internal erosion and rehabilitation of an earth-rock dam

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Internal Erosion and Rehabilitation of an Earth-Rock DamRaúl Flores-Berrones, F.ASCE1; Martín Ramírez-Reynaga2; and Emir José Macari3

Abstract: This paper describes an earth-rock dam whose filters’ characteristics did not comply with the design criterion. In addition, thedegree of compaction and the water content in the impervious core consisted of highly plastic residual clay. Although Terzaghi’s filterdesign criterion was used, the as-built grain size distribution did not comply with such criterion. Several problems arose right after the firstfilling of the dam, including the following: �1� water leak at a rate of 200 L/s was observed along the downstream slope, in the vicinityof the outlet pipeline; the dark color of the observed water implied that the core material was being eroded; �2� two sinkholes near the crest�20 m apart� were observed along the outlet axis, one on the upstream and the other on the downstream slopes; right after the occurrenceof these two sinkholes, the water leakage decreases significantly, noticing a plugged effect of the material that falls down through thesinkholes. To avoid piping, a notch was installed on the spillway to quickly reduce the reservoir level. A site investigation around theaffected zone was performed to find the causes of the observed seepage through the dam. The paper presents a detailed description of thesite investigation. In addition, the paper presents the instrumentation, laboratory, and field tests employed as part of the site investigation.After analyzing the information produced in the site investigation, the following conclusions were derived: �1� large variation in the watercontent was used during the compaction of each lift of the impervious core which resulted in a highly heterogeneous core of the dam; �2�stress analysis of the zones around the outlet pipe demonstrated that the hydrostatic pressure in such zones exceeded the sum of thetransverse normal and tensile stresses, inducing hydraulic fracturing; and �3� because of the grain size segregation during the placementof the filter material, the upstream and downstream filters did not satisfy the design criterion. Description of the stabilization of thedamage zones through a grouting process, together with the construction of a diaphragm wall and an interface grouting, is presented.Finally, this paper reviews existing criteria for designing filters to protect earth and rockfill dams against internal erosion or piping andapplies these criteria to the described dam. A special discussion on this topic is also presented.

DOI: 10.1061/�ASCE�GT.1943-5606.0000371

CE Database subject headings: Dams, earth; Erosion; Dam failures; Rehabilitation.

Author keywords: Earth-rock dam; Internal erosion; Piping; Dam failure; Rehabilitation.

Introduction

The literature has been reported that one of the main causes oflevee, earth dam, and earth-rock dam failures or incidents is thephenomenon known as “piping.” In fact, piping and internal ero-sion are responsible for about 50% of failures in these types ofearthen embankments. To avoid this phenomenon in major dams,Casagrande �1968� recommended the following measures: �1� en-sure good and proper selection of the construction materials; �2�control homogeneity of these materials during constructionstages; �3� construct transition zones between coarse and fine ma-

1Research Engineer, Mexican Institute of Water Technology, PaseoCuauhnahuac 8532, Jiutepec, Mor. 62550, Mexico. E-mail: [email protected]

2Head of Geotechnical Dept., Mexican National Water Commission,Insurgentes Sur 2416, Coyoacan, 04340, Mexico D.F. E-mail: [email protected]

3Dean of Engineering and Computer Science, California State Univ.,Sacramento 6000 J St., Riverside Hall, Suite 2014, Sacramento,CA 95819-6023 �corresponding author�. E-mail: [email protected]

Note. This manuscript was submitted on January 24, 2008; approvedon September 18, 2010; published online on April 19, 2010. Discussionperiod open until July 1, 2011; separate discussions must be submitted forindividual papers. This paper is part of the Journal of Geotechnical andGeoenvironmental Engineering, Vol. 137, No. 2, February 1, 2011.
©ASCE, ISSN 1090-0241/2011/2-150–160/$25.00.
150 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINE

J. Geotech. Geoenviron. Eng

terials; and �4� place properly designed upstream and downstreamfilters.

This paper describes an earth-rock dam whose filters’ charac-teristics did not comply with Terzaghi’s filter criterion, and thedegree of compaction and water content in the impervious corediffered from the design specifications.

First the site and dam characteristics are described, followedby the characterization of the internal erosion that was detectedand the field investigation and rehabilitation work that was per-formed for this dam. Next, aspects of the filter design and con-struction are discussed, and several recommendations areprovided for avoiding piping or internal erosion incidents forearth-rock embankment dams. The incident reported herein oc-curred in the middle of 1991 and the filter’s design was made atthe end of 1988; since then, important contributions in this subjecthave been published. This paper tries to analyze the incident tak-ing into account current filter design criterion.

Site and Dam Characteristics

El Batan dam, located in El Pueblito River, southwest of Quere-taro City, in central Mexico, consists of an earth-rock embank-ment, 207 m long and 45 m high, over fragmentary basalt thatrequired small quantities of grouting during the consolidationtreatment �see Fig. 1�a��; as it is shown in Fig. 1�b�, this dam hasa lateral spillway near the left abutment. In order to control the
water flow during the construction stage, the embankment con-
ERING © ASCE / FEBRUARY 2011

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struction required a two-step process; the first one took place atthe right abutment �see Fig. 2� and the second one at the leftabutment where a diversion trench was constructed with slopes of0.5h:1.0v �see photo in Fig. 3� to place an outlet pipeline forirrigation. Fig. 4 shows some details of the diversion trench wherethe outlet pipe was placed. The spillway was located at the leftbank, as shown in Fig. 1�b�. The total water storage capacity ofthe reservoir was 6.5 h m3. It is important to note that El Pueblitovillage, of 5,000 people, is located 3-km downstream of El Batandam.

The impervious core of the dam consisted of a highly plasticblack residual clay �CH in the unified soil classification system�,with a permeability coefficient k=10−6 cm /s; the imperviouscore was compacted using sheep foot rollers in 20-cm thick liftsparallel to the longitudinal axis of the embankment. The compac-

Fig. 1. �a� Outlet cross section; �b

Fig. 2. Elevation view of the stages of com

JOURNAL OF GEOTECHNICAL AND GEO


tion equipment used at the lower part of the diversion trench wasa manual vibratory compactor, using the same water content anddensity specifications determined in the soil mechanics laboratoryfor the rest of the impervious core; nevertheless, as soon as it wasfeasible, the sheep foot rollers were used at the upper part of thistrench with passes parallel to the diversion trench axis. Table 1synthesizes the soil characteristics of the core clay.

The filter was constructed using sand and gravel materialwhich was dumped from the back of a truck without any compac-tion and was 2 m thick; as it will be mentioned below, this con-struction procedure caused serious material segregation. AlthoughTerzaghi’s criterion for filter design was applied �K. Terzaghi,unpublished report on Bou-Hanifia dam, North Africa, 1929, citedby Bertram �1940��, the constructed grain size distribution did notcomply with Terzaghi’s filter criterion because of the segregation

itudinal soil profile �design stage�

n and lines of equal water content �wt %�

� long

pactio

ENVIRONMENTAL ENGINEERING © ASCE / FEBRUARY 2011 / 151

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factor. This problem will be presented later on in this paper withmore details.

Surrounding the upstream and downstream filters, bouldersand rockfill materials �0.30-m maximum size� were placed withexternal slopes of 2.0h:1.0v; the grain size distribution of thesematerials is not available but it consisted of uniform �but irregularshape� size pieces of the excavated fragmentary basalt with anaverage size of 0.15 m.

Incident Occurrence

The construction of El Batan dam ended in December of 1990and had its first fill, up to elevation of 1,902 m, on July of 1991.Twelve days before the complete fill of the reservoir and as aconsequence of several rainy days, a water leak was observed onthe downstream slope, around the outlet pipeline �see Fig. 5�.

Fig. 3. Photograph of the diversion trench for placing the pipeline,looking downstream

Fig. 4. Cross section of the diversion and outlet pipe trench



Such leak was increasing gradually up to more than 50 L/s, andthe dark color of the water leak denoted that the core material wasbeing eroded. On the morning of July 28th of 1991, a water leakof 200 L/s was observed in the same place as mentioned before,and some hours later, during the same day, two sinkholes near thecrest �20 m apart� were observed along the outlet axis, one up-stream and the other one at the downstream slope, when the waterof the reservoir was at the elevation of 1,902.23 m �3.63 m abovethe spillway elevation�.

Right after the occurrence of these two sinkholes, the waterleakage decreased significantly, up to 15 L/s, likely as a result ofa plugging effect of the material that collapsed into the sinkholes.To prevent further erosion and piping, a notch was constructed inthe spillway to draw down the reservoir level to elevation of1,895 m and some material was placed in the sinkholes �24 h afterthey occurred� to replace the material that had been flushed down;a total of 410 m3 of material �mainly boulders and rockfill, likethose covering the filters in the original design� was used to plugup the sinkholes. Both operations were performed out as soon asit was possible.

Site Investigation

Among the emergency measures applied to prevent a completefailure of the embankment, in addition to the rapid draw down ofthe reservoir, a detailed search was conducted for any other waterleaks along the downstream face of this embankment, and surveil-lance was implemented to follow the evolution of the one aroundthe outlet pipe. The following risk reduction measures were takenaccording to the project emergency plan: �1� control release of thereservoir up to elevation of 1895 m; �2� permanent vigilance con-sisting in periodical dam inspections; �3� subsurface explorationsand instrumentation of the damaged zone; and �4� Make a breachat the crest of the spillway up to elevation of 1898.6 m to preventthe reservoir rising during site investigations and subsequent re-habilitation works.

An immediate soil investigation around the affected zone wasperformed to find out the causes of this incident. This investiga-tion consisted of digging out two braced shafts, drilling nine bor-ings �B3–B11 in Fig. 6� to obtain undisturbed and representativesoil samples, and installing one open piezometer in each of thebraced shafts. Shaft 1 �PCA1� was located at the center of thedam crest near the edge of the projection of the diversion trenchthat was excavated in the bedrock to installation of the outlet pipe�see Fig. 7�. The main purpose of this shaft was to search, alongthe walls, for any anomaly �cracks or traces of differential settle-

Table 1. Soil Characteristics of the Core Clay

Volumetric unit weight 1,650–1,900 kg /m3

Specified volumetric dry unitweight for compaction

1,325 kg /m3

Liquid limit 100–130%

Plastic index 70–105%

Organic matter content 4–7%

Cohesion in UU test 50–100 kPa

Friction angle in UU test 3–11°

Cohesion in CU test 40–70 kPa

Friction angle in CU test 3–22°

Optimum water content 34%

ments� caused by the drastic slope change that occurred on this


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part of the diversion trench. The depth of this shaft was 18 m butno significant anomalies were observed.

The second shaft �PCA2� was located near the intersection ofthe axis along the two sinkholes and the central axis of the crest.It was first excavated down to a 20 m depth, and later on a hollowstem auger was used to reach a depth of 28 m. During this exca-vation, undisturbed and representative soil samples were obtainedand Atterberg’s limits, volumetric unit weights, and water con-tents were determined in the soil mechanics laboratory; Table 1also shows a summary of these laboratory tests. At the depth of 27m �above the interface of the bedrock and clay core�, high waterpressures were observed. It was necessary to install a PVC pipe asa well casing to allow for the completion of the soil exploration inthis shaft.

Using undisturbed soil samples, the shear strength, permeabil-ity, and compressibility of the soils were obtained, along with theconventional index soil properties specified by the ASTM stan-dards. Some additional special tests, like the salt, organic content,and the crumb test �ASTM D6572-06�, were also performed. Thecrumb test, for determining dispersive characteristic of the clay atthe impervious core, resulted negative.

Some findings from the site investigation were the following:1. In boring B4, located near the upstream sinkhole, a 4-m sec-

tion of wet and loose soil, mixed with some boulders, wasfound within the impervious core between the depths of 18and 22 m.

2. The water level inside boring B4 was the same as the one atthe reservoir, indicating that there was a direct link betweenthem.

3. Once all borings were completed, several piezometers wereinstalled to assess, at different depths, the variation of waterlevel with time at various locations within the dam and tocompare these with the water level of the reservoir. Fig. 8

Fig. 5. Preliminary location o

shows this comparison where it can be observed that the



variation of the water level inside these borings was verymuch related to the reservoir level.

Site Investigation Analysis

After analyzing results of the site investigations, the followingconclusions were derived:1. Large variations in the water content during the compaction

of each lift of the impervious core of the embankment pro-duced high heterogeneity in this important element of thedam. Fig. 9 shows the design compaction curve for the em-bankment, and Fig. 10 shows that, between elevations of1,880 and 1,886 m, the water content was below the opti-mum; this fact could have significantly increased the hori-zontal permeability of the core material �up to three orders ofmagnitude�, inducing hydraulic fracturing and, as a conse-quence, internal soil erosion �Alberro 1995; Lofquist 1988�.It was also possible that the zones in the dry side of optimumcollapsed on saturation and caused the initial pathway inwhich erosion initiated. The variation of actual dry unitweight for different depths can be obtained through the in-formation given in Figs. 9 and 10.

2. Because of the steep-sided trench used for the outlet conduit,some differential settlements and arching were formed in thezone, and a hydraulic fracturing took place due to the effec-tive stresses and seepage forces generated by permanent ortransient flows of water. By considering the stress distribu-tion along the excavation for the installation of the outletpipe, zones where hydrostatic pressure exceeded the sum oftransverse normal stresses and tensile stresses were found.Thus, it is believed that in these zones, where the water pres-

holes, filtrations, and borings
f sink
sure exceeded the intergranular pressure, tension developed


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in the soil skeleton and the soil cracked by the processknown as hydraulic fracturing.

3. As it was already mentioned, the original Terzaghi’s criterion�i.e., without the ICOLD recommendations given in Appen-dix� was applied for the design of the upstream and down-stream filters surrounding the impervious core. Nevertheless,because of the grain size segregation during the placement ofthe filter material, such filters did not satisfy the main role:

Fig. 6. Locations of shaft and bori

Fig. 7. Location of shaft, outlet, and spillway



“protect the impervious core against internal erosion”; inother words, the “filters” did not work as it is supposed theyshould. Fig. 11 shows a schematic representation of the pip-ing mechanism within the embankment.

ound the outlet axis and sinkholes

Fig. 8. Piezometric and reservoir levels

ngs ar


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Filter Gradation Analysis

A correlation of the field information related to the core grain sizedistribution �base soil; Table 2� and that related to the installedfilter �Table 3� led to the results presented in Fig. 12. �The grainsize distribution curves of the base soil and the installed filter inthis figure represent the average of the range of such curves.� Thisfigure also shows the range given by Terzaghi’s criterion for thisbase soil. This criterion basically consisted in the following con-ditions:

D15F /D85

B � 4 – 5 � D15F /D15

B

where D15F represents the 15% of the filter material and D85

B andD15

B represent, respectively, the 85 and 15% size of adjacent soilsin place. The left part of the above equation �called piping ratio�is intended to avoid adjacent soil erosion, whereas the right part isto guarantee sufficient permeability to prevent the built up oflarge seepage forces and hydrostatic pressures in filters.

It can be observed that the grain size distribution of the in-stalled filter is not only different from Terzaghi’s criterion �whichwas supposedly applied for the design of this filter�, but the con-cave form of such curve distribution is opposite to the curverecommended by Terzaghi. It is believed that if the recommenda-tions given by the ICOLD �published four years later after theconstruction of this dam� to fulfill the Terzaghi’s filter criteria hadbeen followed, the constructed filter would have worked muchbetter during the occurrence of the incident.

Fig. 13 shows the range of the criteria recommended by theUnited States Soil Conservation Service �USSCS� �1994� for thesoil base of El Batan dam and the filter boundary size D15b �D15b

is the “characteristic” diameter of the filter material correspond-ing to 15% of the grain size curve� that Sherard and Dunnigan

Fig. 9. Design compaction curve of the embankment

�1989� defined through the “no erosion filter” test and used to



separate the successful and unsuccessful filters. Filters with D15

size smaller than D15b are not susceptible to base soil erosion,while filters having D15 sizes larger than D15b might be suscep-tible to some erosion of the base material. Note that Fig. 13 alsopresents the Terzaghi criteria for comparison with the one fromUSSCS.

The shape and concavity of the grain size curves resulted fromapplying Terzaghi and USSCS criteria might look quite differentfrom the curve corresponding to the installed filter; nevertheless,Terzaghi’s range is displaced to the right, indicating a filter de-sign with a more conservation grain size distribution criterion.It can also be observed that the value of D15b determined usingthe Sherard-Dunningan criterion �D15b=0.135 mm� was muchsmaller than the D15Batam�=0.6 mm� value corresponding to 15%of the installed filter in El Batan dam �see also Table 3�.

Using the erosion boundary concept defined in Fig. 14 by Fos-ter and Fell �2001� and applying their criteria for soils with D95

B

Fig. 10. Zones with water contents less than optimal �Wopt

=optimal water content�

�0.3 mm, together with the grain size information of the base


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soil and the installed filter for “El Batam” dam �obtained fromFig. 12 or from Tables 2 and 3�, Table 4 indicates that this casefalls between the “some-erosion” and the “excessive erosion”zones. Furthermore, according to the categories given in Table 5and the results presented by Ramirez-Reynaga �2003�, in connec-tion to El Batan dam’s filter, it can be said that the real classifi-cation for such filter falls into Category 3 of this last table since ithad a partial sealing �a plugging effect was observed from thewater leak around the outlet pipe� and two sinkholes occurred.

Using the criteria recommended by Wan and Fell �2008�,Fig. 15 shows that the constructed filter for this dam falls insidethe stable zone but near of the transition and unstable zones.Nevertheless, it is believed that the suffusion process existed dur-ing the incident of this dam.

Table 3. Grain Size Characteristics of the Installed Filter

Meshnumber

Size�mm�

%finer

0.018 2 According to Sherard and Dunnigan �1989�D15b=0.135 mm and according to USSCS�1994� D15 max

f =0.2 mm andD15 min

f =0.1 mm

40 0.425 12

30 0.6 15

10 2.00 25

8 2.36 30

4 4.75 50

1� / 2 12.50 64

3� / 4 19.00 75

1� 25.40 85

1.5� 37.50 100

of Terzaghi and the in-place filter gradation

Table 2. Grain Size Characteristics of the Base Soil in El Batan Dam

Meshnumber

Size�mm� % finer

0.0012 10

0.002 54

0.004 70

0.01 82

200 0.075 92

40 0.425 99

10 1.651 100

Fig. 11. Internal erosion paths

Fig. 12. Filter design band following the criterion


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From Fig. 12, D15=0.60 mm, D85=0.015 mm, and D95=0.15 mm.

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Adopted Solution

To restore the damaged section of El Batan dam, an imperviousdiaphragm wall was constructed using a cement-bentonite-watermixture. This impervious cutoff wall was 35 m long and 0.8 mwide and its depth varied from 13 to 36 m �bedrock depth plus 1m within the damaged zone to allow penetration into the bed-rock�. Fig. 16�b� shows the location of this barrier. The reservoirlevel that was maintained during the repaired works has an eleva-tion of 1,895,m.

The rehabilitation procedure consisted of the following stages:

Fig. 13. Grain size comparison of the constructed filter and t

Fig. 14. Filter erosion boundaries �Foster and Fell 2001, ASCE�



Table 4. Application of the Criteria of Foster and Fell �2001� for ElBatan Dam

Proposed criteriaby Foster and Fell Boundary

Valuesobtained for

El Batan dama

D15F /D85

B �9 No erosion D15F /D85

B =40��9�D15

F /D95B �9 Excessive or

continuingerosion

D15F /D95

B =4��9�

a F B B

Table 5. Filters’ Classification according to Their Field Performance�Adapted from Foster and Fell 2001�

Categorynumber Filters’ performance

1 Seal with no erosion; rapid sealing of theconcentrated leak, with no potential for damage andno or only minor to moderate increases in leakage

2 Seal with some erosion; sealing of the concentratedleak but with the potential for some damage andminor to moderate increases in leakage

3 Partial or no seal with large erosion; slow sealing orno sealing of the concentrated leak, with the potentialfor large erosion losses, large increases in seepage,and the development of sinkholes on the crest anderosion tunnels through the core

he design following the criteria of Terzaghi and the one of USSCS


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Stage I: Stabilization of the damaged areas using a groutingprocedure. This stage included excavation for the construction ofan impervious diaphragm wall and injecting grout into the soilsand allowing it to harden. The grouting process was carried out intwo steps. The first step consisted of gravity grouting using amixture of 73% water, 24.5% cement, and 2.5% bentonite. Thegravity grouting holes were bored 1 m into the rock �from thecenterline of the crest�, and they were 2 m apart. The averagedrilling fluid taking was 130 L/m and the maximum was 1,100L/m near the cross section 0+060. The second step consisted ofpressure grouting along two parallel lines, spaced 1.4 m from thecenter line and 2 m apart from each other, one upstream line andthe other one downstream �see Fig. 16�a��. To prevent hydraulicfracturing, a maximum grouting pressure of 400 kPa and a maxi-mum intake volume of 400 L were specified.

Fig. 15. Location of Batan’s filter in the graphic recommended byWan and Fell �2008, ASCE�

Fig. 16. �a� Grouting process; �b� cutoff wall



Stage II: Construction of the diaphragm wall �Fig. 16�b��. Thisstage included construction of a diaphragm wall using a slurrytrench method. To construct the wall, 19 alternated panels, 2.2 mlong and 0.8m wide, were excavated to depths varying from 13 to36 m. Drilling fluid for the stabilization of the excavated trencheswas prepared by mixing 75% water, 20.8% cement, and 4.2%bentonite. The construction procedure consisted of excavating al-ternating primary panels, allowing them to cure, and then pro-ceeding with the construction of the secondary panels. Ahydraulic clamshell was used for the excavation into the claycore. A rotary drill rig was used to penetrate into the bedrock.Details of this construction procedure are given by Ramirez-Reynaga et al. �2003�.

Stage III: Interface grouting. After a 28-day curing period,using the grouting injection number �GIN� method �Lombardi andDeere 1993�, a special grout was applied at the interface of thediaphragm wall and the bedrock. This grouting penetrated 5 minto the bedrock; the maximum allowable grouting pressure was500 kPa and the maximum intake volume was 500 L.

Observations of Leakage and PiezometerMeasurements

As shown in Figs. 17 and 18, several piezometers �piezoneumaticstations� and observation wells were installed into the imperviouscore of the dam at the upstream and downstream sides of thediaphragm wall. During a period of 10 years, several measure-ments have been recorded with these devices to assess differencesof water levels within the dam as a function of the reservoir waterlevel. Table 6 presents the computed hydraulic gradients at the toeof the downstream slope of the core for the following conditionsand two different water levels of the reservoir: �1� during thepiping observations; �2� after the occurrence of the sinkholes�plugging effect�; �3� design conditions; and �4� after installationof the cutoff wall. The results shown in Table 6 demonstrate thatthe impervious wall has performed very well during the 10-yearperiod. Fig. 19 presents the outline of the free water surface forthe 1,890-m elevation of the reservoir, for those conditions de-scribed in Table 6. Leakage quantities along the downstream sidesof the outlet pipeline registered a leakage reduction of 65% rightafter the construction of the diaphragm wall.

Conclusions and Recommendations

The main conclusions drawn from this earth-rock dam incidentare the following:1. Zones with sudden slope changes, like those located within

the diversion trench that accommodated the outlet pipe, werevery susceptible to differential settlements and arching, in-ducing cracking and hydraulic fracturing due to the tensionstresses and seepage forces within such zones. It was alsovery difficult to obtain the specified uniform soil compactionnear the transition zones around the outlet pipe, and thus theyconstituted weak areas where internal erosion could easilystart taking place.

2. The grain size distributions of filter zones in El Batan damwere much different than those proposed in Terzaghi’s crite-rion and they did not satisfy the more recent filter criteriaas described by Sherard and Dunnigan �1989� and USSCS�1994�; thus, the installed filters resulted too coarse and didnot work properly to protect against the internal erosion of
the core material.

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3. The field performance of El Batan dam’s filter corresponds tothe Category 3 given by Foster and Fell �2001� because theywere slow to seal, large potential erosion losses were ob-served, and two sinkholes resulted on the crest because oferosion tunneling through the impervious core.

Here, the conclusion is that the constructed filter wasoverly coarse partially due to the applied design criterion butmainly due to the construction processes.

4. Lessons learned from this case study indicate the necessity ofcontinuously updating the filter’s criterion when designingnew earth-rock dams. Unfortunately, many committed errorsare not recognized and published to avoid them in new de-signs.

5. One must do more that embedding filters along the upstreamand downstream sides of the impervious core to avoid thepotential for piping and internal erosion; one must also havegood quality control of the grain size distribution of the filtermaterials and avoid particle segregation during the placementof those filters. In addition, one must have a very strict water

Fig. 17. Location of p

Fig. 18. Plan view of the cutoff w

content quality control during the construction and compac-



tion of each lift of the impervious core as large variations inpermeability can result if soils are compacted in the dry sideof optimum.

6. Special precautions and observations should be taken duringand after first reservoir filling on this type of earth-rock damto apply immediate remedial actions that may be required toavoid a major incident or failure of the dam. The installationof appropriate piezometers at different depths, on both sidesof the impervious core, should show any anomalies of

Table 6. Theoretical Hydraulic Gradients through the Impervious Corefor Different Conditions

Computed at reservoir elevations= 1,902 m 1,890 m

During the piping observations 5.0 2.0

After the occurrence of the sinkholesand due to the plugging effect

2.0 1.0

Design conditions 1.4 0.8

After installation of cutoff wall 1.5 0.8

umatic stations �EPN�

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phreatic surface within the dam during these first or subse-quent reservoir fillings.

7. The construction of the diaphragm walls composed ofbentonite-cement-water mixture used to restore imperviouscores, like the one used in El Batan dam, could provide aviable solution to allow this type of dam to properly operateand fulfill their original objectives.

Acknowledgments

The writers wish to express their gratitude to the reviewers of thispaper for their magnificent comments and recommendations,which help significantly to its quality and comprehension. Wealso to thank the National Water Commission of Mexico for theinformation provided for this work.

Appendix

Recommendations given by International Commission on LargeDams �ICOLD� �1994� to fulfill Terzaghi’s filter criteria:1. Should not segregate during processing, handing, placing

spreading, or compaction. Filter gradation must be suffi-ciently uniform, so that, with appropriate care in the field,segregation is avoided in situ material, especially at the in-terface between adjacent materials.

2. Should not change in gradation �degrade or break down�during processing, handing, placing spreading, and/or com-paction or degrade with time, as might be caused by freeze-thaw or seepage flow. The filter must consist of hard durableparticles.

3. Should not have apparent or real cohesion or the ability tocement as a result of chemical, physical, or biological action.The filter must remain cohesionless so that no tendency tocracking exists even though cracking may have damaged anadjacent core zone.

4. Should be internally stable, that is, the coarser fraction of thefilter with respect to its own finer fraction must meet theretention �piping� criterion.

5. Should have sufficient discharge capacity so that seepage en-tering the system is conveyed safely and readily with little

Fig. 19. Outline of free water surface

head loss. Thus, chimney and blanket filter/drain systems



must be designed with an ample discharge capacity. The de-sign of filters and drains should consider the worst scenario,including a cracked core, hydraulic fracturing, and/or coresegregation

References

Alberro, J. �1995�. “Cracking and piping in earth and earth-rock dams.”Proc., X Pan-American Conf. on Soil Mechanics and Foundation En-gineering, Sociedad Mexicana de Mecánica de Suelos, Guadalajara,México, 1372–1404 �in Spanish�.

Bertram, G. E. �1940�. An experimental investigation of protective filters,Harvard soils mechanics Series No. 7, Harvard Univ., Cambridge,Mass.

Casagrande, A. �1968�. Notes of engineering 262 course, Vol. I, HarvardUniv., Cambridge, Mass./Beijing International Commission on LargeDams �ICOLD�, Paris, 237–260 �Question 76�.

Foster, M. A., and Fell, R. �2001�. “Assessing embankment dam filtersthat do not satisfy design criteria.” J. Geotech. Geoenviron. Eng.,127�5�, 398–407.

International Commission on Large Dams �ICOLD�. �1994�. “Use ofgranular filters and drains in embankment dams.” Bulletin 95, ICOLD,Paris.

Lofquist, B. �1988�. “Discussion.” J. Geotech. Engrg. Div., 114�6�, 740–742.

Lombardy, G., and Deer, D. �1993�. “Design and control of groutingusing GIN method.” Power & Dam Construction, June.

Ramírez-Reynaga, M. �2003�. “Internal erosion at El Batam Dam.” Proc.,XII Pan-American Conf. on Soil Mechanics and Geotechnical Engi-neering, Proceedings Volume 2, Massachusetts Institute of Technol-ogy, Cambridge, Massachusetts.

Ramirez-Reynaga, M., Schmitter, J., and Mendez, R. �2003�. “Rehabili-tation of El Batam dam.” Proc., XII Pan-American Conf. on SoilMechanics and Geotechnical Engineering, Proceedings Volume 2,Massachusetts Institute of Technology, Cambridge, Massachusetts.

Sherard, J. L., and Dunnigan, L. P. �1989�. “Critical filters for impervioussoils.” J. Geotech. Engrg., 115�7�, 927–947.

United States Soil Conservation Service �USSCS�. �1994�. “Gradationdesign of sand and gravel filters.” National engineering handbook,United States Department of Agriculture, Washington, D.C., Chap. 26,Pt. 633.

Wan, C. F., and Fell, R. �2008�. “Assessing the potential of internal in-stability and suffusion in embankment dams and their foundations.”

e 1,890-m elevation of the reservoir
for th
J. Geotech. Geoenviron. Eng., 134�3�, 401–407.


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internal erosion and rehabilitation of an earth-rock dam

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