INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING
Volume 3, No 3, 2013
© Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0
Research article
ISSN 0976 – 4399
Reliability of using standard penetration test (SPT) in predicting
properties of silty clay with sand soil
Mostafa Abdou Abdel Naiem Mahmoud
Associate Professor, Civil Engineering Department, Assiut University, Assiut 71516, Egypt
Civil Eng .Department, Faculty of Engineering, Aljouf University, KSA
mostafaabdo@yahoo.com
doi:10.6088/ijcser.201203013050
ABSTRACT
The properties of soil play an important role in many practices for geotechnical engineering.
To determine the real values of these properties special techniques should be followed such
as undisturbed samples and initial overburden pressures should be taken into consideration.
Actually, it is difficult to obtain 100 % undisturbed samples due to handling, transportation,
release of overburden pressure and poor laboratory conditions. So, prediction of some
properties such as shear strength parameters (c & ϕ) for hard and dry silty clay with sand soil
with the help of standard penetration test (SPT) provides a good opportunity to obtain these
parameters without using of more laboratory tests. Standard penetration tests (SPT), rough
measure the strength of soil. The great merit of this test and the main reason for its
widespread use is that it is simple and inexpensive. The shear strength parameters which can
be inferred are approximate, but may give a useful guide in ground conditions where it may
not be possible to obtain borehole samples of adequate quality like clay containing sand or
gravel. This study was undertaken in order to study the reliability of using standard
penetration test (SPT) in predicting some properties, (such as Atterberg limits LL, PL, PI, and
shear strength parameters (c & φ)), of silty clay with sand soil. Many samples from this soil
have been investigated for this purpose. The results of the research indicated that the shear
strength of soil affects SPT number. Empirical equations to identify the shear strength
parameters of silty clay with sand soil using corrected SPT number (N") have been proposed.
Keyword: Silty clay with sand, (c & φ), LL, PL, PI, SPT, correction coefficient, field tests.
1. Introduction
Numerous methods have been developed for determination the soil properties. These methods
include the field and laboratory tests. Also, to determine the real values of these properties,
special techniques should be followed such as undisturbed samples and initial overburden
pressures should be taken into consideration. On the other hand, field test such as SPT does
not depend on undisturbed sample because it is carried out in original field soil. Most
problems in soils and construction involve either the strength of the in-situ soil or the
compressibility of the soil mass. People tend to have a negative opinion of clay soils. These
soils may not drain quickly after heavy rain and often harden when dry. Most soils contain a
mixture of clay, organic matter, sand and silt. If a soil contains at least 40 percent clay, then
we classify it as a "clay soil" (Tuker, 1999). Clay is made up of tiny particles less than 0.002
mm in diameter. By comparison, silt particles range from 0.002 to 0.06 mm in size and sand
particles from 0.06 to 2.0 mm. For this reason, clay soils are considered to be fine textured;
silt, medium textured; and sand coarse textured. Clay acts as a binding agent between soil
particles. It gives soil elasticity and provides cohesion of soil particles. On the other hand,
sand provides friction of soil particles. Properties of silty clay with sand soil are significantly
Received on January 2013 Published on March 2013
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Reliability of using standard penetration test (SPT) in predicting properties of silty clay with sand soil
Mostafa Abdou Abdel Naiem Mahmoud
associated with stiffness of this soil and give an indication of shear strength of soil. These
properties plays an important role in many practices for geotechnical engineering, such as soil
description or classification of soil, prediction of behavior of soil if it will be subjected to
extensive settlement or swelling. The real value of these properties such as angle of internal
friction (φ) and cohesion (c) require a special care and laboratory technique. So, prediction of
soil properties with the help of field tests such as standard penetration tests (SPT) provides a
good opportunity to obtain this parameter without using of more laboratory tests. Standard
penetration tests (SPT), rough measure the strength of soil. It is currently the most popular
and economical means of obtaining subsurface information. The SPT has been extended to
other areas of soil mechanics and foundations, and it has become a fundamental parameter in
geotechnical engineering. The great merit of the test and the main reason for its widespread
use is that it is simple and inexpensive. In conditions where the quality of the undisturbed
sample is suspect, e.g. silty clay with sand soil, it is often advantageous to alternate the
sampling with standard penetration tests to check the strength.
Standard penetration test (SPT) is one of the field tests of soil. It primary gives idea about
shear strength of soil which it can be expressed in terms of shear strength parameters (c & φ).
Most laboratory tests must be performed on molded samples of soils. These tests are
laborious and time consuming; but sometimes the results are not accurate due to the poor
laboratory conditions (Venkatasubramanian and Dhinakaran, 2011). An aggregate that
contains sufficient fines to fill all voids between aggregate grains will still gain its strength
from grain-to-grain contact but has increased shear resistance (Siswosoebrotho et al., 2005).
Plasticity index (PI) and liquid limit (LL) can be used to determine the swelling
characteristics of expansive clay in general (Subjianto et al., 2011). (Garham, 2012) indicates
that it is a poor ability to get similar results when testing different portions of the same
samples, but the result differences should be below the upper control limit. Sometimes,
Strength values determined from laboratory testing of intact soil cores are recognized as not
being directly applicable to the in-situ soil mass because of the scale effect.
The standard penetration test (SPT) is very widely used for subsurface investigation in many
parts of the world. It measures the resistance of a hollow core being hammered with a 63.5kg
weight. The hollow core receives a soil sample during the process which is logged to soil
identification purposes. The relative density can be estimated from the number of blows used
to drive the SPT a total of distance of 450mm with the total blows for the last 300mm being
the SPT Nf -value (Silva, 2010). To construct a structure, Nf value (SPT value) is an important
parameter to understand the soil condition at different depths. After investigating Nf value of
different depths, foundation depth can be selected according to load of the structure. Then
determine the corrected N" value at the required depth. Angle of internal friction is
determined from corrected SPT N"-value. After that, determine bearing capacity factors to
calculate the bearing capacity of the soil. From this bearing capacity of soil foundation can be
designed (Humyral et al. 2012). When the SPT hammer impacts the drill rod, it creates a
compression stress wave that travels dawn the rod and at the same time propagates in the
hammer (Abou-mater et al., 1997).
Terzaghi, 1940 realized that the penetration resistance of the split-spoon sampler could
provide useful in-site test data that might be correlated with the consistency and density of
the soils encountered. He discussed with Harry Mohr and developed correlations between the
number of blows, N, and a number of salient properties of soils, including the relative density
of sands, consistency and unconfined compressive strength of clays, and allowable bearing
pressure on sands and clays. The first published SPT correlations appeared in Terzaghi and
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Reliability of using standard penetration test (SPT) in predicting properties of silty clay with sand soil
Mostafa Abdou Abdel Naiem Mahmoud
Peck, 1948. These were soon followed by correlations relating SPT blow counts to
consistency for silts and clays and relative density for sands in (Peck et al., 1953). A less
recognized problem is the influence of strata thickness and changes in stiffness, sketched in
Figure 1. As the sample barrel approaches an appreciably stiffer horizon, the penetration
resistance will increase, even though the sampled material remains more or less constant
throughout the softer horizon. This can lead to overestimates of strength, density, and
compressibility based solely on blow-count values.
A big disadvantage of the SPT procedure is that it reports the average blows per foot during
any given sample round, so the measurement would only be valid for horizons .12 in. (30.5
cm) thick, plus the influence area beneath the sampler shoe sketched in Figure 1. This is the
zone of influence in front of the sampler shoe. This influence zone usually equal to four to
seven times the sample barrel diameter. Overall, the SPT procedure tends to ‘‘average’’ the
penetration resistance of the materials sampled, and the material effects of low strength
horizons less than 20–26 in. Soil behavior is greatly influenced by sampling disturbance.
Nevertheless, disturbed samples may be adequate for indices tests such as Atterberg limits
and grain size distribution, but slightly more suspect density and water content because of
densification (Rogers, 2006).
Figure 1: The SPT sample barrel approaches stiff contacts. (Rogers, 2006)
From Figure 1, it can be found that stiff material has increasing resistance to penetration, and
the blow counts will increase markedly, even though sampling in soft material (Rogers, 2006).
This study was undertaken in order to quantify the variation of the values of different
properties of silty clay with sand soil using corrected SPT number (N"). Field and
experimental tests were conducted for this purpose. Empirical equations to identify the shear
strength parameters of silty clay with sand soil using corrected SPT number (N") have been
proposed.
International Journal of Civil and Structural Engineering
Volume 3 Issue 3 2013
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Reliability of using standard penetration test (SPT) in predicting properties of silty clay with sand soil
Mostafa Abdou Abdel Naiem Mahmoud
2. Field and experimental work
The site of this work is, Tabarjal - Al-Jouf, KSA. The field work consisted of drilling and
sampling of more than 100 boreholes to depths between 10 m to 15 m below ground surface.
A standard penetration test (SPT) was carried out according to ASTMD – 1586 – 84. A suit of
in situ testing and sampling within the boreholes was planned including standard penetration
test SPT. Many specimens with different field SPT numbers (Nf) have been taken out. To
carry out the various laboratory tests, the specimens were carefully transported according to
standard classification and techniques. Atterberg limits including liquid limit (LL), plastic
limits (PL) and plasticity index (PI) were determined according ASTMD – 4318 – 84. To
determine the shear strength parameters (c &φ), a direct shear box test was used.
2.1 Description and properties of soil
The borehole logs mainly consisted of surface layer of medium dense and dry poorly graded
sand with silt, then a layer of very dense and dry poorly graded sand. A thick layer of hard
and dry silty clay with sand has appeared below the previous two layers and continued to the
end of borehole logs. This study concerns with the layer of hard and dry silty clay with sand
which lies at depth of 3.0m below ground surface. The specimens were taken at different
depths of mentioned layer. The Field and laboratory results such as SPT, liquid limit (LL),
plastic limit (PL), plasticity index (PI) and shear strength parameters (c & φ) were determined.
2.2 Correction factors for SPT-N-values
There are numerous factors other than hammer type that are permitted by ASTM D 1586-99
and that affect the N value. Correction factors have been proposed by various authors to
account for factors such as the drill stem length and type, the type of anvil, the blow rate, the
use of liners or bore hole fluid and the type of hammer. The standard blow count N' can be
computed from the field measured Nf from the following general equation (excluding the
overburden corrections) (Aggour and Radding, 2001):
N' = Nf n1 n2 n3 n4 n5 n6
(1)
Where n1 = energy correction factor
n2 = rod length correction factor
n3 = liner correction factor
n4 = borehole diameter correction factor
n5 = anvil correction factor
n6 = blow count frequency correction factor
When the length of the drill rod is less than 10ft, a considerable amount of energy is reflected
back in the rod reducing the energy available for driving the sampling tube into the ground,
thus it is recommended that the Nf values should be corrected for short lengths of rods. The
correction factor for length is n2. The barrel is often used without liners. In this case there is
less friction developed inside the sampling tube, which in turn reduces the measured N values.
It has been shown that the use of the ASTM sampler without the liner leads to 10% to 30%
lower Nf values. It is thus recommended that the measured Nf values should be corrected for
the use of the liner. The correction factor is n3. SPT Nf values are corrected if they are made
in boreholes have diameter larger than 4.5 inches. When boreholes are larger than 4.5 inches,
measured Nf values are lower than they would be for a smaller diameter hole. The correction
factor is n4. When the hammer falls during the SPT testing, it strikes an anvil attached to the
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Reliability of using standard penetration test (SPT) in predicting properties of silty clay with sand soil
Mostafa Abdou Abdel Naiem Mahmoud
drill rod stem. The anvil can vary in shape, size and weight. The amount of energy transferred
to the drill rods depends on the weight of the anvil. The correction factor is n5. Another
correction n6 is for blow count frequency that applies for soils including sands below the
water table (Aggour and Radding, 2001). From tables presented by (Aggour and Radding,
2001), and according to field conditions and soil type, average values of correction factors
can be taken as (n1 = 1.67, n2 = 1.0, n3 = 1.0, n4 = 1.0, n5 = 0.7, n6 = 1.0).
Due to overburden, the N' value of SPT for soils including cohesionless soil shall be corrected
for overburden as per Figure 2 and equation (2) (Narian, 1982).
N" = n7 N'
(2)
Where n7 is the overburden correction factor and can be obtained from Figure 2.
The properties for random twenty one chosen specimens were tabulated as shown in Table 1.
Figure 2: Correction due to overburden (Narian, 1982)
2.3 Scope of study
To study the reliability of using SPT test in predicting properties of silty clay with sand soil,
many boreholes and SPT at different depths were carried out. Many samples were obtained to
determine experimentally the properties of soil and the results are tabulated as shown in
Table 1. There are different parameters were taken into consideration such as depth of sample
(D) below ground surface, SPT number (N), liquid limit (LL), Plastic limit (PL), plasticity
index (PI) and shear strength parameters (c & φ) . In this study, the relationships between
corrected SPT number (N") and all previous parameters have been discussed to make sure
that which of these parameters affect the SPT.
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Reliability of using standard penetration test (SPT) in predicting properties of silty clay with sand soil
Mostafa Abdou Abdel Naiem Mahmoud
Table 1: Field and laboratory results
Sample
No.
D
Depth below
(G.S.) (m)
SPT
Nf
Corrected
SPT
N"
LL
(%)
PL
(%)
PI
(%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
4.5
3
3
4.5
3
3
5
7.5
7.5
10.5
3
6
5
4.5
7.5
7.5
10.5
3
6
4.5
4.5
37
44
40
34
34
38
46
77
53
69
51
52
51
46
77
53
69
51
52
64
51
44
57
51
41
44
49
53
77
53
60
61
57
58
55
77
53
60
66
57
76
61
27
25
23
27
29
23
23
25
24
25
24
24
24
23
25
24
25
24
24
24
24
21
21
19
21
25
19
19
20
19
20
19
19
19
18
20
19
20
19
19
19
19
6
4
4
6
4
4
4
5
5
5
5
5
5
5
5
5
5
5
5
5
5
3. Analysis of results and discussions
In the following sections, the results obtained from field and laboratory tests are presented
and discussed in detail. Considering the correlation coefficient (R2), the best fitting between
the results has been plotted. The purpose of use of this statistical method is to give us a
statistic known as the correlation coefficient which is a summary value of a large set of data
representing the degree of linear association between two measured variables. R2 is a statistic
that will give some information about the goodness of fit of a relationship. In regression, the
R2 coefficient of determination is a statistical measure of how well the regression line
approximates the real data points (Taylor, 1990). According to the values of R2, the
relationship between any two parameters can be classified as (R2 <0.30) are considered to
have no correlation, (R2 of 0.30 to 0.499) are considered to be a mild relationship, (R2 of 0.50
to 0.699) are considered to be a moderate relationship and, (R2 of 0.70 to 1.0) are considered
to be a strong relationship.
Many figures have been plotted to analyze and illustrate the relationships between field and
experimental data including corrected SPT number (N"), depth of sample (D) measured from
ground surface, Atterberg limits (LL), (PL), (PI), and shear strength parameters (c & φ).
Three scopes of study to obtain the effect and relationship between these mentioned
parameters have been performed as follows:
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Reliability of using standard penetration test (SPT) in predicting properties of silty clay with sand soil
Mostafa Abdou Abdel Naiem Mahmoud
3.1 Effect of soil depth (D) below ground surface
Figures 3 to 6 show the relationships between depth of sample (D) below ground surface and
different studied parameters including corrected SPT number (N"), Atterberg limits of silty
clay with sand LL, PL and PI, respectively. Figure 3 shows the relationships between depth of
soil below ground surface (D) and corrected SPT number (N"). It can be noticed that
corrected SPT number (N") increase as depth of sample below ground surface increases and
this is due to the effect of overburden pressure which increases the relative density of soil
layer lies deeper below ground surface than others. Considering value of correlation
coefficient (R2 = 0.30), it is considered to be a mild relationship as shown in Figure 3. On the
other hand, Figures 4 to 6 show the relationships between depth of soil (D) and the Atterberg
limits, (LL), (PL) and (PI), respectively. It is clearly seen from these Figures that depth of
sample (D) has no effect on the Atterberg limits of soil because these properties depend on
the physical and mechanical properties of soil particles and are carried out on disturbed
samples. Also, considering the values of correlation coefficient (R2 <0.30), this means that
these relationships are considered to have no correlation and behavior is explained by chance
(see Figures 4, 5 and 6). This leads to conclude that the Atterberg limits do not depend on soil
depth but depend on physical and mechanical properties of soil particles itself.
Figure 3: Depth of Sample below ground surface versus corrected SPT number (N")
Figure 4: Depth of Sample below ground surface versus liquid limit (LL)
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Reliability of using standard penetration test (SPT) in predicting properties of silty clay with sand soil
Mostafa Abdou Abdel Naiem Mahmoud
Figure 5: Depth of Sample below ground surface versus plastic limit (PL)
Figure 6: Depth of Sample below ground surface versus plasticity index (PI)
3.2 SPT versus Atterberg limits LL, PL and PI
Figures 7, 8 and 9 show the relationships between corrected SPT number (N") and Atterberg
limits LL, PL and PI, respectively. From these Figures, it can be noticed that the Atterberg
limits have no effect on SPT number (N") because SPT depends on the relative density of soil
layer. According to the procedures of SPT, it can be found that the test has carried out in the
field on soil as it is (undisturbed soil). Whereas, and according to the procedures of these
laboratory tests, the Atterberg limits of soil depend mainly on the physical and mechanical
properties of soil particles and are carried out on disturbed samples as mentioned in previous
section. Also, considering the values of correlation coefficient (R2 <0.30), this means that
these relationships are considered to have no correlation and behavior is explained by chance.
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Reliability of using standard penetration test (SPT) in predicting properties of silty clay with sand soil
Mostafa Abdou Abdel Naiem Mahmoud
Figure 7: Corrected SPT number (N") versus liquid limit (LL)
Figure 8: Corrected SPT number (N") versus plastic limit (PL)
Figure 9: Corrected SPT number (N") versus plasticity index (PI)
3.3 SPT versus shear strength parameters (c & φ)
Figures 10 and 11 show the relationships between corrected SPT number (N") and shear
strength parameters of silty clay with sand soil, angle of internal friction (ϕ) and cohesion (c),
respectively. From these figures, it can be noticed that corrected SPT number (N") increases
International Journal of Civil and Structural Engineering
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Reliability of using standard penetration test (SPT) in predicting properties of silty clay with sand soil
Mostafa Abdou Abdel Naiem Mahmoud
as shear strength of soil increases. The increasing of shear strength parameters of soil is due
to increasing in relative density which leads to increasing in corrected SPT number (N"). This
may be due to the insertion of fine soil particles in the voids between sand particles to make
the specimens are less voids and the friction surface increases too. Also, This means that the
surface of shear behaves as a general shear failure and leads to shear strength increases to
make SPT number increases too.
From figure 10, a high correlation coefficient (R2 = 0.851) can be an indication of a good
correlation between corrected SPT number (N") and angle of internal friction (ϕ). Also, in
Figure 11, a correlation coefficient (R2 = 0.871) of this figure can be an indication of a good
correlation between corrected SPT number (N") and cohesion (c). This means that the
relationships between SPT number (N") and shear strength parameters (c & ϕ) are considered
to a strong relationships. This leads to conclude that the shear strength parameters (c & ϕ)
have strongly effected on SPT number.
Figure 10: Corrected SPT number (N") versus angle of internal friction (φ)
Figure 11: Corrected SPT number (N") versus cohesion (c)
Also, from the previous analysis, it can be found that the SPT number (N") has strongly
correlated with shear strength of soil which can be expressed in parameters (c & ϕ). So, the
shear strength parameters (c & ϕ) can be reasonably predicted from field with the help of
corrected SPT number (N").
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Reliability of using standard penetration test (SPT) in predicting properties of silty clay with sand soil
Mostafa Abdou Abdel Naiem Mahmoud
3.4 Empirical equations to predict the shear strength parameters (c &ϕ
ϕ)
From the results and relationships shown in Figures 10 and 11 and regression analysis,
empirical equations to estimate the shear strength parameters (c & ϕ) for silty clay with sand
soil with the help of corrected SPT number (N") as follow:
φ = 0.209 N" + 19.68
(3)
c = 0.014 N" – 0.18
(4)
Where, (N") is the corrected SPT number,
ϕ is the angle of internal friction in (degree) and c is the cohesion in (Kg/cm2).
The equations (3) and (4) have correlation coefficients (R2) equal to 0.851and 0.871,
respectively, are considered to be strong relationships. This means that a very good
correlation between SPT number (N") and shear strength parameters (c & ϕ) of silty clay with
sand soil.
4. Conclusions
The following results are concluded based on the results and on the discussion and analysis
presented in this research:
1. The depth of soil (D) below ground surface significantly affect the SPT number, so
that it should be corrected before used.
2. The Atterberg limits LL, PL, and PI do not affect the SPT number.
3. The shear strength of soil strongly affects the SPT number.
4. For identification of shear strength parameters (c & ϕ) of silty clay with sand soil
using SPT is adequate rather than using laboratory tests because SPT carries out in the
field on undisturbed soil.
5. Empirical equations to predict the shear strength parameters (c & ϕ) of silty clay with
sand soil using SPT have been presented as mentioned in equations (3) and (4). The
accuracy of these equations proved to be strong relationships.
6. The standard penetration test (SPT) is considered reliable in predicting of shear
strength parameters (c & ϕ) of silty clay with sand soil.
5. References
1. Abou-mater, H., and Goble, G.G., (1997), SPT dynamic analysis and measurements,
Journal of Geotechnical and Geoenvironmental Engineering, October, pp 921-928.
2. Aggour, M.S., and Radding, W.R., (2001), Standard penetration test (SPT) correction,
Research report, Civil and Environmental Engineering Department, University of
Maryland College Park, Maryland, 20742.
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Reliability of using standard penetration test (SPT) in predicting properties of silty clay with sand soil
Mostafa Abdou Abdel Naiem Mahmoud
3. Garham, J., (2012), TR4 - plasticity index proficiency 2011, CETANZ, Civil
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