companion paper presents the theoretical basis for prioritizations presented herein.     

尾矿堆积坝安全稳定性因素分析及对策

通过对尾矿堆坝的不利因素分析,探讨了安全管理、干滩面长度、堆坝坡度、渗透反滤以及浸润线高度对坝体稳定性的影响.结合生产实际,提出了主控因数,制定了相应提高尾矿堆积坝体稳定性的措施.     

Analysis and Stabilization of Failing Earth Dam Founded on Claystone

A minor slide occurred in the downstream face of an earth dam. Initial observations and limited optical survey data suggested the failure was shallow, but continued measurements from inclinometers in the embankment and survey monuments, as well as visual observations of deformation patterns, revealed a more extensive and severe failure located in the underlying weathered claystone foundation. Deep inclinometers were installed, and inclinometer readings indicated that approximately 1 cm per day of movement was occurring along a very discrete zone in the foundation. Horizontal survey measurements confirmed this displacement, and an emergency soil buttress was constructed at the toe of the dam to reduce movement. The residual strength along the slip surface was estimated by back analysis of the sliding block, assuming a safety factor of 1.0 and incorporating end restraint to assess the lower-bound residual strength. The remediation scheme included installation of an internal drainage system and a soil buttress. Postconstruction monitoring shows that the dam is continuing to move at a decreasing rate and is apparently approaching static equilibrium.     

Earth Dam with Toe Drain on an Impervious Base

The required height of a toe drain for a homogeneous earth dam on an impervious base has been determined considering reservoir water level, capillary rise for the embankment soil, free board, top width of the earth dam, embankment slopes, and tail-water position, such that the surface of seepage does not develop on the downstream sloping face of the earth dam and capillary saturation above phreatic line is contained well within the downstream sloping face. Using Kozeny’s analytic function, exact solution to the unconfined flow through an earth dam having parabolic equipotential boundaries on either side has been obtained. For straight toe drain face, and for various positions of tailwater, approximate toe drain heights and heights of surface of seepage have been determined using the Kozeny’s function and the method of fragments. It has been found that for an earth dam with 1/2 upstream slope, ?1/3 downstream slope, no tailwater, and 2?m capillary rise, capillary saturation is contained within the earth dam and the phreatic line is prevented from emerging on the downstream sloping face by providing a toe drain of height equal to 1/3 of the height of water level in the reservoir.     

Analysis of Arch Dams Using Coupled Trial Load and Block Element Methods

This paper presents the use of the trial load method and the block element method with elastoviscoplastic discontinuities for analysis of arch dams. The arch dam is considered as an arch-cantilever system and the foundation as a block element system. With the displacement compatibility condition at the contact surface of the dam and the foundation (including abutment), the governing equations of the arch dam and foundation are established. These methods are used for the analysis of the double curvature arch dam with complex geology conditions of the Xiaowan Hydroelectric Project in China. The deformation and stress states in both the dam body and the foundation are determined. Furthermore, the stability safety factors of the foundation and the abutment are calculated at the same time, which allows for an optimal design of the arch dam considering the strength, the deformation and the stability of the dam and foundation.     

Seepage Velocity and Piping Resistance of Coir Fiber Mixed Soils

In the context of sustainable watershed management, natural fibers mixed with soil have applications in irrigation and drainage projects such as river levees, contour bunds, temporary canal diversion works, temporary check dams, soil structures, stream restoration, etc., for controlling seepage. In this study, a number of experiments were carried out for determining the seepage velocity and piping resistance of different types of soils mixed randomly with coir fibers. Three types of soils are used in this study. The experiments were carried out for various hydraulic heads, fiber contents, and fiber lengths. Discharge velocity and seepage velocity of flow of water through soil is calculated in each case and compared with plain soil. It is observed that fibers reduce the seepage velocity of plain soil considerably and thus increase the piping resistance of soil. Regression equations based on experiments are developed for quantifying the seepage velocity and piping resistance considering hydraulic gradient, fiber contents, and fiber lengths. Suitability of coir fibers for field applications with typical examples is also highlighted. The results show that coir fiber mixed soil can be used to increase the piping resistance and reduce seepage velocity in the above mentioned applications.     

某尾矿库加高扩容坝体加固处理研究

某尾矿库加高扩容时,对土堤抛石挤淤,土堤周围出现隆起现象。为避免今后尾砂堆高过程中出现此类情况,保证堆积坝及浆砌石坝稳定安全,对堆积坝和浆砌石坝后尾矿区域进行水平加筋,以利尾矿排水固结,并与加筋形成复合坝基。经水平加筋后,效果良好,可供类似工程参考。     

Laboratory Experiments on the Failure of Coarse Homogeneous Sediment Natural Dams on a Sloping Bed

We present the results of a systematic series of 168 laboratory experiments that examine the critical conditions for the failure of landslide dams, which obstruct the course of mountain rivers. The experiments were carried out by using three different sediment materials with a quasiuniform grain size distribution and with a flume bed slope angles that ranged between 0° and 5.71° (0–10%). Three main typologies of dam failure were observed for increasing values of the dam’s downstream-face angle: (1) overtopping; (2) headcutting, which led to the formation of an erosion channel on the dam’s downstream face that progressively migrated up to the dam crest; and (3) initial slide of large part of superficial layer of the dam’s downstream face, which was followed by headcutting. The experiments focused on the second type of failure to provide a safety criterion based on the upstream reservoir level. The quantities that govern this phenomenon have been identified and a functional relationship is proposed based on the dimensional analysis and curve fitting of the minimum level of the upstream reservoir leading to dam failure. A comparison of both the experimental findings and field data that are available in literature shows that the proposed relationship generally provides a conservative estimate for landslide dams in which comminution effects (due to fragmentation of material in the landslide process) are negligible (dam volume lower than 106?m3). Finally, the applicability to larger volume landslide dams or to geometric configurations other than those investigated is discussed.     

Computing the Trajectory of Free Jets

In recent years, design floods have increased beyond spillway capacity at numerous large dams. When additional spillway capacity is difficult or expensive to develop, designers may consider allowing the overtopping of a dam during extreme events. For concrete arch dams, this often raises issues of potential erosion and scour downstream from the dam, where the free jet initiating at the dam crest impacts the abutments and the downstream river channel. A recent review has shown that a commonly cited equation for predicting the trajectory of free jets is flawed, producing jet trajectories that are much too flat in this application. This could lead analysts to underestimate the amount of scour that could occur near a dam foundation, or conversely to overestimate the extent of scour protection required. This technical note presents the correct and incorrect jet trajectory equations, quantifies the errors associated with the flawed equation, and summarizes practical information needed to model the trajectory of free jets overtopping dam crests.