How To Use The Static Cone Penetration Test

Transcription

1 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

2 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 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, q c, and of a surface sleeve defined as local side sleeve friction, f s. 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 10cm 2 with a friction sleeve having an area of 150cm 2. To obtain cone resistance, q c and sleeve friction, f s 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 q c 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 q c and f s 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

3 (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 q c is high in sandy soils and low in clayey soils and the friction ratio R f 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

4 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 q c 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 S u and q c of the form: Su = (qc - бv)/nc.. (1) N c is the bearing capacity factor, sometimes defined as the cone factor of clay and б v 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 N c 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 N c 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 (D r ) 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/cm 2 ) = 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 q c and the friction ratio R f that may be applicable in any type of soil: N (blows/30cm) = (A + B*R f ) q c (kg/cm 2 ).. (3) Where A and B are constants. 4

5 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 б o is the effective overburden pressure. The constant C is also related to the soil constrained modulus (E s ) and the oedometric coefficient of volume change (mv) as follows: C б o = E = 1/m v.. (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 C c (C c = 2.3[(1+e)/C] determined by laboratory consolidation testing. Therefore, the compression index C c may be related to the cone resistance q c using the relationships given by equations 3 and 5 as given below: C c = [2.3(1+e o ) б o ]/( α q c ). (7) Equation 7 is only valid for normally and underconslidated clays i.e. with б o 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 e c. 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

6 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: q a = (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 S c is given by the equation: Sc=C1C2(q- qo) (Iz z/es) (9) Where I z is the strain influence factor, E s 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 E s needed for computing the elastic settlement from equation 8 to the CPT cone penetration resistance qc as follows: E s = 2q c (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 According to the Dutch methods, the ultimate pile bearing capacity (Qu) is the summation of the base resistance (Qb) and the pile shaft resistance (Q s ) and is given by: Qu = Qb +Qs = qc Ab +fs As. (11) Where q c is the average cone resistance measured in the CPT and is calculated from the following equation: qc = 1/2 (qc1+qc2).. (12) q c1 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. A b and A s 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 f s (f s '= 2f s ) 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

7 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: f r = qc'/qc = 1 /tan(45+φ 2 ) 2.. (13) The factor q c ' 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

8 Fig. 2: Typical chart showing variations of CPT data (q c, f s and R f ) 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

9 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 q c versus R f 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 q c -R f 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 *Rf for clays (n = 82).. (14a) X 2 = 0.044*q c *R f for silty and sandy clays (n = 81).. (14b) X 3 = 0.070*q c *R f for clayey sands and silt-sand mixtures (n = 93)... (14c) X4 = 0.10*qc *Rf for sands (n = 62).. (14d) In the above functions the value of q c is in (kg/cm 2 ) units and R f 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 q c and friction ratio R f 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 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

10 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 high low Silty soils plasticity plasticity (CH) (CL) (ML or MH) Clayey or silty sands Clean Sands (SP or SW) Statistical (SC or SM) data Data size Mean q c ( 11 ) (MN/m 2 ) Variance of qc Mean R f ( 12 ) (%) 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: X 1 = 0.35*q c *R f for CH clays (n = 201).. (15a) X 2 = 0.39*q c *R f for CL clays (n = 152).. (15b) X3 = 0.41*qc *Rf for ML and MH silts (n = 184).. (15c) X4 = 0.58*qc *Rf for SC and SM sands (n = 257).. (15d) X 5 = 0.70*q c *R f for SP and SW sands (n = 134).. (15e) The units of qc and Rf in equations (15a) to (15e) are MN/m 2 and % respectively. To classify a soil sample of known q c and R f 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 ( q c and R f ). 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 q c corresponding to the various degrees of consistency and relative density developed for local clayey and sandy soils respectively. With this added feature, 10

11 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 q Clay Soils c Sandy and Equivalent q c values in values in MN/m 2 silty soils MN/m 2 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 < 1.8 < 1.9 < 2.5 Soft Loose 4 to Medium Medium Stiff Dense V. Stiff V. Dense > 50 > 9.4 > 10.5 > 14.8 Hard >30 > 4.7 > 6.0 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 (S u ) 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 c u and angle of internal friction φ u The undraind shear strength (S u ) was determined from measured c u and φ u values using the following expression: S u = c u + tanφ u 2 [R(1-sinφ u ) +(1+sinφ u )] (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 S u 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

12 relationship between qc and Su, both expressed in kg/cm 2, for all soil samples tested with a high correlation coefficient (R 2 = 0.81): q c = 34.9 S u (17) For practical purposes, the constant of 0.16 can be ignored and N c 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-s u 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 N c = Correlation size q c /S u coefficient (R) Silts ML MH All Clays CL CH All All soils 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 S u and q c with N c values ranging from 34.0 for the clay ( CL and CH) soils to 37.2 for the MH soils. These N c values compare favorably to the cone factor N c =35 proposed by Zein (1980) for the same types of soils. Compared to other soil types, a much higher cone factor (N c = 61.5) and a much lower correlation coefficient (R = 0.42) were obtained for the ML soil indicating a poor correlation between S u and q c. The study by Hassan (2004) also showed that for all soil types tested, the stress history has a significant effect on the qc-s u 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

13 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 q c and R f 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 q c and N. In a previous study on local soils, Zein and Ismail (1981) found that the q c /N ratio is dependent on the soil type indicated by the average R f values of the four main soil groups as given in Table 4. Table 4: Relationship between q c /N ratio and friction ratio R f for local soils. Soil type Clays Silty clays and Clayey sands and Sands sandy clays sand-silt mixtures Average Rf (%) q c /N ratio > > 5.0 Therefore, the friction ratio R f of the CPT was chosen as it has been shown in many previous investigations to be a good soil type indicator. To study the q c -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 q c -N relationship trends and the data given in Table 4 indicate that higher q c /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/R f ratio : q c = (N/R f ) (N/R f ) 2 with R 2 = (18) In this equation, qc is expressed in kg/cm 2 units, N in blows/30cm and Rf in percent. The suitability of the q c -R f -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: q c = (N/R f ) (N/R f ) 2 with R 2 = (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 q c value or assumed using the data in Table 4 for the appropriate soil type if q c is to be estimated from known N value. For using the data in Table 4, one needs either to test or uses 13

14 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 q c -R f -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 q c and C c when all the samples were considered in analysis, it was decided to divide the samples of soil types tested into 14

15 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 C c and q c for each soil group and the highest correlation coefficients (R 2 = 0.52 to 0.53) were given by the following two equations for clays and silty and sandy soil types respectively: C c = 0.001q 2 c 0.03q c (20a) C c = 0.002q 2 c 0.05q c (20b) It was noticed that the degree of data scatter was significant in the q c -C c 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 qc qc+2.19]. (21a) C c = 1/FC [0.25 q c q c +48.2]. (21b) The q c values in equations (20) and (21) are expressed in MN/m2 and the PI and FC are in percent. The coefficient of volume compressibility m v (m 2 /MN) and constrained modulus E s (MN/m 2 ) were also related to q c 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 = 0.032R f 1.74 for clay soils (R 2 = 0.61) (22a) α = 0.032Rf Rf for silty and sandy soils (R 2 = 0.56).. (22b) Thus in order to estimate E s or m v from known q c and R f, 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

16 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 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 capacityultimate pile Ultimate pile Allowable pile Discrepancy (%) prediction skin friction end bearing bearing capacity based on Chin s method method Qall value Q s (tons) Q b (tons) Q all (tons) Meyerhof Touma and Reese Schmertmann Dutch Chin 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

17 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. 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. ii. 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 S u of local clayey and silty soils has been correlated to the CPT cone resistance q c 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 q c 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 q c measured in with the compression index C c and the coefficient of volume compressibility mv and the constrained modulus of elasticity Es. 17

18 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 for every penetration depth at which the q c and R f 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. 5 th Pan-American Conf. on SMFE, Buenos Aires, Vol. 1, pp 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): 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 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. 2 nd European Symposium on Penetration Testing, ESOPT-II, Amsterdam, Vol. 2: Chin, F.K. (1970). Estimation of the ultimate load of piles not carried to failure. Proc. 2 nd Southeast Asian Conf. on Soil Engineering, Singapore, : 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:

19 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. 9 th Africa Regional Conf. on SMFE, Lagos: Meyerhof G.G. (1956). Penetration tests and bearing capacity of cohesionless soils. ASCE, J. SMFE Division, Vol. 82, SM1: 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 Robertson, P.K. (1990). Soil classification using the cone penetration test. Canadian Geotechnical Journal, Vol. 3, No. 1: 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: Schmertmann, J.H. (1970). The importance of side friction and lateral stresses to the SPT N value. Proc. 4 th 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: Zein A.K.M. (2002). Development and evaluation of some empirical methods of correlation between CPT and SPT. BRR Journal, Vol. 4: 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): 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: Zein, A.K.M. (2003). Use of cone penetration test for classification of local soils. BRR Journal, Vol. 5: 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