International Journal of Civil Engineering and Technology (IJCIET)
Volume 10, Issue 04, April 2019, pp. 528-535. Article ID: IJCIET_10_04_054
Available online at http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=10&IType=04
ISSN Print: 0976-6308 and ISSN Online: 0976-6316
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IMPROVING THE ACCURACY OF KENNEY
AND LAU METHOD IN ORDER TO ASSESS THE
INTERNAL STABILITY OF GRANULAR SOILS
Mohamed Ahmad ALsakran
Ph.D. candidate, Institute of Geotechnical Engineering, Hohai University, 01 Xikang Road,
Nanjing 210098, P. R. China
Jun-Gao ZHU
Ph.D., Professor, (a) Key Laboratory of Ministry of Education for Geomechanics and
Embankment Engineering, Hohai University, 01 Xikang Road, Nanjing 210098, P. R. China
(b) Guest professor, Key Laboratory of failure mechanism and safety control techniques of
earth-rockfill dam of the Ministry of Water Resources, Nanjing hydraulic research institute,
223# Guangzhou Road, Nanjing, P. R. China
ABSTRACT
Internal instability normally occurs in widely graded or gap graded soils. To be
more specific, it may occur in soils that have a bimodal structure (i.e. the soil has two
components, namely, coarse fraction and loose fine fraction). Internal instability
occurs when the finer fraction particles can be washed out if their size is less than the
size of the constrictions among the coarse fraction particles. A commonly used
approach to evaluating the potential for internal instability in soils is that of Kenney
and Lau. This method is used to assess the internal stability of cohesionless soils based
on the shape of their GSD curves. The main objective of this paper is to propose a
modification on Kenney and Lau method to increase the accuracy of this method using
the critical particle sizes between the groups of the soils. Major type groups of the soils
include; gravel (>4.75mm), sand (0.075-4.75mm) and fines (<0.075mm). The proposed
modification was verified with a large number of experimental tests.
Keywords: Kenney and Lau method, internal stability, suffusion, granular soils, soil
type groups.
Cite this Article: Mohamed Ahmad ALsakran and Jun-Gao ZHU, Improving the
Accuracy of Kenney and Lau Method in Order to Assess the Internal Stability of
Granular Soils, International Journal of Civil Engineering and Technology, 10(4),
2019, pp. 528-535.
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Mohamed Ahmad ALsakran and Jun-Gao ZHU
1. INTRODUCTION
Suffusion is an internal erosion process which involves washing out of loose fine particles
through the voids of the primary soil. The fine particles are removed under the effect of seepage
forces, leaving behind an intact soil skeleton formed by the coarser particles; hence the
permeability and seepage maybe increased, Chang and Zhang (2011), leading to collapse of
the soil skeleton, McCook (2004).
Suffusion may result-in settlement or development of pipes or cracks. Experimental studies
of Ke and Takahashi (2011), (2012) and numerical studies of Scholtes et al (2010) and Wood
et al. (2010) had revealed that internal erosion problems can also cause a strength reduction.
As a result, partial or complete failure of dams and embankments may occur soils (e.g., Fell
etal.,2003; Zhang andChen,2006; Xu and Zhang, 2009; Zhang etal.,2009, 2011;
Fujisawaetal.,2010; Pagano etal.,2010; PengandZhang,2012)..
Suffusion also may occur in the granular filters that were constructed of internally unstable
materials, renders those filters coarser, and accordingly decreases their ability to protect the
core or foundations materials, Wan and Fell (2008).
Many studies investigated the internal stability of cohesionless soils, for example, Kézdi
(1979), Kenney and Lau (1985, 1986), Lafleur et al. (1989), Burenkova (1993), Skempton and
Brogan (1994), Fannin and Moffat (2002), Moffat and Fannin (2006), Li and Fannin (2008),
Ahlinhan et al. (2010), and Andrianatrehina et al. (2012).
2. THE KENNEY-LAU APPROACH
The method of Kenney and Lau (1985, 1986) (KL method) is used to assess the internal
stability of cohesionless soils based on the shape of their GSD curves. The finer percent (F)
corresponding to an arbitrary particle diameter (D) is determined, as shown in Figure 1. (D) is
the particle size that distinguishes the finer particles from the main soil skeleton; it’s called the
delimiting particle size (DPS). The finer percent corresponding to the particle diameter (4D)
is also determined, accordingly, the value of (H) can be easily determined as the difference of
the finer percent between D and 4D. The internal stability is determined by calculating the H/F
ratios in the range of F ≤ 0.2 for widely-graded soils, and in the range of F ≤0.3 for narrowlygraded soils. The soil is considered as internally unstable if the ratio (H/F) lies below the
stability boundary (H/F=1.0) as shown in Figure 2.
Figure 1 Determination of F and H of KL method
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Improving the Accuracy of Kenney and Lau Method in Order to Assess the Internal Stability of
Granular Soils
Figure 2 The stability boundary of KL method
Kenney and Lau (1985) initially defined the stability boundary as H/F = 1.3. Later, it was
revised to be (H/F = 1.0) (Kenney and Lau, 1986) upon comments of Milligan (1986), Sherard
and Dunnigan (1986), and Ripley (1986).
The method assumes that the maximum possible finer content (i.e. erodible particles) for
the widely graded soils (with Cu>3) is 20% and for the narrowly graded soils (with Cu<3) is
30%. For this reason the analysis is performed in the range of F <20% or F <30%.
Li (2008) found out that the method of Kenney and Lau assesses the stability of “unstable
gradations” correctly, while it provides a wrong assessment of some “stable gradations”.
Accordingly, the method is conservative in evaluating the potential for internal stability.
3. MODIFIED KENNEY AND LAU METHOD USING SOIL CLASSES
The range of particle sizes encountered in soil is very large from boulders with a controlling
dimension of over 200mm down to clay particles less than 0.002mm.
In the Unified Soil Classification System, soils are classified into named Basic Soil Type
groups according to size, and the groups further divided into coarse, medium and fine. See
Fig3.
1
Fines
0.8
Gravel
Sand
F%
0.6
0.4
Coar
se
Mediu
m
Fine
0.2
0
0.01
0.1
1
10
100
D mm
Figure 3 Basic Soil Type groups
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Mohamed Ahmad ALsakran and Jun-Gao ZHU
For finding the expected (D) or the expected DPS particles that is located in-between of
coarse and fine particles, see Fig 4. For soils that have three type groups (gravel, sand, fines),
we need to determine two DPS particles (D1, D2). It is suggested to draw a line crosses the
largest grain size and the critical particle size of gravel group and sand group (4.75mm), the
intersection of this line with the diameter axis will determine the first DPS particle (D1). By
drawing another line crosses the largest grain size and the diameter of grain size that represents
the critical particle size of sand group and fines group (0.075mm), the second DPS particle
(D2) will be obtained by intersection of this line with the diameter axis.
1
0.8
F%
0.6
0.4
0.2
0
0.01
D
2
0.075
0.1
mm
D1
4.75mm
1
10
100
D mm
Figure 4 The expected (D) for three type groups
Fig5 shows, for soils that have two type groups (Gravel, Sand), two DPS particles (D1, D2)
are needed. To determine (D1), it is suggested to draw a line crosses the largest grain size and
the critical particle size of coarse aggregate and medium aggregate inside the sand group
(2mm), the intersection of this line with the diameter axis will determine the first DPS particle
(D1). By drawing another line crosses the largest grain size and the diameter of grain size that
represents the critical particle size of gravel group and sand group (4.75mm), the second DPS
particle (D2) will be obtained by intersection of this line with the diameter axis.
As well for soils that have one type group (sand), it is needed to determine one DPS particle
(D), see Fig6. It is suggested to draw a line crosses the largest grain size and the grain size that
represents the critical particle size of sand aggregate and fine aggregate inside the sand group
(0.425mm), the intersection of this line with the diameter axis will determine the DPS particle
(D).
1
0.8
F%
0.6
0.4
0.2
0
0.01
0.1
D
2
D
1
1
D mm
2m
m
10
100
Figure 5 The expected (D) for two type groups
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Improving the Accuracy of Kenney and Lau Method in Order to Assess the Internal Stability of
Granular Soils
1
0.8
F%
0.6
0.4
0.2
0
0.01
D 0.425m
m
0.1
1
10
D mm
Figure 6 The expected (D) for one type group
4. ANALYSIS AND RESULTS
To get a clear idea about the accuracy of modified Kenney and Lau method that proposed here
in this study, data published in the literature have been reanalyzed, namely: Skempton and
Brogan (1994), Lafleur and Nguyen (2007), Wan and Fell (2004) and Wan and Fell (2008),
Kenney and Lau (1985), Lafleur et al. (1989), Aberg (1993), Sadaghiani and Witt (2011), and
Li (2006). In total 53 laboratory tests have been reanalyzed.
In order to verify the modified Kenney and Lau method, the 53 data sets were analyzed and
the delimiting particles size DPS (D) were determined and compared with (4D) to find H/F.
Experimental tests data were analyzed according to the classical Kenney and Lau and
modified Kenney and Lau methods.
Table1 shows analysis soils data published in literature that have three type groups. There
are 5 wrong predictions using the classical Kenney and Lau; where using modified Kenney and
Lau, there are 2 wrong predictions.
By analyzing the data in the literature that have two type groups and one type group, as we
see in table2 and table3, respectively. For two type groups, there are 7 wrong predictions using
the classical Kenney and Lau; where using modified Kenney and Lau, there are 3 wrong
predictions.
As same as for one type group, there are 2 wrong predictions using the classical Kenney
and Lau; where using modified Kenney and Lau, there isn’t any wrong predictions.
Table 1 Internal stability assessment of three type groups soils data
No
Gradat
ion
Soil type
groups
1
X
Gravel - Sand Fines
H/F (DmaxH/F (Dmax4.75mm)
0.075mm)
K-L (1985)
0.64
15.00
Classical
K-L
Modified
K-L
Laborat
ory
U
U
U
Aberg (1993)
2
E
3
G
4
H
Gravel - Sand Fines
Gravel - Sand Fines
Gravel - Sand Fines
1.48
1.20
U
S
S
1.00
3.67
S
S
S
0.37
29.00
U
U
U
Sadaghiani and Witt (2012)
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No
Gradat
ion
5
Soil type
groups
Gravel - Sand Fines
H/F (Dmax4.75mm)
H/F (Dmax0.075mm)
Classical
K-L
Modified
K-L
Laborat
ory
0.33
5.74
U
U
U
1.40
S
S
S
Li (2006)
6
HF10
Gravel - Sand Fines
1.00
Wan and Fell (2004,2008)
7
1,1A
8
2R
9
4R
10
9
11
10
12
A2
13
A3
14
B1
15
B2
16
C1
17
D1
Gravel - Sand Fines
Gravel - Sand Fines
Gravel - Sand Fines
Gravel - Sand Fines
Gravel - Sand Fines
Gravel - Sand Fines
Gravel - Sand Fines
Gravel - Sand Fines
Gravel - Sand Fines
Gravel - Sand Fines
Gravel - Sand Fines
0.34
1.29
U
U
S
0.29
1.375
U
U
S
2.33
1.48
U
S
S
4.47
1.05
U
S
S
0.93
1.83
U
U
U
0.41
1.14
U
U
U
0.71
0.67
U
U
U
0.36
1.31
U
U
U
0.32
0.93
U
U
U
0.60
0.80
U
U
U
0.40
0.88
U
U
U
Table 2 Internal stability assessment of two type groups soils data
No
Gradation
Soil type
groups
18
19
20
21
22
23
24
25
26
27
28
A
Y
YS
AS
DS
1
2
3
20
21
23
Gravel - Sand
Gravel - Sand
Gravel - Sand
Gravel - Sand
Gravel - Sand
Gravel - Sand
Gravel - Sand
Gravel - Sand
Gravel - Sand
Gravel - Sand
Gravel - Sand
29
F
Gravel - Sand
30
31
A
B
Gravel - Sand
Gravel - Sand
H/F (Dmax4.75mm)
H/F (Dmax2mm)
K-L (1985)
0.90
2.20
0.50
0.92
0.76
0.91
1.00
1.08
5.36
4.40
1.15
2.20
2.00
2.25
1.25
1.24
2.33
3.00
8.00
5.67
7.57
10.25
Aberg (1993)
3.47
8.00
Skempton and Brogan (1994)
0.67
13.00
1.17
6.00
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Classical
K-L
Modified
K-L
Laboratory
T
U
U
T
S
S
S
S
S
S
S
U
U
U
S
S
S
S
S
S
S
S
U
U
U
S
S
S
S
S
S
S
S
S
S
S
U
T
U
S
U
U
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Improving the Accuracy of Kenney and Lau Method in Order to Assess the Internal Stability of
Granular Soils
No
Gradation
32
33
C
D
Soil type
groups
Gravel - Sand
Gravel - Sand
34
35
HF01
HF03
Gravel - Sand
Gravel - Sand
36
37
38
39
40
41
42
43
1
2
3
4
11
12
13
14
Gravel - Sand
Gravel - Sand
Gravel - Sand
Gravel - Sand
Gravel - Sand
Gravel - Sand
Gravel - Sand
Gravel - Sand
44
45
M6
M8
Gravel - Sand
Gravel - Sand
H/F (Dmax4.75mm)
2.25
3.50
H/F (Dmax2mm)
3.38
5.36
Li (2006)
3
2.75
0.47
0.33
Burenkova (1993)
0.78
0.18
2.00
0.33
0.33
0.25
0.63
0.31
2.04
3.00
1.92
1.55
1.14
1.14
0.75
0.71
Lafleur et al (1989)
0.47
0.52
1.33
0.84
Classical
K-L
S
S
Modified
K-L
S
S
U
U
S
U
U
U
U
U
U
U
U
U
U
U
U
U
U
U
S
S
S
U
U
U
U
U
S
S
S
S
U
U
U
U
U
U
Laboratory
S
S
Table 3 Internal stability assessment of one type group soils data
No
Gradation
Soil type groups
46
47
FR7
FR8
Sand
Sand
48
49
50
51
52
53
G1-a
G1-b
G3-a
G3-b
G4-a
G4-b
Sand
Sand
Sand
Sand
Sand
Sand
H/F (Dmax-0.425mm)
Li (2006)
0.00
0.07
Honjo et al (1989)
8
2.5
0.75
0
0
0.2
Classical K-L
Modified KL
Laboratory
U
U
U
U
U
U
T
T
U
U
U
U
S
S
U
U
U
U
S
S
U
U
U
U
5. SUMMARY AND CONCLUSIONS
Analysis of a large number of experimental tests showed that the accuracy of classical Kenney
and Lau model is somehow low (14 wrong predictions of 53 data sets). The modification that
has been done on Kenney and Lau method, the internal stability criterion is applied only on
one or two delimiting particle size depending upon the type groups of soils. By this
modification, applying Kenney and Lau method is simpler and faster than the classical method;
as well as, it was effective to enhance the accuracy of Kenney and Lau method to (5 wrong
predictions of 53 data sets). Accordingly, the modified Kenney and Lau method is
recommended to assess the internal stability of the granular soils.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the financial support from National Key R&D Program of
China(2017YFC0404804),a research grant (No. 51479052) from the National Natural Science
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Mohamed Ahmad ALsakran and Jun-Gao ZHU
Foundation of China, and the Fundamental Research Funds for the Central Universities of
China(No. 2017B20614).
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