Engineering Geology 86 (2006) 52 – 69
www.elsevier.com/locate/enggeo
Determination of undrained strength of fine-grained soils by
means of SPT and its application in Turkey
O. Sivrikaya a,⁎, E. Toğrol b
a
Civil Engineering Department, Faculty of Engineering and Architecture, Niğde University, 51100, Niğde, Turkey
b
Civil Engineering Department, Faculty of Engineering, Istanbul University, 34320, Avcilar, Istanbul, Turkey
Received 30 August 2005; received in revised form 20 February 2006; accepted 3 May 2006
Available online 22 June 2006
Abstract
The Standard Penetration Test (SPT) is one of the oldest and the most common in situ test used in soil explorations. In the recent
years with the advent of new technology and techniques in determining the drawbacks in SPT, several researchers have attempted
to correlate corrected field measured values with several soil properties. In this context, corrections applied to field values have
become critical. In this study a questionnaire including the performance of the standard penetration test and equipment used in
practice in Turkey is circulated in order to determine the relevant correction factors. Thus the appropriate corrections are used in
acquiring corrected SPT-N values. The relationships between SPT-N and the undrained shear strength (Su) are examined from the
statistical point of view by taking the test types and SPT corrections into consideration, and comparison is made with previous
studies. It is observed that SPT corrections play an important role on the obtained correlation equations. In addition, the importance
of the effects of test types on the correlations is also emphasized. The Standard Penetration Test is found to be sufficient for reliable
assessment of Su.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Standard Penetration Test; SPT corrections factors; Fine-grained soils; Undrained shear strength; Correlation; Turkey
1. Introduction
In geotechnical engineering, the engineering properties of soil layers must be known down to the required
depths. Engineering properties can be determined by
means of tests carried out in the field and laboratory. In
order to avoid certain difficulties during sampling
processes in coarse-grained soils and the disturbance of
the sampling in fine-grained soils, in situ tests are
frequently used. Therefore, we rely heavily on the results
of the tests performed in field. The Standard Penetration
⁎ Corresponding author. Fax: +90 388 225 0112.
E-mail address: osivrikaya@nigde.edu.tr (O. Sivrikaya).
0013-7952/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.enggeo.2006.05.002
Test (SPT) is one of the oldest and most common in situ
tests used for soil exploration in geotechnical applications
and foundation design. This test is the most commonly
used penetration test in the countries of south Europe,
North and South America, the United Kingdom, Australia, India, Spain, Portugal, South Africa, Turkey, Israel
and Japan (Nixon, 1982; Décourt, 1990). In Turkey, it is a
routine part of almost every soil exploration program as
one of the principle steps (Durgunoğlu and Toğrol, 1974).
Horn (1979) has reported that the SPT has been and is
likely to remain a keystone in soil exploration practice in
North America. According to Mori (1979) more than 90%
of borings during preliminary investigation stage are
performed with the SPT.
O. Sivrikaya, E. Toğrol / Engineering Geology 86 (2006) 52–69
The SPT has the advantages with the easiness of the
test procedure and the simplicity of the equipment
employed. Representative but disturbed samples can be
taken, which is used for classification of different layers.
The test is carried out in various types of soils ranging
from soft clay and loose sand to very stiff clay and dense
sand. SPT is performed by driving a standard splitspoon sampling tube of 30 cm into the ground at the
bottom of the borehole with a 63.5 kg hammer falling
from 760 mm. SPT-N value, which is the number of
hammer blows required to drive the sampler for last the
two 150 mm penetration, is called standard penetration
resistance of soil.
Correlations between SPT-N values and soil
properties are empirical and cannot be considered
particularly accurate in few cases since the SPT is not
completely standardized (Clayton, 1995). However,
the results of the test, SPT-N values, are used to
calculate important engineering properties of soils
such as the internal friction angle (ϕ′), relative density
(Dr), and bearing capacity and settlement of coarsegrained soils. It can also be used for the determination
of the shear wave velocity (vs) of soils, liquefaction
potential of coarse-grained soils and control of
compacted fills. Even though the SPT was originally
developed for coarse-grained soils, it has been applied
to fine-grained soils to estimate engineering properties
such as undrained compressive strength (qu), undrained shear strength (Su), and coefficient of volume
compressibility (mv). However, its applicability for
fine-grained soils is still argued (Broms, 1986;
Décourt, 1990).
Contrary to the implications by its name, the SPT is
not all that standard and SPT-N values may vary even
for identical soil conditions. As would be known, the
SPT is dependent on many factors due to the variations
of applications carried out in the test and some
equipment used in the test. Various factors including
drilling methods, drill rods, borehole sizes and stabilization, sampler, blow count rate, hammer configuration,
energy corrections and test procedure affect the validity
and use of SPT results (Schmertmann and Palacios,
1979; Kovacs et al., 1981; Farrar et al., 1998). The
combined effect of all of these parameters can be
accounted for by knowing the efficiency of system (ER).
While many variables influence the SPT-N values, a
strong relationship is present between the SPT-N values
and the energy transmitted to drill rods. If the energy
transfer characteristics of an SPT system are known, the
SPT-N values obtained by the system may be corrected
to a standardized energy and more appropriately used in
design. In the last 20 years works on the dynamics of the
53
SPT and especially the energy of SPT hammer system
being measured in the field have considerably developed the information on the SPT and its results
(Schmertmann and Palacios, 1979; Kovacs et al.,
1981; Clayton, 1990; Farrar, 1999; Srithar and Ervin,
2001).
Several different types of SPT hammers are used to
perform SPT tests, which influences the SPT-N value
due to their efficiencies. ASTM recommends that a
measured SPT-N value (SPT-Nfield) should be standardized it by ratio (CE) between the measured energy
transferred to the rod (Emeasured) and 60% of the
theoretical potential energy (Etheoretical)
CE ¼ ðEmeasured =Etheoretical Þ=60 ¼ ER=60
ð1Þ
This compensates for variable efficiencies from
different SPT rigs and hammer types and therefore
improves the reliability of soil strength estimates used in
geotechnical designs. Knowing Emeasured permits adjustment of the SPT-Nfield to the normalized SPT-N60 for
standard 60% energy transfer into the rods.
The energy ratio, ER, can be determined by means of
two ways. First, it can be measured directly following
procedures outlined in ASTM D 6066-96. The latter, in
the case of knowing test equipment used and procedure
applied, ER is determined by taking advantage of
previous studies where it is measured. In the current
study, ER was determined by circulating questionnaire
to collect information on the test equipment and
procedure used. According to the results of the
questionnaire, ER, was assumed 45% from previous
studies (Clayton, 1990) and therefore CE 0.75. In
addition, Durgunoğlu et al. (2000) measured ER using
safety hammer with 2 rope turn release in Turkey and
determined that ER was approximately 60%. It was the
same as previous studies (Seed et al., 1985; Clayton,
1990; Youd and Idris, 1997). Determining ER is beyond
the scope of the present paper.
Many factors and variables affect the validity and
usefulness of SPT results (Nixon, 1982; Broms and
Flodin, 1988; Coduto, 1994), and subsequently, measured penetration resistance may be too high or too low.
A measured penetration resistance which is too high
causes unconservative estimates of soil properties and
bearing capacity. A measured penetration resistance
which is too low causes overconservative results.
Therefore, the SPT corrections must be made to be
used in the geotechnical design and the determination of
engineering properties. In the recent years, various
corrections have been developed for measured SPT-N
values taking account of the effect of: rod length,
54
O. Sivrikaya, E. Toğrol / Engineering Geology 86 (2006) 52–69
borehole diameter, sampler type, type of hammer and its
release mechanism, blow count frequency, energy, and
the effects of overburden pressure. The corrections
mainly include overburden correction factor (CN) and
blow count frequency correction factor (CBF) depending
on soil type and underground water level; energy
correction factor (CE), rod length correction factor
(CR), borehole diameter correction factor (CB), liner
correction factor (CS), anvil correction factor (CA) and
hammer cushion correction factor (CC) depending on
test procedure and equipment (McGregor and Duncan,
1998). The correction factors are beyond the scope of
the current paper. Authors are aware that CR and CE are
still under discussion.
In applications of these corrections their use has
generated confusion regarding the correlations used for
corrected and uncorrected SPT-N values. A measured
blow count (SPT-Nfield) value to be used in the
geotechnical engineering applications should be adjusted. SPT-Nfield can be normalized to N60, which is the
blow count corrected to 60% of the theoretical free-fall
hammer energy, and N1,60 which is the blow count
corrected to 100 kPa of effective overburden pressure
and 60% of the theoretical free-fall hammer energy. The
most general equations for N60 and N1,60 are as follows
(McGregor and Duncan, 1998):
N60 ¼ ðCB CC CE CR CBF CS CA ÞNfield
ð2Þ
N1;60 ¼ CN N60
ð3Þ
McGregor and Duncan (1998) have reproduced the
equation of Skempton (1986) taking CBF, CC and CA
into consideration. Though all corrections are made in
coarse-grained soils, the overburden and blow count
frequency correction cannot be made in fine-grained
soils in practice (Saran, 1996; McGregor and Duncan,
1998). As fine-grained soils during penetration are
undrained, the effective vertical stress (overburden)
correction for clays is not normally made. However,
there has been an argument for normalizing the effective
confining pressure (Farrar, 2001). Farrar (2001) states
that although the normalization according to the
overburden correction in the shallow depth is unnecessary, it could be useful at deep conditions.
As it is still argued, whether the effect of the effective
vertical stress particularly for fine-grained soils should
be taken into account or not, and as it is usually not done
in practice, it is dismissed in this study. Therefore the
general equation including corrections for fine-grained
soils is shown in Eq. (4)
N60 ¼ ðCB CC CE CR CS CA ÞNfield
ð4Þ
The SPT-N value is used with many empirical
correlations to determine engineering properties of
soil layer used in design. The relations between the
values of various soil parameters in the field and/or
laboratory conditions both assist the engineer during
the preliminary evaluation of a project and enable
him to check the consistency of the results determined by various methods. The empirical equations
developed in accordance with the soil type by various
researchers can be used mostly in the design stage so
as to obtain the values of engineering parameters
from the results of in situ tests (Terzaghi and Peck,
1967; Sanglerat, 1972; Stroud, 1974; Sowers, 1979;
Nixon, 1982; Kulhawy and Mayne, 1990; Sivrikaya
and Toğrol, 2002). Although the equations in the
literature are known, there is not available information about whether they covered the SPT corrections
or had the statistical results of the regression analysis.
Therefore while these corrections are used, their use
has led to confusion regarding the correlations used
for corrected SPT-N values and for use of uncorrected SPT-N values, and causes erroneous results and
designs.
2. Previous correlative works on SPT-N and
engineering properties of fine-grained soils
In engineering applications, the information concerning soil types and soil conditions obtained is
limited due to the difficulties encountered in sampling,
testing, and the time and costs involved. Therefore it is
useful to use the correlations by using a small number
of soil parameters that can be easily obtained. For finegrained soils, certain useful relations can be determined
between the SPT-N value and undrained compressive
strength (qu), undrained shear strength (Su), coefficient
of volume compressibility (mv) for fine-grained soils.
Correlations are important to estimate engineering
properties of soils particularly for a project where there
is a financial limitation, lack of test equipment or limited
time. The correlations with SPT-N are commonly used
in the preliminary stage of a project. However, the use of
correlation equations given in the literature is not clear
since there are generally four uncertainties, arisen in
their use, which are not well defined. They have
considerable effects on the correlation equations and
they are as follows:
• whether the correlation includes SPT corrections or
not; and if it does, then which corrections have been
made;
• whether the correlation has a statistical meaning;
O. Sivrikaya, E. Toğrol / Engineering Geology 86 (2006) 52–69
Table 1
Correlations between SPT-N and qu according to soil types in finegrained soils
Author(s)
Soil type
Sanglerat (1972)
Clay
Silty clay
Terzaghi and Peck Fine-grained soil
(1967)
Sowers (1979)
Highly plastic clay
Medium plastic clay
Low plastic clay and silt
Nixon (1982)
Clay
Kulhawy and Mayne Fine-grained soil
(1990)
Sivrikaya and Toğrol Highly plastic clay (CH)
(2002)
n = 113
Low plastic clay (CL)
n = 72
Fine-grained soil n = 226
Fine-grained soil n = 30
qu (kPa)
25N
20N
12.5N
25N
15N
7.5N
24N
58N 0.72
9.70Nfield, r = 0.83
13.63N60, r = 0.80
6.70Nfield, r = 0.76
9.85N60, r = 0.73
8.64Nfield, r = 0.80
12.36N60, r = 0.78
(0.19I p + 6.20)N 60 ,
N60 < 25
• which test results are used in the correlation; and
• which type of soil the correlation is valid for.
Therefore the correlation equations with SPT-N value
should be used taking into consideration these effects
mentioned above.
2.1. SPT-N and undrained compressive strength (qu)
The undrained compressive strength (qu) is an
important characteristic for fine-grained soils and it
gives an idea about their consistency. In addition, it is
used to estimate both the undrained shear strength (Su)
and the sensitivity of clays. qu is determined by means of
the unconfined compression (UC) test. Despite some
disadvantages, the unconfined compression test is
commonly used for determination of the undrained
shear strength.
Many researchers have recommended the relationships between SPT-N value and undrained compressive
strength in accordance with the soil type in fine-grained
soils as shown in Table 1. The correlation equations
obtained from the studies by 1990 do not include any
information regarding whether there were any corrections made or not. However, McGregor and Duncan
(1998) suggest that it seems sound to use N60 instead of
N with those correlations since hammers transmitting
60% of the theoretical energy have been the most
commonly used hammers for SPT. Moreover, the
statistical information about them has not been encountered. Sivrikaya and Toğrol (2002) have recently
55
proposed the correlation equations in accordance with
soil types and corrections. They have performed linear
regression analysis with a large number of data (n) from
statistical point of view with coefficient of determination (r2). They have shown that the corrections are quite
important on the correlation equations obtained. As can
be seen, the correlation equation (qu = 12.5N) for finegrained soils proposed by Terzaghi and Peck (1967) and
frequently used in practice is compatible with that
(qu = 12.36N60) proposed by Sivrikaya and Toğrol
(2002) and they give quite close results. Thus, when
Terzaghi and Peck's correlation equation is used,
corrections should be considered. In addition, they
have examined “a” regression coefficient and developed
“a” as a function of plasticity index (Ip). It gives
compatible and good results with Terzaghi and Peck's
correlation equation for N60 less than 25 (Sivrikaya and
Toğrol, 2002). The values based on the SPT-N value and
the consistency by Terzaghi and Peck (1967) for finegrained soils and commonly practiced are shown in
Table 2.
2.2. SPT-N and undrained shear strength (Su)
It is required to know the undrained shear strength
(Su) in order to make stability analysis for structures and
slopes laying on fine-grained soils. Su is determined by
means of the laboratory and field vane (FV) tests,
unconsolidated undrained (UU) compression test. In
addition, for saturated fine-grained soils undrained shear
strength (Su = qu / 2) can be obtained by taking the half of
unconfined compressive strength by the unconfined
compression (UC) test (Table 1).
It is possible to estimate the undrained shear strength
of fine-grained soils from SPT data. It is indicated that
sensitivity of clays may raise lower SPT-N values for a
given undisturbed strength due to strength loss during
the penetration of the sampler (De Mello, 1971;
Schmertmann, 1971; Mitchell et al., 1978). Using the
results of UU compression tests, Stroud (1974) and
Décourt (1990) have proposed the relationships between
Table 2
Relations between SPT-N and qu for fine-grained soils in accordance
with consistency (Terzaghi and Peck, 1967)
Consistency
SPT-N
qu, (kPa)
Very soft
Soft
Medium
Stiff
Very stiff
Hard
<2
2∼4
4∼8
8∼15
15∼30
>30
<25
25∼50
50∼100
100∼200
200∼400
>400
56
O. Sivrikaya, E. Toğrol / Engineering Geology 86 (2006) 52–69
Table 3
Relations between SPT-N and Su for fine-grained soils
Author
Soil type
Su (kPa)
Clay Su = f1N, f1 = f(Ip) Medium plastic clay, (4∼5)N
Ip < 20 (6∼7)N
Ip > 30 ≅ 4.2N
Décourt (1990) Clay
12.5N
15N60
Stroud (1974)
SPT-N value and undrained shear strength in accordance
with the plasticity index and corrections, respectively in
overconsolidated, insensitive clays (Table 1).
Stroud (1974) has developed the relation between
SPT-N and undrained shear strength depending on the
plasticity index and has found that the ratio of Su to the
SPT-N value, called f1, decreases with increasing
plasticity index. f1 (= Su / N) is a constant parameter,
dependent on plasticity index, Ip, and varies approximately between 4 and 7. It is taken nearly 4–5 for
medium plastic clay, 6–7 or higher for plasticity index
less than 20 and 4.2 for plasticity index more than 30.
In contrast, Sowers (1954) found that f1 increases with
an increase in plasticity of clays on the homogenous
clays. On the relations obtained from Stroud's and
Sower's studies, there is no information regarding both
statistics and whether they are corrected or not.
Décourt (1990) has established the relations between
SPT-N and Su for S. Paulo clays taking account of the
corrections. As can be seen from Table 3, the
correlation equation proposed by Décourt (1990)
gives approximately twice as much as the result of
Stroud (1974). For Décourt's correlations equation, no
statistical information is given either.
As shown in Table 4, Tschebotarioff (1973), Parcher
and Means (1968), and Terzaghi and Peck (1967)
suggest approximate undrained shear strength for finegrained soils based on the SPT-N value and consistency.
The values in both Tables 2 and 4 are valid for
insensitive clays.
3. Material and method
For this study, the laboratory test results and boring
logs obtained from borings performed at various parts
of Turkey are collected from private companies,
universities and one public institutions which carried
out the boring, field and laboratory tests performed on
undisturbed and disturbed samples recovered from
field at the different regions in Turkey. In this study
the correlations between the SPT-N values and Su
values obtained from similar depths are developed by
taking account of the fine-grained soil types and SPT-
N corrections. Furthermore, a questionnaire (Table 5)
is prepared to collect information on the SPT
procedure and equipment so that the reliable corrections and correlations can be established. It is
necessary for geological and geotechnical engineers
to know the procedure and equipment used in order to
interpret the SPT results correctly. To develop the
correlations between SPT-N and Su values, the linear
regression analysis is performed on the data.
The profile of the SPT procedure and equipment
types used in Turkey, from the results of the questionnaire, is summarized below:
• SPT is conducted by using mostly Crealius (D500,
D750, D900) made in Turkey, Acker (I, II, ACE),
Mobil Drill and Foremost Mobile drill rigs. Rotary
wash boring method is the most commonly used
drilling method. However, continuous flight solid
and hollow-stem auger methods are used, too.
While drilling mud is used rarely bentonite is
utilized when unstable soils or water leakages are
present. Drilling diameters varies between 68 mm
and 216 mm depending on the soil type and
depth.
• The outside and inside diameters of casings vary
between 89 mm–114 mm and between 76 mm–
101 mm, respectively.
• While the types of drill rods used are AW, BW and
NW, their lengths result from 1 m, 1.5 m and 3 m.
Mostly AW drill rods are used.
• While the donut hammer is most common and
frequently used in Turkey, safety and automatic or
trigger hammers are rarely used.
• Releasing hammer mechanism composes of lifting
and dropping the hammer of 63.5 kg from the height
of 76 cm on the anvil with approximately a diameter
of 90 mm and height of 150 mm, and none wood
cushion block by means of 1 or 2 turns of rope with
the diameter of 20 mm–30 mm around the cathead
Table 4
Relations between SPT-N and Su for fine-grained soils in accordance
with consistency
SPT-N Consistency Undrained shear strength Su (kPa)
Tschebotarioff Parcher and
Terzaghi and
(1973)
Means (1968) Peck (1967)
<2
2∼4
4∼8
8∼15
15∼30
>30
Very soft
Soft
Medium
Stiff
Very stiff
Hard
15
15∼30
30∼60
60∼120
120
>225
<12
12∼25
25∼50
50∼100
100∼200
>200
<12.5
12.5∼25
25∼50
50∼100
100∼200
>200
O. Sivrikaya, E. Toğrol / Engineering Geology 86 (2006) 52–69
57
Table 5
Table of questionnaire including SPT procedure and equipment
Sampler
Shoe
Tube
Head
Drill rod
Anvil
Hammer
Boring
SPT
Detail of cathead and rope method
Type
Releasing mechanism
Dimensions
Outside diameter (mm)
Inside diameter (mm)
Tip thickness (mm)
Tip angle (°)
Split barrel
Liner
Retainer
Whole barrel
Ball (mm)
Type
Diameter (mm)
Length (mm)
Hammer cushion
Diameter (mm)
Height (mm)
Outside diameter (mm)
Inside diameter (mm)
Length (mm)
Vent (mm)
Outside diameter (mm)
Inside diameter (mm)
Height (mm)
Type
Diameter (mm)
Casing
Crew number
Blow count frequency
Rope
Cathead
Pulley
with the diameter of 90 mm–112 mm. The blow
count frequency is 20–40 blows per minute.
• The sampler consists of three separated parts. The
first part is an open shoe made from hard steel with
the outside diameter of 50.8 mm, inner diameter of
35 mm and length of about 90 mm. The second part
is a split barrel tube with the constant inner diameter
of 36–38 mm, outside diameter of 50.8 mm, and
length of 600 mm–740 mm without the liner.
Generally various retainers are used in this part
depending on the soil type. Third part is a head with 2
or 4 vents and a ball.
• SPT-N value is taken as the sum of the blow counts
for the second 15 cm and third 15 cm penetration
after first 15 cm penetration, which is assumed to
pass disturbed soils.
• During testing, crew consists of 2 or 3 people.
According to the results of the questionnaire, it is
assumed that CE is 0.75 due to the type of donut hammer
with releasing 2 turns of rope (Clayton, 1990; Seed et
Outside diameter (mm)
Inside diameter (mm)
Boring mud
Water
Type
Diameter (mm)
Releasing type
Diameter (mm)
Numbers of turns of rope
Diameter (mm)
al., 1984 and 1985), CB is 1.00 due to the change of the
borehole diameter from 65 mm to 115 mm (Skempton,
1986), CC is 1.00 due to not using hammer cushion
(Décourt, 1990), CS is 1.20 due to not using liners
(Skempton, 1986), CA is 0.85 due to using donut
hammer and small anvil (Tokimatsu, 1988), and CR is
0.75, 0.85, 0.95, 1.00 depending on the rod length
(Skempton, 1986) for the data used in this study. Under
the light of the results of the questionnaire, Eq. (4)
becomes Eq. (5):
N60 ¼ ðCE CR CS CA ÞNfield
ð5Þ
Since CSCA is about 1, Eq. (5) becomes:
N60 ¼ 0:75CR Nfield
ð6Þ
In order to determine the correlation functions
between Su and SPT-N value depending on both soil
types and the SPT-N corrections the linear regression
analysis is performed from statistical point of view. First
58
O. Sivrikaya, E. Toğrol / Engineering Geology 86 (2006) 52–69
of all, Student t-test is performed to establish whether
there is any relation between Su and SPT-N or not. The
significance tests of each statistical parameters and lines
are done for 5% meaningfulness level. As a result of ttests a relation is observed between Su and SPT-N. To
develop relations between Su and SPT-N, the linear
Fig. 1. Soil profiles of data obtained for CU, UU and FV tests.
O. Sivrikaya, E. Toğrol / Engineering Geology 86 (2006) 52–69
(Su = b + aN) regression analysis by the method of least
squares is performed. The significance of the regression
coefficients, a and b, is examined by means of t-test and
it is found that while the regression coefficient, a, has a
significance, the regression coefficient, b, does not have
any significance. Thus the linear regression equation
becomes Su = aN. Also, the standard errors (s) are
determined for each regression equation obtained. For
these relations, the coefficient of determination (r2) is
established.
59
Table 7
The numbers and soil types of specimens used for the relations
between Su determined by CU, UU and FV tests and SPT-N
Soil
type
Number of data (n)
UC
UU
FV
CH
CL
ML
OH
MH
SM
CL–ML
113
72
4
–
13
–
24
80
66
24
–
14
–
3
13
11
13
13
7
5
–
4. Results and discussions
While the empirical equations are developed, SPTN value and plasticity index (Ip) are taken as
independent variables and Su as a dependent variable.
The values of Su are obtained from Unconfined
Compression (UC), Unconsolidated Undrained Triaxial
Compression (UU) and Field Vane (FV) tests.
Analyses are performed on the data divided into the
subgroups which are highly plastic clays (CH), low
plastic clays (CL), all clays (CH–CL) and fine-grained
soils by taking the corrections into account. Unified
soil classification system is used in grouping the data.
Thus, the results obtained from this study are
compared with the equations suggested by previous
researchers by examining the effects of the soil type
and SPT corrections. In addition, effects of test type on
the obtained regression equation are investigated for
undrained shear strength by means of UC, UU and FV
tests.
The data for fine-grained soils are obtained from
many parts of Turkey. They are non-sensitive clays with
liquid limit (wL) in the range of 22 to 110 and plastic
limit (wp) in the range of 14 to 44. The soil profiles
belonging to data obtained from different two sites for
each UC, UU and FV tests are shown in Fig. 1. The
SPTs were performed to the depth of 30 m.
All data are divided into four subsoil groups, which are
highly plastic clays (CH), low plastic clays (CL), clays
(CH, CL) and fine-grained soils (CH, CL, CL–ML, ML,
MH) (Table 6). In this paper while clays refer to both CH
Table 6
Soil types and numbers analyzed for relations between SPT-N and Su
Soil type
Highly plastic clay (CH)
Low plastic clay (CL)
Clay
Fine-grained soil
Number of data (n)
UC
UU
FV
113
72
185
226
80
67
147
190
13
11
24
62
and CL soils, fine-grained soils refer to cohesive soils
including CH, CL, CL–ML, ML, MH soils.
4.1. SPT-N and Su
In order to develop valid correlations between SPT-N
value and Su, SPT-Nfield values are obtained from the
different sites where SPTs are performed in Turkey, and
Su values are determined by means of UC, UU tests
performed on the undisturbed specimens. These specimens were taken from Shelby tube samples recovered
from depths close to where SPTs were performed and
FV tests performed close SPT. For Su obtained from FV,
FV tests were conducted up to depth of 30 m by field
vanes with two type blades. One of the blades had a
diameter of 55 mm – height of 130 mm and the diameter
of 65 mm and height of 110 mm. The number of data
pairs (SPT-N and Su) is 226 for UC tests, 190 for UU
tests and 62 for FV tests. The soil types and numbers of
the specimens are given in Table 7.
The linear regression analyses are conducted with
and without SPT corrections. The results obtained from
linear analyses, which are the functions with its
coefficients of determination (r2) and its standard errors
(s), are shown for each soil type and SPT corrections in
accordance with the test types in Table 8. Charts of these
correlations are given in Figs. 2A–C, 3A–C and 4A–C
and 5A–C. In the analysis of FV tests, Su⁎(= μSu)
represents undrained shear strength from FV test
including field correction. The field correction factor
(μ) depending on Su(FV) / σ′v0 proposed by Aas et al.
(1986) is used in this study.
As would be seen from Table 9, the coefficients of
determination from uncorrected data (SPT-Nfield) and
corrected data (SPT-N60) for all soil types are in the
range of 0.58–0.74 and 0.53–0.64 for UC tests,
0.52–0.64 and 0.52–0.64 for UU tests, 0.53–0.74
and 0.56–0.76 for FV tests, respectively. The
coefficient of determination varies between 0.52 and
60
O. Sivrikaya, E. Toğrol / Engineering Geology 86 (2006) 52–69
Table 8
Results of the analysis performed for relations between SPT-N and Su
Soil type
Su⁎(μSu) kPa
Su kPa
Su kPa
UC
UU
FV
5.90Nfield,
r = 0.80, s = ±39
8.76N60, r = 0.80,
s = ±39
3.97Nfield,
r= 0.75, s = ±21
5.82N60, r = 0.75,
s = ±21
5.13Nfield,
r = 0.76, s = ±36
7.57N60, r = 0.76,
s = ±36
4.68Nfield,
r = 0.72, s = ±35
6.97N60, r = 0.71,
s = ±35
6.17Nfield,
r = 0.86, s = ±10
8.27N60, r = 0.87,
s = ±10
3.58Nfield,
r = 0.82, s = ±6
4.88N60, r = 0.83,
s = ±5
4.97Nfield,
r = 0.73, s = ±12
6.72N60, r = 0.75,
s = ±11
4.18Nfield,
r = 0.76, s = ±10
5.77N60, r = 0.79,
s = ±10
4.85Nfield,
r = 0.83, s = ±80
6.82N60, r = 0.80,
s = ±86
CL
3.35Nfield,
r = 0.76, s = ±64
4.93N60, r = 0.73,
s = ±68
Clay
4.33Nfield,
r = 0.82, s = ±82
6.19N60, r = 0.77,
s = ±85
Fine4.32Nfield,
grained r = 0.80, s = ±78
soil
6.18N60, r = 0.78,
s = ±81
CH
0.76. While the coefficient of determination (r2) is
found to be 0.52 as the lowest value for fine-grained
soils with SPT-N60 in the UU test, the highest value
is 0.76 for CH with SPT-N60 in FV test. The highest
coefficients of determination (r 2 = 0.64–0.76) are
established for CH in all test types. The standard
error (s) is a measure of the amount of error in the
prediction of Su for individual SPT-N value. Statistical analysis indicated that the standard errors (s) of
the correlation equations vary in the range of ± 64–
84 kPa for UC test, ±21–39 kPa for UU test and
± 5–12 kPa for FV test. While standard errors of
obtained correlations are the highest for UC test, they
are quite low for UU and considered due to the
disadvantages and assumptions of UC tests. In
addition, the standard errors found from FV tests
are the lowest values for all soil types. This is
attributed to the data obtained from only one site in
which the soil is homogeneous with depth. It should
be noted that test type (UC, UU and FV tests) plays
an important role on the coefficients of determination
and standard errors depending on the homogeneity of
soil and advantages or disadvantages of the test
types.
Taking three different tests (UC, UU, FV) data into
consideration, the proposed mean correlations for each
type soils from the current study are given below in
terms of SPT-Nfield and SPT-N60, and they are plotted in
Figs. 2–5.
Su ¼ 5:50Nfield
for CH
ð7Þ
Su ¼ 7:80N60
for CH
ð8Þ
Su ¼ 3:70Nfield
for CL
ð9Þ
Su ¼ 5:35N60
for CL
ð10Þ
Su ¼ 4:75Nfield
for Clays
ð11Þ
Su ¼ 6:90N60
for Clays
ð12Þ
Su ¼ 4:45Nfield
for Fine−grained soils
ð13Þ
Su ¼ 6:35N60
for Fine−grained soils
ð14Þ
For highly plastic clays (CH) the results obtained
from this study for UC, UU and FV tests and the
previous studies are shown in terms of SPT
corrections (SPT-Nfield) and without SPT corrections
(SPT-N60) in Fig. 2A–B. The results obtained from
the current study fall between those obtained from
the equations proposed by Sowers (1979) and Stroud
(1974). As can be seen in Fig. 2, while the equation
proposed by Sowers (1979) for highly plastic clays
gives highly over-estimated values, the equation
proposed by Stroud (1974) gives very close results
to the current study for UC. Therefore, the authors
recommend that the equation suggested by Sowers
(1979) for highly plastic clays should not be used in
practice. Fig. 2C also shows that the SPT corrections
play an important role on the correlations and the
variations indicate the magnitude of the corrections
on SPT-N. “a” coefficient in “Su = aN” equation
determined from linear regression analysis varies
approximately between 4.5 and 9. The upper and
lower limits based on both SPT raw data (SPTNfield) and on test types (UC, UU, FV) are 6.17 and
4.85, respectively. In addition, the upper and lower
limits based on both SPT corrections (SPT-N60) and
test types are 8.76 and 6.82, respectively (Table 8,
Fig. 2C).
The results obtained for low plastic clays (CL) in
this study for various test types with and without SPT
corrections and those of the previous studies are
shown in Fig. 3A–B. The correlations obtained from
the current study for each test type give very close
results, and are compatible with each other. The values
of proposed mean correlation from the current study
for SPT raw data (SPT-Nfield) for CL are approximately the same as Sowers (1979) for CL (Fig. 3A).
The regression line obtained by Stroud (1974) for CL
O. Sivrikaya, E. Toğrol / Engineering Geology 86 (2006) 52–69
constitutes the upper limit and is approximately
identical with the line obtained from the current
study based on SPT corrections and UU test data. “a”
coefficient varies approximately between 3 and 6. The
upper and lower limits based on both SPT raw data
and test types are 3.97 and 3.35, respectively. In
addition, the upper and lower limits based on both
SPT corrections and test types are 5.82 and 4.88,
respectively (Table 8, Fig. 3C).
For clays the results obtained from this study for
various test types with and without SPT corrections
and those obtained from the previous studies are
shown in Fig. 4A–B. The results obtained from the
equations proposed by Sanglerat (1972), Nixon (1982)
and Décourt (1990) for clays are much higher than
those obtained from the current study. The previous
studies give approximately as twice as the values
from this study. Therefore, the authors recommend
that the equations suggested by the previous studies
for clays should not be used in practice. “a”
coefficient varies approximately between 4 and 8.
The upper and lower limits based on both SPT raw
data and test types are 5.13 and 4.33, respectively. In
addition, the upper and lower limits based on both
SPT corrections and test types are 7.57 and 6.19,
respectively (Table 8, Fig. 4C).
For fine-grained soils the results obtained from this
study and the previous studies for test types are shown
with and without SPT corrections in Fig. 5A–B. The
correlation by Kulhawy and Mayne (1990) has given
much higher results than that of the current study for
fine-grained soils in terms of both test types and SPT
correction effects (Fig. 5A, B). The correlation of
Kulhawy and Mayne (1990) are suggested not to be
used in practice by the authors. As regards, the SPT
corrections it is found that the results of proposed
mean correlation from this study are approximately the
same as the correlation suggested by Terzaghi and
Peck (1967) commonly used in practice for finegrained soils (Fig. 5A). Therefore, it seems necessary
that SPT-N values suggested by Terzaghi and Peck
(1967) must be used after the SPT corrections applied.
“a” coefficient varies approximately between 4 and 7.
The upper and lower limits based on both SPT raw
data and test types are 4.68 and 4.18, respectively. In
addition, the upper and lower limits based on both SPT
corrections and test types are 6.97 and 5.77, respectively (Table 8, Fig. 5C).
For FV, UC and UU tests in all soil types, the
results of correlations obtained with SPT-Nfield never
exceeded the values suggested by Terzaghi and Peck
(1967) as would be seen in Table 8. For the UC tests,
61
the range of the variation for clay and fine-grained
soils is found to be the same. It is thought that this is
due to the number of data available for ML, MH and
CL–ML subsoil types which are quite few in
comparison with CH and CL clays as shown in Tables
6 and 7. The range of variations for fine-grained soils
from this study is nearly the same as that (4 < a < 7)
proposed by Stroud (1974) without taking plasticity
index (Ip) into consideration. However, it is found that
the range of the variation suggested by Stroud (1974)
for all fine-grained soils remains between the limits
obtained for SPT-N60 in this study.
It can be concluded from Figs. 2C 3C 4C and 5C
that whether or not the correlation equation based on
SPT corrections is important and lack of that
information may mislead an engineer. Thus, unexpected situations are encountered at the design stage.
Therefore, it is necessary to check whether or not the
correlation equation covers the corrections before it is
used. In addition, in this study it is observed as
expected that “a” coefficient increases for soil types
such as highly plastic clays (CH), clays, fine-grained
soils and low plastic clays (CL) in all test types
regardless of whether SPT corrections are made or
not.
4.2. Test Type Effect on SPT-N and Su
In general, there are two uncertainties with correlations including SPT-N value, which has considerable
effect on the correlation equations. First is whether a
correlation includes the SPT corrections or not and the
latter is the test type effect. In particular, unless which
type of test results is used for the correlation is known,
the correlation used may cause overestimation or
underestimation of the design. In this study using the
results (Su) of three different tests types, UC, UU and
FV tests, linear regression analysis (Su = aN) is made
and the effects of the test types on the regression
coefficient, a, are studied. Table 9 summarizes the
variation of “a” coefficient based on the SPT
corrections and test types.
The variation intervals of “a” is larger for N60 than
for Nfield in each soil type as seen Table 9. “a”
coefficient varies approximately between 4.5 and 9 for
CH, between 3 and 6 for CL, between 4 and 8 for
clays and between 4 and 7 for fine-grained soils based
on both SPT correction and test type effects. Table 9
and Figs. 3 4 and 5 shows that the SPT corrections
play an important role on the correlations and the
variations indicate the magnitude of the corrections on
SPT-N for all soil types.
62
O. Sivrikaya, E. Toğrol / Engineering Geology 86 (2006) 52–69
Fig. 2. A. Comparison of previous studies with the present study for highly plastic clays (CH) in terms of SPT-Nfield. B. Comparison of previous
studies with the present study highly plastic clays (CH) in terms of SPT-N60. C. Effect of SPT corrections on Su = aN highly plastic clays (CH).
O. Sivrikaya, E. Toğrol / Engineering Geology 86 (2006) 52–69
63
Fig. 3. A. Comparison of previous studies with the present study for low plastic clays (CL) in terms of SPT-Nfield. B. Comparison of previous studies
with the present study for low plastic clays (CL) in terms of SPT-N60. C. Effect of SPT corrections on Su = aN for low plastic clays (CL).
64
O. Sivrikaya, E. Toğrol / Engineering Geology 86 (2006) 52–69
Fig. 4. A. Comparison of previous studies with the present study for clays in terms of SPT-Nfield. B. Comparison of previous studies with the present
study for clays (CH) in terms of SPT-N60. C. Effect of SPT corrections on Su = aN for clays.
O. Sivrikaya, E. Toğrol / Engineering Geology 86 (2006) 52–69
65
Fig. 5. A. Comparison of previous studies with the present study for the fine-grained soils in terms of SPT-Nfield. B. Comparison of previous studies
with the present study for the fine-grained soils in terms of SPT-N60. C. Effect of SPT corrections on Su = aN for fine-grained soils.
66
O. Sivrikaya, E. Toğrol / Engineering Geology 86 (2006) 52–69
Table 9
Test type effects on regression coefficient, a, in regression equation of Su = aN
Nfield
N60
CH
CL
Clays
Fine-grained soils
4.85 ≤ a ≤ 6.17
6.82 ≤ a ≤ 8.76
3.35 ≤ a ≤ 3.97
4.88 ≤ a ≤ 5.82
4.33 ≤ a ≤ 5.13
6.19 ≤ a ≤ 7.57
4.18 ≤ a ≤ 4.68
5.77 ≤ a ≤ 6.97
Fig. 6 shows the correlations for each soil type from
different test types point of view. It is observed that
while the results of correlations including UC test data
constitute lowest limits, those including UU test data
constitute uppermost limit for each soil type. It is
thought to be due to the advantages of UU test in
accordance with UC test. Generally, the results of
correlations including FV test data remain between the
other two test type limits despite some small variations.
It is because of using a small data in the regression
analysis. In the correlations including SPT-Nfield, “a”
coefficient varies approximately between 4.5 and 6.5
for CH, between 3 and 4 for CL, between 4 and 5.5
for clays and between 4 and 5 for fine-grained soils
based on test type. In the correlations including SPTN60, it varies approximately between 6.5 and 9 for
CH, between 4.5 and 6 for CL, between 6 and 8 for
clays and between 5.5 and 7 for fine-grained soils
based on the test type.
4.3. Relation of Su /N and Ip
The data of undrained shear strength (Su) obtained
from UU test versus plasticity index (IP) is plotted in
Fig. 7. It is quite difficult to determine any relation and
make any comment between f1 (= Su / N) and plasticity
index as seen from Fig. 7. However, it seems as if f1(Su /
N) decreases with the increase in plasticity index in
contrast to the relation proposed by Stroud (1974) and
used in practice commonly, which f1 decreases with the
increases in plasticity index. The study verifying and
backing up the same finding is performed on homogenous clayey soils by Sowers (1954) (Table 10). But the
information is brief and any further data are not given
about any indication of how Su is determined. The
comparisons of studies on the variation of f1 are made in
accordance with soil types or plasticity index, IP as it is
shown in Table 10. It should be noted that for IP less than
20, Su values in f1 are open to question due to
discontinuities in sampling, the difficulties of sampling
and testing on this sort of samples.
In addition to test results using FV obtained from one
site which has homogenous soil with depth, the relations
between f1 and Ip with 25 number of data pairs are
examined and a linear relationship is determined with
and without SPT corrections made (Fig. 7) and the
highest correlation equation is obtained as follows:
Su⁎ ¼ ð0:12Ip þ 2:08ÞNfield n ¼ 25; r2 ¼ 0:38
ð15Þ
It is observed that even though the obtained correlation
has the low coefficient of determination of 0.38, f1(Su /
N) increases with the increase in plasticity index, Ip. The
same trend is determined by Sivrikaya and Toğrol
(2002) and Sowers (1954), except Stroud (1974).
The SPT procedure and equipment must be known so
that the interpretation of the SPT test results can be made
correctly. The equipment used for SPT during the period
at which many studies (Tables 1 and 3) are carried out
has varied from one country to another. Although the
SPT was a routine test in the USA, it was not sufficiently
standardized. Furthermore different equipment was used
in South America and Europe (Sanglerat, 1972).
Therefore, it is thought that different measured SPT-N
values are obtained due to differences in equipment and
procedures followed. Thus, this could cause high “a”
coefficient and subsequently different correlations.
However, it is remarkable that the correlation recommended by Terzaghi and Peck (1967) used frequently in
practice is very close to that determined by this study
(Tables 1 and 3 and Fig. 5).
5. Summary and conclusions
Correlations between SPT-N values and soil
properties are empirical and cannot be considered
particularly accurate in few cases since the SPT is not
completely standardized. This study has attempted to
develop correlation between SPT-N and Su. Statistical
approach has been applied to find the best linear
correlation result with number of data (n), coefficient
of determination (r 2) and standard errors (s). A
questionnaire is circulated to collect information on
the SPT procedure and equipment so that the reliable
corrections and correlations can be attempted. Thus the
profile of the performance and equipment used to
determine SPT in Turkey are exposed. As a result of
the questionnaires, based on the equipment and
application standards of SPT, SPT correction factors,
CB, CC, CA, CS, CE can be taken as 1.00, 1.00, 0.85,
O. Sivrikaya, E. Toğrol / Engineering Geology 86 (2006) 52–69
1.20 and 0.75, respectively in Turkey. The equation
including all SPT corrections for fine-grained soils can
be used as N60 = (0.75 ⁎ CR) ⁎ Nfield in Turkey.
67
In this study the relation between SPT-N value and
Su used to determine the undrained shear strength of
clayey soils obtained from UC, UU and FV tests is
Fig. 6. Test type effect on Su = aN for each soil type.
68
O. Sivrikaya, E. Toğrol / Engineering Geology 86 (2006) 52–69
Table 10
Comparison of studies on f1 = Su / N and Ip
Author
Soil type
f1 range
f1 average
Sowers
(1954)
Highly plastic clay (CH)
Medium plastic clay
Low plastic clay (CL)
and Plastic silts
35 < IP < 65
IP < 20
Fine-grained soil, n = 190
CH, n = 80
Clay, n = 147
CL, n = 67
ML, n = 26
MH, n = 14
7.1∼16.5
4.7∼9.5
2.4∼4.7
–
–
–
4∼5
≥6
2∼17.5
2.25∼17.5
2.12∼17.5
2.12∼13
2.68∼6.67
2∼6.88
–
–
6.09
7.52
6.38
4.98
4.22
3.80
Stroud
(1974)
Current
study
examined. The results are quite consistent with the
linear equation proposed by Terzaghi and Peck (1967)
used commonly in practice fine-grained soils and
Sowers (1979) for low plastic clays (CL). However,
the correlations suggested by other researchers give
fairly higher results than those of this study. Different
equipment and procedure were used in the South
America and Europe. Therefore, it is considered that
different results may come from differences in
equipment and procedures employed in the test. In
all soil types and test types, the correlations obtained
with SPTNfield never exceed the values suggested by
Terzaghi and Peck (1967). Their equation always
gives the upper limit. Based on this, it is fair to say
that SPT-N values suggested by Terzaghi and Peck
(1967) must be used after making the SPT corrections.
It is found that even though it is difficult to say
anything about the relationship between Su/N ratio and
plasticity index from UU tests, Su/N ratio increases with
the increase of plasticity index as a general trend from
UC and FV tests. This result is compatible with that of
Sowers (1954) and Sivrikaya and Toğrol (2002), but not
with the results of Stroud (1974).
The range of “a” coefficient in Su = aN equation is
determined for each soils type and SPT corrections
from the different test types. These variations indicate
the magnitude of effect of the SPT corrections, soil
and test types on Su = aN. Therefore it is seen how
much SPT corrections and test types affect the
correlation equations for soil types in fine-grained
soils. The range of the variation (3.5 < a < 6) for finegrained soils from UU test is approximately the same
as that (4 < a < 7) proposed by Stroud (1974) without
taking into consideration plasticity index for SPTNfield. A variation of “a” coefficient for fine-grained
soils depending on the SPT corrections and test types
is approximately in the range of 4 to 7. It is exactly
the same as that by Stroud (1974). “a” coefficient
increases for soil types such as highly plastic clays
(CH), clays, fine-grained soils and low plastic clays
(CL) in all test types regardless of whether SPT
corrections are made or not.
The undrained shear strength appears to be determined most correctly by using SPT. However, geological and geotechnical engineers should be aware that the
obtained relations from this study should be used only in
the preliminary design stage of any project.
Acknowledgments
We would like to express our sincere thanks to the
companies of Yüksel, Toker, STFA, Istanbul Technical
University and General Directorate of Highways of
Turkey for providing the borehole logs and laboratory
test data.
Fig. 7. Variation between Su/N and IP by UU test.
O. Sivrikaya, E. Toğrol / Engineering Geology 86 (2006) 52–69
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