On the Reliability of SPT-N value as an Indication of Consistency of Clayey Soils


Işık Yılmaz

Cumhuriyet University, Department of Geological Engineering, Sivas, Turkey


Yusuf Erzin

Cumhuriyet University, Department of Civil Engineering, Sivas, Turkey



Investigation of the shear strength properties and consistency distributions of the clayey alluviual soils of the Niksar basin located at the right bank of the Kelkit River and evaluation of the shear strength of the clays, and correlation of them with the consistency characteristics obtained from SPT-N values from Standard Penetration Tests is aimed in this study. Investigations of the study were programmed consisting Standard Penetration Tests in the field and index and shear strength tests in the laboratory. Undrained shear strength values obtained indicate the stiff consistency; classification due to the SPT blow counts in the same depths gives the same consistency properties, and the determination of the consistency of clayey soils by SPT-N value appears to be very reliable. The section constructed by using the SPT results from 24 boreholes 15 m in depth along the Niksar basin showed the widespread distribution of the clayey soils with stiff consistency.

Keywords:Alluvial soils, clay, consistency, shear strength, standard penetration test.


The area selected for this study is at the right bank of the Kelkit River in Niksar Basin in seismically active North Anatolian Fault zone (Fig. 1). The objective of this study was to determine the shear strength of the clays and correlate them with the consistency characteristics deduced from N values from Standard Penetration Tests.


Figure 1. Location map of the study area.

The investigation comprised two stages: field work and laboratory testing. Initially, geological mapping was carried out, and undisturbed soil samples were obtained from sample pits in various representative locations near to the bore holes where the SPT-N values obtained. 18 samples (3 samples from each sample pit) of were taken from 6 sample pits having the depths of 4-6 m. Based on field observations and drill logs, soil profiles and SPT-N value distributions were then constructed. Thereafter, index properties and shear strength of the clays were determined by means of laboratory testing. After the field and laboratory work, the geological and geotechnical characteristics of the study area soils were reviewed, and geotechnical parameters obtained from laboratory tests were assessed from the point of view of engineering. This study showed that the determination of the consistency of clayey soils by SPT-N value is very reliable.


The Niksar Basin has been affected by the earthquakes, and is characterised by a very large alluvial soils. In the study area, there are six units ranging in age from Upper Jurassic-Lower Cretaceous to Quaternary. They are from oldest to youngest, the Doğdu formation, the Kırandağ formation, the Kabaklik formation and Yolüstü Volcanics, debris and alluvium, respectively (Fig. 2).

Partly well-bedded, but also crushed and jointed via tectonism, the yellowish Doğdu Formation crops out little in the study area. This formation comprises limestone and is of Late Jurassic-Early Cretaceous age. The white-gray, thinly bedded Kırandağ formation occurs as an alternation of marl, sandstone and limestone. The age of this unit is Late Upper Cretaceous according to Tatar (1993). The Eocene Kabaklik formation is formed of gray unconsolidated tuff, breccia, agglomerate and dark andesitic lava. Yolüstü volcanics consist of andesite, basalt and tuff. Alluvial units crop out most extensively in the study area. They comprise gravelly, sandy, silty clay in the flood-affected areas and gravelly clay in the non-flood affected areas along the Kelkit River Valley. The alluvial units consist of stratified materials having varied grain sizes, and derived from the various geological units in the vicinity. Their continuity cannot be established laterally and vertically. Wedges and lenses therein were observed locally. A derived soil profile cross-section shows that clayey soils are widespread and sandy-silty levels can be observed.

Figure 2. Geological and documentation map of the study area.


The North Anatolian Fault (NAF) is one of many large strike-slip faults, striking at low angles to the general trend of the Alpine-Himalayan system of Eurasia and formed late in the orogenic history of the segments they cut (Tatar, 1993). Seismically active North Anatolian Fault Zone (NAFZ) is a right-lateral fault zone taking up the relative motion between the Black Sea and the Anatolian Block and is sub-parallel to the Black Sea coast of Anatolia, running some 1200 km from Karlıova in the east to the Gulf of Saros in the west (McKenzie, 1972; Şengör, 1977; Şengör and Kidd, 1979).

Niksar Basin is a Z type pull-apart basin in the NAFZ (Mann et al., 1983). It has a length of about 16-20 km and a width of 8-13 km, trending parallel to the North Anatolian Fault Zone. The 1942 and 1939 earthquake faults bound the northern and southern margin of the basin (Ambraseys, 1970). And, the estimate of the total right-lateral displacement of the fault is given as 25±5 km, giving an average rate of movement of 0.4-0.5 cm/year (Barka, 1984).

The Niksar Basin is in the first seismic zone according to the seismic zoning map of Turkey. The plot of earthquake epicentres in the basin indicate that earthquakes are generally associated with known active faults existing in the area. The region is known to be sesmically active, and large earthquakes (M>7) are historically known and expected.


Specific gravity, Atterberg limits, natural water content, degree of saturation, void ratio, porosity and grain-size distributions were determined on 18 undisturbed samples collected from sample pits near to the bore holes locations.

Coarse sieve, fine sieve and hydrometer methods were used for grain size analyses of the soils. The range of grain-size distribution is given in Fig. 3; all grain-size distribution curves indicate the fine-grained nature of the soils. Grain-size analyses showed that the fine-grained soils are composed, an average, of 13% sand, 44% silt and 43% clay-size particles.

Figure 3. Grain-size distribution of the soils.

The average values of liquid limit, plastic limit and plasticity index are 50.4%, 25.5%, 24.8%, respectively, and samples are defined as “plastic soils” according to the classification of Leonards (1962). Results show a distribution above the A-line of the plasticity chart (Fig. 4). According to this distribution, 50% of the samples are identified as CL-group (inorganic clay, low plasticity) clay and 50% are CH-group (inorganic clay, high plasticity) clay, according to the Unified System of Soil Classification (USBR, 1974).

Figure 4. Distribution of the soil samples on the plasticity chart.

The specific gravity of the soils varies from 2.64 to 2.72. Void ratio changes from 66% to 82%, with an average value of 77%. While the average value of natural unit weight (density) was 1.83 g/cm3, it changes from 1.77 g/cm3 to 1.90 g/cm3 (Table 1). The average natural water content (w) of the study-area soils was found to be 21.5%, with a range extending from 19.1% to 24.5% (Table 1). Obtained natural water content values and plastic limit values are similar. Consequently, there is evidence of plastic behavior of the soils during sampling. Samples will not show the viscous flow behavior during sampling because their water contents are less than their liquid limits.

Table 1. Index and geotechnical properties of the soils.
  NU 1 NU 2 NU 3 NU 4 NU 5 NU 6
Grain Size
a. % < sieve #4 100 100 100 100 100 100
b. % < sieve #200 86 87 92 97 80 93
Atterberg Limits
a. Liquid Limit (%) 42 60 41 63 43 53
b. Plastic limit (%) 23 28 24 31 22 25
c. Plasticity Index (%) 19 32 17 32 21 28
Specific Gravity (Gs) 2.64 2.72 2.68 2.70 2.69 2.68
Natural unit weight,g (g/cm3) 1.90 1.82 1.81 1.85 1.77 1.85
Natural water content, wn (%) 19.5 24.5 21.2 23.2 19.1 21.5
Void ratio, e .66 .82 .81 .80 .81 .73
Porosity, n (%) 40 45 45 44 45 42
Degree of saturation, Sr (%) 78 81 70 79 63 79
Cohesion, cu (kg/cm2) 1.10 1.90 1.25 1.70 1.30 1.80
Internal friction angle, () 8.5 5.5 7.4 6.0 6.5 6.0
Soil group CL CH CL CH CL CH
All values given above are averages of 3 samples from each sample pit.


Undrained triaxial compression tests were carried out on 18 undisturbed samples to determine the shear strength properties of the soils. Prepared samples were placed in the triaxial cell and tested (ASTM D-2850, 1987). In this test, until the specimen fails; the cell pressure was then raised and the axial loading continued until a new peak deviator stress is obtained. In this way 3 number of Mohr circles were obtained enabling a shear strength envelope to be drawn.

Figure 5 and Table 1 shows the variation in the shear strength for soils. The slope of the Mohr-Coulomb failure envelopes seems to vary in a narrow range from 5.5° to 8.5°. This internal friction angle is a result of a little quantity of sand size material mixed with clay. The respective cohesion values for CL and CH group soils are 1.22 and 1.80 kgf/cm2. Soils were evaluated as stiff clayey soils according to their shear strength (Anon, 1979).

Figure 5. Typical shear strength test results.


Determination of the clay shear strength on the basis of penetration resistance can be very unreliable. The SPT- N values do give useful preliminary indications of consistency for clay, and the information is in some cases sufficient for final design (Al-Khafaji and Andersland, 1992). Obtained blow counts from the bore holes near to the sample pits varied from 15 to 50 (Fig. 6). These blow counts indicates the consistency classes of “stiff” and “hard” according to the classification proposed by Terzaghi and Peck (1948) (Terzaghi and Peck, 1967). This consistency classes obtained from SPT- N values is largely supported by the stiff characteristics obtained from the shear strength tests. The distribution of the consistency in the Niksar basin had been shown in a cross section constructed along the basin up to 15 m in depth (Fig. 7). This distribution indicates that SPT- N values are generally 15-50 which shows the “stiff” clays. Besides, it can be seen that the clayey levels are very spread from the cross section along the basin (Yılmaz, 1999) (Fig. 8).

Figure 6. Typical graphs of SPT results in bore holes near the sample pits.

Figure 7. Consistency distribution profile along the basin.

Figure 8. Soil profile of the basin (Yılmaz, 1999).


Undrained shear strength tests showed that the internal friction angle vary in a narrow range from 5.5° to 8.5°. The respective cohesion values for CL and CH group soils are 1.22 and 1.80 kgf/cm2. Soils were evaluated as “stiff” clayey soils according to their shear strength (Anon, 1979).

Blow counts from the bore holes near to the sample pits varied from 15 to 50, and these blow counts indicated the consistency classes of “stiff” and “hard” according to the classification proposed by Terzaghi and Peck (1948) (Terzaghi and Peck, 1967). Constructed section by using the SPT-N results from 24 boreholes 15 m in depth along the Niksar basin showed the widespread distribution of the clayey soils with stiff consistency. Consistency classes obtained from SPT- N values is largely supported by the “stiff” characteristics obtained from shear strength tests. Obtained undrained shear strength values indicates the “stiff consistency” and also classification due to the obtained blow count from SPT in the same levels gives the same consistency properties. Thus, the determination of the consistency of clayey soils by SPT-N value was found to be very reliable.


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