10th Asian Regional Conference of IAEG (2015)

Research on the Influence of Material Permeability to Landslide Dam Seepage Stability
Zhenming SHI(1)(2), Shenggong GUAN(1)(2), Ming PENG (1)(2) and Xi XIONG(1)(2)(1) Department of Geotechnical Engineering, Tongji University, China
E-mail: shi_tongji@tongji.edu.cn(2) Ministry of Education Key Laboratory of Geotechnical and Underground Engineering, Tongji University,
China
Abstract
Landslide dams, often formed by quick deposits of landslide materials, are lack ofsufficient consolidation, and loose in dam structure and material composition. Lacking of
seepage control and flood drainage facilities, big water head difference can trigger seepage
deformation of soil and influence landslide dam stability with the rise of landslide lakewater level, which may lead to dam breach. The paper reports the experimental study on theseepage characteristics of landslide dam material, which takes the differences of grain
grading into consideration. It was observed that piping and soil flow are the two seepage
failure modes of landslide dam material. The experiments showed seepage deformation
mode and permeability coefficient mainly depend on the condition of fine particles filling
the pores between the coarse particle and the compactness. The paper provides a kind ofcritical hydraulic gradient equation of piping, which can be used in the critical hydraulicgradient calculation of the landslide dam materials with different densities.
Keywords: landslide dam, permeability, seepage stability, piping
1.

Introduction

has low permeability. Grain-supported material, of
which coarser particles are in contact with each other,has higher permeability. Wang and Yang (2003) and
Hu et al. (2010) obtained the permeability coefficientof landslide dam materials by field tests. However,landslide dam sites are mostly complicated and it isdifficult to do field tests.
In the study of landslide dam seepage stability,
Meyer et al. (1994) concluded the modes of landslide
dam seepage failure and provide methods to analyze
the stability of landslide dams under seepage by using
seepage stability criterion of soil. He et al. (2009)
estimated the failure mode of Xiaojiaqiao Landslide
Dam is piping and analyzed the seepage stability
using critical hydraulic gradient. Hu et al. (2010)
found that critical hydraulic gradient is different withdifferent soil layers and seepage failure can occur inlocal region. However, critical hydraulic gradient inlandslide dam seepage stability analysis is mainly
calculated by the formula of general soil, and there
are some differences between the grain gradation oflandslide dam material and that of general soil.
The main objective of this research is to do
typical landslide dam material seepage tests, of which
result can be used to discuss the influence factors of
the landslide dam material permeability and provide aformula of piping critical hydraulic gradient toLandslide dams are natural dams, formed byquick deposits of landslide materials, which are lackof sufficient consolidation and loose in dam structure
and material composition. Landslide dams canintercept upstream runoff to form landslide lakes,
which lead to water head difference and makeseepage in the dams. Because landslide dams arelacking of seepage control and flood drainage
facilities, big water head difference can trigger
seepage deformation of soil and influence landslide
dam stability with the rise of landslide lake waterlevel, which may lead to dam breach. Breach floodcan cause huge losses of downstream life andproperty. Therefore, the seepage stability analysis oflandslide dam is important.
Seepage problem study of landslide dams
includes material permeability and seepage stability.
Landslide dam material particle gradation has great
influence on permeability. Casagli et al. (2003)
divided landslide dam material structures into
matrix-supported and grain-supported types. Thesematerials have difference inpermeability.
Matrix-supported material, of which fine particles fill
the pores between the coarse particle material andcoarser particles are not in contact with each other,1

10th Asian Regional Conference of IAEG (2015)

analyze the seepage stability of landslide dam.
120

2.

Experimental methods

100

Percent finer by weight

Because the particle sizes of landslide dam
material are great various, exiting testing apparatus
do not apply to it. Referring to the principle ofvertical seepage deformation apparatus in Code forgeotechnical testing (Ministry of Water Resources ,PRC, 1992), a testing apparatus for landslide dam
material was developed (Fig. 1). It is composed of apermeameter, a water supply system, a suction
system and a measurement system. Permeameter isdesigned to be 30 cm in diameter and 60 cm in height.
If the landslide dam material contains sticky particles
and the permeability is small, it can be saturated bysuction method, which can reduce the use of time bywater-head saturation method.

80

1#
2#
3#
4#

60
40
20
0
1E-3

0.01

0.1

1

10

100

Particle size (mm)Fig. 2 Grading curves of landslide dam materials
Because Darcy’s Law only can be applied tolaminar flow conditions, the water head was loaded
for multiple times in the tests, and the whole processof seepage from the beginning to failure can beobserved. If the hydraulic gradient is nearly thecritical hydraulic gradient, the horizontal fractures or
springs may appear on the top surface of specimens,
which can develop to the bottom, and the seepage
flow can continually increase. When the fine particle
move occurs on the two-thirds of the specimen top
surface, the specimen can be considered to seepage
failure and the test is stopped. The linear section of
hydraulic gradient- flow velocity curve can be used to
calculate permeability coefficient of the landslide
dam material.

Fig. 1 Sketch of the testing apparatus

3.

Four typical particle gradations were chosen to
prepare specimens and the grading curves are shownin figure 2. The particle gradations of landslide dam
material are influenced by the causes of the dams,
which can be divided into three categories as highspeed and long distance sturzstrom, large-scalecollapse and bedding landslide. Donghekou landslide
dam caused by high speed and long distancesturzstrom mainly composed of loose soil and rubble
and the particle gradation was wide (Zhao, 2013),
which was used to prepare specimens group 1 with
continuous gradation. Collapse-caused Xiaogangjian
Landslide Dam mainly composed of gravel (Qiu andLi, 2008), and specimens group 2 with coarse grain in
majority was prepared according to it. The
permeability of bedding landslide dam material, like
Tangjiashan Landslide Dam, depends on the lithologyof landslide (Hu, 2010). Specimens group 3 with finegrain in majority and Specimens group 4 gap-graded
were prepared according to particle gradations ofdifferent soil layers of Tangjiashan Landslide Dam.
Each group included four specimens with different
dry densities, from 1.90 g/cm3 to 1.78 g/cm3, of
which the parameters are shown in table 1.

Results and analyses

3.1 Tests result
It was found from the tests result that the seepage
failure mode of specimens group 3 was piping. Whenthe hydraulic gradient rose to a certain value, fineparticles of specimens’ surfaces were washed away
by water and concentrated seepage channels occurredin the specimens, which finally developed to thebottom (Fig. 3(a)). The piezometric level began
decreasing, when hydraulic gradient exceed thecritical hydraulic gradient. The I-V curves (Fig. 4)
show that the hydraulic gradient remains stable, while
the flow velocity increases significantly at thatmoment.
However, the seepage failure mode of specimens
group 1 and 4 was soil flowing. With the rising of
hydraulic gradient, there was slightly movement offine particles on the specimens’ surfaces, but no
seepage channel. Finally, some horizontal fractures
occurred in the specimens (Fig. 3(b)), the upper
portions floated, flow velocity increased sharply andsoil damaged.

2

10th Asian Regional Conference of IAEG (2015)

Table 1 Characteristic indexes of landslide dam materials

Specimennumber

d10
(mm)

d30
(mm)

d60
(mm)

1-1
1-2
1-3
1-4
2-1
2-2
2-3
2-4
3-1
3-2
3-3
3-4
4-1
4-2
4-3
4-4

0.220
0.220
0.220
0.220
0.950
0.950
0.950
0.950
0.106
0.106
0.106
0.106
0.005
0.005
0.005
0.005

0.56
0.56
0.56
0.56
7.97
7.97
7.97
7.97
0.67
0.67
0.67
0.67
0.106
0.106
0.106
0.106

1.2
1.2
1.2
1.2
27.3
27.3
27.3
27.3
6.5
6.5
6.5
6.5
0.57
0.57
0.57
0.57

Nonuniform
coefficientCu
5.454545.454545.454545.4545428.7368
28.7368
28.7368
28.7368
61.320761.320761.320761.3207
114
114
114
114

CurvaturecoefficientCc
1.18787
1.18787
1.18787
1.18787
2.449232.449232.449232.449230.65152
0.65152
0.65152
0.65152
3.94245
3.94245
3.94245
3.94245

Dry density
ρd（g/cm3）
1.78
1.82
1.86
1.90
1.78
1.82
1.86
1.90
1.78
1.82
1.86
1.90
1.78
1.82
1.86
1.90

Void ratioePorosityn0.5168539
0.4835164
0.4516129
0.4210526
0.5168539
0.4835164
0.4516129
0.4210526
0.5168539
0.4835164
0.4516129
0.4210526
0.5168539
0.4835164
0.4516129
0.4210526

0.3407407
0.3259259
0.3111111
0.2962962
0.3407407
0.3259259
0.3111111
0.2962962
0.3407407
0.3259259
0.3111111
0.2962962
0.3407407
0.3259259
0.3111111
0.2962962

0.16
1-1
1-2
1-3
1-4

I=1.0683

Flow velocity V (cm/s)

0.12
I=1.1167

I=1.4650

0.08

I=1.7716

0.04

0.00
0.0

0.5

1.0

1.5

2.0

2.5

3.0

Hydraulic gradient I(a)(b)
Fig. 3 Piping and soil flowing

Fig. 5 The I-V curves for specimen group 1
0.04

0.16
3-1
3-2
3-3
3-4

Flow velocity V (cm/s)

0.12
I=0.9533

I=0.8317

I=1.1967

0.10
0.08
0.06
I=1.4017

0.04

4-1
4-2
4-3
4-4

0.03
Flow velocity V (cm/s)

0.14

I=1.0533

I=1.2417

I=1.3383

I=1.3667

0.02

0.01

0.02
0.00
0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.00
0.0

3.5

0.5

1.0

1.5

2.0

Hydraulic gradient I

Hydraulic gradient I

Fig.4 The I-V curves for specimen group 3

Fig. 6 The I-V curves for specimen group 43

2.5

10th Asian Regional Conference of IAEG (2015)

soil flowing occurs (as specimens group 1 and 4).
The compaction of landslide dam material has
influence on the seepage stability and permeability
coefficient. The critical hydraulic gradients Ik
increase and the permeability coefficients K reducewith the increasing of compaction. Comparing theresults of specimens group 1 and 4tests, the Ik and K
change of the former is larger significantly than thoseof latter combining with the dry density  d risingfrom 1.78 g/cm3 to 1.90 g/cm3. Because the content
of the fine particles of the specimens group 1 is larger,
the rising of dry density  d makes the arrangementof fine particles closer, the anti-permeability strength
increases, and then the changes of Ik and K are clear.
However, because the shortage of intermediate grain
diameter, the dry density  d rising of specimens
group 4 just changes the arrangement of coarse
particles structure and the fine particles are still easily
washed away by water, and then the changes of Ik andK are not significant.

0.4
2-1
2-2
2-3
2-4

Flow velocity V (cm/s)

0.3

0.2

0.1

0.0
0.00

0.05

0.10

0.15

0.20

0.25

Hydraulic gradient I

Fig. 7 The I-V curves for specimen group 2
The I-V curves are linear before the seepage
deformation occurs. When hydraulic gradients exceed
the critical hydraulic gradients, the slopes of thecurves start to change and curves are nonlinear. The
critical hydraulic gradient I k can be calculated by
the equation:Ik 

I 2  I1
2

3.3 The discrimination method of seepage failuremodes
As the results of tests, the content of the fineparticles decides the seepage failure modes. Liu andXie (2012) studied the seepage stability of gravelly
soil and put forward a discrimination method of
seepage failure modes according to the optimal fineparticles content:(1)

Where I2 is the hydraulic gradient that triggers piping
and I1 is the hydraulic gradient just before the piping
occurs. Damage hydraulic gradient can be calculatedby the same method.
The results of specimens group 2 tests were notideal. Even the water head reached the maximum thatthe water supply system can offer, there was no
seepage failure been observed. The Piezometric level
difference remained a low value as the whole seepage
process, which meant the hydraulic gradients in thespecimens were lower. The phenomenon may be
result from the grain gradation of specimens group 2.
The pores of coarse particles are so large that watercan easily flow away. As a result, when the waterhead goes up, the hydraulic gradients increases
slightly.

P  1.1Pop，Soil flow typeP  (0.9 ~ 1.1) Pop，Transitional type P  0.9 Pop，Piping type(2)

Where P is the fine particles content (%). Specially,
the geometry mean particle size dq  d 70 d10 is usedas the particle size to distinguish the fine and coarse
particles (Liu, 2012). Pop is optimal fine particles
content, which can be calculated as the following
equation:Pop 

3.2 Influencing factors
It can be found from the tests results that the
seepage failure modes and permeability coefficientsof landslide dam material mainly depend on thecondition of fine particles filling the pores between
the coarse particle and the compactness. When the
pores between the coarse particle are large and thecontent of the fine particles is less, fine particles
cannot fill the pores, the water can easily wash themaway and piping occurs (as specimens group 3).
However, when the content of the fine particles islager, they can be constrained in the pores and thenc  3n 2  n
1 n

(3)

Where n is the soil porosity and nc porosity without
the fine particles, which relates to the uniformity
coefficient of coarse particles.
The method was used to analyze the seepage
failure modes of the four landslide dam materials
types. It is found that the analyze results basicallyequates with the tests results (table 6). Therefore, themethod applies to landslide dam material and using
the geometry mean particle size dq to distinguish
the fine and coarse particles is reasonable.

410th Asian Regional Conference of IAEG (2015)

Table 2 Test results for specimen group 3

SpecimennumberNonuniform
coefficient
Cu

CurvaturecoefficientCc

Dry density
ρd(g/cm3)Porosity
n

3-1
3-2
3-3
3-4

61.32061.32061.32061.320

0.651
0.651
0.651
0.651

1.78
1.82
1.86
1.90

0.340
0.325
0.311
0.296

Criticalhydraulicgradient
Ik
0.738
1.003
1.033
1.267

Failurehydraulicgradient
IF
1.548
1.624
1.925
2.843

Permeability
coefficientk(m/s)
3.331E-04
2.734E-04
2.498E-04
2.203E-04

Table 3 Test results for specimen group 1

SpecimennumberNonuniform
coefficient
Cu

CurvaturecoefficientCc

Dry density
ρd(g/cm3)Porosity
n

1-1
1-2
1-3
1-4

5.454
5.454
5.454
5.454

1.187
1.187
1.187
1.187

1.78
1.82
1.86
1.90

0.340
0.325
0.311
0.296

Criticalhydraulicgradient
Ik
0.928
1.063
1.32
1.619

Failurehydraulicgradient
IF
1.458
1.519
2.291
2.541

Permeability
coefficientk(m/s)
8.724E-04
6.295E-04
2.073E-04
1.031E-04

Table 4 Test results for specimen group 4

SpecimennumberNonuniform
coefficient
Cu

CurvaturecoefficientCc

Dry density
ρd(g/cm3)Porosity
n

4-1
4-2
4-3
4-4

114
114
114
114

3.942
3.942
3.942
3.942

1.78
1.82
1.86
1.90

0.340
0.325
0.311
0.296

Criticalhydraulicgradient
Ik
0.903
1.078
1.176
1.204

Failurehydraulicgradient
IF
1.468
1.611
1.771
2.101

Permeability
coefficientk(m/s)
1.289E-04
9.942E-05
7.845E-05
6.080E-05

Table 5 Test results for specimen group 2

SpecimennumberNonuniform
coefficient
Cu

CurvaturecoefficientCc

Dry density
ρd(g/cm3)Porosity
n

2-1
2-2
2-3
2-4

28.73628.73628.73628.736

2.449
2.449
2.449
2.449

1.78
1.82
1.86
1.90

0.340
0.325
0.311
0.296

Criticalhydraulicgradient
Ik/

FailurehydraulicgradientIF

Permeability
coefficientk(m/s)/

2.482E-02
2.024E-02
1.556E-02
9.846E-03

phenomenon that fine particles are washed away
through the pores of coarse particles is piping, which
relates to the problem of particle move starting.
Referring to Chen and Ming (2001), this paper
provides a new critical hydraulic gradient equation
for piping:

3.4 Critical hydraulic gradient equation of piping
Costa and Schuster (1988) found that overtopping,landslide failure and piping failure are the threebreaching modes of landslide dam. Therefore, it isimportant to provide a critical hydraulic gradientequation of piping for landslide dam materials. The

5

10th Asian Regional Conference of IAEG (2015)

Table 6 Analysis results and the tests results of seepage failure modes

Specimennumber

3-1
3-2
3-3
3-4
4-1
4-2
4-3
4-4

Geometrymean particlesize
dq (mm)Porosity
n

1.08
1.08
1.08
1.08
0.14
0.14
0.14
0.14

0.34070.32590.31110.29630.34070.32590.31110.2963

Optimal fine particles
content Pop

0.9 PopPop

1.1 Pop

0.38170.35350.32830.30580.34030.31310.28870.26700.4241
0.39280.36480.33980.37810.34790.32080.2967

0.46650.43210.40130.37380.41590.38270.35290.3264Fine
particles
content
P (%)

Seepage failure modes

Analysis
results

Tests
results

PipingPiping0.3583
Transitional

0.5725

Soil flow

Transitional

Soil flow

Table 7 Tests results and calculation results of critical hydraulic gradient of piping

Critical hydraulic gradient I kSpecimennumberPorosityn3-10.3407407

3-2

0.3259259

3-3

0.3111111

3-4

0.2962962

Fine content

Pf

0.3583 d
I k  (0.721 ~ 1.30)  s  1 1
  w  2 D0

Tests
resultCalculation
result by author

Calculation
result by Liu

Calculation
result by Mao

0.738

0.806

0.783

0.398

1.003

0.870

0.819

0.416

1.033

0.937

0.855

0.434

1.267

1.006

0.892

0.453

I k  (2.60 ~ 4.68) (4)

Where 1 is drag coefficient, d is particle sizethat is washed away by the water and D0 is averageporous diameter of particles, which can be calculated
as the following equation: n  3n 2  n 
D0 = d q  c p po (1  n) d  p f (1  n) d q  B  3n 2  n (6)The equation was used to calculate the criticalhydraulic gradient I k of landslide dam materials andthe results were compared with tests results andcalculation results of other equations (Liu, 1992; Mao,
2005) shown in table 7. The calculation results ofequation provided in this paper and tests results arequite consistent.

(5)

4.

Conclusions
Seepage characteristics of landslide dam materials
are experimentally studied. The tests measurements
can be used to study the influence of thecharacteristics on the seepage stability of landslide
dams.

Where dq is the particle size of grain boundary,and 1 =4.24， s =2.7， w =1，d  0.20 in this paper.
The equation (5) can be changed to:6

10th Asian Regional Conference of IAEG (2015)

Chinese Journal of Rock Mechanics andEngineering, Vol. 28, pp.:3626-3631.
Hu, X.W., Luo, G. and Wang, J.Q. et al. (2010):
Seepage stability analysis and dam-breaking modeof Tangjiashan barrier dam, Chinese Journal ofRock Mechanics and Engineering, Vol. 29, No. 7,
pp. 1409-1417.
Liu, J. and Xie, D. S. (2012): Research on seepage
stability experiment of gravelly soil, Rock and Soil
Mechanics, Vol. 33, No. 9, pp. 2632-2638.
Liu, J. (1992): Soil’s seepage stability and control,
Water Resources and Electric Power Press, pp.
52-59.
Mao, C. X. (2005): Study on piping and filters: part Iof piping, Rock and Soil Mechanics, Vol. 26, No. 2,
pp. 209-215.
Meyer, W., Schuster, R. L. and Sabol, M. A. (1994):
Potential for seepage erosion of landslide dam,
Journal of geotechnical engineering, Vol. 120, No. 7,
pp. 1211-1229.
Ministry of Water Resources, PRC (1992): Test
methods of soils (SL237-1999), China WaterPower
Press, pp. 434-435.
Qiu, C. X. and Li, S. J. (2008): Emergency treatment
for risk-elimination of Xiao-gangjian Landslide
Dam on Mianyuan River, Water Resources andHydropower Engineering, Vol. 39, No. 8, pp. 36-38.
Schuster, R. L. and Evans, S. G. (2006):
Risk-reduction measures for landslide dams, Italian
J Eng Geol Environ, Special Issue, Vol. 1, pp. 9-13.
Wang Z. Z. and Yang S. P. (2003): On
Characteristics
of
Seepagefrom
theEarthquake-induced Barrier Bar of the Xiaonanhai
Reservoir, Acta Geologica Sichuan, Vol. 23, No. 1,
pp. 26-30.
Zhao, H. F., Zhang, L. M. and Xu, Y., et al. (2013):
Variability of geotechnical properties of a fresh
landslide soil deposit, Engineering Geology, Vol.
166, No. 8, pp. 1-10.

Piping and soil flowing are the two seepage
failure modes of landslide dam material. Before theseepage deformation occurs, the I-V curves are linear.
The seepage failure modes and permeability
coefficients of landslide dam material mainly dependon the condition of fine particles filling the pores
between the coarse particle and the compactness.
Using the geometry mean particle size dq todistinguish the fine and coarse particles of landslide
dam materials is reasonable. A new critical hydraulicgradient equation for piping is provided in this paper
and its calculation results are quite consistent with thetests results.
Acknowledgements

The research reported in this paper was
substantially supported by the Natural Science
Foundation of China (Nos. 41372272, 41402257 and
51208218), the Shanghai Pujiang Program(14PJ1408200),and the Fundamental Research Funds
for the Central Universities (20131940).
References

Casagli, N., Ermini, L. and Rosati, G. (2003):
Determining grain size distribution of the material
composing landslide dams in the Northern
Apennines: sampling and processing methods,
Engineering Geology, Vol. 69, No. 1, pp. 83-97.
Chen, L. and Ming, Z. F. (2001): River Dynamics,
Wuhan University Press, pp.2001: 28-35.
Costa, J. E. and Schuster, R. L. (1988): The formationand failure of natural dams, Geological Society ofAmerica Bulletin, No. 100, pp. 1054-1068.
He, B.S., Ding, L. Q. and Wang, Y. J. (2009):
Preliminary evaluation of stability of Xiaojiaqiao
dammed lake in Anxian county, Sichuan province,7