USE AND APPLICATIONS OF THE STATIC CONE PENETRATION TEST (CPT) METHOD FOR
THE CHARACTERISATION AND PREDICTION OF LOCAL SOILS BEHAVIOUR; THE
BUILDING AND ROAD RESEARCH INSTITUTE (BRRI) EXPERIENCE.
Abdul Karim Mohammad Zein
Building and Road Research Institute (BRRI), University of Khartoum
ABSTRACT
The static cone penetration test (CPT) method was introduced to Sudan as a geotechnical site
investigation tool in 1977 and since then has successfully been applied for undertaking many
research projects as well as designing foundations for various engineering structures. A
considerable amount of experience and knowledge have been gained on the value of this method
in the understanding and evaluation of the local soils behaviour. This paper reviews the several
studies carried out by the researchers and academicians at BRRI and presents the main findings
drawn from these studies on the use and application of this simple and reliable site investigation
method for local soils. The areas of research work covered during the last two decades included
the use of CPT to estimate geotechnical parameters which can be used as input in analysis such
as (i) soil classification (ii) shear strength of fine and coarse grained sandy soils and (iii)
compressibility characteristics of fine grained soils. In the direct application of the CPT to an
engineering problem the research work was focused on soil profiling and the evaluation of the
bearing capacity and settlements of shallow and deep bored piles foundations. Several methods
and correlation relationships have been developed between the CPT data and some geotechnical
characteristics of local soils from analyses of results of the studies undertaken at BRRI.
INTRODUCTION
The cone penetration test “CPT” is one of the most commonly used site investigation tools in the
field of geotechnical engineering for the classification and characterization of soils. Other site
investigation techniques normally used in site investigations for engineering projects include the
conventional drilling and laboratory testing methods, dynamic cone penetration (DCP) test,
standard penetration test (SPT), field vane shear test, pressuremeter test and plate loading test.
The advantages of the CPT method as a soil investigation tool which makes it in many cases
superior to other techniques include the following:
The test equipment can be easily and quickly mobilized to the site
The test is relatively quick, simple and economical
The test results provide information on soils in their undisturbed or natural conditions
The test provides a continuous record of data measurement for the whole investigated
soil depth
The test provides repeatable and reliable data i.e. not operator dependent, and
There are strong theoretical basis for CPT data interpretation.
The main disadvantages of the CPT are that no soil samples could be retrieved during testing and
the penetration can be restricted in gravelly and highly cemented soil layers.
1
The research work on the use of CPT method for the classification and evaluation of the
geotechnical properties of local soils was first initiated at the Building and Road Research Institute
(BRRI), University of Khartoum in 1972. This came as a response to a request placed at that time
to BRRI from the Sudanese-Egyptian Permanent Joint Technical Commission ( PJTC) for Jonglei
canal project to advice on the suitability of the CPT method for local Sudanese soils. The first
research study on the use and application of the CPT method for local soils was carried out by the
author in 1980 (Zein, 1980) and since that time several studies have subsequently been
undertaken at BRRI by researchers and postgraduate students to cover aspects which have not
been dealt with in the study by Zein (1980).
This paper aims at reviewing and compiling in one document the scattered results of all previous
research studies on the use of CPT for classifying and evaluating the geotechnical behavior of
local soils. The paper also presents and discusses the most important findings and conclusions
drawn from research works carried out on the uses and applications of the CPT in local soils.
USES AND APPLICATIONS OF CONE PENETRATION TEST (CPT) IN GEOTECHNICAL
ENGINEERING, A REVIEW
The Static Cone Penetrometer
The use and application of the static cone penetration test, CPT is being more and more frequently
considered for the insitu investigation of soils for engineering purposes. In this `test, a cone on the
end of a series of rods is pushed into the ground at a constant rate and continuous measurements
are made of the resistance to penetration of the cone defined in terms of cone resistance, qc, and
of a surface sleeve defined as local side sleeve friction, fs. There are a variety of shapes and sizes
of penetrometers being used for site investigation. However, the one that is standard in most
countries is the cone with an apex angle of 60˚ and a base area of 10cm2 with a friction sleeve
having an area of 150cm2. To obtain cone resistance, qc and sleeve friction, fs a mechanical
“friction jacket” cone developed in1953 (Begemann, 1969) shown in Fig. 1(a) can be advanced
separately by means of sounding rods pushed vertically into the soil at a constant rate of 2cm/sec.
Initially, the cone is pushed through a distance of 5cm to measure qc and with further advancement
of the cone, a flange engages the friction jacket to measure both qc and fs. Subtracting qc from the
latter reading gives fs value at the corresponding depth.
A further development is the “electric cone” in which qc and fs can be measured independently and
continuously with penetration by means of load cells installed the body of the probe. Cone
penetrometers that could also measure pore water pressure (piezocone) were introduced in 1974
(Holden, 1974) with the filter element placed close behind the cone as shown in Fig. 1(b).
2
(a) Mechanical adhesion jacket cone
(b) Electrical cone tip
Fig.1: Static cone penetrometer device
Interpretation of CPT Data
The development and wide application of the CPT method is mostly due to the fact that the test
has yielded a considerable amount of valuable information needed in the design of foundations.
The results of the CPT have been applied in various ways such as soil classification, the
determination of physical and mechanical soil properties, the estimation of soil bearing capacity,
prediction of soil settlements and the design of shallow and deep foundations. Numerous empirical
methods and semi-empirical correlations have been developed to estimate geotechnical
parameters from the CPT data for wide ranges of soil types and conditions. The most important of
these methods and correlations are briefly reviewed here.
Soil Classification and Profiling
The major application of the CPT is for soil classification and description of soil strata penetrated
i.e. soil profiling. Typically, the cone resistance qc is high in sandy soils and low in clayey soils and
the friction ratio Rf is low in sandy soils and high in clayey soils. It has been reported by many
authors that the basic CPT parameters of cone resistance qc, skin friction fs and friction ratio, Rf
may be used for soil classification. The most popular and commonly used soil classification
methods based on CPT data are probably those proposed by Begemann (1969), Schmertmann
(1977), Robertson (1990) and Fellenius and Eslami (2000). The CPT soil classification charts or
methods cannot be expected to provide accurate predictions of soil type based on grain size
distribution but provide a guide to the mechanical characteristics of the soil, or the soil behavior.
These CPT classification methods may prove to be quite useful when applied in some soils
different from those for which they have been developed but differences may well be indicated in
other locations because of their empirical nature. It is therefore recommended to examine the
3
validity of any system before being used in countries where the experience on the interpretation of
CPT data is not adequate.
Prediction of Some Geotechnical Parameters of Soils
a) Undrained Shear Strength of Cohesive Soils
Various authors suggested formulae based on correlation studies between the cone resistance qc
and the undrained shear strength of cohesive soils Su mostly based on the bearing capacity theory
using the classical Terzaghi’s equation. All theories result in a relationship between Su and qc of
the form:
Su = (qc - бv)/Nc ……………………….. (1)
Nc is the bearing capacity factor, sometimes defined as the cone factor of clay and бv is the
effective overburden pressure. Schmertmann (1977) showed that the Nc factor cannot be imagined
as a simple constant but depends on several factors such as the shape and roughness of the cone
and the physical and mechanical soil properties. Due to these factors, the Nc values reported in
literature varied over a wide range of 5 to 70 and thus, the use of a certain value for all soils and
penetrometers leads to a serious error. Despite this variation in Nc values, equation (1) may be
used by researchers and geotechnical engineers to make their own correlations for Nc to match
their local clay soils.
b) Standard Penetration Test (SPT)or Relative Density of Cohesionless Soils
The standard penetration test (SPT) is used in many countries as a routine test for estimating the
relative density (Dr) of cohesionless soils which measures the compactness of sands and has a
decisive effect on their angle of internal friction, bearing capacity and settlement.
According to Robertson and Robertson (2006), despite continued efforts to standardize the SPT
procedure and equipment there are still problems associated with repeatability and reliability.
Because of the widespread use of the SPT in the field of foundation engineering, many attempts
have been made to establish the relationship between the dynamic SPT N-value and the static
CPT qc. The first and most popular qc-N correlation was developed by Meyerhof (1956) for fine or
silty, loose to medium dense sand as follows:
qc (kg/cm2) = 4N (blows/30cm) ………….. (2)
Sanglerat (1972) received test data from various sources in different countries showing that
indiscriminative use of equation (2) without taking into consideration the types of penetrometer
used and soils tested might lead to a serious error. As a result, a more flexible relationship has
been proposed in which the Meyerhof’s figure of 4 was replaced by a constant “n” varying widely
from 2 to 18 as reported in literature. The variation in ‘n’ values was mainly attributed to variations
in soil type, equipment and method of testing. Schmertmann (1970) developed the following
correlation equation which gives N as a function of qc and the friction ratio Rf that may be
applicable in any type of soil:
N (blows/30cm) = (A + B*Rf) qc (kg/cm2) ……….. (3)
Where A and B are constants.
4
c) Soil Compressibility Characteristics
The first correlation between soil compressibility and CPT data using the Dutch cone penetrometer
was probably the one proposed by Buisman (1940) for loose sands. Subsequent research works
have indicated that the constant value of 1.5 in his equation must be modified by using a variable
denoted as α which depends on the nature of soil tested to be as follows:
C = α (qc/ бo) ……….. (4)
Where C is a constant of compressibility of the layer being compressed and бo is the effective
overburden pressure. The constant C is also related to the soil constrained modulus (Es) and the
oedometric coefficient of volume change (mv) as follows:
C бo = E = 1/mv ……….. (5)
From equations 4 and 5, E and mv may be related to qc by the equation:
mv = 1/E = 1/α*qc ……….. (6)
The Buisman’s method originally developed for cohesionless soils has been extended for cohesive
soils by applying equation (4) and using the relationship between C, void ratio e and the
compression index Cc (Cc = 2.3[(1+e)/C] determined by laboratory consolidation testing. Therefore,
the compression index Cc may be related to the cone resistance qc using the relationships given by
equations 3 and 5 as given below:
Cc = [2.3(1+eo) бo]/( α qc) ……. (7)
Equation 7 is only valid for normally and underconslidated clays i.e. with бo values within the linear
portion of the consolidation curve. For over-consolidated clay soils, the equation was modified by
some authors by replacing the initial values of бo and eo by the over-consolidation pressure бc and
the corresponding void ratio ec.
Equations 4 through 7 furnish simple mathematical forms that can be verified experimentally by
comparison of the qc measured by the CPT method and Cc (or C) and mv determined from
laboratory compressibility tests. Following this approach, investigations were carried out in different
countries and several correlation relationships have been developed between soil compressibility
parameters and CPT data for various soil types. However, the results of previous studies indicate
that it is not possible to establish a simple and reliable relationship between CPT data and soil
compressibility. This suggests that the applicability of any of the methods developed in a specific
area to soils from other regions would be questionable.
Applications of CPT Results
In addition to using CPT results to estimate geotechnical parameters needed as input in analysis,
they may be directly applied to an engineering problem without the need for soil parameters.
Typical examples of this approach are the CPT application in predicting bearing capacity and
settlements of shallow and deep foundations and evaluation of compaction control and liquefaction
behavior of soils. Some of these aspects are briefly presented here.
Bearing Capacity and Settlement of Shallow Foundations
5
For shallow footings of commonly used dimensions, the net allowable bearing capacity (qa) may be
estimated from the following empirical equation based on CPT data (Meyerhof, 1956) for width of
footing B>1.22m and settlement of 25.4mm:
qa = (qc/25)[(3.28B+1)/(3.28B)]2 ………. (8)
T he qa value given by equation (8) may be doubled for raft foundations.
The elastic settlement of granular soils can be estimated by the use of the semi-empirical strain
influence factor proposed by Schmertmann et al (1978). According to this method, the immediate
or elastic settlement Sc is given by the equation:
Sc=C1C2(q- qo)∑(Iz∆z/Es) ……… (9)
Where Iz is the strain influence factor, Es is the Young’s modulus of elasticity, ∆z is soil layer
thickness, C1 is a correction factor for foundation depth, C2 is a correction factor for soil creep, q is
the stress at foundation level and qo is the overburden pressure.
Several authors (e.g. Schmertmann, 1970) have correlated Es needed for computing the elastic
settlement from equation 8 to the CPT cone penetration resistance qc as follows:
Es = 2qc ………… (10)
Estimation of settlements for shallow foundations resting on clayey soils from CPT results has
been studied by some researchers (Sanglerat, 1972). However, the general trend for such cases is
to depend mainly on the results of laboratory tests and the conventional settlement computation
methods.
Bearing Capacity of Piled Foundations
The development of the static CPT is strongly connected with its application to the pile foundation
design for buildings and other structures and several Dutch and Belgian authors have suggested
methods to estimate pile capacity and embedment as early as 1950. According to the Dutch
methods, the ultimate pile bearing capacity (Qu) is the summation of the base resistance (Qb) and
the pile shaft resistance (Qs) and is given by:
Qu = Qb +Qs = qc Ab +fs As ………. (11)
Where qc is the average cone resistance measured in the CPT and is calculated from the following
equation:
qc = 1/2 (qc1+qc2) …….. (12)
qc1 is the average of the envelope of minimum cone resistance above the pile toe over a hight of
8D (D= pile diameter) above the largest section of the pile base and qc2 is the mean of the average
of the cone resistance below the pile toe over a depth range 0.7D to 4D below toe level and the
minimum cone resistance value recorded within this depth range. Ab and As are the pile base and
peripheral areas and fs' is the peripheral shear or skin friction of the pile. According to Sanglerat
(1972), the fs'value may be estimated from the CPT cone resistance qc (fs' = qc/200) or from skin
friction fs (fs'= 2fs) measured by the adhesion jacket cone.
The application o the Dutch bearing capacity calculation method is restricted to driven piles only.
De Beer (1964) concluded that some reduction factor has to be applied to those of driven piles in
6
order to determine their ultimate bearing capacity of bored and cast-in-situ piles. He proposed a
reduction factor fr that may be estimated from the two shear strength parameters c (cohsion) and φ
(angle of internal friction) of mixed soils which is given by the following expression in cohesinless
soils:
fr
=
qc'/qc = 1 /tan(45+φ2)2
………….. (13)
The factor qc' being the cone resistance value to be used for the bearing capacity calculations of
bored piles instead of qc of driven piles.
Besides the Dutch and Belgian experience, an important experience has been gained elsewhere
and several authors from various countries have reported the value of the CPT method in the
prediction of pile bearing capacity for driven and bored reinforced concrete piles. However, field
trials to correlate the CPT cone resistance with pile bearing capacity estimated from loading test
results are necessary in any locality where there is no previous experience to establish the
relationship between the soil parameters.
CPT METHOD USED AND FIELDS OF APPLICATIONS FOR SOME SUDANESE SOILS
As stated in the previous section, the importance of establishing relationships between the soil
types and characteristics determined from the conventional testing methods and the static CPT for
local soils is that some theoretical and empirical solutions of foundation engineering problems are
based on the CPT. This test has proved its reliability in solving quickly and successfully some of
these foundation problems in the regions where a sufficient experience has been gained in the
interpretation of the CPT results.
The static cone penetrometer types used in all studies were mechanically operated deep sounding
machines with rated capacities of 100 and 200 kN. The type of cone regularly used throughout the
testing programs in all studies was the adhesion jacket cone known as “Begeman’s tip” shown in
Fig. 1(a).
For the purpose of making good and sound comparisons for the various soil parameters studied,
the CPT soundings were made at test points very close to the locations of the conventional
boreholes drilled to obtain soil samples required for testing and the locations of the pile load tests
in the studies on the bearing capacity of bored piles. A typical graph showing the variations of CPT
results with depth measured at one site in Khartoum State is shown in Fig. 2. The boreholes were
drilled by a truck mounted Acker rotary rig in all investigated sites using continuous augers for
advancement of borings. Most of the sites investigated in the various previous works are mainly
located within Khartoum State territory but some areas in other parts of the country were
considered in few studies.
7
Fig. 2: Typical chart showing variations of CPT data (qc, fs and Rf) with depth
The main research topics covered in the studies undertaken at the BRRI since the time of
importing the first CPT machine to Sudan in 1977 include the following:
Soil classification and profiling
Evaluation of the undrained shear strength of cohesive soils
Correlation with the Standard penetration test (SPT)
Estimation of the compressibility characteristics of fine grained soils, and
Prediction of the bearing capacity of bored piles
The main results findings of the research works accomplished so far on the use and application of
the CPT for prediction and evaluation of the engineering behavior of local soils are presented in the
following sections.
USE AND APPLICATION OF CPT METHOD TO CLASSIFY AND EVALUATE THE BEHAVIOR
OF LOCAL SOILS
Soil Classification and Profiling
On the basis of a comprehensive study, a soil classification method was developed at BRRI by
Zein (1980) for local soils from analysis of CPT and standard laboratory test results for various soil
samples from Khartoum State and other sites in Jonglei and Upper Nile States in southern Sudan.
A detailed description of the developed CPT soil classification method is given elsewhere ( Zein
and Ismail, 1981) but a brief account on the same is outlined here. Zein (1980) analysed a large
size of CPT data points pertaining to soil types that had been tested in the laboratory to determine
8
their grain size distribution and consistency characteristics. All the soil samples tested were
classified using laboratory test results according to the USCS(Unified System for Classifying Soils)
scheme and divided into four main groups namely; clays, silty and sandy clays, clayey sands and
silt-sand mixtures, and sands.
The cone resistance (qc) and friction ratio (Rf) were obtained by the two mechanical CPT machines
equipped with adhesion jacket cones at the corresponding depths of the soil samples considered in
the analysis. It was noted from plotting of the soil types on a combined qc versus Rf graph that each
soil group tends to occupy a certain region in the plot, though overlap between the groups can
however be observed. To enable classification of a soil sample according to the CPT only, the
specific zone occupied by each soil group should be defined.
A statistical approach of data analysis known as the “discriminant method” was used to
differentiate in mathematical terms between the zones corresponding to the four soil groups in the
qc-Rf plot. In this method, the term “soil population” which in this case has the same meaning of
“soil group” is used to describe one set of data having similar characteristics. Each soil group has a
certain function known as “decriminant function, Xl” of which parameters have to be derived from
statistical analysis of the CPT data that is known for certain to come from that group as described
by Zein (1980).
The following descriminant functions were developed for the four soil groups considered in the
study:
X1 = 0.041*qc + 4.04*Rf - 12.6 for clays (n = 82)
…….. (14a)
X2 = 0.044*qc + 3.18*Rf - 8.3 for silty and sandy clays (n = 81)
…….. (14b)
X3 = 0.070*qc + 2.50*Rf - 7.4 for clayey sands and silt-sand mixtures (n = 93) ... (14c)
X4 = 0.10*qc + 1.40*Rf - 7.9 for sands (n = 62)
…….. (14d)
In the above functions the value of qc is in (kg/cm2) units and Rf in (%) whereas ‘n’ denotes the
sample size used for analysis in each soil group. According to the developed classification method,
a soil sample of known cone resistance qc and friction ratio Rf but of uncertain type is allocated to
the nearest population where “nearness” here is a measure of probability. The nearest population
is that from which a greater likelihood of the sample is coming and therefore the sample should be
allocated to whichever population gives the greatest value of Xl in equations 14a to 14e.
Zein (2003) introduced major modifications to the formerly developed CPT classification method to
meet the current requirements of research workers and practicing engineers by satisfying the
following objectives:
To improve the degree of classification accuracy by including in the analysis the soil test data
from research works and site investigation reports for various engineering projects made available
between the years 1980 and 2003.
To consider new grouping of soil types by splitting and rearranging so as to be more specific in
the soil classification.
To develop computer software that simplifies and speeds up the computations involved in the
application of the analytical procedure; and
To incorporate in the classification method some important information on the degree of
compactness (relative density) in cohesionless soils and the degree of consistency in cohesive
soils.
9
Five main soil groups based on the same terminology of the USCS scheme were considered in the
2003 study for the purpose of statistical analysis using the descriminant method for the subsequent
classification of soils using the CPT data only. These were:
a) Clays of high plasticity (CH)
b) Clays of low plasticity (CL)
c) Silty soils of low to high compressibility (ML and MH)
d) Clayey and silty sands (SC and S M), and
e) Poorly and well graded clean sands (SP and SW)
The measured CPT data pertaining to these five soil groups were used for the calculations of the
statistical parameters as shown in Table 1.
Table1: Summary of CPT statistical data used as input for analysis
Soil group Clays
of Clays of
Clayey or
Silty
soils
high
low
silty sands
plasticity plasticity
(ML or MH) (SC or SM)
Statistical
(CH)
(CL)
data
Data size
201
152
184
257
2
Mean qc ( 11) (MN/m )
4.49
6.10
6.60
10.54
Variance of qc
1094.48
2639.05
6045.44
6377.16
Mean Rf ( 12) (%)
6.10
4.48
4.07
3.50
Clean Sands
(SP or
SW)
134
13.69
3596.24
2.09
The data in Table 1 were subsequently used input for the derivation of the five different
discriminant functions corresponding to the different soil groups as follows:
X1 = 0.35*qc + 2.40*Rf
X2 = 0.39*qc + 1.87*Rf
X3 = 0.41*qc + 1.73*Rf
X4 = 0.58*qc + 1.59*Rf
X5 = 0.70*qc + 1.12*Rf
+ 8.31
+ 5.39
+ 4.86
+ 5.87
+ 5.99
for CH clays (n = 201)
for CL clays (n = 152)
for ML and MH silts (n = 184)
for SC and SM sands (n = 257)
for SP and SW sands (n = 134)
…….. (15a)
…….. (15b)
…….. (15c)
…….. (15d)
…….. (15e)
The units of qc and Rf in equations (15a) to (15e) are MN/m2 and % respectively.
To classify a soil sample of known qc and Rf it should be allocated to the soil group the
descriminant function of which gives the highest numerical value when substituted in equations
(14a) through (14e). Important and useful information have been incorporated in the revised and
updated CPT soil classification method to roughly evaluate the degree of consistency and relative
density in cohesive and cohesionless soils respectively using only the CPT data ( qc and Rf). The
widely accepted correlations developed by Terzaghi and Peck (1948) between the standard
penetration test (SPT) N-value on one hand and the relative density of sandy soils and consistency
of cohesive soils on the other were adopted as basis of comparison along with using an empirical
correlation developed for local soils between the SPT’s N value and the CPT parameters qc and Rf
(see Section 3.2.3).
Table 2 gives the proposed ranges of qc corresponding to the various degrees of consistency and
relative density developed for local clayey and sandy soils respectively. With this added feature,
10
the developed CPT soil classification method may be used not to predict the soil type of local soils
only but moreover to roughly evaluate some of its physical and engineering properties.
Table 2: Estimation of soil consistency and relative density from CPT data
Equivalent
qc Sandy and
Equivalent qc values in
Clay Soils
2
values in MN/m
silty soils
MN/m2
Consistenc
Relative
ML/M
SC
N value CH
CL
N value
SP/ SW
y
density
H
/SM
V. Soft
<2
< 1.3
< 1.4
V. Loose < 4
Soft
2-4
1.3 - 1.6
1.4- 1.7 Loose
Medium
4-8
1.6- 2.1
1.7- 2.4 Medium 10- 30
Stiff
8 - 15
2.1- 2.9
2.4- 3.6 Dense
V. Stiff
Hard
15- 30 2.9 - 4.7
>30
> 4.7
4 to 10
30- 50
3.6- 6.0 V. Dense > 50
> 6.0
< 1.8
1.82.9
2.96.4
6.49.4
> 9.4
< 1.9
1.9 3.2
3.2 7.2
7.210.5
> 10.5
< 2.5
2.5 - 4.6
4.6
10.6
10.614.8
> 14.8
To facilitate a continuous profiling of the soil strata at any CPT point in investigated site, an
interactive computer software was developed by a research student to enable computations of the
discriminant values according to equations (14a) to (14e) for every penetration depth at which the
qc and Rf values are measured (normally every 200mm intervals). The application of this computer
program enables fast classification of the penetrated soil layers and provides rough evaluation of
their degrees of consistency of clay soils or the relative density of sandy soils based ranges of the
qc values listed in Table 2.
Undrained Shear Strength of Cohesive Soils
The first study to estimate the undrained shear strength (Su) of Sudanese cohesive soils directly
from CPT data was reported by Zein (1980) who tested alluvial silty clay and clayey silt deposits
located near the Blue Nile left bank (Khartoum city side) in Khartoum State. Fifty undisturbed soil
samples mostly representing the CH and MH soil groups were taken at different depths from
boreholes drilled near the CPT soundings where conditions of full and partial saturation existed.
Being the most commonly used type of shear strength tests, the undrained unconsolidated (UU)
was adopted in this study to determine the soil shear strength parameters; cohesion cu and angle
of internal friction φu The undraind shear strength (Su) was determined from measured cu and φu
values using the following expression:
Su = cu + tanφu2 [R(1-sinφu) +(1+sinφu)] ……… (16)
R is the ratio of normal failure stress бf and the minor principal stress б3.
A statistical regression analysis was carried out to correlate the Su determined according to
equation 16 and the average CPT cone resistance qc measured at the corresponding sample
depths to determine the cone factor Nc defined in equation 1.The analysis yielded the following
11
relationship between qc and Su, both expressed in kg/cm2, for all soil samples tested with a high
correlation coefficient (R2 = 0.81):
qc = 34.9 Su + 0.16 …………. (17)
For practical purposes, the constant of 0.16 can be ignored and Nc is assumed to be 35. Hassan
(2004) carried out a research for a larger sample size (187 samples) including those reported in
Zein’s study to investigate the effects of soil type and stress history evaluated in terms of the soil
over-consolidation ratio (OCR) factors on the qc-Su relationship. To study the effects of these
factors on undrained shear strength, the soils tested were divided into two main groups; clay soils
(subdivided into CL and CH types) and silty soils (subdivided into ML and MH types). Each soil
type was further divided into three categories; normally or slightly consolidated (OCR <2),
moderately over-consolidated (2<OCR< 6) and heavily over-consolidated (OCR ≥ 6).
Analysis for comparison of the Su and qc data sets was carried out and a concise summary of the
study results is listed in Table 3 for all samples considered.
Table 3: Cone factor Nc obtained from qc-Su correlations for local soils (Hassan, 2004)
Soil type
Soil designation Sample
Cone factor Nc = Correlation
size
qc/Su
coefficient (R)
Silts
ML
37
61.5
0.42
MH
36
37.2
0.72
All
73
51.5
0.57
Clays
CL
32
34.0
0.59
CH
79
34.5
0.56
All
113
34.4
0.32
All soils
187
38.7
0.27
As may be noted from the data inTable 3, rather poor correlations were found from analysis when
all the soil samples were considered. However for the CL, CH and MH soil types which represent
about 79% of all samples tested reasonable to fairly good correlations (R = 0.56 to 0.72) were
established between Su and qc with Nc values ranging from 34.0 for the clay ( CL and CH) soils to
37.2 for the MH soils. These Nc values compare favorably to the cone factor Nc =35 proposed by
Zein (1980) for the same types of soils. Compared to other soil types, a much higher cone factor
(Nc = 61.5) and a much lower correlation coefficient (R = 0.42) were obtained for the ML soil
indicating a poor correlation between Su and qc.
The study by Hassan (2004) also showed that for all soil types tested, the stress history has a
significant effect on the qc-Su relationship and thus on the cone factor such that for a given soil
type the Nc value decreases when the OCR ratio increases.
Correlation between Cone and Standard Penetration (CPT-SPT) Methods
The results of the insitu static cone penetration test (CPT) and the dynamic standard penetration
test (SPT) methods are widely used in the prediction of bearing capacity and the settlements of the
foundations of engineering structures. As stated in section 2.2.2(b), correlation relationships have
been proposed by several authors between the cone resistance qc of the CPT and the blow counts
12
N of the SPT to enable estimating either soil parameter from available data of the other. In Sudan,
the first comparison study was undertaken at BRRI by Zein (1980) and has been updated (Zein,
2003) to examine the validity of some published qc-N relationships and to search into the
possibility of developing a sound correlation for Sudanese soils. The CPT soundings were
performed with the adhesion jacket cone and the SPT was done following the ASTM standard
procedure. The qc and Rf were determined at approximately the same borehole depths were the
SPT had been made. The soils in the different sites investigated in 1980 which are located in
Khartoum and Jonglei States covered a variety of types including silty, clayey and sandy soil
deposits.
Since widely different soil types and conditions were considered in the two studies it was deemed
important to introduce a parameter or index to account for soil variability in order to establish a
reliable correlation between qc and N. In a previous study on local soils, Zein and Ismail (1981)
found that the qc/N ratio is dependent on the soil type indicated by the average Rf values of the four
main soil groups as given in Table 4.
Table 4: Relationship between qc/N ratio and friction ratio Rf for local soils.
Soil type
Clays
Silty clays and
Clayey sands and
sandy clays
sand-silt mixtures
Average Rf (%)
5.8
4.5
3.5
qc/N ratio
> 2.0
2.0-3.0
3.5-4.5
Sands
1.7
> 5.0
Therefore, the friction ratio Rf of the CPT was chosen as it has been shown in many previous
investigations to be a good soil type indicator. To study the qc-N relationship more closely, they
were plotted against each other for the soil types of approximately constant Rf values and a linear
relationship was found to exist between the two parameters (Ismail and Zein, 1987). The observed
qc-N relationship trends and the data given in Table 4 indicate that higher qc/N ratio values and
lower Rf values correspond to cohesionless soils where their opposites correspond to cohesive
soils.
In a more recent study (Zein, 2002), a statistical analysis was carried out on 138 CPT and SPT
data points assuming many mathematical forms to establish the best qc-Rf-N correlation
relationship for local soils and the following empirical polynomial equation was obtained between
qc and N/Rf ratio :
qc = 10.3 + 1.6(N/Rf) – 0.0038(N/Rf)2
with R2 = 0.64
………. (18)
2
In this equation, qc is expressed in kg/cm units, N in blows/30cm and Rf in percent.
The suitability of the qc-Rf-N correlation given by equation (18) was examined using data published
in literature for American soils (Bennet et al, 1979) in which the same CPT and SPT methods were
followed and as a result the following correlation was obtained:
qc = -1.23 + 11.56(N/Rf) – 0.0865(N/Rf)2
with R2 = 0.76 ………. (19)
This implies the suitability of the mathematical form and soil variables used in equations (18) and
(19) for describing the qc-N relationship for soils of different origins.
A graphical solution of equation (18) was made as shown in Fig. 3 to enable estimating N directly
from known qc values or vice versa for soils from known or arbitrarily assumed Rf values. The Rf
value needed to be substituted in equation (18) is directly taken from CPT data for estimating the N
from a known qc value or assumed using the data in Table 4 for the appropriate soil type if qc is to
be estimated from known N value. For using the data in Table 4, one needs either to test or uses
13
his judgment and experience to identify the type of soil from the visual inspection of the soil sample
recovered inside the SPT sampler tube.
Fig. 3: Combined qc-Rf-N correlation chart for local soils (Zein, 2002)
Therefore, either the data presented in Table 4, the charts shown in Fig. 3 or the correlation
relationship given by equation (18) can be used to estimate either qc or N if information is available
on the other for local soil types. In this manner, it would be possible to apply the theoretical and
empirical solutions of the foundation engineering problems which have been based on the results
of the CPT and SPT methods.
Soil Compressibility Characteristics
A research study was undertaken by Eltahir (1994) under the supervision of the author on local
soils aiming at investigating the possibility of developing useful correlation between CPT and soil
compressibility characteristics. An experimental testing program was performed on 76 undisturbed
soil samples representing clayey soils (CL and CH types), silty soils (ML and MH types) and sandy
soils (SC and SM types) obtained from different sites located in four different Sudanese states;
Khartoum, Northern and southern Kurdofan and White Nile. The CPT was made at points located
adjacent to the boreholes from which the soil samples had been taken. Consolidation tests were
performed in the laboratory following the BS1377(1990) procedure on soil samples soaked to
saturate and the compression index Cc and coefficient of volume compressibility mv were
determined from the results of these tests for each sample. The CPT data (qc and Rf) were also
determined at the borehole depths corresponding to those from which the soil samples were
collected. Further details on this study were published by Zein and El Tahir (2002) but the main
findings and conclusions of the study are presented here.
Because no particular trend was observed in the relationship between qc and Cc when all the
samples were considered in analysis, it was decided to divide the samples of soil types tested into
14
plastic and nonplastic soil groups. The plastic group included the clay soils where the non-plastic
group included the silty and sandy soils. The least square regression method was used to establish
the relationship between Cc and qc for each soil group and the highest correlation coefficients (R2 =
0.52 to 0.53) were given by the following two equations for clays and silty and sandy soil types
respectively:
Cc = 0.001qc2 – 0.03qc + 0.38
……. (20a)
2
Cc = 0.002qc – 0.05qc + 0.47
….. (20b)
It was noticed that the degree of data scatter was significant in the qc -Cc relationships represented
by the above two equations and therefore a new parameter was introduced to reflect the effect of
soil type in an attempt to improve the correlation and thus the accuracy of the Cc-qc relationships.
After several trials of data analysis, it was found that the best correlations would be obtained by
using the plasticity index, PI, and the fines content FC (soil fraction passing No. 200 test sieve), as
indicative indices for the clay and silty-sandy soil samples respectively. The following correlation
equations were derived upon introducing the PI and FC indices, to describe the Cc versus qc
relationships for the clays and the silty and sandy soils respectively:
Cc = 1/PI [0.007 qc2+ 0.28 qc+2.19]
……. (21a)
Cc = 1/FC [0.25 qc2- 6.67 qc+48.2]
……. (21b)
The qc values in equations (20) and (21) are expressed in MN/m2 and the PI and FC are in
percent.
The coefficient of volume compressibility mv (m2/MN) and constrained modulus Es (MN/m2) were
also related to qc for an assumed consolidation pressure increment from 100 to 200kN through
evaluation of the coefficient α (defined in equation 6) to the soil friction ratio Rf (%) as given below:
α = 0.032Rf1.74 for clay soils (R2 = 0.61)
……… (22a)
2
2
α = 0.032Rf - 5.8 Rf + 2.77 for silty and sandy soils (R = 0.56) …….. (22b)
Thus in order to estimate Es or mv from known qc and Rf , the coefficient α is firstly obtained from
equations (22a) or (22b) and then the values of α and qc are substituted in equation 6 for the soil
type under consideration.
Prediction of Bearing Capacity of Bored Piles
The application of the CPT to predict the bearing capacity of piles in Sudan was limited to the case
of bored or drilled shaft piles, being the foundation system that has received wide acceptance by
local foundation design and contracting engineers and executed during the construction of several
engineering projects. Two different research studies have been carried out at BRRI to assess the
reliability of some published CPT based methods proposed for predicting the bearing capacity of
bored piles developed in other countries for local soils.
The first study was made at the site of Gerief-Manshia bridge on the Blue Nile in Khartoum State in
which a 1.50m diameter and 21.5m long bored pile was constructed and tested by a slow
maintained load method. One deep borehole and one CPT were made close to pile test location to
determine the types and characteristics of soil strata and CPT data. The soil profile was
predominantly comprised of alluvial silt and sand deposits resting on highly to moderately
weathered Nubian sandstone or mudstone formations. A comparison was made between the pile’s
bearing capacity estimated from the pile load test results as well as the results of testing soil
15
samples using two empirical methods and that predicted according to three methods based on the
CPT data, namely the methods developed by Schmertmann (1977), the Dutch Engineers
(Sanglerat, 1972) and Meyerhof (1956) and one empirical method by Touma and Reese (1974).
The Chin’s method (1970) was adopted to evaluate the bearing capacity from the pile load test
data. A summary of the results of comparison of the pile bearing capacity end bearing and skin
friction predicted according to the different methods considered in this study is given inTable 5. As
may be noted in Table 5, there is a good comparison of the total bearing capacity of the bored pile
estimated according to the five different methods. Taking Chin’s method as a basis for comparison
the discrepancy in the predicted total bearing capacity was -15.7 to 23.6%. However, some
differences were noted in comparing the pile end bearing and pile skin friction components of the
pile capacity calculated according to the five different methods. An exception is the method
proposed by Touma and Reese which compared favorably with Chin’s method for both pile bearing
capacity components while the method by Schmertmann gave a good comparison with Chin’s
method for estimating the pile skin friction only.
Table 5: Comparison of bearing capacity values predicted according to CPT and other methods for
a bored pile foundation.
Pile
capacity Ultimate
pile Ultimate pile Allowable
pile Discrepancy
(%)
prediction
skin friction
end bearing bearing capacity
based on Chin’s
method
method Qall value
Qs(tons)
Qb(tons)
Qall (tons)
Meyerhof
149.3
133.5
494.6
-6.9
Touma and Reese 777.9
565.5
447.8
-15.7
Schmertmann
816.2
1153.6
656.6
23.6
Dutch
230.6
1159.2
463.3
-12.8
Chin
929.9
664.1
531.3
In a recent study (Babikir, 2006), a research work which involved drilling borehole, performing CPT
soundings and carrying out pile load tests was undertaken under the author’s supervision to
compare the bearing capacity values estimated according to different approaches for eight bored
piles of variable lengths and diameters at five different sites located in Khartoum State. The
bearing capacity results were obtained for the tested piles according to five different prediction
methods including two based on CPT data (Bustamante and Gianeselli, 1982 and Aoki and De
Alencer, 1975), two based on interpretation of pile load test results (De Beer, 1964 ,Chin 1970)
and Meyerhof’s method. However, the results obtained from this study indicated that the bearing
capacity values predicted according to the five different methods considered were inconsistent and
significantly different for practically all the piles tested. Based on the findings of this study, none of
the two methods based on the CPT was reliable in estimating the bearing capacity of bored piles
constructed in local soils. The differences in pile bearing capacity prediction may be attributed to
several factors, the most important of which is the scale effects, the characteristics of the soils at
the investigated site and the procedure used to determine the pile load capacity from the load test.
Despite these differences, the CPT is still believed to give the closest simulation to a pile
foundation system. Superiority of the CPT methods over non CPT methods has been confirmed by
some authors (as cited by Robertson and Robertson, 2006).
In this respect, a study has recently been started at BRRI to search into developing a sound CPT
based method that can be used for estimating the bearing capacity of bored piles with acceptable
16
accuracy when applied for local soil types. This would require performing small and large scale
loading tests with separate pile end bearing and pile skin friction measurements and comparing
their values with the cone resistance and skin friction measured in CPT soundings made close to
the pile test locations.
CONCLUSIONS
This paper is meant to reflect the local experience gained within the last four decades on the use of
the static cone penetration test CPT for estimating the geotechnical characteristics of some
Sudanese soils and its applications for the solution of some problems related to the design and
construction of foundations of engineering structures.
Several studies have been undertaken at BRRI by researchers and research students on the main
basic and applied research topics and aspects for which the CPT has proved to be useful and
reliable worldwide in order to examine its value in the understanding and evaluation of the
engineering behavior of Sudanese soils. The CPT methodology regularly used in all studies was
that originally developed in the Netherlands using a hydraulic operated machine equipped with a
standard mechanical friction jacket cone. A great effort was made in these studies to carry out
investigations and collect relevant soil data for samples representing various soil types and
conditions obtained from many sites distributed in widely different parts of the country.
The following conclusions have been drawn from the results of the various research works
reviewed in this paper on the use and applications of the CPT for local soils:
i.
ii.
The experience which has been gained at BRRI with the CPT has proved that this method
can be used to solve quickly and successfully some of the foundation problems in local
soils. The test is fast, reliable, economical and has strong theoretical basis for data
interpretation.
Sound correlation relationships and methods have been derived or developed between
the CPT data and the following characteristics of local soils from the results of the various
studies reported in this paper including the following:
a) A mathematical soil classification method has been developed to enable prediction
the nature and some basic physical properties of subsoil strata from the CPT results
only. The method was based on the statistical decriminant analysis carried out on the
assumption that any considered soil sample will fall within one of five main soil groups
designated according to the world-wide accepted USCS scheme for soil classification.
b) The undrained shear strength Su of local clayey and silty soils has been correlated to
the CPT cone resistance qc and thus the bearing capacity of such soils may be
estimated using a cone factor Nc =qc/Su = 35.
c) A reasonable correlation was found to exist between the cone resistance qc of the
CPT and the N value of the standard penetration test SPT but their relationship is soil
type dependent. A combined empirical graphical qc-soil type-N correlation method has
been developed to enable estimating either parameter from knowledge of the other for a
given soil type.
d) The compressibility characteristics of local clayey, silty and slightly sandy soils may
be roughly estimated from CPT data using correlation equations developed to relate the
cone resistance qc measured in with the compression index Cc and the coefficient of
volume compressibility mv and the constrained modulus of elasticity Es.
17
iii. Successful applications of the CPT method has been considered in some geotechnical
design and research oriented works on local soils for the provision of information and parameters
required for the foundation design to:
a) Facilitate a continuous profiling of the soil strata at any CPT point in any investigated
site, using an interactive computer software based on the developed soil classification
method described in section 3.2.1 for every penetration depth at which the qc and Rf
values are measured. The application of this computer program enables fast
classification of the penetrated soil layers and provides rough evaluation of their degrees
of consistency and relative density of clayey and sandy soils respectively.
b) Predict the bearing capacity of bored piles drilled at various lengths through the
depths of local soil strata. This foundation system has received wide acceptance by
foundation designers and has been used for the construction of the superstructures for
several large engineering projects in Sudan.
ACKNOWLEDGEMENT
The author acknowledges with gratitude the assistance offered to him by the former and present
M.Sc. students at BRRI Asher Rifaat,Mostafa Hasan, Haitham A. Babikir and Samah B.
Mohammed for collection some of the data used for analysis in this study and Hisham Osman, for
the preparation of computer program.
REFERENCES
Aoki, N. and De Alencer, D. (1975). An approximate method to estimate the bearing capacity of
piles. Proc. 5th Pan-American Conf. on SMFE, Buenos Aires, Vol. 1, pp. 367-376.
Babikir H.A. (2006). Unpublished M.Sc. Thesis, BRRI.
Begemann H.K.S. (1969). The Dutch static penetration test with the adhesion jacket cone.
Lab. Groundmech., Delft, Netherlands, 13(10): 1-86.
Bennet M.J., Youd T.L., Harp E.L. and Wieczorek G.F. (1979). Subsurface investigation for
liquefaction, Imperial Valley Earthquake. Calif. US Geological Survey Report 81-502.
BuismanA.S.K. (1940). Gronndmechanica, Waltman, Delft.
Bustamante, M. and Gianeselli, L. (1982). Pile bearing capacity prediction by means of static cone
penetrometer CPT. Proc. 2nd European Symposium on Penetration Testing, ESOPT-II,
Amsterdam, Vol. 2: 493-500.
Chin, F.K. (1970). Estimation of the ultimate load of piles not carried to failure. Proc. 2nd Southeast
Asian Conf. on Soil Engineering, Singapore, : 81-90.
De Beer, E.E. (1964). Some considerations concerning the point bearing capacity of bored piles.
Proc. Symp. On Bearing Capacity, Roorkee, India, Vol. 1, p.178.
El Tahir M.M. (1994). Use of the static cone penetration data for the prediction of compressibility
characteristics for some local soils. Unpublished M.Sc. Thesis, BRRI, Univ. of Khartoum.
Fellenius, B. H. and Eslami, A. (2000). Soil Profile interpreted from CPTu data. Proc. “Year 2000
Geotechnics” Geotchnical Engineering Conference, Asian Institute of Technology, Bangkok, 18p.
Hassan M.A. (2004). Evaluation of undrained shear strength from CPT data for local fine grained
soils. Unpublished M.Sc.Thesis, BRRI, Univ. of Khartoum
Holden, J. (1974). Penetration testing in Australia. Proc. European Symp. On Penetration Testing,
Stockholm, Vol. 1: 155-162.
18
Ismail H.A.E. and Zein A.K.M (1987). Prediction of the undrained shear strength and standard
penetration test using the static cone penetration test data. Proc. 9th Africa Regional Conf. on
SMFE, Lagos: 185-192.
Meyerhof G.G. (1956). Penetration tests and bearing capacity of cohesionless soils. ASCE, J.
SMFE Division, Vol. 82, SM1: 1-12.
Mohammed S.B. (2009) Personal contact.
Robertson P.K. and Robertson K.L. (2006). Guide to cone penetration testing and its application to
geotechnical engineering. Gregg Drilling and Testing Incl. Report, July 2006.
Robertson, P.K. (1990). Soil classification using the cone penetration test. Canadian
Geotechnical Journal, Vol. 3, No. 1: 151-158.
Sanglerat G.G.J. (1972). The penetrometer and soil exploration. Development in Geotechnical
Engineering. Elsevier Publishing Co., Amsterdam.
Schmertmann J.H., Hartman J.P. and Brown, P.R. (1978). Improved strain influence factor
diagrams. Proc. ASCE, J. GE Division, Vol. 104, GT8: 1131-1135.
Schmertmann, J.H. (1970). The importance of side friction and lateral stresses to the SPT N value.
Proc. 4th Pan-American Conf. on SMFE, Puerto Rico.
Schmertmann, J.H. (1977). Guidelines for CPT performance and design. Report prepared for
Fedral Highway Administration, Washington D.C.
Terzaghi K. and Peck R.B. (1948). Soil Mechanics in Engineering Practice. J. Wiley and Sons,
Inc., NY.
Touma, F.T. and Reese L.C. (1974). Behavior of bored piles in sand. Proc. ASCE, J. GE Division,
Vol. 100, GT7:749-761.
Zein A.K.M. (2002). Development and evaluation of some empirical methods of correlation
between CPT and SPT. BRR Journal, Vol. 4: 16-28
Zein A.K.M. and El Tahir M.M. (2002). Prediction of compressibility of some Sudanese soils using
cone penetration test method. Proc. Geotechnical and Geo-environmental Engineering in Arid
Lands. Alawaji(ed): 121-126.
Zein, A. K. M. (1980). Correlation between static cone penetration and recognized standard
test results for some local soils. M.Sc. Thesis, Civil Eng. Dept., U. of K.
Zein, A. K. M. (2002). Development and evaluation of some empirical methods of correlation
between CPT and SPT. BRR Journal, Vol. 4: 16-28.
Zein, A.K.M. (2003). Use of cone penetration test for classification of local soils. BRR Journal, Vol.
5: 23-29.
Zein, A.K.M., and Ismail, H.A.E. (1981). Use of static cone penetration test for soil
classification. BRRI Current Paper Publication CP1/81.
19