Reliability of using standard penetration test (SPT) in ... · PDF fileReliability of using standard penetration test (SPT) in predicting properties of silty clay with sand soil Mostafa

INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING

Volume 3, No 3, 2013

© Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0

Research article ISSN 0976 – 4399

Received on January 2013 Published on March 2013 545

Reliability of using standard penetration test (SPT) in predicting

properties of silty clay with sand soil Mostafa Abdou Abdel Naiem Mahmoud

Associate Professor, Civil Engineering Department, Assiut University, Assiut 71516, Egypt

Civil Eng .Department, Faculty of Engineering, Aljouf University, KSA

[email protected]

doi:10.6088/ijcser.201203013050

ABSTRACT

The properties of soil play an important role in many practices for geotechnical engineering.

To determine the real values of these properties special techniques should be followed such

as undisturbed samples and initial overburden pressures should be taken into consideration.

Actually, it is difficult to obtain 100 % undisturbed samples due to handling, transportation,

release of overburden pressure and poor laboratory conditions. So, prediction of some

properties such as shear strength parameters (c & ϕ) for hard and dry silty clay with sand soil

with the help of standard penetration test (SPT) provides a good opportunity to obtain these

parameters without using of more laboratory tests. Standard penetration tests (SPT), rough

measure the strength of soil. The great merit of this test and the main reason for its

widespread use is that it is simple and inexpensive. The shear strength parameters which can

be inferred are approximate, but may give a useful guide in ground conditions where it may

not be possible to obtain borehole samples of adequate quality like clay containing sand or

gravel. This study was undertaken in order to study the reliability of using standard

penetration test (SPT) in predicting some properties, (such as Atterberg limits LL, PL, PI, and

shear strength parameters (c & φ)), of silty clay with sand soil. Many samples from this soil

have been investigated for this purpose. The results of the research indicated that the shear

strength of soil affects SPT number. Empirical equations to identify the shear strength

parameters of silty clay with sand soil using corrected SPT number (N") have been proposed.

Keyword: Silty clay with sand, (c & φ), LL, PL, PI, SPT, correction coefficient, field tests.

1. Introduction

Numerous methods have been developed for determination the soil properties. These methods

include the field and laboratory tests. Also, to determine the real values of these properties,

special techniques should be followed such as undisturbed samples and initial overburden

pressures should be taken into consideration. On the other hand, field test such as SPT does

not depend on undisturbed sample because it is carried out in original field soil. Most

problems in soils and construction involve either the strength of the in-situ soil or the

compressibility of the soil mass. People tend to have a negative opinion of clay soils. These

soils may not drain quickly after heavy rain and often harden when dry. Most soils contain a

mixture of clay, organic matter, sand and silt. If a soil contains at least 40 percent clay, then

we classify it as a "clay soil" (Tuker, 1999). Clay is made up of tiny particles less than 0.002

mm in diameter. By comparison, silt particles range from 0.002 to 0.06 mm in size and sand

particles from 0.06 to 2.0 mm. For this reason, clay soils are considered to be fine textured;

silt, medium textured; and sand coarse textured. Clay acts as a binding agent between soil

particles. It gives soil elasticity and provides cohesion of soil particles. On the other hand,

sand provides friction of soil particles. Properties of silty clay with sand soil are significantly

Reliability of using standard penetration test (SPT) in predicting properties of silty clay with sand soil

Mostafa Abdou Abdel Naiem Mahmoud

International Journal of Civil and Structural Engineering 546

Volume 3 Issue 3 2013

associated with stiffness of this soil and give an indication of shear strength of soil. These

properties plays an important role in many practices for geotechnical engineering, such as soil

description or classification of soil, prediction of behavior of soil if it will be subjected to

extensive settlement or swelling. The real value of these properties such as angle of internal

friction (φ) and cohesion (c) require a special care and laboratory technique. So, prediction of

soil properties with the help of field tests such as standard penetration tests (SPT) provides a

good opportunity to obtain this parameter without using of more laboratory tests. Standard

penetration tests (SPT), rough measure the strength of soil. It is currently the most popular

and economical means of obtaining subsurface information. The SPT has been extended to

other areas of soil mechanics and foundations, and it has become a fundamental parameter in

geotechnical engineering. The great merit of the test and the main reason for its widespread

use is that it is simple and inexpensive. In conditions where the quality of the undisturbed

sample is suspect, e.g. silty clay with sand soil, it is often advantageous to alternate the

sampling with standard penetration tests to check the strength.

Standard penetration test (SPT) is one of the field tests of soil. It primary gives idea about

shear strength of soil which it can be expressed in terms of shear strength parameters (c & φ).

Most laboratory tests must be performed on molded samples of soils. These tests are

laborious and time consuming; but sometimes the results are not accurate due to the poor

laboratory conditions (Venkatasubramanian and Dhinakaran, 2011). An aggregate that

contains sufficient fines to fill all voids between aggregate grains will still gain its strength

from grain-to-grain contact but has increased shear resistance (Siswosoebrotho et al., 2005).

Plasticity index (PI) and liquid limit (LL) can be used to determine the swelling

characteristics of expansive clay in general (Subjianto et al., 2011). (Garham, 2012) indicates

that it is a poor ability to get similar results when testing different portions of the same

samples, but the result differences should be below the upper control limit. Sometimes,

Strength values determined from laboratory testing of intact soil cores are recognized as not

being directly applicable to the in-situ soil mass because of the scale effect.

The standard penetration test (SPT) is very widely used for subsurface investigation in many

parts of the world. It measures the resistance of a hollow core being hammered with a 63.5kg

weight. The hollow core receives a soil sample during the process which is logged to soil

identification purposes. The relative density can be estimated from the number of blows used

to drive the SPT a total of distance of 450mm with the total blows for the last 300mm being

the SPT Nf -value (Silva, 2010). To construct a structure, Nf value (SPT value) is an important

parameter to understand the soil condition at different depths. After investigating Nf value of

different depths, foundation depth can be selected according to load of the structure. Then

determine the corrected N" value at the required depth. Angle of internal friction is

determined from corrected SPT N"-value. After that, determine bearing capacity factors to

calculate the bearing capacity of the soil. From this bearing capacity of soil foundation can be

designed (Humyral et al. 2012). When the SPT hammer impacts the drill rod, it creates a

compression stress wave that travels dawn the rod and at the same time propagates in the

hammer (Abou-mater et al., 1997).

Terzaghi, 1940 realized that the penetration resistance of the split-spoon sampler could

provide useful in-site test data that might be correlated with the consistency and density of

the soils encountered. He discussed with Harry Mohr and developed correlations between the

number of blows, N, and a number of salient properties of soils, including the relative density

of sands, consistency and unconfined compressive strength of clays, and allowable bearing

pressure on sands and clays. The first published SPT correlations appeared in Terzaghi and





Peck, 1948. These were soon followed by correlations relating SPT blow counts to

consistency for silts and clays and relative density for sands in (Peck et al., 1953). A less

recognized problem is the influence of strata thickness and changes in stiffness, sketched in

Figure 1. As the sample barrel approaches an appreciably stiffer horizon, the penetration

resistance will increase, even though the sampled material remains more or less constant

throughout the softer horizon. This can lead to overestimates of strength, density, and

compressibility based solely on blow-count values.

A big disadvantage of the SPT procedure is that it reports the average blows per foot during

any given sample round, so the measurement would only be valid for horizons .12 in. (30.5

cm) thick, plus the influence area beneath the sampler shoe sketched in Figure 1. This is the

zone of influence in front of the sampler shoe. This influence zone usually equal to four to

seven times the sample barrel diameter. Overall, the SPT procedure tends to ‘‘average’’ the

penetration resistance of the materials sampled, and the material effects of low strength

horizons less than 20–26 in. Soil behavior is greatly influenced by sampling disturbance.

Nevertheless, disturbed samples may be adequate for indices tests such as Atterberg limits

and grain size distribution, but slightly more suspect density and water content because of

densification (Rogers, 2006).

Figure 1: The SPT sample barrel approaches stiff contacts. (Rogers, 2006)

From Figure 1, it can be found that stiff material has increasing resistance to penetration, and

the blow counts will increase markedly, even though sampling in soft material (Rogers, 2006).

This study was undertaken in order to quantify the variation of the values of different

properties of silty clay with sand soil using corrected SPT number (N"). Field and

experimental tests were conducted for this purpose. Empirical equations to identify the shear

strength parameters of silty clay with sand soil using corrected SPT number (N") have been

proposed.





2. Field and experimental work

The site of this work is, Tabarjal - Al-Jouf, KSA. The field work consisted of drilling and

sampling of more than 100 boreholes to depths between 10 m to 15 m below ground surface.

A standard penetration test (SPT) was carried out according to ASTMD – 1586 – 84. A suit of

in situ testing and sampling within the boreholes was planned including standard penetration

test SPT. Many specimens with different field SPT numbers (Nf) have been taken out. To

carry out the various laboratory tests, the specimens were carefully transported according to

standard classification and techniques. Atterberg limits including liquid limit (LL), plastic

limits (PL) and plasticity index (PI) were determined according ASTMD – 4318 – 84. To

determine the shear strength parameters (c &φ), a direct shear box test was used.

2.1 Description and properties of soil

The borehole logs mainly consisted of surface layer of medium dense and dry poorly graded

sand with silt, then a layer of very dense and dry poorly graded sand. A thick layer of hard

and dry silty clay with sand has appeared below the previous two layers and continued to the

end of borehole logs. This study concerns with the layer of hard and dry silty clay with sand

which lies at depth of 3.0m below ground surface. The specimens were taken at different

depths of mentioned layer. The Field and laboratory results such as SPT, liquid limit (LL),

plastic limit (PL), plasticity index (PI) and shear strength parameters (c & φ) were determined.

2.2 Correction factors for SPT-N-values

There are numerous factors other than hammer type that are permitted by ASTM D 1586-99

and that affect the N value. Correction factors have been proposed by various authors to

account for factors such as the drill stem length and type, the type of anvil, the blow rate, the

use of liners or bore hole fluid and the type of hammer. The standard blow count N' can be

computed from the field measured Nf from the following general equation (excluding the

overburden corrections) (Aggour and Radding, 2001):

N' = Nf n1 n2 n3 n4 n5 n6 (1)

Where n1 = energy correction factor

n2 = rod length correction factor

n3 = liner correction factor

n4 = borehole diameter correction factor

n5 = anvil correction factor

n6 = blow count frequency correction factor

When the length of the drill rod is less than 10ft, a considerable amount of energy is reflected

back in the rod reducing the energy available for driving the sampling tube into the ground,

thus it is recommended that the Nf values should be corrected for short lengths of rods. The

correction factor for length is n2. The barrel is often used without liners. In this case there is

less friction developed inside the sampling tube, which in turn reduces the measured N values.

It has been shown that the use of the ASTM sampler without the liner leads to 10% to 30%

lower Nf values. It is thus recommended that the measured Nf values should be corrected for

the use of the liner. The correction factor is n3. SPT Nf values are corrected if they are made

in boreholes have diameter larger than 4.5 inches. When boreholes are larger than 4.5 inches,

measured Nf values are lower than they would be for a smaller diameter hole. The correction

factor is n4. When the hammer falls during the SPT testing, it strikes an anvil attached to the





drill rod stem. The anvil can vary in shape, size and weight. The amount of energy transferred

to the drill rods depends on the weight of the anvil. The correction factor is n5. Another

correction n6 is for blow count frequency that applies for soils including sands below the

water table (Aggour and Radding, 2001). From tables presented by (Aggour and Radding,

2001), and according to field conditions and soil type, average values of correction factors

can be taken as (n1 = 1.67, n2 = 1.0, n3 = 1.0, n4 = 1.0, n5 = 0.7, n6 = 1.0).

Due to overburden, the N' value of SPT for soils including cohesionless soil shall be corrected

for overburden as per Figure 2 and equation (2) (Narian, 1982).

N" = n7 N' (2)

Where n7 is the overburden correction factor and can be obtained from Figure 2.

The properties for random twenty one chosen specimens were tabulated as shown in Table 1.

Figure 2: Correction due to overburden (Narian, 1982)

2.3 Scope of study

To study the reliability of using SPT test in predicting properties of silty clay with sand soil,

many boreholes and SPT at different depths were carried out. Many samples were obtained to

determine experimentally the properties of soil and the results are tabulated as shown in

Table 1. There are different parameters were taken into consideration such as depth of sample

(D) below ground surface, SPT number (N), liquid limit (LL), Plastic limit (PL), plasticity

index (PI) and shear strength parameters (c & φ) . In this study, the relationships between

corrected SPT number (N") and all previous parameters have been discussed to make sure

that which of these parameters affect the SPT.





Table 1: Field and laboratory results

Sample

No.

D Depth below

(G.S.) (m)

SPT

Nf

Corrected SPT N"

LL

(%)

PL

(%)

PI

(%)

1 4.5 37 44 27 21 6

2 3 44 57 25 21 4

3 3 40 51 23 19 4

4 4.5 34 41 27 21 6

5 3 34 44 29 25 4

6 3 38 49 23 19 4

7 5 46 53 23 19 4

8 7.5 77 77 25 20 5

9 7.5 53 53 24 19 5

10 10.5 69 60 25 20 5

11 3 51 61 24 19 5

12 6 52 57 24 19 5

13 5 51 58 24 19 5

14 4.5 46 55 23 18 5

15 7.5 77 77 25 20 5

16 7.5 53 53 24 19 5

17 10.5 69 60 25 20 5

18 3 51 66 24 19 5

19 6 52 57 24 19 5

20 4.5 64 76 24 19 5

21 4.5 51 61 24 19 5

3. Analysis of results and discussions

In the following sections, the results obtained from field and laboratory tests are presented

and discussed in detail. Considering the correlation coefficient (R2), the best fitting between

the results has been plotted. The purpose of use of this statistical method is to give us a

statistic known as the correlation coefficient which is a summary value of a large set of data

representing the degree of linear association between two measured variables. R2 is a statistic

that will give some information about the goodness of fit of a relationship. In regression, the

R2 coefficient of determination is a statistical measure of how well the regression line

approximates the real data points (Taylor, 1990). According to the values of R2, the

relationship between any two parameters can be classified as (R2 <0.30) are considered to

have no correlation, (R2 of 0.30 to 0.499) are considered to be a mild relationship, (R

2 of 0.50

to 0.699) are considered to be a moderate relationship and, (R2 of 0.70 to 1.0) are considered

to be a strong relationship.

Many figures have been plotted to analyze and illustrate the relationships between field and

experimental data including corrected SPT number (N"), depth of sample (D) measured from

ground surface, Atterberg limits (LL), (PL), (PI), and shear strength parameters (c & φ).

Three scopes of study to obtain the effect and relationship between these mentioned

parameters have been performed as follows:





3.1 Effect of soil depth (D) below ground surface

Figures 3 to 6 show the relationships between depth of sample (D) below ground surface and

different studied parameters including corrected SPT number (N"), Atterberg limits of silty

clay with sand LL, PL and PI, respectively. Figure 3 shows the relationships between depth of

soil below ground surface (D) and corrected SPT number (N"). It can be noticed that

corrected SPT number (N") increase as depth of sample below ground surface increases and

this is due to the effect of overburden pressure which increases the relative density of soil

layer lies deeper below ground surface than others. Considering value of correlation

coefficient (R2 = 0.30), it is considered to be a mild relationship as shown in Figure 3. On the

other hand, Figures 4 to 6 show the relationships between depth of soil (D) and the Atterberg

limits, (LL), (PL) and (PI), respectively. It is clearly seen from these Figures that depth of

sample (D) has no effect on the Atterberg limits of soil because these properties depend on

the physical and mechanical properties of soil particles and are carried out on disturbed

samples. Also, considering the values of correlation coefficient (R2 <0.30), this means that

these relationships are considered to have no correlation and behavior is explained by chance

(see Figures 4, 5 and 6). This leads to conclude that the Atterberg limits do not depend on soil

depth but depend on physical and mechanical properties of soil particles itself.

Figure 3: Depth of Sample below ground surface versus corrected SPT number (N")

Figure 4: Depth of Sample below ground surface versus liquid limit (LL)





Figure 5: Depth of Sample below ground surface versus plastic limit (PL)

Figure 6: Depth of Sample below ground surface versus plasticity index (PI)

3.2 SPT versus Atterberg limits LL, PL and PI

Figures 7, 8 and 9 show the relationships between corrected SPT number (N") and Atterberg

limits LL, PL and PI, respectively. From these Figures, it can be noticed that the Atterberg

limits have no effect on SPT number (N") because SPT depends on the relative density of soil

layer. According to the procedures of SPT, it can be found that the test has carried out in the

field on soil as it is (undisturbed soil). Whereas, and according to the procedures of these

laboratory tests, the Atterberg limits of soil depend mainly on the physical and mechanical

properties of soil particles and are carried out on disturbed samples as mentioned in previous

section. Also, considering the values of correlation coefficient (R2 <0.30), this means that

these relationships are considered to have no correlation and behavior is explained by chance.





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





as shear strength of soil increases. The increasing of shear strength parameters of soil is due

to increasing in relative density which leads to increasing in corrected SPT number (N"). This

may be due to the insertion of fine soil particles in the voids between sand particles to make

the specimens are less voids and the friction surface increases too. Also, This means that the

surface of shear behaves as a general shear failure and leads to shear strength increases to

make SPT number increases too.

From figure 10, a high correlation coefficient (R2 = 0.851) can be an indication of a good

correlation between corrected SPT number (N") and angle of internal friction (ϕ). Also, in

Figure 11, a correlation coefficient (R2 = 0.871) of this figure can be an indication of a good

correlation between corrected SPT number (N") and cohesion (c). This means that the

relationships between SPT number (N") and shear strength parameters (c & ϕ) are considered

to a strong relationships. This leads to conclude that the shear strength parameters (c & ϕ)

have strongly effected on SPT number.

Figure 10: Corrected SPT number (N") versus angle of internal friction (φ)

Figure 11: Corrected SPT number (N") versus cohesion (c)

Also, from the previous analysis, it can be found that the SPT number (N") has strongly

correlated with shear strength of soil which can be expressed in parameters (c & ϕ). So, the

shear strength parameters (c & ϕ) can be reasonably predicted from field with the help of

corrected SPT number (N").





3.4 Empirical equations to predict the shear strength parameters (c &ϕϕϕϕ)

From the results and relationships shown in Figures 10 and 11 and regression analysis,

empirical equations to estimate the shear strength parameters (c & ϕ) for silty clay with sand

soil with the help of corrected SPT number (N") as follow:

φ = 0.209 N" + 19.68 (3)

c = 0.014 N" – 0.18 (4)

Where, (N") is the corrected SPT number,

ϕ is the angle of internal friction in (degree) and c is the cohesion in (Kg/cm2).

The equations (3) and (4) have correlation coefficients (R2) equal to 0.851and 0.871,

respectively, are considered to be strong relationships. This means that a very good

correlation between SPT number (N") and shear strength parameters (c & ϕ) of silty clay with

sand soil.

4. Conclusions

The following results are concluded based on the results and on the discussion and analysis

presented in this research:

1. The depth of soil (D) below ground surface significantly affect the SPT number, so

that it should be corrected before used.

2. The Atterberg limits LL, PL, and PI do not affect the SPT number.

3. The shear strength of soil strongly affects the SPT number.

4. For identification of shear strength parameters (c & ϕ) of silty clay with sand soil

using SPT is adequate rather than using laboratory tests because SPT carries out in the

field on undisturbed soil.

5. Empirical equations to predict the shear strength parameters (c & ϕ) of silty clay with

sand soil using SPT have been presented as mentioned in equations (3) and (4). The

accuracy of these equations proved to be strong relationships.

6. The standard penetration test (SPT) is considered reliable in predicting of shear

strength parameters (c & ϕ) of silty clay with sand soil.

5. References

1. Abou-mater, H., and Goble, G.G., (1997), SPT dynamic analysis and measurements,

Journal of Geotechnical and Geoenvironmental Engineering, October, pp 921-928.

2. Aggour, M.S., and Radding, W.R., (2001), Standard penetration test (SPT) correction,

Research report, Civil and Environmental Engineering Department, University of

Maryland College Park, Maryland, 20742.





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

11-16.

5. Narian, J., (1982), Method for standard penetration test for soil, Bureau of Indian

standard, 3rd

edition, New Delhi 110002.

6. 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 161-179.

8. 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 427-

436.

9. 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

845-856.

10. 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 142-148.

11. Taylor, R., (1990), Interpretation of the correlation coefficient, A basic review,

Journal of Diagnostic Medical Sonography, JDMS, 6, pp 35-39.

12. 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

151-160.

13. 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 614-620.

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Reliability of using standard penetration test (SPT) in ... · PDF fileReliability of using standard penetration test (SPT) in predicting properties of silty clay with sand soil Mostafa