the statistics of embankment dam failures and accidents

The statistics of embankment dam failures andaccidents

Mark Foster, Robin Fell, and Matt Spannagle

Abstract: The paper describes the results of a statistical analysis of failures and accidents of embankment dams, spe-cifically concentrating on those incidents involving piping and slope instability. The compilation of dam incidents in-cludes details on the characteristics of the dams, including dam zoning, filters, core soil types, compaction, foundationcutoff, and foundation geology. An assessment of the characteristics of the world population of dams was also carriedout. By comparing the characteristics of the dams which have experienced failures and accidents to those of the popu-lation of dams, it was possible to assess the relative influence of particular factors on the likelihood of piping andslope instability.

Key words: dams, failures, piping, instability database.

Rsum : Cet article dcrit les rsultats dune analyse statistique des ruptures et accidents dans les barrages en terre,se concentrant spcifiquement sur ces incidents impliquant la formation de renard et linstabilit des talus. La compila-tion des incidents de barrages inclut des dtails sur les caractristiques des barrages incluant le zonage du barrage, lesfiltres, les types de noyau, le compactage, le rideau dtanchit de fondation, et la gologie de la fondation. Unevaluation des caractristiques de la population mondiale des barrages a galement t ralise. En comparant lescaractristiques des barrages qui ont t affects par des ruptures et des accidents avec celles de la population des bar-rages, il a t possible dvaluer linfluence relative que des facteurs particuliers ont sur la vraisemblance de renards etde linstabilit des talus.

Mots cls : barrages, ruptures, renard, base de donnes sur linstabilit.

[Traduit par la Rdaction] Foster et al. 1024

Introduction

Embankment dam engineering has evolved over manycenturies, with the major developments occurring since the1940s with the development of soil mechanics andgeotechnical engineering. Some aspects are now readily ana-lysed, e.g., the stability of the embankment slopes. Others,e.g., piping failure through a dam foundation, remain moredifficult to quantify, and the measures taken in design andconstruction are more experience based. It is particularly dif-ficult to assess the safety of dams which do not meet moderndesign and construction criteria, e.g., dams with no or inade-quate filters.

Recognising the value of the historic performance of damsin assessing dam safety, the International Commission onLarge Dams (ICOLD) has carried out extensive surveys ofdam incidents (ICOLD 1974, 1983, 1995). The ICOLD sur-veys are for large dams, a large dam being defined as a damwhich is more than 15 m in height (measured from the low-est point in the general foundations to the crest of the dam)

or any dam between 10 and 15 m in height which meets oneof the following conditions: (i) the crest length is not lessthan 500 m, (ii) the capacity of the reservoir formed by thedam is not less than 106 m3, (iii) the maximum flood dis-charge 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 deter-mine the most common cause of dam incidents. Others, in-cluding USCOLD (1975, 1988), USCOLD Committee onDam Safety (1996), ANCOLD (1992), Charles and Boden(1985), Olwage and Oosthuizen (1984), and Gomez et al.(1979), have compiled data on incidents for various coun-tries. There have been attempts to use the statistical analysisof 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 ofthese analyses are limited to the statistics of height, year ofconstruction, and only basic descriptions of the dam type.For example, the embankment zoning classification is re-stricted to two categories in the ICOLD dam incident anddam population databases, namely earthfill embankment(TE) and rockfill embankment (ER).

Embankment zoning would be expected to have a signifi-cant influence on the likelihood of failure, particularly forstructural modes of failure which include slope stability andpiping. As is well recognised in dam engineering, and de-scribed in Fell et al. (1992), different embankment zoningtypes have varying degrees of control of embankment seep-age. These give varying degrees of control of the potential

Can. Geotech. J. 37: 10001024 (2000) 2000 NRC Canada

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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.

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for piping failure through the embankment and foundationand the pore pressures which affect slope stability.

This paper presents the results of a statistical analysis ofembankment dam incidents, specifically concentrating on in-ternal erosion and slope instability. While the data can beused in a quantitative risk assessment (QRA) framework,they also provide useful insights into the factors which con-tribute to dam incidents and can therefore be used in a non-QRA, dam safety context. The study has been done as partof a larger research project studying methods for estimatingthe probability of failure of embankment dams for use inQRA. This paper describes only part of the research project.Foster et al. (2000) describe the application of these data toestimating the relative likelihood of failure of embankmentdams by internal erosion and piping, Foster and Fell (2000a)discuss the assessment of filters which do not satisfy moderndesign criteria, and Foster and Fell (2000b) use event treesto estimate the probability of failure of embankment damsby internal erosion and piping. All components are describedin Foster (1999).

The principal components of the study were as follows:(i) extension of the existing compilations of dam incidents toinclude more details on embankment zoning, including thepresence or absence of filters, foundation geology, and em-bankment material characteristics such as core soil types andcompaction; and (ii) analysis of the dam incident databaseand comparison to a dam population database to estimatehistoric frequencies of failure for different modes of failureand dam zoning types and identify factors, such as founda-tion geology types and core embankment characteristics, thathave an influence on the likelihood of embankment dam fail-ure for piping and slope instability modes of failure.

Establishment of databases

Compilation of dam incident databaseA list of dam incidents was compiled primarily from the

three ICOLD studies (ICOLD 1974, 1983, 1995) supple-mented with additional incidents from the other existingcompilations, from the literature, and from the project spon-sors. The criteria set for the selection of the dam incidents tobe entered into the database are (i) embankment dam failuresfor all modes of failure for large dams; (ii) failures of em-bankment dams (not necessarily large) by piping and slopeinstability; and (iii) accidents involving piping, slope insta-bility, 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 anddam population datasets consistent for proper statisticalanalysis. The other two criteria were set to maximise theamount of data for the detailed analysis of piping and slope-instability failures.

Data on the dam and incident details were obtained from(i) incident descriptions in ICOLD (1974, 1983) and othercompilations of dam incidents; (ii) published data from anextensive search through the literature; and (iii) reports col-lected from the sponsor organisations and from the UnitedStates Bureau of Reclamation (USBR), British ColumbiaHydroelectric and Power Corporation (BC Hydro), and theNorwegian Geotechnical Institute.

Information on the dam and incident details was extractedfrom the data gathered and entered into a database calledERDATA1.

The ERDATA1 database is divided into seven main cate-gories: (i) dam details, e.g., dam name, country, height, yearconstructed; (ii) dam zoning category, including dam zoningand description of filters; (iii) foundation cutoff category;(iv) foundation geology; (v) earthfill core characteristics;(vi) incident details; and (vii) references. Sketches of thedam zoning categories used are shown in Fig. 1. A full de-scription of the variables in the ERDATA1 database is givenin Foster et al. (1998) and Foster (1999).

Population of embankment dams databaseThe population of embankment dams database is required

to determine whether an overrepresentation of a particulardam characteristic in the incident cases, such as a particularzoning type, is due to this characteristic being common in allexisting dams, or whether dams with this characteristic tendto be more susceptible to dam incidents. The data from thepopulation of dams database are combined with those fromthe incident database to estimate the frequencies of failureand accidents for the various dam zoning types.

The ERDATA1 classification system has been used forcollating data on the population of dams, so the databasesare consistent. The ideal situation for a sound statisticalanalysis would be to have the characteristics of theERDATA1 classification system for all the existing embank-ment dams as listed in the World register of dams (ICOLD1984). However, there are insufficient data in the ICOLDregister to obtain data on dam zoning, foundation geology,and other characteristics in the ERDATA1 classification sys-tem, so it was necessary to select sample populations of em-bankment dams in an attempt to represent the characteristicsof the world population. Sample populations of embankmentdams which were used include 356 dams in Australia, 44 inNew Zealand, 246 in the United States (from the USBR),174 in Norway, and 642 described in papers in the ICOLDcongresses up to 1982, giving a total of 1462 embankmentdams, or about 13% of the total population.

These datasets were primarily selected due to the avail-ability of sufficient data required for the ERDATA1 classifi-cation system. Information for the Australian and NewZealand dams was obtained from the project sponsors andfrom questionnaires sent out to dam owning authorities. Thezoning characteristics for the dams in the United States wereobtained from USBR (1994) and the foundation geologycharacteristics from the Safety Evaluation of Existing Dams(SEED) databooks held at the USBR office in Denver, Colo-rado. Data on the Norwegian dams were limited to the damzoning category which was obtained from K. Senneset (per-sonal communication, 1996). Dams described in the ICOLDcongress papers from 1933 to 1982 were used to obtain arepresentative set of dams to take account of the range oftrends in dam construction with time and for various coun-tries.

Information was also collected from the literature, provid-ing additional data on the distribution of dam zoning categoriesin different parts of the world and describing general trendsin dam design. Literature sources included Snethlage et al.(1958), Leps et al. (1978), Building Research Establishment

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(1990), Sherard (1953), ICOLD (1984), Skempton (1990),Cooke (1984), ICOLD (1989), and Schnitter (1994).

The following approach was used for each of the maincategories: (i) dam zoning; (ii) foundation geology; and(iii) core material geological origin, soil classification, andcompaction.

Dam zoningThe general approach used to make the estimates of the

world distribution of zoning types was to subdivide the datainto tables according to the country, dam height range(50 m), and constructionyear period (before 1900, 19001929, 19301949, 195069,and 19701986).

The dam zoning distribution estimates were made forAustralia, France, India, Japan (post-1950 only), New Zea-land, Norway, United Kingdom, and United States. Thesecountries had sufficient data in the sample population avail-able to give reasonably reliable estimates of the zoning char-acteristics of the population. The dams from remainingcountries were grouped into a category called other coun-tries. Dams constructed in China, and in Japan prior to 1930,were excluded from the analysis of the population due to thelack of information in the literature on these dams and lowreported failure rates despite them making up a significantproportion of the dam population.

The number of earthfill (TE) and rockfill (ER) dams foreach of the countries, construction periods, and height cate-gories were obtained from the ICOLD register (ICOLD1984). The information from the sample populations anddata gathered from the literature were used as a basis tomake estimates of the number of dams for each of the dam

zoning categories. A considerable degree of judgement wasrequired to make the estimates, but this was facilitated bybreaking down the data into the smaller units of constructionyears and dam height ranges. The analysis was by a trial anderror process in which the estimated percentages were modi-fied so that they reflected the expected trends in dam designwith time and dam height.

Table 1 shows the assessed distribution of dam zoning forthe world population of dams accounting for constructionperiod. Rockfill dams, comprising zoned earth and rockfilldams, central core earth and rockfill dams, concrete facerockfill 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 rockfilldams (ER), namely 9%, given by the ICOLD register(ICOLD 1984). This difference is attributed to the differ-ences in classification of rockfill dams of the ERDATA1 andICOLD systems. Zoned earth and rockfill dams with lessthan 50% rockfill by volume are classified as TE by ICOLD.

Foundation geologyThe distribution of foundation geology types was assumed

to be dependent on the spatial distribution of dams. Esti-mates were made for countries where sufficient data wereavailable from the sample population, namely United States,India, United Kingdom, Canada, Australia, and New Zea-land. This subset of countries was carried through into theanalysis of the dam incidents when comparing the distribu-tion of foundation geology types of the accident and failurecases to the population. The distributions of geology typesfrom the sample population database were determined foreach of the countries and were used as a basis together with

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Fig. 1. Dam zoning categories.



the use of geology maps of the countries to make estimatesof the percentage distribution of foundation geology types.

Core material geological origin, soil classification, andcompaction

Estimation of the distribution of the geological origin ofthe core material was made on the assumption that the coregeology would be influenced by the regional geology andwas therefore analysed by country. The distributions of coregeological origin types from the population database and thefoundation geology distributions described above were usedas a basis to make estimates of the core geology distribu-tions for United States, Australia, New Zealand, UnitedKingdom, Canada, India, Norway, and other countries. Thefinal world distribution was then obtained by applying aweighted sum of the distributions from the countries basedon the number of large embankment dams in each country in1982 (from ICOLD 1984).

The distribution of core soil types, classified by theUnified Soil Classification System (USCS), was determineddirectly from the percentages of core soil types from thepopulation database. It was assumed that there were suffi-cient data in the population database to give a representativesample of core soil types. Typically, the core material com-prises more than one soil type and so the percentages in thedistribution do not necessarily sum to 100%. The percent-ages were normalised such that they sum to 100% for com-parison with the incident statistics.

Estimates of the distribution of the degree of compactionof the core materials were made using the population data-base, assuming it to be dependent mainly on the period ofconstruction. Hydraulic fill and puddle core dams were notincluded in the analysis because their compaction is inherentin 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 abundanceof the presence of dispersive soils in the cores of embank-ment dams due to their prominence in the piping failurecases. There was insufficient information in the populationdatabase on which to make such an estimate and so informa-tion from the literature was used. ICOLD (1990) lists partsof the world which have experienced problems withdispersive soils, and combined these countries have approxi-mately 35% of large embankment dams. There is no basis on

which to estimate the proportion of dams in these countrieswith dispersive soils actually present in the core, but thevalue is probably less than 25%. If it is assumed that say 510% of the dams in these countries have dispersive soilspresent in the core, then a very approximate estimate of theproportion of dams in the world with dispersive soils is 24%.

Other dam characteristicsEstimates for the distributions of the other dam character-

istics, such as foundation cutoff details, embankment andfoundation filters, and the location of conduits, were deter-mined directly from the population database. Details aregiven in Foster et al. (1998) and Foster (1999). The statisticsof 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 aregiven in Foster (1999) and Foster et al. (1998).

Analysis methodology

There are two components of the analysis of theERDATA1 database, namely analysis of the overall statisticsof failure, and detailed analysis of piping and slope-instabilityfailures.

Mode of failureThe philosophy of the analysis of the ERDATA1 data was

to categorise dam accidents and failures into modes of fail-ure as opposed to causes of failure. This is compatible withthe methods used in event-tree analysis.

The failure mode categories used are flood overtopping,gatespillway failure, piping, slope instability, and earth-quake. Piping failures were further subdivided into pipingthrough the foundation and piping from the embankmentinto the foundation. Slope-instability failures were subdi-vided into upstream slides and downstream slides.

Dam incidents sometimes involve more than one failuremode, so for example, development of piping through theembankment may cause saturation of the downstream slopewhich then initiates a downstream slide. In these cases, allthe modes of failure that were involved in the dam incidentare assigned in the database.

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Zoning category Before 1900 19001930 19301950 19501970 19701986 All yearsHomogeneous earthfill 16 14 16 9 6 10Earthfill with filter 0 1 11 18 18 15Earthfill with rock toe 5 5 6 7 6 6Zoned earthfill 7 18 37 37 40 36Zoned earth and rockfill 0 7 8 10 10 9Central core earth and rockfill 0 0 5 8 12 8Concrete face earthfill 5 4 5 5 3 4Concrete face rockfill 1 5 2 2.5 3 3Puddle core earthfill 58 24 4 0.5 0 5Earthfill with corewall 5 11 4 2 1 2Rockfill with corewall 0.5 3 1 1 1 1Hydraulic fill 2 8 1 0 0 1Number of embankment dams 370 819 1167 4436 4400 11 192

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



Overall statistics of failureThe frequencies of failure are estimated from the number

of dam incidents compared with either the total number oflarge (i.e., >15, high) embankment dams (or dams of a par-ticular zoning) up to 1986 to give the average frequencies offailure over the life of the dam, or the total number of em-bankment dam-years, allowing for the estimated average ageof each zoning type (up to 1986) to give annual frequenciesof failure.

Analysis of piping and slope instability incidentsThis analysis involved comparing the frequency of occur-

rence of the dam characteristics such as dam zoning type,foundation geology, and embankment core type in the damincidents to that in the dam population. An overrepre-sentation of a particular feature, such as a particular damzoning or foundation geology type, in the dam incidents rel-ative to the dam population suggests dams with that particu-lar feature are possibly more vulnerable to that failure mode.

The analysis method utilises the concept that piping andslope-instability failure modes can be broken down into sev-eral stages of development. Typically these stages are takento be initiation, progression, and breaching, as shown inFig. 2 for piping through the embankment. This concept hasbeen suggested by several authors, including Von Thun(1996). Accidents involve initiation of piping, but the pro-gression stage is limited and breaching does not occur.Therefore, by comparing the characteristics of the accidentcases to the failure cases of a particular failure mode, it maybe possible to identify factors that influence the progressionstage of the failure mode.

The analysis of the data has kept all failures as onedataset, 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 fromother piping failures. An initial assessment of the datashowed there did not appear to be big differences in the geo-logical and core material characteristics for these sets, butthis was not proven in a rigorous statistical way. Separationof the data had the major disadvantage that already smallsamples (of the dams which had failed or had accidents)would become smaller.

Overall statistics of failure of embankmentdams

Table 2 gives the overall statistics of failure for all failuremodes, separating for all failures and failures during opera-

tions. The historical average frequency of failure of largeembankment dams is estimated to be 1.2% over the life ofthe dam (136 embankment dam failures out of 11 192 largeembankment dams constructed up to 1986, excluding Chinaand Japan pre-1930). This reduces slightly to 1.1% over thelife of the dam for dam failures occurring only while thedam was in operation. The historical annual probability offailure of large embankment dams is estimated to be 4.5 104 per dam per year (136 embankment dam failures in anestimated 300 400 embankment dam-years up to 1986). Thisreduces slightly to 4.1 104 per dam year if failures occur-ring during construction are excluded. These figures wouldreduce by about 30% if none of the dams in the 11 192 hadfailed up to 1999.

Table 3 presents the statistics of failure by failure modeand dam zoning. These overall statistics are useful if onemakes the assumption that the performance of dams in thepast is a reasonable prediction of what may happen in the fu-ture. It is apparent from the statistics combined with the con-ventional understanding of dam stability and piping thatfactors such as dam zoning and core material properties havean influence on the likelihood of failure or accidents and canbe used to get an idea of which dams are more or less likelyto experience stability and piping problems.

An analysis was carried out of the frequency of embank-ment dam failures for each mode of failure for the failuresoccurring before and after 1950 (excluding failures duringconstruction). The failure statistics for dams constructed be-fore and after 1950 are shown for all modes of failure andstructural modes of failure in Table 4. Structural modes offailure are those involving piping, slope instability, or earth-quake. The analysis showed the proportion of failures bypiping increases from 43% before 1950 to 54% after 1950.Over the same period, the proportion of failures by floodovertopping and appurtenant works modes of failure de-creases from 53% to 41%. There is a significant reduction inthe proportion of failures due to sliding with time, reducingfrom 7% before 1950 to only 1.5% after 1950.

The following sections present the outcomes of furtheranalysis of the incidents for piping and slope instability tofurther assess the factors affecting the frequency of incidentsof piping through the embankment and slope instability. Thepopulation database consists of over 11 000 dams, with over300 000 dam-years of performance, so the analysis of the re-lationship of zoning to incidents is based on a large sample.For the assessment of other factors, e.g., core material prop-erties, there is a greater reliance on the characteristics of thedams that have experienced incidents, and the sample size is

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Fig. 2. Stages of development of piping failure.



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

Factors affecting the frequency of incidentsof piping through the embankment

Incidents have been classified as piping through the em-bankment if the incident involved any type of internal ero-sion process occurring primarily through the embankmentdam. Cases of piping along and into conduits through damsare included. Cases where piping initiated at the embank-mentfoundation contact are not included but analysed sepa-rately under piping from the embankment into thefoundation.

Dam zoningThe failure and accident statistics for piping through the

embankment are summarised in Table 5. It is evident that thedam zoning categories with high average frequencies of fail-ure by piping through the embankment tend to be the zoningtypes with inherently poor control of seepage through theembankment. Homogeneous earthfill dams, which have nozoning of materials, have the highest frequency of failure,nearly five times higher than the average of all dams com-bined. Other dam zoning categories with higher than averagefrequencies of failure by this mode of piping are earthfillwith rock toe, concrete face earthfill, and puddle coreearthfill dams. These four zoning categories combined makeup nearly 80% of the failure cases but only 25% of the pop-ulation. For homogeneous earthfill dams, earthfill dams withrock toe, and concrete face earthfill dams, there are the samenumber or more failures than accidents. It is possible thatmany piping accidents have not been reported to the ICOLD

studies or in the literature, but this trend suggests that thesedams are more likely than other dams to fail (i.e., breach)once piping initiates.

Embankment dams with downstream rockfill zones have aparticularly low incidence of failure due to piping throughthe embankment. There is only one dam failure due to pip-ing through the embankment for zoning categories withdownstream rockfill. This was Avalon Dam, a zoned earthand rockfill dam which failed in 1904. It had no filter be-tween the core and the dumped rockfill. The large number ofpiping accidents but no failures of central core earth androckfill dams indicates that these dams have a low frequencyof failure because they are less likely to progress to breach-ing if piping initiates compared with dams with earthfill ma-terials in the downstream zones. Review of the descriptionsof the accidents to rockfill dams suggest that this is probablydue to the inherent stability of the downstream rockfill zonesunder 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 conduitspassing through the dam (in nine cases), piping throughpoorly compacted fill materials (in 12 cases), and pipingthrough dispersive fill materials (in four cases). The averagefrequency of failure of homogeneous earthfill dams con-structed prior to 1900 is about 10 times higher than that fordams constructed after 1950.

(2) The failures of earthfill dams with filter drains havegenerally been associated with piping through dispersive fillmaterials around outlet conduits passing through the dam (inthree cases) or at the contact with concrete spillway struc-tures (one case). Embankment filters were present in two ofthe failures, and in both cases failure was attributed to pipingaround the outlet conduit where locally there were no filters

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No. of cases% failures(where known)

Average frequency offailure (103)

Mode of failureAllfailures

Failures inoperation

Allfailures


Allfailures


Overtopping and appurtenantOvertopping 46 40 35.9 34.2 4.1 3.6Spillwaygate 16 15 12.5 12.8 1.4 1.3Subtotal 62 55 48.4 47.0 5.5 4.9

PipingThrough embankment 39 38 30.5 32.5 3.5 3.4Through foundation 19 18 14.8 15.4 1.7 1.6From embankment into foundation 2 2 1.6 1.7 0.18 0.18Subtotal 59 57 46.1 48.7 5.3 5.1

SlidesDownstream 6 4 4.7 3.4 0.54 0.36Upstream 1 1 0.8 0.9 0.09 0.09Subtotal 7 5 5.5 4.3 0.63 0.45Earthquakeliquefaction 2 2 1.6 1.7 0.18 0.18Unknown mode 8 7Total no. of failures 136 124 12.2 (1.2%) 11.1 (1.1%)Total no. of failures where mode of failure known 128 117No. of embankment dams 11 192 11 192

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

Table 2. Overall failure statistics for large embankment dams up to 1986, excluding dams constructed in Japan pre-1930 and in China.



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Mode of failurePiping Slope instability

SpillwaygatefailureDam zoning type

% ofpopulation

No. offailurecases

% offailurecases

Throughembankment

Throughfoundation

Fromembankmentinto foundation

Downstreamslide

Upstreamslide Earthquake Overtopping Unknown

Homogeneous earthfill 9.5 23 (17) 28 (32) 14 2 0 1 0 1 6 0 0Earthfill with filter 15 4 (2) 5 (4) 2 0 0 0 0 0 2 0 0Earthfill with rock toe 6.1 9 (9) 11 (17) 5 3 1 0 0 0 0 0 0Zoned earthfill 35.9 7 (5) 9 (9) 4 1 0 0 0 0 2 0 0Zoned earth and

rockfill9.3 4 (3) 5 (6) 1 0 1 1 0 0 1 0 0

Central core earthfilland rockfill

8.4 4 (1) 5 (2) 0 0 0 1 0 0 3 0 0

Concrete face earthfill 4.1 4 (4) 5 (8) 2 3 0 0 0 0 0 0 0Concrete face rockfill 2.8 1 (0) 1 (0) 0 0 0 0 0 0 1 0 0Puddle core earthfill 4.7 5 (4) 6 (8) 4 0 0 0 0 0 0 1 0Earthfill with concrete

corewall2.4 11 (3) 13 (6) 0 2 0 0 1 0 4 3 1

Rockfill with concretecorewall

0.9 0 (0) 0 (0) 0 0 0 0 0 0 0 0 0

Hydraulic fill 0.9 5 (3) 6 (6) 0 1 0 2 0 0 0 2 0Other 5 (3) 6 (6) 1 2 0 0 0 0 1 1 0Unknown 54 (13) 6 5 0 1 0 1 26 9 7Total 100 136 (66) 100 (100) 39 19 2 6 1 2 46 16 8

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 sumto the total number of failure cases because some dams were classified as multiple modes of failure.

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



provided. For the other three failures, only foundation filterswere present.

(3) Earthfill dams with a rock toe have one of the highestfrequencies of failure by piping through the embankment.The failure and accident cases have generally been the resultof outlet conduits passing through the dam, piping of the fillmaterials into coarse rockfill materials, or piping throughcracks which formed through the dam over irregularities inthe foundation or steep abutments. Zoned earthfill dams havea relatively low probability of failure; three of the four damsfailed on first filling and the other dam (Walter BouldinDam) failed after 8 years operation. The construction peri-ods for the dams that failed range from 1947 to 1975.

(4) In four of the seven accidents to zoned earth androckfill dams, no embankment filter was provided and corematerials were eroded into the downstream rockfill. The pip-ing incidents at McMillan and Scofield dams demonstratedthe high discharge capacity of rockfill zones (in both cases,dumped rockfill zones). At Scofield Dam, leakage during thepiping incident was estimated to be in the range 1400

5000 L/s through the rockfill. It took 2 days to stabilise theflow through the dam by placing sandbags on the crest.

(5) There are a total of 21 piping accidents to central coreearth and rockfill dams, 15 of these involving piping ofbroadly graded core materials of glacial origin into coarse orsegregated filters. At Matahina and Bullileo dams, no down-stream filters were present and the core and transition mate-rials were eroded into the downstream rockfill zone.

(6) In all three cases of failures of concrete face earthfilldams, piping occurred around the conduits. The cause offailure was attributed to problems associated with the con-nection of the conduit to the upstream face in two of thecases and in the other case as a result of settlement and rup-ture of the conduit within the embankment.

(7) Concrete (or other impervious) face rockfill dams havea low incidence of piping through the embankment, with nofailures and only two piping accident cases. Both accidentcases were attributed to internal erosion of the bedding layerof the upstream face into the rockfill zone. Neither of thetwo dams had concrete upstream faces: one was constructed

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Class of damsDams constructedbefore 1950

Dams constructedafter 1950 All dams

No. of large embankment dams constructed 2356 8836 11 192No. of large embankment dam failures by all modes of failure 88 48 136No. of embankment dam failures by structural modes of failure 41 25 66Average frequency of failure over life of dam 3.8102 0.5102 1.2102Average frequency of failure over life of dam by structural modes of failure 1.7102 0.3102 0.6102Average annual frequency of failure by all modes of failure 8.6104 2.7104 4.1104Average annual frequency of failure by structural modes of failure 3.6104 1.6104 2.0104

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

Averagefrequencyof failure(103)

Averagefrequency ofaccident(103)

Average annual frequency offailure (106)*

Zoning categoryNo. offailures

No. ofaccidents

First 5 yearsof operation

After 5 yearsof operation

Homogeneous earthfill 14 9 16.0 9.2 2086 188Earthfill with filter 2 1 1.5 0.6 189 37Earthfill with rock toe 5 5 8.9 8.0 1160 158Zoned earthfill 4 9 1.2 2.4 158 25Zoned earth and rockfill 1 7 1.2 7.3 152 24Central core earth and rockfill 0 (1) 19 (

with a plastic membrane (Martin Gonzalo Dam), and theother with a bituminous concrete membrane (Scotts PeakDam). There are also 11 cases involving leakages throughthe concrete face in the dam incident database, but these areclassified as seepage accidents because they did not involvepiping of materials. An upper bound average frequency offailure of

It is evident that dams with core materials of glacial originhave experienced more piping accidents (i.e., initiation ofpiping) than those built from other materials but have experi-enced fewer failures. The erodibility of some glacial soilshas been shown by pinhole tests (Ravaska 1997) to be simi-lar to that of highly dispersive clays (category D1). It is pos-sible that these glacial soils are more erodible because theirfine silt and clay size fractions are finely ground rock, ratherthan more common clay minerals. The piping accidents in-volving glacial core materials have mostly occurred in damswhere embankment filters were present. Review of thesecases indicates problems have been attributed to the use ofcoarse or segregated filters adjacent to the broadly gradedglacial core materials in central core earth and rockfill dams.Failures involving piping of glacial core materials have onlyoccurred where no filters were present.

Dams with core materials of alluvial origin have experi-enced more than the average number of piping failures butan average number of piping accidents. In all but one of thenine failure cases involving alluvial soils, low-plasticity silts(ML) or silty and clayey sands (SM, SC) were present.Dams constructed of residual soils experience an averagenumber of piping failures. However, in three of the 10 pipingfailures, the residual soils were dispersive, and therefore thefrequency of piping failures in nondispersive residual soilstends to be less than the average.

Unified Soil Classification and dispersivityThe relationship between the classification of the embank-

ment core materials and the incidence of piping through theembankment was analysed. Dams constructed of low-plasticity silts (ML) experienced more piping failures thanaverage, those constructed of low-plasticity clays (CL) mar-ginally less failures than average, and those constructed ofhigh-plasticity clays marginally less incidents than average,but if dispersive soils are excluded dams experienced muchfewer incidents than average.

Dams constructed of dispersive clays are particularly sus-ceptible to piping failures. Dispersive clays are recorded aspresent in 18% (nine out of 51 cases) of the piping failures.However, the actual proportion is likely to be higher, as themajority of piping failures occurred prior to knowledge ofthe nature of dispersive clays. All nine of the piping failureswhere dispersive clays were known to be present occurredon first (and usually rapid) filling of the reservoir. In six ofthe cases, piping occurred around conduits or adjacent tospillways. In one spectacular case, La Escondida Dam, 50independent piping tunnels and eight breaches formedthrough the dam on first filling.

Compaction of the core materialTables 7 and 8 present the statistics of the incidence of

piping related to the compaction of the core material. Thepiping incidents for hydraulic fill or puddle dams are notconsidered in the analysis because these forms of compac-tion are directly related to the zoning of the dams, which isanalysed separately.

It is concluded that dams with no or poor compaction ofthe core material experience many more piping incidentsthan the average, and those with good compaction somewhatless than the average. Dams with limited control of erosionthrough the dam and with no formal compaction are morelikely to fail than experience accidents.

Factors affecting the frequency of incidentsof piping through the foundation

Incidents are classified as piping through the foundationwhere they involved any type of internal erosion process oc-curring primarily through the foundation of the dam. The in-cidents have been classified into categories depending on thenature of the internal erosion as shown in Table 9. The ratioof the number of accidents to the number of failures by pip-ing through the foundation (77 accidents to 19 failures) issignificantly higher than that for piping through the embankment,

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Foster et al. 1009

No. of cases in the pipingincidents

% ofpipingfailures

% ofpipingaccidents

% of totalpopulationof damsMethod of compaction Failures Accidents

No formal compaction 11 8 38 17 8Rolled, modest control 13 25 45 52 32Rolled, good control 5 15 17 31 60Unknown 17 35Total (where known) 29 48 100 100 100

Table 7. Statistics of compaction of the core for the dams experiencing piping incidents.

Class of dams % of piping failures % of the populationAll dams (excluding hydraulic fill and puddle core)Well compacted 38 60No or poor compaction 62 40Dams with limited internal erosion controlWell compacted 24 60No or poor compaction 76 40

Note: Dam types with limited zoning are homogeneous earthfill, earthfill with foundation filter only, and earthfill with rock toe.

Table 8. Summary of data relating piping incidents to internal erosion control and compaction of the core material.



suggesting that this mode of piping is less likely to progressfrom the initiation stage of piping to breaching of the dam.Self-healing of the piping processes by collapse of the pipe,or self-limitation by the finite width of open joints in thecase of rock foundations, may partly explain the relativelylow number of failures by this mode of piping.

Dam zoningTable 10 shows the statistics of failures and accidents by

piping through the foundation for each of the zoning catego-ries and for all embankment dams combined. The low num-ber of failure cases for foundation piping makescomparisons between the dam zoning categories somewhatdoubtful, but the dam zoning categories with above-averagefrequencies of failure are homogeneous earthfill, earthfill

with rock toe, concrete face earthfill, earthfill with corewall,and hydraulic fill. These zoning categories would be ex-pected to have relatively poor control of seepage and porepressures within the downstream foundation, and therefore itis concluded that the zoning of the dam does appear to havesome influence on the frequency of failure by foundationpiping. The high incidence of piping failures for these zon-ing categories is not reflected in the accidents, so it is likelythe poor zoning of these dams contributes to the progres-sion of piping. However, as discussed in the following sec-tions, it is evident from the failures that other factors such asthe type of cutoff and foundation geology are more influen-tial.

The significant proportion of rockfill dams in the founda-tion piping accidents (25%) but not in the failures suggests

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Averagefrequency offailure (103)

Averagefrequency ofaccident (103)

Average annual frequency offailure (103)

Zoning categoryNo. offailures

No. ofaccidents

First 5 yearsof operation

After 5 yearsof operation

Homogeneous earthfill 2 9 3.0 11.2 447 25Earthfill with filter 0 5 3.9Earthfill with rock toe 3 2 7.0 3.9 1044 88Zoned earthfill 1 14 0.4 4.6 59 6Zoned earth and rockfill 0 6 7.6Central core earth and rockfill 0 7 9.8Concrete (or other) face earthfill 3 2 10.4 5.8 1553 105Concrete (or other) face rockfill 0 0Puddle core earthfill 0 0Concrete corewall, earthfill 2 1 11.8 4.9 1768 68Concrete corewall, rockfill 0 0Hydraulic fill 1 7 15.7 91.8 2358 61Unknown 7 17All dams 19 70 1.7 6.2 255 19

Note: The percentage of failures by piping through the embankment occurring at the different times after construction are as follows: 25% during firstfilling, 50% during first 5 years of operation, and 25% after 5 year of operation. Calculations of annual frequencies of failure are made as follows: annualfrequency of failure (all years) = (average frequency of failure)/(average age), annual frequency of failure (first 5 years) = (average frequency offailure)0.75/5, and annual frequency of failure (after 5 years) = (average frequency of failure)0.25/(average age 5).

Table 10. Average frequency of failure due to piping through the foundation by dam zoning types for large dams up to 1986.

Type of piping No. of failures No. of accidentsPiping through soil foundation

Dam foundation 7 16Abutment 1 10Reservoir foundation 1 3Subtotal 9 (43) 29 (34)

Piping foundation soil into foundation rockDam foundation 0 5Reservoir foundation 0 1Subtotal 0 (0) 6 (7)

Piping through rock foundation 6 (28) 7 (8)Piping spillway foundation 1 (5) 10 (12)Piping foundation soil into relief wells toe drains 0 (0) 9 (11)Sand boils in foundation 0 (0) 22 (26)Unknown 5 (24) 2 (2)Total 21 (100) 85 (100)

Note: Percentages are given in parentheses.

Table 9. Incidence of piping through the foundation.



the high seepage gradients allow initiation of piping, but therockfill prevents progression of piping to breaching.

Foundation filtersThe effect of the presence of foundation filters is summa-

rised in Table 11. Comparison with the population suggeststhey are important in preventing failures but not accidents.

Foundation cutoffFoundation cutoff descriptors and other cutoff types used

in the ERDATA1 classification system have been simplifiedfor analysis of the foundation piping incidents into twobroad categories, namely partially penetrating and fully pen-

etrating. Here cutoff has been taken as cutoff trenches andother cutoff types such as cutoff walls as shown in Fig. 3.The number of foundation piping incidents in which the twogeneral types of foundation cutoff were present is listed inTable 12.

The presence of grouting does not appear to have had asignificant influence in reducing the likelihood of initiationof piping through rock foundations or piping of foundationsoils into foundation rock, with grouting carried out in ap-proximately 50% of the incidents. There are five accidentswhere piping occurred through the soil foundation despitefully penetrating cutoffs being present. In these, foundationerosion occurred through fractured foundation rock belowthe cutoff (in two cases), due to an improper seal betweenthe cutoff wall and bedrock (in one case), or due to seepagethrough the abutment under the cutoff (in two cases).

Foundation geology

Piping in soil foundationsFoundation soil geology types that were involved in inci-

dents of piping through soil foundations are shown inTable 13 with the distribution of foundation geology types

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Foster et al. 1011

Presence of foundation filter No. of failures No. of accidentsNo foundation filter 13 33One foundation filter 1 23Two foundation filters 0 2Unknown 7 27Total 21 85

Table 11. Presence of foundation filters in piping through foun-dation accident and failure cases.

Fig. 3. Foundation cutoff categories: (a) partially penetrating cutoff where cutoff does not extend to bedrock; (b) fully penetrating cut-off where cutoff extends to bedrock.

No. of failures No. of accidents

Type of piping

Partiallypenetratingcutoff

Fullypenetratingcutoff

Partiallypenetratingcutoff

Fullypenetratingcutoff

Piping through soil foundation 9 0 23 5Piping of foundation soil into rock 0 0 1 5Piping through rock foundation 1 4 0 7Foundation sand boils 0 0 14 5Piping of foundation soil into drainage systems 0 0 7 2Piping in spillway foundation 0 0 5 1Unknown 1 0 0 0Total 11 4 50 25% of piping incidents 73 27 67 33% of population 15 85 15 85

Table 12. Incidence of piping through the foundation related to foundation cutoff types.



for the selected group of countries in the embankment dampopulation.

The data suggest dams founded on glacial and colluvialsoils are more likely to experience piping accidents thanother dams but less likely to experience piping failures.Therefore it appears that these soils are more susceptible tothe initiation of piping, but the soil properties and structuregive them good self-healing characteristics or a high likeli-hood of collapse of the pipes, so the progression of piping isless likely when compared to other soils.

Table 13 also suggests dams with residual soils in thefoundation are more likely to experience piping failures thanthe average. However, there are limited data, and in two ofthe three failures involving piping of residual soils, namelyBaldwin Hills Reservoir and Laguna Dam, the residual soilswere dispersive. Residual soils are not common in the pipingaccidents. Therefore dams with residual soils present in thefoundation are less likely to experience piping incidents inthe foundation unless the soils are dispersive.

Piping in rock foundationsNot unexpectedly, dams constructed on limestone rock

foundations appear to be particularly susceptible to pipingincidents. Review of the accident and failure cases withlimestone foundations shows that seven of the eight cases in-volved piping through infilled solution channels, joints, orkarstic cavities in the limestone. The other incidents were insandstone (one failure) and shale (one failure, one accident).Piping of foundation soil into foundation rock

Foundation soils were piped into granite rock in three ofthe five incident cases involving piping of foundation soilinto foundation rock. These three cases occurred in glaciatedvalleys. Stress-relief effects on granite rocks due to glaci-ation may have contributed to fracturing of the bedrock inthese cases. These are described in Fell et al. (1992) and dis-cussed further in this paper in relation to piping of embank-ment soils into rock foundations. In the other two cases, thefoundation soils were piped into a karstic limestone founda-tion (at Walter F. George Lock Dam) and into open fracturedsandstone (at Bad Axe Watershed Structure No. 33). In the

latter case, vertical fractures, with joint openings 50300 mmwide, were present in the foundation sandstone in the abut-ments of the dam.

Factors affecting the frequency of incidentsof piping from the embankment into thefoundation

Incidents have been classified as piping from the embank-ment into the foundation in the cases where embankmentmaterials, including the cutoff trench fill, have been erodedinto the foundation of the dam due to seepage through thefoundation or embankment. Table 14 summarises the statis-tics of failures and accidents for piping from the embank-ment to the foundation. There are insufficient failure cases(four in total) for this type of piping to assess the signifi-cance of the various factors on failures alone, so the analysisuses failures and accidents and is likely to identify the fac-tors that have the most influence on the frequency of initia-tion of piping.

The statistics given in Table 15 may underestimate the ac-tual frequencies of piping from the embankment into thefoundation. All of the failure cases for this mode of failureoccurred after 1976, and it is likely that failures occurringprior to this were classified by the investigators of the fail-ures as piping through the embankment or foundation. Thewell-investigated and well-publicised case of Teton Damfailure in 1976 highlighted this mode of piping as a possiblemode of piping for other failures occurring after 1976.

Piping of embankment materials into rock foundations ismore common (24 accidents, three failures) than piping intosoil foundations (seven accidents, one failure), possibly

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Soil geology typeNo. offailures

No. ofaccidents

Population(%)

Alluvial 5 (50) 10 (32) 56Glacial 1 (10) 9 (29) 19Aeolian 0 (0) 3 (10) 6Colluvial 0 (0) 5 (16) 7Lacustrine 0 (0) 2 (6) 3Residual 3 (30) 1 (3) 8Volcanic 1 (10) 1 (3)

because of the presence of continuous, open joints in somerock foundations. It is also likely to be easier to identify thanpiping into soils.

Dam zoningThe analysis of incidents shows that dam zoning has little

influence on the initiation of piping. However, it would beexpected that dams with good downstream discharge capac-ity, e.g., free-draining rockfill, would be less likely to prog-ress to breach compared with dams with downstream zoneswith poor discharge capacity (e.g., zoned earthfill with siltysandy gravel in the downstream zone).

Foundation filtersFoundation filters were present in six of the 31 accidents

and one of the four failures. In two of the accidents withfoundation filters, the accident was partly attributed to thepresence of coarse filters. All cases where foundation filterswere present involved piping into rock foundations, so it ap-pears the seepage bypassed the filters, or the filters werewashed into the joints.

Foundation cutoffAll accidents and failures involving piping of embank-

ment materials into rock foundations have fully penetratingcutoff trenches to and into rock. The characteristics of thefoundation cutoff present in these incidents are summarisedin Table 15. The presence of deep, narrow cutoff trenchesand poorly treated open joints were believed to have beenimportant contributing factors in the failure of Teton Damand in accidents involving six puddle core dams. In all ofthese cases, it was considered highly likely that hydraulicfracturing within the cutoff trenches had contributed to theinitiation of piping. Even without fracturing, the gradientsare often high. These conditions can also make compactionof the earthfill materials difficult and thereby potentially giveareas of poorly compacted earthfill materials against thefoundation.

The majority of incidents involving piping from the em-bankment into soil foundations have occurred in dams withpartially penetrating cutoff trenches with no other cutoffpresent. Two accidents with fully penetrating cutoff trenchesinvolved puddle core earthfill dams with deep and narrowtrenches.

Erosion control at the corefoundation contactErosion-control measures, in the form of either concrete

covering or filter protection, at the embankmentfoundationcontact are present in only two of the incidents involvingpiping from the embankment into the foundation. Both ofthese are accidents involving piping into fractured bedrock.At Brodhead Dam, glacial core materials were eroded into acoarse foundation blanket layer which covered an area ofhighly weathered and fractured shale bedrock. At HallbyDam, glacial core materials were eroded into granite bed-rock despite the presence of foundation treatment in theform of sealing of cracks and partial covering with concreteslabs. The cause of this incident was attributed to the flush-ing out of montmorillonitic clay material in the open jointsof the granite bedrock.

Foundation geologyTable 16 presents the geology of the rock foundations into

which piping occurred. Interbedded sandstone and shalefoundations are relatively common in the incidents involvingpiping into rock foundations. It is known that the presenceof interbedded layers of sandstone and shale can give rise toopen joints due to stress-relief effects, as described in Fell etal. (1992). All six of the incident descriptions involving pip-ing into interbedded sandstone and shale foundations notedthe fractured nature of the bedrock and the presence of openjoints. At Fontenelle Dam, open stress-relief joints in theabutment were up to 300 mm wide, and at Cowm Dam verywide (estimated to be at least 0.5 m), open stress-relief jointsare visible in the construction photographs of the foundationbedrock. Piping of embankment materials into limestonefoundations was also relatively common. In two of the cases,Wolf Creek Dam and Apa Dam, embankment materials werepiped into solution channels in the limestone foundation. AtTaibilla Dam, core materials were piped into open joints, upto 100 mm wide, in the limestone foundation.

At least 10 of the 24 accidents involving piping into rockfoundations occurred in geological environments previouslyaffected by glaciation. The presence of highly fractured bed-rock and open joints is noted in all 10 of these cases. Fell etal. (1992) describe the open fractured nature of bedrock be-neath glacial deposits at several sites. The formation of thesefeatures is attributed to the effects of stress relief orglacitectonic thrusting. The four accidents involving granite

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Foster et al. 1013

Foundation rockNo. offailures

No. ofaccidents

% of failuresand accidents

% inpopulation

Sandstone 3 14339

Sandstone and shale 1 6 21*Sandstone and limestone 2Limestone 3 14 7Granite 3 14 7Quartzite 1 4 3Schist 1 4 7Tuff 1 4 2Basalt 1 4 5Unknown 1 4

* For sandstone.

Table 16. Foundation rock into which core material piped.

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foundations were in geological environments that had beenaffected by glaciation. The presence of horizontal stress-relief joints in the granite bedrock was found to be the causeof the piping incident at Churchill Falls GJ-11A Dyke.

The three soil foundation types present in piping into soilfoundation, namely glacial (four accidents), colluvial (threeaccidents), and alluvial (four accidents), are all characterisedby the potential for great variability, high permeability, and alarge range in grain sizes. In two of the accidents, the pres-ence of open-work gravel layers in the foundation wasnoted. Soil foundations of glacial and colluvial origin appearto be overrepresented, which is not surprising because theyoften have cobbles and boulders with large voids.

Embankment core characteristicsThe effects of the geological origin, Unified Soil Classifi-

cation, and compaction on the frequency of piping incidentsare similar to those for piping through the embankment.

Factors affecting the frequency of incidentsof downstream sliding

Incidents have been classified as downstream slides wherethe incident has involved any form of sliding movement ofthe downstream slope of the dam. Dams which have shownsigns of incipient sliding of the downstream slope have alsobeen included. Downstream slides that have occurred todams during construction have been classified as accidentsunless they have resulted in uncontrolled release of reservoirwater. This is consistent with the definition used in theICOLD (1995) study.

The downstream slide incidents have been classified ac-cording to the location and type of sliding movement. Theseare sloughing (progressive sliding of the downstream slopedue to seepage through the embankment), through the em-bankment (slide surface passes through the embankmentonly), and through the embankment and foundation (base ofslide surface passes through the foundation).

Table 17 summarises the statistics of failures and acci-dents for downstream slides. The fact that there are rela-tively many accidents compared to failures may reflect thefact that movement usually occurs slowly, giving warning ofa slope-instability problem and allowing remedial action ordrawing down of the reservoir, or the slide is simply tooshallow to directly release the reservoir.

Dam zoning and type of slidingTable 18 summarises the types of sliding and how these

relate to the dam zoning. There are very few failures in total,and of these only one (Utica Dam) involved failure throughthe embankment, and one (Fruitgrowers Dam) failurethrough the foundation. As might be expected, the type ofembankment zoning appears to have a significant influenceon the frequency of initiation of sliding of the downstreamslope. Dam zoning types with poor control of pore pressuresand seepage within the dam and foundation, such as homo-geneous earthfill, earthfill with foundation filter only,earthfill with rock toe, and earthfill with concrete corewalldams, all have average frequencies of initiation of slidinggreater than the average of all the dams combined. Theoverrepresentation of puddle core earthfill, earthfill withconcrete corewall, and hydraulic fill dams in the slide

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No. ofcases

Averagefrequency

Average annual frequencyCategory First 5 years After 5 yearsFailures 6 5.4104 4105 1.5105Incidents (failures and accidents) 59 5.3103 5.2104 1.2104Incidents in operation 50 4.4103 4.4104 1.0104

Table 17. Statistics of failures and accidents for downstream slides on large dams up to 1986.

Zoning type Sloughing EmbankmentEmbankmentand foundation Unknown

Homogeneous earthfill 5 (1) 12 6 (1) 3 (2)Earthfill with filter 1 4 4 0Earthfill with rock toe 1 3 0 0Zoned earthfill 1 2 3 0Zoned earthfill and rockfill 1 (1) 0 0 0Central core earth and rockfill 1 (1) 0 2 0Concrete face earthfill 0 0 0 0Concrete face rockfill 0 0 0 0Puddle core earthfill 0 1 3 0Earthfill with concrete corewall 1 4 0 0Rockfill with concrete corewall 0 0 0 0Hydraulic fill 3 (1) 2 0 1 (1)Other 0 1 0 1 (1)Unknown 0 5 (1) 0 2 (1)Total 14 (4) 34 (1) 18 (1) 7 (5)

Note: Values in parentheses are the number of failure cases.

Table 18. Incidence of downstream slide type and dam zoning (failure and accident cases combined).

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incidents may also be a reflection of the lower standards ofdesign and construction (particularly the lack of compactionby rollers) of these older dams compared with more moderndam types. Dam types which inherently have relatively goodcontrol of pore pressures and high, reliable shear strength inthe downstream zone, such as zoned earthfill, zoned earthand rockfill, central core earth and rockfill, and concreteface rockfill, tend to have a relatively low frequency ofdownstream slide incidents compared with the dam popula-tion.

There is a recorded case of a failure of a recently con-structed concrete face rockfill dam, Gouhou Dam, which isbelieved to have failed by sliding of the downstream slope.The incident is not included in the analysis because dams inChina are excluded from the accident and failure statistics inthis study. The cause of the failure is attributed to leakagethrough the poorly constructed connection between the crestwave wall and the concrete face which led to saturation ofthe gravel fill downstream slope and probable sliding. Poorzoning of the sandy gravel fill forming the body of the damis believed to have been a major contributing cause of thefailure.

Foundation geologyThe relationship of downstream sliding incidents to foun-

dation geology has been assessed and shows that slidingthrough the foundation has occurred mainly on soil founda-tions (16 out of 18 cases). Sliding through rock has onlybeen noted in two cases. The type of rock, or origin of thesoil, has little influence, but the presence of high-plasticityclays in the foundation does seem important, with these be-ing present in 12 of the 18 incidents involving slidingthrough the foundation. High-plasticity clays are generallycharacterised by relatively low shear strength and, probablymore importantly, a large reduction from peak to residualshear strength. The presence of fissured high-plasticity clayswas noted in two of the incidents involving sliding throughthe foundation.

The presence of soft sedimentary rocks and residual soilsof sedimentary origin in the foundation appears to be a com-mon feature of both upstream and downstream slide inci-dents where sliding has occurred through the foundation;40% of upstream and downstream foundation slides com-bined (i.e., 14 out of 35 cases) involved sliding throughthese materials. The presence of low-strength bedding fea-tures such as surface bedding shears in this geological envi-

ronment, particularly in interbedded sandstone and shales,may explain why foundation slide incidents are so common.

Embankment core characteristics

Geological originThe geological origin of the core material appears to have

some influence on the frequency of slide incidents, withcores of lacustrine origin more susceptible, cores of glacialorigin less susceptible, and cores of residual and alluvialsoils neutral. Sloughing incidents are more likely with soilsof alluvial and glacial origin, and rotational slides are morelikely to occur in cores built of residual, glacial, and lacus-trine soils.

Unified Soil ClassificationDams embankments comprised of high-plasticity materials

are much more likely than average to experience rotational-type slide incidents. Half of the high-plasticity core materi-als were of either glacial or lacustrine origin, with the otherhalf unknown. No cases of sloughing-type incidents areknown to have occurred in dams with high-plasticity materi-als. Dams with embankment materials of low-plasticity claysand silts (CL, ML) are also more likely to experience slideincidents by both rotational- and sloughing-type slides. Ap-proximately 70% of the cases with low-plasticity core mate-rials are of either residual or alluvial origin.

Embankment core materials comprised of clayey or siltysands and gravels (SC, SM, GC, GM) are shown to be lesslikely to experience slide incidents. The lower proportion ofsandy materials in the embankment core materials of theslide incidents is probably indicative of the higher shearstrength of these materials compared to the more clayey corematerials. They may be generally more permeable, facilitat-ing dissipation of pore pressures and giving less chance ofcontractive behaviour in poorly rolled material.

CompactionTable 19 summarises the compaction of dams that have

experienced downstream sliding incidents. Hydraulic fill andpuddle core dams are not included because the core compac-tion is directly related to the zoning of the dam.

As might be expected, the degree of compaction of thecore materials appears to have a strong influence on the like-lihood of initiation of sliding. Relatively poor compactionconditions exist in the majority of the downstream slide

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Foster et al. 1015

Compaction of core materials

No. ofrotational-typeslides

No. ofsloughing-typeslides

% ofincidents

% ofpopulation

Hydraulic fill 2 3 Puddle 6 0 No formal compaction 10 6 48 8Rolled, modest control 8 2 30 32Rolled, good control 7 0 21 60Unknown 19 3 Total no. of slide incidents 52 14 Total no. of incidents (where known) 33 11 100 100

Table 19. Summary of data relating downstream slide incidents to the method of compaction of thecore material.



incident cases. The ratio of the percentage of incidents tothat of the population suggests dams which have no formalcompaction of the core materials are about seven times morelikely to experience slides than the average and about 25times more likely than dams with good compaction of thecore. Poorly compacted core materials would be expected tohave considerably lower shear strengths and be more perme-able compared with materials with poorly compacted clays.The increased permeability of poorly compacted soils canpotentially allow wetting and softening of the core materialsand failure in undrained loading. Slides through the founda-tion appear to be less influenced by the degree of compac-tion of the embankment.

Factors affecting the frequency of incidentsof upstream sliding

Incidents have been classified as upstream slides wherethe incident has involved any form of sliding of the upstreamslope of the dam. The upstream slide incidents have beenclassified according to the location and type of sliding move-ment in the same way as that for downstream slides. In addi-tion, upstream slide types have been classified according towhether or not the slide was initiated by a drawdown of thereservoir.

Table 20 summarises the statistics of failures and acci-dents for upstream slides. There is only one large embank-ment dam that is known to have failed (i.e., breached) due tosliding of the upstream slope. The incident description ofKaila Dam (ICOLD 1974) describes settlement and pipingaround the conduit giving rise to the slip of the entire up-stream bank up to the corewall. It has been assumed the in-cident at Kaila Dam is a failure (i.e., was breached), whichis consistent with the classification given by both ICOLD(1974) and ICOLD (1995).

The average frequency of failure occurring once sliding ofthe upstream slope has initiated is one failure out of 46cases, giving a ratio of approximately 2%. This is five timeslower than that for downstream slides. This is considered tobe an upper bound, as one might expect that a large numberof upstream sliding accident cases have not been reported inthe ICOLD studies or in the literature and the failure ofKaila Dam might just as well be classified as a piping fail-ure.

A total of 57% of all the upstream slide incidents wereinitiated by a drawdown of the reservoir. The one failurecase does not appear to be a drawdown slide. Of the 19 inci-dent cases which are not drawdown slides, 10 occurred dur-ing construction prior to reservoir filling. Therefore, 73% ofupstream slides which occurred in operation (i.e., excludingconstruction slides) were initiated by drawdown of the reser-voir. This helps explain why so few upstream slides havecaused breaches: the water level in the reservoir is low, so

freeboard is large. The number of cases for each of the threedifferent types of downstream slides are listed for the acci-dent and failure cases in Table 21.

Sloughing-type slides are relatively uncommon in up-stream slide incidents, with only one accident to HolmesCreek Dam, which was constructed of very uniform, fine,cohesionless sand. In this case, progressive sloughing of theupstream face was initiated by an explosion and within 5 hthe sloughing had cut 9 m into the crest of the dam (Sherard1953).

Dam zoning and type of slidingAn analysis of the relationship of dam zoning to upstream

slides shows the following embankment dam zoning catego-ries which have average frequencies of initiation of upstreamslides higher than or similar to the average of all dams com-bined and the approximate ratios of the average frequenciesof the dam zoning categories to the average frequency of alldams (for example, homogeneous earthfill dams are aboutthree times more likely to experience upstream slides thanall dams combined): homogeneous earthfill 3, earthfill withrock toe 1, concrete face earthfill 1.5, puddle core earthfill2, earthfill with concrete corewall 2.5, and hydraulic fill12. All of these dam zoning types generally have earthfillmaterial in the upstream slope, making them more suscepti-ble to upstream slides under drawdown conditions. The olderdams are also likely to have been poorly compacted.

All of the dams with rockfill zoning have a low incidenceof upstream slide incidents. This is related to the relativelyhigh shear strength and high permeability of rockfill materi-als. Only one of these three incidents was initiated bydrawdown of the reservoir (old Eildon Dam), which was aconcrete corewall dam with an earthfill core and rockfillshoulders. The other two cases were a construction slide of acentral core earth and rockfill dam founded on soft fatclays (Clendening Dam) and an upstream slide initiated byan earthquake (La Calera Dam).

Zoned earthfill dams are less likely to experience up-stream slide incidents than the population of dams by a fac-tor of approximately 0.5. The majority of upstream slideincidents to zoned earthfill dams have been foundationslides, making up six of the nine incidents to this dam type.

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No. ofcases

Averagefrequency

Average annual probabilityCategory First 5 years After 5 yearsFailures 1 9105 3106 all yearsIncidents (failures and accidents) 47 4.2103 4.1104 1.0104

Table 20. Statistics of failures and accidents for upstream slides on large dams.

Type of slideNo. offailures

No. ofaccidents

Sloughing 0 1Through embankment only 0 26Through embankment and foundation 0 17Unknown 1 2Total 1 46

Table 21. Incidence of upstream slide incidents.



Foundation geologyThe relationship of upstream sliding to incidents to foun-

dation geology has been assessed and shows that the major-ity of the foundation slide incidents involve sliding throughsoil foundations, making up 13 of the 17 cases. Sliding oc-curred through rock foundations in only four incidents,namely at Bear Gulch Dam, where sliding was initiated onthree occasions, and at Fort Peck Dam.

Embankment core characteristicsThe relationship between geological origin and compac-

tion of the materials and incidence of slides is similar forboth upstream and downstream slides. Dams with core mate-rials composed of clay materials are more likely to experi-ence upstream slide incidents than the average. This isevident for dams with low-plasticity clays (CL), which arenearly two times more likely to experience slides than theaverage. Embankment core materials comprised of clayey orsilty sands and gravels (SC, SM, GC, GM) are less likely toexperience slide incidents by both upstream and downstreamslides. This is probably indicative of the higher shearstrength of these materials compared with the more clayeycore materials.

Summary of the factors affecting thefrequency, timing, and location of pipingand sliding

Factors affecting the frequency of piping and slidingTables 2224 summarise the factors influencing the fre-

quency of piping, and Tables 25 and 26 summarise the fac-tors affecting the frequency of sliding. These tables havebeen prepared from the analysis of the data, but also take ac-

count of general dam engineering principles and the natureof soil and rock environments. The allocation into the cate-gories much more likely, more likely, neutral, lesslikely, and much less likely is somewhat judgmental. It isrecognised that some factors may be surrogates for the other,e.g., the geological environment and soil classification prop-erties may be linked. However, in some cases, particularlyfor older dams, there may be little other than the geologicalenvironment available to assess the dams, so we have in-cluded both factors.

The analysis of dam zoning, filters, core properties, andcompaction from the dam failure statistics, as described inthe preceding sections, has assessed the factors individually.An analysis was also carried out to assess what combinationof factors is more likely to lead to piping failure.

Dams which failed generally had several poor characteris-tics, such as limited zoning, no filters, poor compaction, anderodible soils. Dams which suffered accidents commonlyhad fewer of the poor characteristics and more of the goodcharacteristics. The results of this simple analysis indicatethat dams which have a combination of several poor charac-teristics are much more likely to fail by piping than thosewith only one or two poor characteristics. As an example,the presence of dispersive soils in itself may not necessarilyinfluence the likelihood of failure but is more likely to influ-ence a dam with poor compaction around a conduit and ho-mogeneous zoning.

Timing of incidentsThe timing of piping incidents is summarised in Ta-

bles 2729 and Fig. 4. The frequency of piping failures issignificantly higher on first filling and early in the life of thedam. There does, however, appear to be a trend for olderdams to also experience piping incidents. This may reflect

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Foster et al. 1017

General factors influencing frequency of failureFactor Much more likely More likely Neutral Less likely Much less likelyZoning Homogeneous earthfill,

earthfill with rocktoe, puddle coreearthfill

Concrete faceearthfill

Earthfill withconcrete corewall,hydraulic fill

Earthfill withfilter, zonedearthfill,zoned earthand rockfill

Central core earthand rockfill;concrete facerockfill; rockfillwith concretecorewall

Embankment filters No embankmentfilter

Embankment filterpresent

Core geologicalorigin

Alluvial Aeolian, colluvial Residual, lacustrine,marine, volcanic

Glacial

Core soil type Dispersive clays; low-plasticity silts (ML);poorly graded andwell-graded sands(SP, SW)

Clayey and siltysands (SC, SM)

Well-graded andpoorly gradedgravels (GW,GP); high-plastic-ity silts (MH)

Clayey andsilty gravels(GC, GM);low-plasticityclays (CL)

High-plasticity clays(CH)

Compaction No formal compaction Rolled, modestcontrol

Puddle, hydraulicfill

Rolled, good control

Conduits and otherlocations of piping

Conduit through theembankment

Irregularities infoundation orabutment, steepabutments

No conduit throughthe embankment

Table 22. Summary of the factors influencing the frequency of failure by piping through the embankment.



deterioration or the design and construction of the olderdams.

The time of the incident of piping in the embankment foreach of the dam zoning categories has been analysed forfailures and accidents. The results suggest that the time ofincident for failures may be related to dam zoning. Homoge-neous earthfill, earthfill with filter, earthfill with rock toe,and zoned earthfill dams all show a relatively high propor-tion of failures on first filling, ranging from 50% failures forhomogeneous earthfill dams up to 7080% for the otherthree dam zoning types. Less than 25% of the failures inconcrete face earthfill and puddle core dams occurred on

first filling. For the accidents, less than half of the piping in-cidents occurred on first filling for most of the dam zoningcategories. Piping accidents of puddle core earthfill damstend to occur generally after many years of operation.

The time of sliding incidents is summarised in Tables 30and 31. An assessment of the different failure types showsthat slides through the foundation tend to occur at an earlierage than slides through the embankment; 44% of the founda-tion slides occurred during construction or on first fillingcompared with only 15% of embankment slides. Sloughing-type slides also tend to be more frequent in the early stages.The potential for preexisting shear surfaces or other weak

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General factors influencing frequency of failureFactor Much more likely More likely Neutral Less likely Much less likelyZoning Homogeneous earthfill,

earthfill with rock toe,concrete face earthfill,earthfill with corewall,hydraulic fill

Puddle coreearthfill

Earthfill with filter,zoned earthfill

Zoned earth androckfill, central coreearth and rockfill,concrete face rockfill,rockfill with corewall

Filters No foundation filterpresent when required

No foundationfilter

Foundation filter(s)present

Foundation type(below cutoff)

Soil foundation Erodible rock Non-erodible rock

Foundation cutoff(soil foundation)

Shallow or no cutofftrench

Upstream blanket,partially penetratingcutoff wall

Foundation cutoff(rock foundation)

Fully penetrating cutoffwall

Cutoff trench

Soil geology type(below cutoff)

Dispersive soils,volcanic ash

Residual Aeolian,colluvial,lacustrine,marine

Alluvial Glacial

Rock type(below cutoff)

Limestone,dolomite,soluble rocks(gypsum), basalt

Tuff, rhyolite, marble,quartzite

Sandstone, shale,siltstone,claystone,mudstone,hornfels,agglomerate,volcanic breccia

Conglomerate,andesite, gabbro,granite, gneiss,schist, phyllite, slate

Table 23. Summary of the factors influencing the frequency of failure by piping through the foundation.

Fig. 4. Time after construction of incidents of piping through the embankment.



zones in the foundation of low shear strength may explainthe tendency of foundation slides to initiate at an early age.

The effect of pore pressures generated in high-plasticityclay foundations during construction may also explain thetendency of foundation soils to initiate sliding early. In sixof the eight foundation slide incidents that occurred duringconstruction or on first filling, sliding took place throughhigh-plasticity and (or) soft clays in the foundation. High-plasticity and soft clays are generally characterised by lowpermeability (giving slow dissipation of pore pressures), low

peak shear strength, and a large reduction in shear strengthfrom peak to residual once sliding is initiated. These all giveconditions conducive to initiation and progression of sliding.

Location of the initiation of pipingThe location where piping originated for the piping

through the embankment and piping from the embankmentto the foundation was noted from the incident descriptions.The number of cases where piping initiated for the differenttypes of location for both accidents and failures are plotted

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Foster et al. 1019

General factors influencing frequency of initiation of pipingFactor Much more likely More likely Neutral Less likely Much less likelyZoning Appears to be

independent ofzoning

Appears to beindependent ofzoning




Filters Appears to be inde-pendent ofpresenceabsence ofembankment orfoundation filters

Appears to be inde-pendent ofpresenceabsenceof embankmentor foundationfilters

Appears to be inde-pendent ofpresenceabsenceof embankment orfoundation filters

Appears to beindependent ofpresenceabsence ofembankment orfoundationfilters

Appears to beindependent ofpresenceabsence ofembankment orfoundationfilters

Foundation cutofftrench

Deep and narrowcutoff trench

Average cutoff trenchwidth and depth

Shallow or nocutoff trench

Foundation type Founding on orpartly on rockfoundations

Founding on orpartly on soilfoundations

Erosion-controlmeasures of corefoundation

No erosion-controlmeasures, openjointed bedrock, oropen-work gravels

No erosion-controlmeasures

Erosion-controlmeasures

present

Grouting offoundations

No grouting onrock foundations

Soil foundation only,not applicable

Rock foundationsgrouted

Soil geology type Colluvial Glacial Residual Alluvial, aeolian,lacustrine,marine, volcanic

Rock type Sandstone interbeddedwith shale or lime-stone; limestone,gypsum

Dolomite, tuff,quartzite, rhyo-lite, basalt,marble

Agglomerate, volcanicbreccia, granite,andesite, gabbro,gneiss

Sandstone, con-glomerateschist, phyllite,slate, hornfels

Shale, siltstone,mudstone,claystone


Glacial Aeolian, alluvial, col-luvial lacustrine,marine, volcanic

Residual

Core soil type Dispersive clays; low-plasticity silts (ML);poorly graded andwell-graded sands(SP, SW)

Clayey and siltysands (SC, SM)

Well-graded andpoorly gradedgravels (GW, GP);high-plasticity silts(MH)

Clayey and siltygravels (GC,GM); low-plasticity clays

High-plasticityclays (CH)

Core compaction Appears to be inde-pendent ofcompaction

Appears to be inde-pendent ofcompaction

Appears to be inde-pendent ofcompaction

Appears to beindependent ofcompaction

Appears to beindependent ofcompaction

Foundationtreatment

Untreated verticalfaces or overhangsin core foundation

Irregularities infoundation orabutment, steepabutments

Careful slopemodification bycutting, fillingwith concrete

Careful slopemodification bycutting, fillingwith concrete

Note: The ranking is designed to place those rocks which commonly have open joints, e.g., due to stress relief, as much more likely and those with alow likelihood of open joints as much less likely. Some rock types, assumed to occur in large masses, e.g., andesite, sometimes occur as flows, so maybe better in a much more likely category in that case.

Table 24. Summary of the factors influencing the frequency of piping accidents and failures from the embankment into the foundation.



in Figs. 5 and 6. The presence of conduits through the em-bankment and (or) through the trenches in the foundationhas an important influence on the initiation of pipingthrough the embankment for the reasons discussed earlier.

Other locations where piping has initiated, but much lessfrequently than conduits, are at the contact between the em-

bankment and concrete structures such as spillways, over ir-regularities in the foundation or abutments, and over steepabutments. These are all locations in the embankment whichare particularly susceptible to hydraulic fracturing and (or)differential settlement and where difficulties can be experi-enced with compaction of the core materials.

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General factors influencing frequency of initiation of downstream slidesFactor Much more likely More likely Neutral Less likely Much less likelyZoning Homogeneous

earthfill, earthfillwith corewall,hydraulic earthfill

Earthfill with rocktoe

Earthfill with filter Concrete faceearthfill, puddlecore earthfill

Zoned earthfill, zonedearth and rockfill,central core earthand rockfill,concrete facerockfill, rockfillwith corewall

Foundation type Soil foundations Rock foundationsGeology type

(foundationslides)

High-plasticity claysin foundation, i.e.,marine, lacustrine

Residual soils ofsedimentaryorigin and softsedimentary rocks

All other geologytypes (due to lownumber of founda-tion slide cases)


Lacustrine Residual, alluvial,colluvial, volcanic

Glacial, aeolian

Core soil type High-plasticity claysand silts (for rota-tional slides)

Low-plasticity siltsand clays (ML,CL)

Clayey sands (SC) Clayey gravels(GC)

Silty sands andgravels (SM, GM)

Core compaction No formalcompaction

Rolled, modestcontrol

Puddle, hydraulic(accounted for byzoning)

Rolled, wellcompacted (forfoundationslides)

Rolled, wellcompacted(particularly forembankment slidesand sloughing)

Table 25. Summary of the factors influencing the frequency of downstream slides, accidents and failures.

Fig. 5. Piping initiation location of the incidents of piping through the embankment.



Conclusions

Overall statisticsThe analysis of the dam incidents in the ERDATA1 data-

base has shown that structural modes of failure, i.e., thoseinvolving piping, slope instability, or an earthquake, account

for approximately half of the failures to large embankmentdams. Piping failure account for most of these. The inci-dence of piping through the embankment is two times higherthan piping through the foundation an

the statistics of embankment dam failures and accidents

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