The Statistics of Embankment Dam Failures and Accidents

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The statistics of embankment dam failures and accidents Mark Foster, Robin Fell, and Matt Spannagle

Abstract: The paper describes the results of a statistical analysis of failures and accidents of embankment dams, specifically concentrating on those incidents involving piping and slope instability. The compilation of dam incidents includes details on the characteristics of the dams, including dam zoning, filters, core soil types, compaction, foundation cutoff, and foundation geology. An assessment of the characteristics of the world population of dams was also carried out. By comparing the characteristics of the dams which have experienced failures and accidents to those of the population of dams, it was possible to assess the relative influence of particular factors on the likelihood of piping and slope instability. Key words: dams, failures, piping, instability database. Résumé : Cet article décrit les résultats d’une analyse statistique des ruptures et accidents dans les barrages en terre, se concentrant spécifiquement sur ces incidents impliquant la formation de renard et l’instabilité des talus. La compilation des incidents de barrages inclut des détails sur les caractéristiques des barrages incluant le zonage du barrage, les filtres, les types de noyau, le compactage, le rideau d’étanchéité de fondation, et la géologie de la fondation. Une évaluation des caractéristiques de la population mondiale des barrages a également été réalisée. En comparant les caractéristiques des barrages qui ont été affectés par des ruptures et des accidents avec celles de la population des barrages, il a été possible d’évaluer l’influence relative que des facteurs particuliers ont sur la vraisemblance de renards et de l’instabilité des talus. Mots clés : barrages, ruptures, renard, base de données sur l’instabilité. [Traduit par la Rédaction]

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Introduction Embankment dam engineering has evolved over many centuries, with the major developments occurring since the 1940s with the development of soil mechanics and geotechnical engineering. Some aspects are now readily analysed, e.g., the stability of the embankment slopes. Others, e.g., piping failure through a dam foundation, remain more difficult to quantify, and the measures taken in design and construction are more experience based. It is particularly difficult to assess the safety of dams which do not meet modern design and construction criteria, e.g., dams with no or inadequate filters. Recognising the value of the historic performance of dams in assessing dam safety, the International Commission on Large Dams (ICOLD) has carried out extensive surveys of dam incidents (ICOLD 1974, 1983, 1995). The ICOLD surveys are for large dams, a large dam being defined as a dam which is more than 15 m in height (measured from the lowest point in the general foundations to the crest of the dam) Received February 5, 1999. Accepted February 10, 2000. Published on the NRC Research Press website on October 6, 2000. M. Foster. URS, Level 3, 116 Miller St., North Sydney, Australia 2060. R. Fell. School of Civil and Environmental Engineering, University of New South Wales, Sydney, Australia 2052. M. Spannagle. Department of Land and Water Conservation, GPO Box 39, Sydney, Australia 2001. Can. Geotech. J. 37: 1000–1024 (2000)

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or any dam between 10 and 15 m in height which meets one of the following conditions: (i) the crest length is not less than 500 m, (ii) the capacity of the reservoir formed by the dam is not less than 106 m3, (iii) the maximum flood discharge dealt with by the dam is not less than 2000 m3/s, or (iv) the dam is of unusual design. ICOLD carried out analyses of the data compiled to determine the most common cause of dam incidents. Others, including USCOLD (1975, 1988), USCOLD Committee on Dam Safety (1996), ANCOLD (1992), Charles and Boden (1985), Olwage and Oosthuizen (1984), and Gomez et al. (1979), have compiled data on incidents for various countries. There have been attempts to use the statistical analysis of dam incidents to predict the likelihood of failure of dams, including those by Silveira (1984, 1990), Blind (1983), Serafim (1981a, 1981b), Tavares and Serafim (1983), Ingles (1988), Gruner (1963, 1967), and Von Thun (1985). All of these analyses are limited to the statistics of height, year of construction, and only basic descriptions of the dam type. For example, the embankment zoning classification is restricted to two categories in the ICOLD dam incident and dam population databases, namely earthfill embankment (TE) and rockfill embankment (ER). Embankment zoning would be expected to have a significant influence on the likelihood of failure, particularly for structural modes of failure which include slope stability and piping. As is well recognised in dam engineering, and described in Fell et al. (1992), different embankment zoning types have varying degrees of control of embankment seepage. These give varying degrees of control of the potential © 2000 NRC Canada

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Foster et al.

for piping failure through the embankment and foundation and the pore pressures which affect slope stability. This paper presents the results of a statistical analysis of embankment dam incidents, specifically concentrating on internal erosion and slope instability. While the data can be used in a quantitative risk assessment (QRA) framework, they also provide useful insights into the factors which contribute to dam incidents and can therefore be used in a nonQRA, dam safety context. The study has been done as part of a larger research project studying methods for estimating the probability of failure of embankment dams for use in QRA. This paper describes only part of the research project. Foster et al. (2000) describe the application of these data to estimating the relative likelihood of failure of embankment dams by internal erosion and piping, Foster and Fell (2000a) discuss the assessment of filters which do not satisfy modern design criteria, and Foster and Fell (2000b) use event trees to estimate the probability of failure of embankment dams by internal erosion and piping. All components are described in Foster (1999). The principal components of the study were as follows: (i) extension of the existing compilations of dam incidents to include more details on embankment zoning, including the presence or absence of filters, foundation geology, and embankment material characteristics such as core soil types and compaction; and (ii) analysis of the dam incident database and comparison to a dam population database to estimate historic frequencies of failure for different modes of failure and dam zoning types and identify factors, such as foundation geology types and core embankment characteristics, that have an influence on the likelihood of embankment dam failure for piping and slope instability modes of failure.

Establishment of databases Compilation of dam incident database A list of dam incidents was compiled primarily from the three ICOLD studies (ICOLD 1974, 1983, 1995) supplemented with additional incidents from the other existing compilations, from the literature, and from the project sponsors. The criteria set for the selection of the dam incidents to be entered into the database are (i) embankment dam failures for all modes of failure for large dams; (ii) failures of embankment dams (not necessarily large) by piping and slope instability; and (iii) accidents involving piping, slope instability, and seepage. The definitions of failures, accidents, and incidents used are consistent with ICOLD (1983). The first criterion was used to keep the dam failure and dam population datasets consistent for proper statistical analysis. The other two criteria were set to maximise the amount of data for the detailed analysis of piping and slopeinstability failures. Data on the dam and incident details were obtained from (i) incident descriptions in ICOLD (1974, 1983) and other compilations of dam incidents; (ii) published data from an extensive search through the literature; and (iii) reports collected from the sponsor organisations and from the United States Bureau of Reclamation (USBR), British Columbia Hydroelectric and Power Corporation (BC Hydro), and the Norwegian Geotechnical Institute.

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Information on the dam and incident details was extracted from the data gathered and entered into a database called ERDATA1. The ERDATA1 database is divided into seven main categories: (i) dam details, e.g., dam name, country, height, year constructed; (ii) dam zoning category, including dam zoning and description of filters; (iii) foundation cutoff category; (iv) foundation geology; (v) earthfill core characteristics; (vi) incident details; and (vii) references. Sketches of the dam zoning categories used are shown in Fig. 1. A full description of the variables in the ERDATA1 database is given in Foster et al. (1998) and Foster (1999). Population of embankment dams database The population of embankment dams database is required to determine whether an overrepresentation of a particular dam characteristic in the incident cases, such as a particular zoning type, is due to this characteristic being common in all existing dams, or whether dams with this characteristic tend to be more susceptible to dam incidents. The data from the population of dams database are combined with those from the incident database to estimate the frequencies of failure and accidents for the various dam zoning types. The ERDATA1 classification system has been used for collating data on the population of dams, so the databases are consistent. The ideal situation for a sound statistical analysis would be to have the characteristics of the ERDATA1 classification system for all the existing embankment dams as listed in the World register of dams (ICOLD 1984). However, there are insufficient data in the ICOLD register to obtain data on dam zoning, foundation geology, and other characteristics in the ERDATA1 classification system, so it was necessary to select sample populations of embankment dams in an attempt to represent the characteristics of the world population. Sample populations of embankment dams which were used include 356 dams in Australia, 44 in New Zealand, 246 in the United States (from the USBR), 174 in Norway, and 642 described in papers in the ICOLD congresses up to 1982, giving a total of 1462 embankment dams, or about 13% of the total population. These datasets were primarily selected due to the availability of sufficient data required for the ERDATA1 classification system. Information for the Australian and New Zealand dams was obtained from the project sponsors and from questionnaires sent out to dam owning authorities. The zoning characteristics for the dams in the United States were obtained from USBR (1994) and the foundation geology characteristics from the Safety Evaluation of Existing Dams (SEED) databooks held at the USBR office in Denver, Colorado. Data on the Norwegian dams were limited to the dam zoning category which was obtained from K. Senneset (personal communication, 1996). Dams described in the ICOLD congress papers from 1933 to 1982 were used to obtain a representative set of dams to take account of the range of trends in dam construction with time and for various countries. Information was also collected from the literature, providing additional data on the distribution of dam zoning categories in different parts of the world and describing general trends in dam design. Literature sources included Snethlage et al. (1958), Leps et al. (1978), Building Research Establishment © 2000 NRC Canada

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Fig. 1. Dam zoning categories.

(1990), Sherard (1953), ICOLD (1984), Skempton (1990), Cooke (1984), ICOLD (1989), and Schnitter (1994). The following approach was used for each of the main categories: (i) dam zoning; (ii) foundation geology; and (iii) core material geological origin, soil classification, and compaction. Dam zoning The general approach used to make the estimates of the world distribution of zoning types was to subdivide the data into tables according to the country, dam height range (50 m), and construction year period (before 1900, 1900–1929, 1930–1949, 1950–69, and 1970–1986). The dam zoning distribution estimates were made for Australia, France, India, Japan (post-1950 only), New Zealand, Norway, United Kingdom, and United States. These countries had sufficient data in the sample population available to give reasonably reliable estimates of the zoning characteristics of the population. The dams from remaining countries were grouped into a category called other countries. Dams constructed in China, and in Japan prior to 1930, were excluded from the analysis of the population due to the lack of information in the literature on these dams and low reported failure rates despite them making up a significant proportion of the dam population. The number of earthfill (TE) and rockfill (ER) dams for each of the countries, construction periods, and height categories were obtained from the ICOLD register (ICOLD 1984). The information from the sample populations and data gathered from the literature were used as a basis to make estimates of the number of dams for each of the dam

zoning categories. A considerable degree of judgement was required to make the estimates, but this was facilitated by breaking down the data into the smaller units of construction years and dam height ranges. The analysis was by a trial and error process in which the estimated percentages were modified so that they reflected the expected trends in dam design with time and dam height. Table 1 shows the assessed distribution of dam zoning for the world population of dams accounting for construction period. Rockfill dams, comprising zoned earth and rockfill dams, central core earth and rockfill dams, concrete face rockfill dams, and rockfill with corewall dams, make up 21% of the world population based on the dam zoning categories. This is significantly higher than the proportion of rockfill dams (ER), namely 9%, given by the ICOLD register (ICOLD 1984). This difference is attributed to the differences in classification of rockfill dams of the ERDATA1 and ICOLD systems. Zoned earth and rockfill dams with less than 50% rockfill by volume are classified as TE by ICOLD. Foundation geology The distribution of foundation geology types was assumed to be dependent on the spatial distribution of dams. Estimates were made for countries where sufficient data were available from the sample population, namely United States, India, United Kingdom, Canada, Australia, and New Zealand. This subset of countries was carried through into the analysis of the dam incidents when comparing the distribution of foundation geology types of the accident and failure cases to the population. The distributions of geology types from the sample population database were determined for each of the countries and were used as a basis together with © 2000 NRC Canada

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Table 1. Estimated dam zoning (%) for the world population of embankment dams by construction period. Zoning category

Before 1900

1900–1930

1930–1950

1950–1970

1970–1986

All years

Homogeneous earthfill Earthfill with filter Earthfill with rock toe Zoned earthfill Zoned earth and rockfill Central core earth and rockfill Concrete face earthfill Concrete face rockfill Puddle core earthfill Earthfill with corewall Rockfill with corewall Hydraulic fill Number of embankment dams

16 0 5 7 0 0 5 1 58 5 0.5 2 370

14 1 5 18 7 0 4 5 24 11 3 8 819

16 11 6 37 8 5 5 2 4 4 1 1 1167

9 18 7 37 10 8 5 2.5 0.5 2 1 0 4436

6 18 6 40 10 12 3 3 0 1 1 0 4400

10 15 6 36 9 8 4 3 5 2 1 1 11 192

the use of geology maps of the countries to make estimates of the percentage distribution of foundation geology types. Core material geological origin, soil classification, and compaction Estimation of the distribution of the geological origin of the core material was made on the assumption that the core geology would be influenced by the regional geology and was therefore analysed by country. The distributions of core geological origin types from the population database and the foundation geology distributions described above were used as a basis to make estimates of the core geology distributions for United States, Australia, New Zealand, United Kingdom, Canada, India, Norway, and other countries. The final world distribution was then obtained by applying a weighted sum of the distributions from the countries based on the number of large embankment dams in each country in 1982 (from ICOLD 1984). The distribution of core soil types, classified by the Unified Soil Classification System (USCS), was determined directly from the percentages of core soil types from the population database. It was assumed that there were sufficient data in the population database to give a representative sample of core soil types. Typically, the core material comprises more than one soil type and so the percentages in the distribution do not necessarily sum to 100%. The percentages were normalised such that they sum to 100% for comparison with the incident statistics. Estimates of the distribution of the degree of compaction of the core materials were made using the population database, assuming it to be dependent mainly on the period of construction. Hydraulic fill and puddle core dams were not included in the analysis because their compaction is inherent in the dam zoning. Information provided by Sherard (1953) and Skempton (1990) was used to adjust the database values. An attempt was made to estimate the relative abundance of the presence of dispersive soils in the cores of embankment dams due to their prominence in the piping failure cases. There was insufficient information in the population database on which to make such an estimate and so information from the literature was used. ICOLD (1990) lists parts of the world which have experienced problems with dispersive soils, and combined these countries have approximately 35% of large embankment dams. There is no basis on

which to estimate the proportion of dams in these countries with dispersive soils actually present in the core, but the value is probably less than 25%. If it is assumed that say 5– 10% of the dams in these countries have dispersive soils present in the core, then a very approximate estimate of the proportion of dams in the world with dispersive soils is 2– 4%. Other dam characteristics Estimates for the distributions of the other dam characteristics, such as foundation cutoff details, embankment and foundation filters, and the location of conduits, were determined directly from the population database. Details are given in Foster et al. (1998) and Foster (1999). The statistics of year of construction, cumulative dam embankment years, and dam heights were calculated from ICOLD (1984, 1983) and used in the analysis. More details on the databases are given in Foster (1999) and Foster et al. (1998).

Analysis methodology There are two components of the analysis of the ERDATA1 database, namely analysis of the overall statistics of failure, and detailed analysis of piping and slope-instability failures. Mode of failure The philosophy of the analysis of the ERDATA1 data was to categorise dam accidents and failures into modes of failure as opposed to causes of failure. This is compatible with the methods used in event-tree analysis. The failure mode categories used are flood overtopping, gate–spillway failure, piping, slope instability, and earthquake. Piping failures were further subdivided into piping through the foundation and piping from the embankment into the foundation. Slope-instability failures were subdivided into upstream slides and downstream slides. Dam incidents sometimes involve more than one failure mode, so for example, development of piping through the embankment may cause saturation of the downstream slope which then initiates a downstream slide. In these cases, all the modes of failure that were involved in the dam incident are assigned in the database. © 2000 NRC Canada

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Fig. 2. Stages of development of piping failure.

Overall statistics of failure The frequencies of failure are estimated from the number of dam incidents compared with either the total number of large (i.e., >15, high) embankment dams (or dams of a particular zoning) up to 1986 to give the average frequencies of failure over the life of the dam, or the total number of embankment dam-years, allowing for the estimated average age of each zoning type (up to 1986) to give annual frequencies of failure. Analysis of piping and slope instability incidents This analysis involved comparing the frequency of occurrence of the dam characteristics such as dam zoning type, foundation geology, and embankment core type in the dam incidents to that in the dam population. An overrepresentation of a particular feature, such as a particular dam zoning or foundation geology type, in the dam incidents relative to the dam population suggests dams with that particular feature are possibly more vulnerable to that failure mode. The analysis method utilises the concept that piping and slope-instability failure modes can be broken down into several stages of development. Typically these stages are taken to be initiation, progression, and breaching, as shown in Fig. 2 for piping through the embankment. This concept has been suggested by several authors, including Von Thun (1996). Accidents involve initiation of piping, but the progression stage is limited and breaching does not occur. Therefore, by comparing the characteristics of the accident cases to the failure cases of a particular failure mode, it may be possible to identify factors that influence the progression stage of the failure mode. The analysis of the data has kept all failures as one dataset, rather than, for example, separating dams by zoning, separating first-filling failures from those which occur later, or separating failures due to piping around conduits from other piping failures. An initial assessment of the data showed there did not appear to be big differences in the geological and core material characteristics for these sets, but this was not proven in a rigorous statistical way. Separation of the data had the major disadvantage that already small samples (of the dams which had failed or had accidents) would become smaller.

Overall statistics of failure of embankment dams Table 2 gives the overall statistics of failure for all failure modes, separating for all failures and failures during opera-

tions. The historical average frequency of failure of large embankment dams is estimated to be 1.2% over the life of the dam (136 embankment dam failures out of 11 192 large embankment dams constructed up to 1986, excluding China and Japan pre-1930). This reduces slightly to 1.1% over the life of the dam for dam failures occurring only while the dam was in operation. The historical annual probability of failure of large embankment dams is estimated to be 4.5 × 10–4 per dam per year (136 embankment dam failures in an estimated 300 400 embankment dam-years up to 1986). This reduces slightly to 4.1 × 10–4 per dam year if failures occurring during construction are excluded. These figures would reduce by about 30% if none of the dams in the 11 192 had failed up to 1999. Table 3 presents the statistics of failure by failure mode and dam zoning. These overall statistics are useful if one makes the assumption that the performance of dams in the past is a reasonable prediction of what may happen in the future. It is apparent from the statistics combined with the conventional understanding of dam stability and piping that factors such as dam zoning and core material properties have an influence on the likelihood of failure or accidents and can be used to get an idea of which dams are more or less likely to experience stability and piping problems. An analysis was carried out of the frequency of embankment dam failures for each mode of failure for the failures occurring before and after 1950 (excluding failures during construction). The failure statistics for dams constructed before and after 1950 are shown for all modes of failure and structural modes of failure in Table 4. Structural modes of failure are those involving piping, slope instability, or earthquake. The analysis showed the proportion of failures by piping increases from 43% before 1950 to 54% after 1950. Over the same period, the proportion of failures by flood overtopping and appurtenant works modes of failure decreases from 53% to 41%. There is a significant reduction in the proportion of failures due to sliding with time, reducing from 7% before 1950 to only 1.5% after 1950. The following sections present the outcomes of further analysis of the incidents for piping and slope instability to further assess the factors affecting the frequency of incidents of piping through the embankment and slope instability. The population database consists of over 11 000 dams, with over 300 000 dam-years of performance, so the analysis of the relationship of zoning to incidents is based on a large sample. For the assessment of other factors, e.g., core material properties, there is a greater reliance on the characteristics of the dams that have experienced incidents, and the sample size is © 2000 NRC Canada

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Table 2. Overall failure statistics for large embankment dams up to 1986, excluding dams constructed in Japan pre-1930 and in China. No. of cases

% failures (where known)

Average frequency of failure (×10–3)

Mode of failure

All failures

Failures in operation

All failures

Failures in operation

All failures

Overtopping and appurtenant Overtopping Spillway–gate Subtotal

46 16 62

40 15 55

35.9 12.5 48.4

34.2 12.8 47.0

4.1 1.4 5.5

3.6 1.3 4.9

Piping Through embankment Through foundation From embankment into foundation Subtotal

39 19 2 59

38 18 2 57

30.5 14.8 1.6 46.1

32.5 15.4 1.7 48.7

3.5 1.7 0.18 5.3

3.4 1.6 0.18 5.1

Slides Downstream Upstream Subtotal Earthquake–liquefaction Unknown mode Total no. of failures Total no. of failures where mode of failure known No. of embankment dams

6 1 7 2 8 136 128 11 192

4 1 5 2 7 124 117 11 192

4.7 0.8 5.5 1.6

3.4 0.9 4.3 1.7

0.54 0.09 0.63 0.18

0.36 0.09 0.45 0.18

Failures in operation

12.2 (1.2%)

11.1 (1.1%)

Note: Subtotals and totals do not necessarily sum to 100%, as some failures were classified as multiple modes of failure.

smaller. For this reason it has been necessary to group all the dams experiencing incidents together, rather than keeping them separate, for this part of the analysis.

Factors affecting the frequency of incidents of piping through the embankment Incidents have been classified as piping through the embankment if the incident involved any type of internal erosion process occurring primarily through the embankment dam. Cases of piping along and into conduits through dams are included. Cases where piping initiated at the embankment–foundation contact are not included but analysed separately under piping from the embankment into the foundation. Dam zoning The failure and accident statistics for piping through the embankment are summarised in Table 5. It is evident that the dam zoning categories with high average frequencies of failure by piping through the embankment tend to be the zoning types with inherently poor control of seepage through the embankment. Homogeneous earthfill dams, which have no zoning of materials, have the highest frequency of failure, nearly five times higher than the average of all dams combined. Other dam zoning categories with higher than average frequencies of failure by this mode of piping are earthfill with rock toe, concrete face earthfill, and puddle core earthfill dams. These four zoning categories combined make up nearly 80% of the failure cases but only 25% of the population. For homogeneous earthfill dams, earthfill dams with rock toe, and concrete face earthfill dams, there are the same number or more failures than accidents. It is possible that many piping accidents have not been reported to the ICOLD

studies or in the literature, but this trend suggests that these dams are more likely than other dams to fail (i.e., breach) once piping initiates. Embankment dams with downstream rockfill zones have a particularly low incidence of failure due to piping through the embankment. There is only one dam failure due to piping through the embankment for zoning categories with downstream rockfill. This was Avalon Dam, a zoned earth and rockfill dam which failed in 1904. It had no filter between the core and the dumped rockfill. The large number of piping accidents but no failures of central core earth and rockfill dams indicates that these dams have a low frequency of failure because they are less likely to progress to breaching if piping initiates compared with dams with earthfill materials in the downstream zones. Review of the descriptions of the accidents to rockfill dams suggest that this is probably due to the inherent stability of the downstream rockfill zones under large seepage flows. Some specific points about the failures are as follows: (1) Failures of homogeneous earthfill dams have generally been associated with one or more of piping around conduits passing through the dam (in nine cases), piping through poorly compacted fill materials (in 12 cases), and piping through dispersive fill materials (in four cases). The average frequency of failure of homogeneous earthfill dams constructed prior to 1900 is about 10 times higher than that for dams constructed after 1950. (2) The failures of earthfill dams with filter drains have generally been associated with piping through dispersive fill materials around outlet conduits passing through the dam (in three cases) or at the contact with concrete spillway structures (one case). Embankment filters were present in two of the failures, and in both cases failure was attributed to piping around the outlet conduit where locally there were no filters © 2000 NRC Canada

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Mode of failure Piping

Dam zoning type

9.5 15 6.1 35.9 9.3 8.4 4.1 2.8 4.7 2.4 0.9 0.9 — — 100

23 4 9 7 4

(17) (2) (9) (5) (3)

4 (1) 4 1 5 11

(4) (0) (4) (3)

0 (0) 5 5 54 136

(3) (3) (13) (66)

% of failure cases 28 5 11 9 5

(32) (4) (17) (9) (6)

Through embankment

Slope instability Through foundation

From embankment into foundation

Downstream slide

Upstream slide

Earthquake

Overtopping

Spillway– gate failure

Unknown

14 2 5 4 1

2 0 3 1 0

0 0 1 0 1

1 0 0 0 1

0 0 0 0 0

1 0 0 0 0

6 2 0 2 1

0 0 0 0 0

0 0 0 0 0

0

0

0

1

0

0

3

0

0

(8) (0) (8) (6)

2 0 4 0

3 0 0 2

0 0 0 0

0 0 0 0

0 0 0 1

0 0 0 0

0 1 0 4

0 0 1 3

0 0 0 1

0 (0)

0

0

0

0

0

0

0

0

0

6 (6) 6 (6)

0 1 6 39

1 2 5 19

0 0 0 2

2 0 1 6

0 0 0 1

0 0 1 2

0 1 26 46

2 1 9 16

0 0 7 8

5 (2) 5 1 6 13

100 (100)

Note: The values in parentheses refer to statistics for structural modes of failure, comprising piping, slope instability, and earthquake modes of failure. The number of failure cases for the modes of failure do not necessarily sum to the total number of failure cases because some dams were classified as multiple modes of failure.

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Homogeneous earthfill Earthfill with filter Earthfill with rock toe Zoned earthfill Zoned earth and rockfill Central core earthfill and rockfill Concrete face earthfill Concrete face rockfill Puddle core earthfill Earthfill with concrete corewall Rockfill with concrete corewall Hydraulic fill Other Unknown Total

% of population

No. of failure cases

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Table 3. Failure statistics for large embankment dams by dam zoning categories (up to 1986).

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Table 4. Average frequency of failure for large embankment dams constructed before and after 1950. Class of dams

Dams constructed before 1950

Dams constructed after 1950

All dams

No. of large embankment dams constructed No. of large embankment dam failures by all modes of failure No. of embankment dam failures by structural modes of failure Average frequency of failure over life of dam Average frequency of failure over life of dam by structural modes of failure Average annual frequency of failure by all modes of failure Average annual frequency of failure by structural modes of failure

2356 88 41 3.8×10–2 1.7×10–2 8.6×10–4 3.6×10–4

8836 48 25 0.5×10–2 0.3×10–2 2.7×10–4 1.6×10–4

11 192 136 66 1.2×10–2 0.6×10–2 4.1×10–4 2.0×10–4

Table 5. Average frequency of failure due to piping through the embankment by dam zoning categories for large dams up to 1986.

Zoning category

No. of failures

No. of accidents

Homogeneous earthfill Earthfill with filter Earthfill with rock toe Zoned earthfill Zoned earth and rockfill Central core earth and rockfill Concrete (or other) face earthfill Concrete (or other) face rockfill Puddle core earthfill Concrete corewall, earthfill Concrete corewall, rockfill Hydraulic fill Zoning type unknown All dams

14 2 5 4 1 0 (1) 2 0 4 0 0 0 7 39

9 1 5 9 7 19 1 1 (11)‡ 10 2 2 3 6 75

Average frequency of failure (×10–3)

Average frequency of accident (×10–3)

Average annual frequency of failure (×10–6)* First 5 years of operation

After 5 years of operation

16.0 1.5 8.9 1.2 1.2 (