INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 3, No 3, 2013

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1 INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 3, No 3, 2013 Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0 Research article ISSN Reliability of using standard penetration test (SPT) in predicting properties of silty clay with sand soil Associate Professor, Civil Engineering Department, Assiut University, Assiut 71516, Egypt Civil Eng.Department, Faculty of Engineering, Aljouf University, KSA mostafaabdo@yahoo.com doi: /ijcser ABSTRACT The properties of soil play an important role in many practices for geotechnical engineering. To determine the real values of these properties special techniques should be followed such as undisturbed samples and initial overburden pressures should be taken into consideration. Actually, it is difficult to obtain 100 % undisturbed samples due to handling, transportation, release of overburden pressure and poor laboratory conditions. So, prediction of some properties such as shear strength parameters (c & ϕ) for hard and dry silty clay with sand soil with the help of standard penetration test (SPT) provides a good opportunity to obtain these parameters without using of more laboratory tests. Standard penetration tests (SPT), rough measure the strength of soil. The great merit of this test and the main reason for its widespread use is that it is simple and inexpensive. The shear strength parameters which can be inferred are approximate, but may give a useful guide in ground conditions where it may not be possible to obtain borehole samples of adequate quality like clay containing sand or gravel. This study was undertaken in order to study the reliability of using standard penetration test (SPT) in predicting some properties, (such as Atterberg limits LL, PL, PI, and shear strength parameters (c & φ)), of silty clay with sand soil. Many samples from this soil have been investigated for this purpose. The results of the research indicated that the shear strength of soil affects SPT number. Empirical equations to identify the shear strength parameters of silty clay with sand soil using corrected SPT number (N") have been proposed. Keyword: Silty clay with sand, (c & φ), LL, PL, PI, SPT, correction coefficient, field tests. 1. Introduction Numerous methods have been developed for determination the soil properties. These methods include the field and laboratory tests. Also, to determine the real values of these properties, special techniques should be followed such as undisturbed samples and initial overburden pressures should be taken into consideration. On the other hand, field test such as SPT does not depend on undisturbed sample because it is carried out in original field soil. Most problems in soils and construction involve either the strength of the in-situ soil or the compressibility of the soil mass. People tend to have a negative opinion of clay soils. These soils may not drain quickly after heavy rain and often harden when dry. Most soils contain a mixture of clay, organic matter, sand and silt. If a soil contains at least 40 percent clay, then we classify it as a "clay soil" (Tuker, 1999). Clay is made up of tiny particles less than mm in diameter. By comparison, silt particles range from to 0.06 mm in size and sand particles from 0.06 to 2.0 mm. For this reason, clay soils are considered to be fine textured; silt, medium textured; and sand coarse textured. Clay acts as a binding agent between soil particles. It gives soil elasticity and provides cohesion of soil particles. On the other hand, sand provides friction of soil particles. Properties of silty clay with sand soil are significantly Received on January 2013 Published on March

2 associated with stiffness of this soil and give an indication of shear strength of soil. These properties plays an important role in many practices for geotechnical engineering, such as soil description or classification of soil, prediction of behavior of soil if it will be subjected to extensive settlement or swelling. The real value of these properties such as angle of internal friction (φ) and cohesion (c) require a special care and laboratory technique. So, prediction of soil properties with the help of field tests such as standard penetration tests (SPT) provides a good opportunity to obtain this parameter without using of more laboratory tests. Standard penetration tests (SPT), rough measure the strength of soil. It is currently the most popular and economical means of obtaining subsurface information. The SPT has been extended to other areas of soil mechanics and foundations, and it has become a fundamental parameter in geotechnical engineering. The great merit of the test and the main reason for its widespread use is that it is simple and inexpensive. In conditions where the quality of the undisturbed sample is suspect, e.g. silty clay with sand soil, it is often advantageous to alternate the sampling with standard penetration tests to check the strength. Standard penetration test (SPT) is one of the field tests of soil. It primary gives idea about shear strength of soil which it can be expressed in terms of shear strength parameters (c & φ). Most laboratory tests must be performed on molded samples of soils. These tests are laborious and time consuming; but sometimes the results are not accurate due to the poor laboratory conditions (Venkatasubramanian and Dhinakaran, 2011). An aggregate that contains sufficient fines to fill all voids between aggregate grains will still gain its strength from grain-to-grain contact but has increased shear resistance (Siswosoebrotho et al., 2005). Plasticity index (PI) and liquid limit (LL) can be used to determine the swelling characteristics of expansive clay in general (Subjianto et al., 2011). (Garham, 2012) indicates that it is a poor ability to get similar results when testing different portions of the same samples, but the result differences should be below the upper control limit. Sometimes, Strength values determined from laboratory testing of intact soil cores are recognized as not being directly applicable to the in-situ soil mass because of the scale effect. The standard penetration test (SPT) is very widely used for subsurface investigation in many parts of the world. It measures the resistance of a hollow core being hammered with a 63.5kg weight. The hollow core receives a soil sample during the process which is logged to soil identification purposes. The relative density can be estimated from the number of blows used to drive the SPT a total of distance of 450mm with the total blows for the last 300mm being the SPT N f -value (Silva, 2010). To construct a structure, N f value (SPT value) is an important parameter to understand the soil condition at different depths. After investigating N f value of different depths, foundation depth can be selected according to load of the structure. Then determine the corrected N" value at the required depth. Angle of internal friction is determined from corrected SPT N"-value. After that, determine bearing capacity factors to calculate the bearing capacity of the soil. From this bearing capacity of soil foundation can be designed (Humyral et al. 2012). When the SPT hammer impacts the drill rod, it creates a compression stress wave that travels dawn the rod and at the same time propagates in the hammer (Abou-mater et al., 1997). Terzaghi, 1940 realized that the penetration resistance of the split-spoon sampler could provide useful in-site test data that might be correlated with the consistency and density of the soils encountered. He discussed with Harry Mohr and developed correlations between the number of blows, N, and a number of salient properties of soils, including the relative density of sands, consistency and unconfined compressive strength of clays, and allowable bearing pressure on sands and clays. The first published SPT correlations appeared in Terzaghi and International Journal of Civil and Structural Engineering 546

3 Peck, These were soon followed by correlations relating SPT blow counts to consistency for silts and clays and relative density for sands in (Peck et al., 1953). A less recognized problem is the influence of strata thickness and changes in stiffness, sketched in Figure 1. As the sample barrel approaches an appreciably stiffer horizon, the penetration resistance will increase, even though the sampled material remains more or less constant throughout the softer horizon. This can lead to overestimates of strength, density, and compressibility based solely on blow-count values. A big disadvantage of the SPT procedure is that it reports the average blows per foot during any given sample round, so the measurement would only be valid for horizons.12 in. (30.5 cm) thick, plus the influence area beneath the sampler shoe sketched in Figure 1. This is the zone of influence in front of the sampler shoe. This influence zone usually equal to four to seven times the sample barrel diameter. Overall, the SPT procedure tends to average the penetration resistance of the materials sampled, and the material effects of low strength horizons less than in. Soil behavior is greatly influenced by sampling disturbance. Nevertheless, disturbed samples may be adequate for indices tests such as Atterberg limits and grain size distribution, but slightly more suspect density and water content because of densification (Rogers, 2006). Figure 1: The SPT sample barrel approaches stiff contacts. (Rogers, 2006) From Figure 1, it can be found that stiff material has increasing resistance to penetration, and the blow counts will increase markedly, even though sampling in soft material (Rogers, 2006). This study was undertaken in order to quantify the variation of the values of different properties of silty clay with sand soil using corrected SPT number (N"). Field and experimental tests were conducted for this purpose. Empirical equations to identify the shear strength parameters of silty clay with sand soil using corrected SPT number (N") have been proposed. International Journal of Civil and Structural Engineering 547

4 2. Field and experimental work The site of this work is, Tabarjal - Al-Jouf, KSA. The field work consisted of drilling and sampling of more than 100 boreholes to depths between 10 m to 15 m below ground surface. A standard penetration test (SPT) was carried out according to ASTMD A suit of in situ testing and sampling within the boreholes was planned including standard penetration test SPT. Many specimens with different field SPT numbers (N f ) have been taken out. To carry out the various laboratory tests, the specimens were carefully transported according to standard classification and techniques. Atterberg limits including liquid limit (LL), plastic limits (PL) and plasticity index (PI) were determined according ASTMD To determine the shear strength parameters (c &φ), a direct shear box test was used. 2.1 Description and properties of soil The borehole logs mainly consisted of surface layer of medium dense and dry poorly graded sand with silt, then a layer of very dense and dry poorly graded sand. A thick layer of hard and dry silty clay with sand has appeared below the previous two layers and continued to the end of borehole logs. This study concerns with the layer of hard and dry silty clay with sand which lies at depth of 3.0m below ground surface. The specimens were taken at different depths of mentioned layer. The Field and laboratory results such as SPT, liquid limit (LL), plastic limit (PL), plasticity index (PI) and shear strength parameters (c & φ) were determined. 2.2 Correction factors for SPT-N-values There are numerous factors other than hammer type that are permitted by ASTM D and that affect the N value. Correction factors have been proposed by various authors to account for factors such as the drill stem length and type, the type of anvil, the blow rate, the use of liners or bore hole fluid and the type of hammer. The standard blow count N' can be computed from the field measured N f from the following general equation (excluding the overburden corrections) (Aggour and Radding, 2001): N' = N f n 1 n 2 n 3 n 4 n 5 n 6 (1) Where n 1 = energy correction factor n 2 = rod length correction factor n 3 = liner correction factor n 4 = borehole diameter correction factor n 5 = anvil correction factor n 6 = blow count frequency correction factor When the length of the drill rod is less than 10ft, a considerable amount of energy is reflected back in the rod reducing the energy available for driving the sampling tube into the ground, thus it is recommended that the N f values should be corrected for short lengths of rods. The correction factor for length is n 2. The barrel is often used without liners. In this case there is less friction developed inside the sampling tube, which in turn reduces the measured N values. It has been shown that the use of the ASTM sampler without the liner leads to 10% to 30% lower N f values. It is thus recommended that the measured N f values should be corrected for the use of the liner. The correction factor is n 3. SPT N f values are corrected if they are made in boreholes have diameter larger than 4.5 inches. When boreholes are larger than 4.5 inches, measured N f values are lower than they would be for a smaller diameter hole. The correction factor is n 4. When the hammer falls during the SPT testing, it strikes an anvil attached to the International Journal of Civil and Structural Engineering 548

5 drill rod stem. The anvil can vary in shape, size and weight. The amount of energy transferred to the drill rods depends on the weight of the anvil. The correction factor is n 5. Another correction n 6 is for blow count frequency that applies for soils including sands below the water table (Aggour and Radding, 2001). From tables presented by (Aggour and Radding, 2001), and according to field conditions and soil type, average values of correction factors can be taken as (n 1 = 1.67, n 2 = 1.0, n 3 = 1.0, n 4 = 1.0, n 5 = 0.7, n 6 = 1.0). Due to overburden, the N' value of SPT for soils including cohesionless soil shall be corrected for overburden as per Figure 2 and equation (2) (Narian, 1982). N" = n 7 N' (2) Where n 7 is the overburden correction factor and can be obtained from Figure 2. The properties for random twenty one chosen specimens were tabulated as shown in Table Scope of study Figure 2: Correction due to overburden (Narian, 1982) To study the reliability of using SPT test in predicting properties of silty clay with sand soil, many boreholes and SPT at different depths were carried out. Many samples were obtained to determine experimentally the properties of soil and the results are tabulated as shown in Table 1. There are different parameters were taken into consideration such as depth of sample (D) below ground surface, SPT number (N), liquid limit (LL), Plastic limit (PL), plasticity index (PI) and shear strength parameters (c & φ). In this study, the relationships between corrected SPT number (N") and all previous parameters have been discussed to make sure that which of these parameters affect the SPT. International Journal of Civil and Structural Engineering 549

6 Sample No. Table 1: Field and laboratory results D Depth below (G.S.) (m) SPT N f Corrected SPT N" LL (%) PL (%) Analysis of results and discussions In the following sections, the results obtained from field and laboratory tests are presented and discussed in detail. Considering the correlation coefficient (R 2 ), the best fitting between the results has been plotted. The purpose of use of this statistical method is to give us a statistic known as the correlation coefficient which is a summary value of a large set of data representing the degree of linear association between two measured variables. R 2 is a statistic that will give some information about the goodness of fit of a relationship. In regression, the R 2 coefficient of determination is a statistical measure of how well the regression line approximates the real data points (Taylor, 1990). According to the values of R 2, the relationship between any two parameters can be classified as (R 2 <0.30) are considered to have no correlation, (R 2 of 0.30 to 0.499) are considered to be a mild relationship, (R 2 of 0.50 to 0.699) are considered to be a moderate relationship and, (R 2 of 0.70 to 1.0) are considered to be a strong relationship. Many figures have been plotted to analyze and illustrate the relationships between field and experimental data including corrected SPT number (N"), depth of sample (D) measured from ground surface, Atterberg limits (LL), (PL), (PI), and shear strength parameters (c & φ). Three scopes of study to obtain the effect and relationship between these mentioned parameters have been performed as follows: PI (%) International Journal of Civil and Structural Engineering 550

7 3.1 Effect of soil depth (D) below ground surface Figures 3 to 6 show the relationships between depth of sample (D) below ground surface and different studied parameters including corrected SPT number (N"), Atterberg limits of silty clay with sand LL, PL and PI, respectively. Figure 3 shows the relationships between depth of soil below ground surface (D) and corrected SPT number (N"). It can be noticed that corrected SPT number (N") increase as depth of sample below ground surface increases and this is due to the effect of overburden pressure which increases the relative density of soil layer lies deeper below ground surface than others. Considering value of correlation coefficient (R 2 = 0.30), it is considered to be a mild relationship as shown in Figure 3. On the other hand, Figures 4 to 6 show the relationships between depth of soil (D) and the Atterberg limits, (LL), (PL) and (PI), respectively. It is clearly seen from these Figures that depth of sample (D) has no effect on the Atterberg limits of soil because these properties depend on the physical and mechanical properties of soil particles and are carried out on disturbed samples. Also, considering the values of correlation coefficient (R 2 <0.30), this means that these relationships are considered to have no correlation and behavior is explained by chance (see Figures 4, 5 and 6). This leads to conclude that the Atterberg limits do not depend on soil depth but depend on physical and mechanical properties of soil particles itself. Figure 3: Depth of Sample below ground surface versus corrected SPT number (N") Figure 4: Depth of Sample below ground surface versus liquid limit (LL) International Journal of Civil and Structural Engineering 551

8 Figure 5: Depth of Sample below ground surface versus plastic limit (PL) Figure 6: Depth of Sample below ground surface versus plasticity index (PI) 3.2 SPT versus Atterberg limits LL, PL and PI Figures 7, 8 and 9 show the relationships between corrected SPT number (N") and Atterberg limits LL, PL and PI, respectively. From these Figures, it can be noticed that the Atterberg limits have no effect on SPT number (N") because SPT depends on the relative density of soil layer. According to the procedures of SPT, it can be found that the test has carried out in the field on soil as it is (undisturbed soil). Whereas, and according to the procedures of these laboratory tests, the Atterberg limits of soil depend mainly on the physical and mechanical properties of soil particles and are carried out on disturbed samples as mentioned in previous section. Also, considering the values of correlation coefficient (R 2 <0.30), this means that these relationships are considered to have no correlation and behavior is explained by chance. International Journal of Civil and Structural Engineering 552

9 Figure 7: Corrected SPT number (N") versus liquid limit (LL) Figure 8: Corrected SPT number (N") versus plastic limit (PL) Figure 9: Corrected SPT number (N") versus plasticity index (PI) 3.3 SPT versus shear strength parameters (c & φ) Figures 10 and 11 show the relationships between corrected SPT number (N") and shear strength parameters of silty clay with sand soil, angle of internal friction (ϕ) and cohesion (c), respectively. From these figures, it can be noticed that corrected SPT number (N") increases International Journal of Civil and Structural Engineering 553

10 as shear strength of soil increases. The increasing of shear strength parameters of soil is due to increasing in relative density which leads to increasing in corrected SPT number (N"). This may be due to the insertion of fine soil particles in the voids between sand particles to make the specimens are less voids and the friction surface increases too. Also, This means that the surface of shear behaves as a general shear failure and leads to shear strength increases to make SPT number increases too. From figure 10, a high correlation coefficient (R 2 = 0.851) can be an indication of a good correlation between corrected SPT number (N") and angle of internal friction (ϕ). Also, in Figure 11, a correlation coefficient (R 2 = 0.871) of this figure can be an indication of a good correlation between corrected SPT number (N") and cohesion (c). This means that the relationships between SPT number (N") and shear strength parameters (c & ϕ) are considered to a strong relationships. This leads to conclude that the shear strength parameters (c & ϕ) have strongly effected on SPT number. Figure 10: Corrected SPT number (N") versus angle of internal friction (φ) Figure 11: Corrected SPT number (N") versus cohesion (c) Also, from the previous analysis, it can be found that the SPT number (N") has strongly correlated with shear strength of soil which can be expressed in parameters (c & ϕ). So, the shear strength parameters (c & ϕ) can be reasonably predicted from field with the help of corrected SPT number (N"). International Journal of Civil and Structural Engineering 554

11 3.4 Empirical equations to predict the shear strength parameters (c &ϕ) From the results and relationships shown in Figures 10 and 11 and regression analysis, empirical equations to estimate the shear strength parameters (c & ϕ) for silty clay with sand soil with the help of corrected SPT number (N") as follow: φ = N" (3) c = N" 0.18 (4) Where, (N") is the corrected SPT number, ϕ is the angle of internal friction in (degree) and c is the cohesion in (Kg/cm 2 ). The equations (3) and (4) have correlation coefficients (R 2 ) equal to 0.851and 0.871, respectively, are considered to be strong relationships. This means that a very good correlation between SPT number (N") and shear strength parameters (c & ϕ) of silty clay with sand soil. 4. Conclusions The following results are concluded based on the results and on the discussion and analysis presented in this research: 1. The depth of soil (D) below ground surface significantly affect the SPT number, so that it should be corrected before used. 2. The Atterberg limits LL, PL, and PI do not affect the SPT number. 3. The shear strength of soil strongly affects the SPT number. 4. For identification of shear strength parameters (c & ϕ) of silty clay with sand soil using SPT is adequate rather than using laboratory tests because SPT carries out in the field on undisturbed soil. 5. Empirical equations to predict the shear strength parameters (c & ϕ) of silty clay with sand soil using SPT have been presented as mentioned in equations (3) and (4). The accuracy of these equations proved to be strong relationships. 6. The standard penetration test (SPT) is considered reliable in predicting of shear strength parameters (c & ϕ) of silty clay with sand soil. 5. References 1. Abou-mater, H., and Goble, G.G., (1997), SPT dynamic analysis and measurements, Journal of Geotechnical and Geoenvironmental Engineering, October, pp Aggour, M.S., and Radding, W.R., (2001), Standard penetration test (SPT) correction, Research report, Civil and Environmental Engineering Department, University of Maryland College Park, Maryland, International Journal of Civil and Structural Engineering 555

12 3. Garham, J., (2012), TR4 - plasticity index proficiency 2011, CETANZ, Civil Engineering Testing Association of New Zealand. 4. Humyral, T., Awall, M. R., Mofiz, S. A., and Sobhan M. A., (2012), Preparation of SPT contour Map of Rajshahi City Area and its application to foundation design, International Journal of Civil & Environmental Engineering IJCEE-IJENS 12(2), pp Narian, J., (1982), Method for standard penetration test for soil, Bureau of Indian standard, 3 rd edition, New Delhi Peck, R. B., Hanson, W.E., and Thornburn, T. H., (1953), Foundation Engineering, Joun Wiley, & Sons, New York. 7. Rogers, J. D., (2006), Subsurface exploration using the standard penetration test and the cone penetrometer test, The Geological Society of America, Environmental & Engineering Geoscience, XII (2), pp Silva, S. D., Wightman, E. N. R. and Kamruzzaman, M., (2010), Geotechnical ground investigation for the Padma main bridge, IABCE-JSCE Joint Conference on Advances in Bridge Engineering-II, Dhaka, Bangladesh, Amin, Okui, Bhuiyan (eds.), pp Siswosoebrotho, B.I., Widodo, P. and Augusta, E., (2011), The influence of fines content and plasticity on the strength and permeability of aggregate for base course material, Proceedings of the Eastern Asia Society for Transportation Studies, (5), pp Subjianto, A.T., Suryolelono, K.B., Rifa'i, A., and Mochtar, I.B., (2011), The effect of variation index plasticity and activity in swelling vertical of expansive soil, International Journal of Engineering & Technology IJET-IJENS, 11(06), pp Taylor, R., (1990), Interpretation of the correlation coefficient, A basic review, Journal of Diagnostic Medical Sonography, JDMS, 6, pp Terzaghi, K., (1940), Sampling, testing and averaging, In Proceedings of the Purdue Conference on Soil Mechanics and its Applications, September 2-6, Section III, pp Tuker, M. R., (1999), Soil fertility note 13 - Clay minerals: Their importance and function in soils, Soil Testing Section of the NCDA and CS Agronomic Division. 14. Venkatasubramanian. C. and Dhinakaran.G., (2011), ANN model for predicting CBR from index properties of soils, International Journal of Civil and Structural Engineering, 2(2), pp International Journal of Civil and Structural Engineering 556