Proceedings of Indian Geotechnical Conference December 15-17, 2011, Kochi (Invited Talk-11)

CONTROLLING INTERNAL EROSION IN EARTH DAMS AND THEIR FOUNDATIONS:

CASE STUDIES

A. Soroush, Associate Professor, Amirkabir University of Technology, President of Iranian Geotechnical Society (IGS)

P. T. Shourijeh, Formerly PhD Student, Amirkabir University of Technology, Member of Iranian Geotechnical Society (IGS)

A. Mohammadinia, Formerly MSc Student, Amirkabir University of Technology, Tehran, Iran

ABSTRACT: This treatment reviews case histories of a handful of embankment dams recently completed or under

construction in Iran. Special attention is devoted to characteristics of core materials, in relation to internal erosion, and the

substantiation of filters through NEF (No Erosion Filter) tests. Filter design and proportioning for fine-grained low-plasticity

soils (viz. CL, CL-ML, and ML) is critically elaborated and exemplary comparison with contemporary filter design criteria is

provided. Guides concerning the NEF testing procedure are also recommended.

INTRODUCTION

Internal erosion and piping present serious risks to the

stability of embankment dams. An excellent review of dam

incidents up to 1986 by Foster et al. [1] revealed that 48% of

earth and rockfill dam failures were caused by piping and

internal erosion. Contemporary researches have indicated that

even in modern zoned dams internal erosion and piping are

still major threats of damage that may eventuate to dam

failure [2].

The sequences of internal erosion through a zoned

embankment are clearly described by Fell et al. [3]. The

process of internal erosion can be broken into four phases;

initiation of erosion, continuation of erosion, progression of

erosion and formation of a breach. While initiation of erosion

to some extent depends on characteristics of the core, filters

act as barriers to stop continuation of erosion. If the filter

fails, erosion will progress and may lead to breaching. The

importance of filters in dam safety along with the usually

high costs of filter production makes filter design and

substantiation a contentious issue.

Heretofore, numerous filter design criteria have been

proposed, from which a few are more accepted and

implemented. Filters in modern dams generally respect the

criteria presented in Table 1 that were proposed by Sherard

and Dunnigan [4]. Based on analysis of an extensive NEF test

database, Shourijeh and Soroush [5] suggested minor

modifications to Sherard and Dunnigan [4] criteria (cf. Table

1). Although criteria of Table 1 have lead to proven

performances, filter testing still provides the most confident

and reliable method for selection of filters [6 & 7]. The No

Erosion Filter (NEF) test is recognized as a competent filter

test especially for fine grained soils. Many researchers have

repeated NEF testing to substantiate appropriate filters and to

assess filter criteria credibility [8, 9, 10, 11 & 12].

Fine-grained low-plasticity silty and silty-sandy soils, such as

CL, CL-ML, ML and SC, are considered as competent core

materials given that they satisfy permeability requirements of

central sealing (i.e. core) elements. The use of these materials

has been reported in numerous cases [13, 14 & 15]. However,

from the viewpoint of internal erosion these soils are

disadvantageous, as they have feeble erosion resistance [16,

17 & 18]. In analysis of dam incidents, Foster et al. [1]

noticed that 34% of cores which had experienced erosion

damages or failures were consisted of CL soils and 18% were

ML soils. For such soils, special attention should be devoted

to internal erosion; propensity of core cracking should be

minimized and proper critical filters should be executed.

Besides, cohesionless soils impose practical difficulties

during dam construction, especially in core compaction [19].

Many embankment dams have been completed or currently

are under construction in Iran, and fortunately, aspects of

modern dam engineering are considered in their design

features. Specifically speaking, great emphasis is placed on

designing appropriate critical filters to prevent internal

erosion. Filters are strictly delimited according to criteria and

in most cases confirmed/delineated via NEF tests. In the last

decade extensive experimental investigations have focused

on internal erosion and piping in Amirkabir University of

Technology. These efforts have served as a platform for both

state of the art researches in geotechnical engineering, and

professional consultancy to numerous dam projects. This

paper deals mainly with some of the authors’ experiences

regarding filter design for low plasticity core soils.

NEF TESTING

Since NEF testing has played a pivotal role in the selection of

filter materials for the dam case histories, a complete yet brief

description of NEF testing practiced by the authors is

presented. Experiences by Soroush et al. [20], and Soroush

and Shourijeh [21] resulted in semi-standard procedures for

NEF testing.

For NEF tests herein, the main container of the apparatus,

illustrated in Figure 1, is a Plexiglas cylinder with internal

diameter of 11 cm and height of 30 cm. Hence, specimen

fabrication and testing procedure is as follows:

64

- An appropriate drainage layer is placed at cylinder bottom.

- Filter materials are blended from fractions of washed sands

(with uniform sizes) in four equal portions. Every portion is

thoroughly mixed with 3% moisture content and carefully

placed in the cylinder to prevent segregation. The amount of

compaction for each layer is determined by trial and error,

such that the final filter thickness produces the desired

relative density (Dr) required.

- A plastic or rubber ring is forced to intrude the final filter

layer. This ring is located flush with internal walls of the

cylinder and thus is water-tight. The plastic ring is used in

lieu of granular side materials.

- Base soil materials are compacted in a special mold at 1-

2% wet of optimum moisture content, and to 0.98( d)max. The

compacted base soil specimen (3 cm thick) is detached from

the mold and pushed into the apparatus cylinder to sit on top

of filter materials. A few mild strokes of a tamper will

enhance attachment of the base specimen to cylinder walls

and also to the upper filter face.

- A 1 mm hole is punched throughout the middle of the base

specimen such that it extends 1 cm into filter materials.

- A wire screen separator is placed on top of the base

specimen. The space remained on top of the cylinder is filled

with gravels. The voids of gravel are filled with water.

- The air vent is closed and the inlet valve is fully opened; the

outlet valve is opened and the test is started.

- During the test, the out-coming effluent is collected in

graduated cylinders. The time interval for collecting the flow

varies generally from 30 seconds at the start and the end of

the test and 1 minute for the mid-duration of the test. The test

is usually continued for at least 20 minutes until flow rate and

turbidity generally stabilize.

The test is judged successful if there is no visible erosion of

the performed hole in the base specimen. Besides the effluent

flow rate and turbidity should be carefully monitored for

supportive information regarding the test behaviour. The

NEF test results define the boundary filter- designated by

D15b- that is the coarsest filter that prevents base soil erosion.

During NEF testing on fine grained low plasticity soils there

is a chance that upper regions of the hole soften/slake leading

to hole closure. For such instances no flow emerges through

the hole (i.e. out of the apparatus); hence filter functionality

can not be tested. To circumvent this problem Soroush and

Shourijeh [21] recommend application of a truncated cone

(nipple) that intrudes the base specimen and supports the hole

during testing. This detail for NEF testing, shown in Fig. 2, is

similar to Pinhole Tests [22].

Example conditions of the hole in base specimens before and

after successful/unsuccessful NEF tests, for cases with and

without the nipple are depicted in Fig. 3.

Fig. 1 (a) Photograph of NEF apparatus, and (b) schematic

illustration of soil layers during NEF testing; Note:1-air vent,

2-plexiglas cylinder, 3-base specimen (3 cm), 4- hole in base

specimen (1 mm), 5- filter material (12 to 14 cm), 6- wire

screen, 7- outlet pipe, 8- drainage layer, 9- water tight plastic

ring, 10- wire screen, 11- top gravel layer, 12- inlet pipe, 13-

pressure gauge.

Fig. 2 Schematic illustration of hole details implementing

nipple in NEF testing; (1) nipple, (2) filter, (3) 1mm hole, (4)

base specimen, (5) wire screen, and (6) top gravel

2

1

3

4

5

6

(a) (b)

Abbas Soroush, Piltan Tabatabaie Shourijeh & Alireza Mohammadinia

Table 1 Proposed filter criteria based on NEF testing

Base soil designation Investigator

Group 1 Group 2 Group 3 Group 4

%<75 m* 85 40-85 < 15 15-40 Sherard &

Dunnigan [4] Criterion D15 9d85** D15 0.7 mm D15 4d85

Intermediate between value for group 2

and 3 based on %<75 m

%<75 m 85 80 35-80 < 15 15-35 Shourijeh &

Soroush [5] Criterion D15 9d85ק

D15 minimum of

(0.7 mm and 6.4d85)§

D15 0.7

mm‡ D15 4d85

Intermediate between value for group 2

and 3 based on %<75 m

Notes: * % finer than 0.075 mm in the gradation with maximum size of 4.75 mm, ** d85 for gradation passing 4.75 mm, ‡ D15 0.5 mm for

highly dispersive soils, × D15 6d85 for ML and CL-ML soils, § D15 7.5d85 for highly dispersive soils.

65

Controlling internal erosion in earth dams and their foundations: case studies

(a) (b)

(c) (d)

(e) (f)

Fig. 3 Photographs of base specimens in NEF tests; (a) 1 mm

hole before testing, (b) enlarged hole in unsuccessful test, (c)

hole after successful test, (d) base specimen after removal of

nipple, (e) hole under nipple in successful test, and (f)

enlarged hole under nipple after unsuccessful test

CASE HISTORIES

Dams A and B

The A and B embankment dams are located in southern Iran.

The impounding of these dams started in 2008 and they have

a common reservoir. The main intention of the dams’

construction has been controlling destructive seasonal floods

besides fulfilling irrigation and municipal water needs in the

arid region. Fig. 4 illustrates the typical cross sections of

Dams A and B. Both dams are similar in design and are

constructed on an alluvial sediment foundation. Table 2

presents some general features of the dams.

Table 2 General specifications of dam case histories

Crest Dimensions (m) Dam Height*

Length Width

Reservoir

A 32.3 1200 8

B 28.3 510 8

115

C 32.5 352 9 17

D 86 1820 12 126

E 86 807 10 250 * Elevation from river bed.

The construction materials of dams A and B are the same.

The borrow areas and naturally occurring strata in vicinity of

the dam sites generally comprised rounded to sub-rounded

sedimentary soils. These materials encompassed boulders,

gravels and sands with low fine contents which provided

Fig. 4 Typical cross sections of Dams A and B

Dam A

Dam B

66

Abbas Soroush, Piltan Tabatabaie Shourijeh & Alireza Mohammadinia

excellent materials for shell and transition zones. Rich sands

with minor processing for filter material were readily

available in the area.

However, the only affordable core material borrow area near

the dam site consisted of silty soils. The core materials finally

selected for the dam construction comprised a mixture of CL,

ML, and CL-ML soils. To recognize features of the core

materials, a number of 113 samples were taken from the

borrow area and analyzed. Most of the samples were limited

to the maximum size of 2mm. The percent of CL, ML, and

CL-ML soils in the core materials were 26.9%, 6.7% and

66.4% respectively. Fig. 5 and Fig. 6 illustrate respectively

the core material gradation range and Plasticity Index (PI)

distribution of core samples. Most core material samples had

low plastic indices less than 8, and permeability ranging from

5.2×10-8 to 2.1×10-7 cm/s.

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Core Range

B4

B6

B8

B9

B10

F1 (Fine Filter)

F2 (D15=0.3mm)

F3 (D15=0.4mm)

F4 (D15=0.5mm)

Sand

FineClay and Silt

Fine Medium Coarse

Gravel

Coarse Co

bb

les

Fig. 5 Core and filter material ranges for Dams A and B

Fig. 6 PI distribution for 113 core samples of Dams A and B

The natural erosion patterns (gullies, water scours, etc.) in the

core material borrow area suggested that the materials are

sensitive to erosion. A comprehensive study of dispersivity

for 50 samples by the pinhole test [22] revealed that the core

materials are mostly ND1 and ND2 with few samples

categorized as ND3 and ND4. Double hydrometer dispersion

tests [23] on 40 samples indicated that 75% of the samples

are not dispersive and 25% have medium dispersive

tendencies. The results of crumb dispersion tests indicated

that core samples do not show chemical reaction with water

and from this standpoint they are not dispersive.

The differences between results of pinhole dispersion and

Emerson crumb tests suggested that the core material is

highly erodible but not chemically dispersive. This means

that the core material is very sensitive to hydro mechanical

erosion by seeping water, whilst it does not show symptoms

of dispersive erosion. Ravaska [24] and Foster and Fell [9]

have alluded to the existence of soils which are highly

erodible yet not chemically dispersive. Foster and Fell [9]

also state that the erosion of highly erodible soils is easier

than dispersive soils; from their viewpoint a soil is considered

dispersive if it shows dispersive-ness in both pinhole and

Emerson crumb tests. That is, very low shear stresses induced

by seeping water may cause erosion of highly erodible soils.

Since the core materials dominantly consisted of CL and ML

soils, possessing symptoms of highly erodible soils, a

comprehensive NEF testing program was carried out on core

samples to substantiate the appropriate filter. NEF tests were

performed with water pressure of 400 kPa and filters having

Dr=70%. A nipple (i.e. truncated cone) was used whenever

required. Three filter gradations, i.e. F2, F3 and F4, were

incorporated in NEF tests. Test results, Table 3, manifest that

F2 (D15=0.3 mm) was capable of preventing erosion in

almost all cases. This is interesting since for all base soils

tested, D15b/d85 is 6.1 to 9.1, that is to say the criterion of

D15/d85 9 would not guarantee no-erosion for all tested soils.

In Fig. 7 the scattering of D15b/d85 with PI is plotted for 10

tested core samples. Accordingly, D15b/d85 increases with

increase in PI.

Table 3 Specifications of base soils and NEF test results for

Dams A and B

Sample PI (%)

USCS ( d)max

(gr/cm3) wopt

(%) D15b

** (mm)

D15b/d

85

B4 8.3 CL 1.74 17.33 0.3 9.1

B6 6.7 CL-ML 1.73 17.60 0.3 8.8

B8* 4.5 CL-ML 1.84 14.29 0.4 7.4

B9* 3.9 ML 1.80 14.90 0.3 6.7

B10* 3.3 ML 1.72 16.18 0.3 6.1 * Nipple use; ** D15 of coarsest filter with no-erosion of base soil.

2 4 6 8 10 12 144

6

8

10

12

14

16

D1

5b/d

85

PI (%)

9

Fig. 7 Variation of D15b/d85 versus PI for NEF tests on 10

core samples of Dams A and B

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Controlling internal erosion in earth dams and their foundations: case studies

Fig. 8 Cross section of Dam C at maximum elevation

Dam C

This central core embankment dam is currently under

construction in eastern Iran. Important specifications of the

dam are reported in Table 2, while Fig. 8 illustrates the

typical cross section. The dam site is located at the arid desert

outskirts having hot temperatures and very low relative

humidity, that increases propensity of core materials

desiccation and cracking. The only affordable material for the

impervious core in the dam proximity comprised fine grained

CL, ML and CL-ML soils having traces (about 2 to 3%) of

gypsum (CaSO4), and average PI of 8%. The design

gradation range of core materials is shown in Fig. 9. The filter initially designed for the core range, i.e. F7 in Fig.

9, satisfied criteria of Table 1 for the fine core envelope. It

was decided to recheck the filter design by filtration tests, and

30 core samples were collected from the borrow area.

Analysis of new core samples suggested that the core range

had not been precisely defined in design phase and many core

samples were in fact finer than the projected fine envelope.

NEF tests were conducted on core specimens and filters F6

and F7 (cf. Fig. 9) having D15 equal to 0.3 mm and 0.4 mm

respectively. River water was used as influent flow with

pressure of 400 kPa in NEF tests. Filter material had Dr=70%

in all tests, and a nipple was used to support the 1 mm hole

whenever required.

Selected NEF test results in Table 5 reveal that F6

(D15=0.3mm) was successful in preventing erosion of core

base specimens. In the case of NC26 with PI=5.2% a no-

erosion filer required that D15/d85=5.9 that is much lower than

the criteria of D15/d85 9 for group 1 base soils (see Table 1).

As for NC1 (PI=19.7%) the no-erosion filter had

D15/d85=11.5. This corroborates the notion that fine grained

low plasticity soils require special attention in filtration

related problems, as they have both small particle sizes and

weak erosion resistance.

Table 5 NEF test results for Dam C

Sample PI (%) Filter D15/d85 Test Result

F7 15.4 Unsuccessful NC1 19.7

F6 11.5 Successful

F7 11.6 Unsuccessful NC14* 6.1

F6 8.7 Successful

F7 11.8 Unsuccessful NC20 8.5

F6 8.8 Successful

F7 7.8 Unsuccessful NC26* 5.2

F6 5.9 Successful * Hole in base specimen supported by nipple.

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NC1

NC14

NC20

NC26

F5 (Fine Filter)

F6 (D15=0.3mm)

F7 (D15=0.4mm)

Sand

FineClay and Silt

Fine Medium Coarse

Gravel

Coarse Co

bb

les

Fig. 9 Core and filter material ranges for Dam C

Dam D

Dam D is an embankment dam currently under construction

intending to store water for irrigation and municipal water

supplies. The general specifications of the dam are presented

in Table 2. Also Fig. 10 illustrates gradation ranges of

core/filter materials. Owing to (1) large reservoir volume, and

(2) flat valley profile and long crest length, special attention

was devoted to reducing the internal erosion risk. Thus a

conservative critical filter material with D15max=0.3 mm, see

gradation F9 in Fig. 10, was designed according to criteria of

Table 1. NEF tests were performed on some core samples and

the coarse filter envelope (F9) in order to experimentally

68

Abbas Soroush, Piltan Tabatabaie Shourijeh & Alireza Mohammadinia

confirm filter design. In all tests the filter materials had

Dr=70%, while the base soil specimen was compacted to

d=1.76 gr/cm3 contributing to 0.95( d)max (based on standard

Proctor compaction), and w=15% which was 2% higher than

core average optimum water content.

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TP3

TP4

TP7

F8 (Fine Filter)

F9 (D15=0.3mm)

Sand

FineClay and Silt

Fine Medium Coarse

Gravel

Coarse Co

bb

les

Fig. 10 Core and filter material ranges for Dam D

A concise selection of the NEF test results is presented in

Table 6. Accordingly F9 is successful in preventing erosion

of core specimens. Test results were indifferent to testing

time for durations of 10 to 60 minutes. The NEF test on TP7

repeated at main’s pressure of 700 kPa was unsuccessful,

showing that water pressure and in turn velocity had a severe

intensifying effect on soil erosion.

Table 6 NEF test results for Dam D

Base

Soil

PI

(%) D15/d85

* Pw

(kPa)

Time

(min) Test Result

400 20 Successful TP3 16.7 8.1

400 40 Successful

400 20 Successful TP4 18.0 7.7

400 60 Successful

400 20 Successful TP7 12.8 8.3

700 20 Unsuccessful

Note: * D15=0.3 mm; d85 measured for base soil passing 4.75 mm.

Dam E

Dam E is an embankment dam constructed in the southern

slopes of the Alborz Mountains with the purpose of supplying

municipal water supplies for the nearby megacity. The

general specifications of the dam are presented in Table 2,

and Fig. 11 illustrates gradation ranges of core/filter

materials. The relatively conservative filter (D15max=0.4 mm)

satisfied the criterion of D15 0.7 mm in Table 1.

During planning phases the volume of available core material

was overestimated; hence as the dam erection progressed and

reached high elevations, the source and in turn gradation of

core materials had to be changed. A mixture of quarry-run

rock and available clayey soils was used as GC material for

the core. Examples of this mixed core materials are soils M2

and M3 in Fig. 11. Investigations suggested that the mixed

core samples were both broadly graded and finer than the

core envelope, or had a gap in the sand particles range.

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M3

F10 (Fine Filter)

F11 (D15=0.4mm)

Sand

FineClay and Silt

Fine Medium Coarse

Gravel

Coarse Co

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Fig. 11 Core and filter material ranges for Dam E

Complementary test on core samples revealed that; PI>15%,

( d)max=1.9-2.0 gr/cm3 and wopt=10-13% based on standard

Proctor compaction. Furthermore, large scale tri-axial

permeability tests (cf. Fig. 12-a) suggested permeability

values ranging from 2.5×10-8 to 1.2×10-7 cm/s.

NEF tests were conducted on core samples M2 and M3 and

filter F11. Owing to broad/gap-gradation of base soils and the

existence of large grains it was decided to perform tests on

the complete gradation without sifting-out large gravel sizes.

Therefore, NEF tests were performed in a large scale

apparatus with the internal diameter of 25.4 cm, see Fig. 12-

b. Details of this apparatus (flow exit-system, auxiliary

equipment, etc.) is available in Soroush and Shourijeh [21].

The large scale NEF tests were performed with inflow

pressure of 400 kPa and filter Dr of 70%. The initial hole

diameter was 5 mm. Based on test results the filter was

successful and capable of preventing erosion of base

specimens. Nonetheless, modifications to the GC mixtures

were suggested in order to prevent formation of gap-graded

mixtures which possess possible long-term problems

associated with self-clogging at the core-filter interface [25].

(a) (b)

Fig. 12 Pictures of; (a) sample M2 after tri-axial permeability

test, and (2) large scale NEF test on M3

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Controlling internal erosion in earth dams and their foundations: case studies

FILTER COHESION AND SELF-HEALING

It is well understood that filter materials should have

sufficient permeability to allow passage of seeping water

without buildup of excessive pore water pressure.

Furthermore in embankment dams filter materials should be

non-cohesive and collapsible to remain intact in the event of

core cracking [7]. Fell et al. [26] state that the fines content

and their plasticity along with the compaction amount,

govern cracking propensity of soils. Fell et al. [27] suggest

that limiting the fines content to 7% would be reasonable to

allow for collapsibility and self-healing of filters.

The sand castle test, devised by Vaughan and Soares [28], is

recommended for recognition of filter collapsibility and self-

healing [7]. Soroush et al. [12] have presented a detailed sand

castle testing procedure. In general sand castle test results are

qualitative, and companion proficient engineering judgment

is needed to verify its authenticity and reliability.

For case histories discussed in the paper, sand castle tests

have been performed to assess filter collapsibility and self-

healing. Fig. 13 depicts a sand castle test in its initial stages.

In-situ sand castle tests are also recommended for constructed

filter layers in a dam, and are particularly applicable to filters

over-compacted under construction equipment traffic,

segregated filters, etc. [29].

Fig. 13 Photograph of sand castle testing

CONCLUSIONS

Filter materials are one of the most important/expensive

elements of an embankment dam body. In developed

countries, with a large population of dams built before

presentation of modern filter criteria, problems related to

internal erosion and filter design are mostly viewed from a

standpoint of dam upgrading/rehabilitation, and assessment

of internal erosion risks in aging dams. Contrarily, in many

developing countries, especially in Asia, where design and

construction of earth and rockfill dams has gained

widespread popularity, design/proportioning of filters

requires great attention to prevent internal erosion problems.

In many regions of world, particularly Middle East and

Central Asia, fine grained low-plasticity soils are abundant

and comprise the only affordable core materials for dam

construction. Problems related to piping of such soils pose

serious threats for embankment dams; hence special attention

should be devoted to selection of proper filters to prevent

internal erosion.

The precise knowledge of basic core characteristics

(gradation envelope, range of PI, permeability, compaction

properties etc.) predicated on statistical study of sufficient

core samples plays an important role in contriving safety

measures against internal erosion.

The case histories outlined in this paper suggest that filter

design criteria may not always ensure the safe filter action for

protecting fine grained low plasticity soils, viz. CL, CL-ML

and ML. In the authors’ experience, a filter criterion of

D15/d85 6 may be required for safe filtration of low-plasticity

group 1 base soils.

In Iran many embankment dams have been constructed

successfully with fine low-plasticity soils, and NEF testing is

common practice for substantiation of filters. This study

indicates that NEF testing is helpful in determining safe

filters. For problematic soils and major projects conduction of

NEF tests are vital for filter design. It should be noted

however that the NEF test is a specialist test that requires

proficient conduction and interpretation. The procedures and

details recommended in this paper can be used for authentic

NEF testing.

Sand castle test is an inexpensive method for assessing the

collapsibility and self-healing of filter materials and it is

applicable in field conditions. Conducting regular in-situ sand

castle tests is recommended to control the quality of filter

materials.

ACKNOWLEDGEMENT

The authors would like to express their gratitude to Dr. F.

Jafarzadeh and Dr. A. Akhtarpour. Also kind efforts of

Messrs Taghi Bahrami and Reza Javadi, the staff of the

Geotechnical Laboratory of the Amirkabir University of

Technology, for providing help and assistance during the

testing are appreciated.

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1. Foster, M., Spannagle, M., and Fell, R., (1998)

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Australia.

2. Flores-Berrones, R., Ramírez-Reynaga, M. and Macari,

E.J., (2011) “Internal Erosion and Rehabilitation of an

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pp. 150–160.

3. Fell, R., Wan, C. F., Cyganiewicz, J., and Foster, M.,

(2003), “Time for development of internal erosion and

piping in embankment dams.” Journal of Geotechnical

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Abbas Soroush, Piltan Tabatabaie Shourijeh & Alireza Mohammadinia

and Geoenvironmental Engineering, ASCE, Vol. 129,

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4. Sherard, J.L. and Dunnigan, L.P., (1985) “Filters and

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Impoundments, (Volpe, R.L. and Kelly, W.E. (eds)),

1985, ASCE, pp. 1-30.

5. Shourijeh, P.T., and Soroush, A., (2009) “Statistical

study of no-erosion filter (NEF) test results”,

Proceedings of the Institution of Civil Engineers-

Geotechnical Engineering, ICE, Vol. 162, Issue. 3, pp.

165-174.

6. U.S. Bureau of Reclamation (USBR), (1987) ‘‘Filters.’’

Chapter 5, Design standards No.13-Embankment dams,

Denver.

7. International Commission on Large Dams (ICOLD),

(1994) ‘‘Bulletin 95, embankment dams granular filters

and drains’’, 1994, Paris.

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