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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Core Envelope
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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Particle Size (mm)
Core Envelope
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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M2
M3
F10 (Fine Filter)
F11 (D15=0.4mm)
Sand
FineClay and Silt
Fine Medium Coarse
Gravel
Coarse Co
bb
les
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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Abbas Soroush, Piltan Tabatabaie Shourijeh & Alireza Mohammadinia
and Geoenvironmental Engineering, ASCE, Vol. 129,
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