Standard Penetration Testing (SPT)

Introduction

A standard penetration test (SPT) is one of the most popular in-situ tests carried out worldwide. The test was developed in the United States in the 1920s (Das, 2019).

Standard Penetration Test (SPT) is a simple and low-cost testing procedure widely used in geotechnical investigation to determine the relative density and angle of shearing resistance of cohesionless soils and also the strength of stiff cohesive soils.

Driving sequence

Firstly, a borehole is extended to a predetermined depth. The drill tools are removed, and the sampler is lowered to the bottom of the hole. The sampler is driven into the soil by hammer blows to the top of the drill rod (see Figure 1). The standard weight of the hammer is 63.5 kg (140 lbs), and for each blow, the hammer drops a distance of 760 mm (30 inches).

The number of blows required for a spoon penetration of three consecutive 150 mm (6 inches) intervals is recorded (see Figure 2). The number of blows required to penetrate the first 150 mm is called “seating drive” and the total number of blows required to penetrate the remaining 300 mm depth is known as the “standard penetration resistance”, or otherwise, the “N-value”. If the N-value exceeds 50 then the test is discontinued and is called a “refusal”. The interpreted results, with several corrections, are used to estimate the geotechnical engineering properties of the soil.

The sampler is then withdrawn, and the shoe and coupling are removed. Finally, the soil sample recovered from the tube is placed in a glass bottle and transported to the laboratory. 

Sampling is part of a standard penetration test (SPT). The split-spoon sampler is the most common type of sampler that is used in the field. Split-spoon samplers can be used to obtain soil samples that are generally disturbed, but still representative. A section of a standard split-spoon sampler is shown in Figure 3. Figures 3a and b show a split-spoon sampler unassembled before and after sampling (Das, 2019).

Corrections

In the field, the magnitude of the SPT hammer efficiency can vary from 30 to 90%. The standard practice now is to express the N-value to an average energy ratio of 60%, N₆₀.

Also, the SPT hammer efficiency, borehole diameter, sampling method, and rod length contribute to the variation of the standard penetration number N at a given depth for similar soil profiles.

Therefore, the N-value corrected to account for these factors can be written as

 

where

N₆₀ – standard penetration number, corrected for field conditions

N – measured penetration number

η_H – hammer efficiency (%)

η_B – correction for borehole diameter

η_S – sampler correction

η_R – correction for rod length

 

Variations of η_H, η_B, η_S, and η_R, based on recommendations by Seed et al. (Seed et al., 1985) and Skempton (Skempton, 1986) are given in Tables 1-4 below.

Table 1. Variations of η_H (Das, 2019)

Country	Hammer type	Hammer release	ηH (%)
Japan	Donut	Free fall	78
	Donut	Rope and pulley	67
United States	Safety	Rope and pulley	60
	Donut	Rope and pulley	45
Argentina	Donut	Rope and pulley	45
China	Donut	Free fall	60
	Donut	Rope and pulley	50

 

Table 2. Variations of η_B (Das, 2019)

Diameter, mm	ηB
60-120	1
150	1.05
200	1.15

 

Table 3. Variations of η_S (Das, 2019)

Variable	ηS
Standard sampler	1
With liner for dense sand and clay	0.8
With liner for loose sand	0.9

 

Table 4. Variations of η_R (Das, 2019)

Rod length, m	ηR
>10	1.0
6-10	0.95
4-6	0.85
0-4	0.75

 

 

Correction for N₆₀ in granular soil

 

The value of N₆₀ obtained from field exploration under different effective overburden pressures should be changed to correspond to a standard value of σ’₀. That is,

 

where

(N₁)₆₀ – value of N₆₀ corrected to a standard value of σ’₀ = p_a (p_a ≈ 100 kN/m²)

C_N – correction factor

N₆₀ – value of N obtained from field exploration

 

A number of empirical relations were proposed for C_N in the past.

Liao and Whitman’s relationship (Liao and Whitman, 1986):

 

Skempton’s relationship (Skempton, 1986):

- for normally consolidated fine sand

 

- for normally consolidated coarse sand

 

- for overconsolidated sand

 

Seed et al.’s relationship (Seed et al., 1975):

 

 

Correlations

 

Correlation between standard penetration number N60 and relative density of granular soil Dr

 

Marcuson and Bieganousky (Marcuson and Bieganousky, 1977), Kulhawy and Mayne (Kulhawy and Mayne, 1990):

 

where

D_r – relative density

σ’₀ – effective overburden pressure

C_u – uniformity coefficient of sand

OCR = preconsolidation pressure, σ’_c / effective overburden pressure, σ’₀

p_a – atmospheric pressure

 

Cubrinovski and Ishihara (Cubrinovski and Ishihara, 1999):

 

where

N₆₀ – standard penetration number

D₅₀ – sieve size through which 50% of the soil will pass (mm)

σ’₀ – effective overburden pressure

p_a – atmospheric pressure (p_a ≈ 100 kN/m²)

 

Kulhawy and Mayne (Kulhawy and Mayne, 1990):

 

where

(N₁)₆₀ – value of N₆₀ corrected to a standard value of σ’₀ = p_a (p_a ≈ 100 kN/m²)

C_P – grain-size correlations factor, C_P = 60 + 25 log D₅₀

C_A – correlation factor for aging, C_A = 1.2 + 0.05 log (t/100)

C_OCR – correlation factor for overconsolidation, C_OCR = OCR^0.18

D₅₀ – diameter through which 50% soil will pass through (mm)

t – age of soil since deposition (years)

OCR – overconsolidation ratio

 

Correlation between angle of friction φ’ and standard penetration number N₆₀

 

Peck et al. (Peck et al., 1974), Wolff (Wolff, 1989):

 

where

(N₁)₆₀ – value of N₆₀ corrected to a standard value of σ’₀ = p_a (p_a ≈ 100 kN/m²)

φ’ – soil friction angle

 

Schmertmann (Schmertmann, 1975), Kulhawy and Mayne (Kulhawy and Mayne, 1990):

 

where

N₆₀ – field standard penetration number

σ’₀ – effective overburden pressure

p_a – atmospheric pressure in the same unit as σ’₀

φ’ – soil friction angle

 

Correlation between modulus of elasticity E_s and standard penetration number N₆₀

 

Kulhawy and Mayne (Kulhawy and Mayne, 1990):

 

where

p_a – atmospheric pressure

α – α = 5 for sands with fines, α = 10 for clean normally consolidated sand, α = 15 for clean overconsolidated sand

N₆₀ – standard penetration number

 

Correlations for standard penetration number N₆₀ in cohesive soil

 

The approximate correlation among the consistency index (CI), N₆₀, and the unconfined compression strength (q_u) is given in Table 5.

 

Table 5. Approximate correlation among CI, N₆₀, and q_u (Das, 2019)

Standard penetration number, N60	Consistency	CI	Unconfined compression strength, qu (kN/m2)
<2	Very soft	<0.5	<25
2-8	Soft to medium	0.5-0.75	25-100
8-15	Stiff	0.75-1.0	100-200
15-30	Very stiff	1.0-1.5	200-400
>30	Hard	>1.5	>400

Correlations between CPT and SPT

Correlations between CRR-values and adjusted N-indices

Correlations for calculating the ultimate point resistance, q_p, of a pile with SPT results in granular soil

 

Meyerhof (Meyerhof, 1976):

where

p_a – atmospheric pressure (p_a ≈ 100 kN/m²)

N₆₀ – the average value of the standard penetration number near the pile point (about 10D above to 4D below the pile point)

L – the actual embedment length of the pile

D – the width of the pile

 

Briaud et al. (Briaud et al., 1985):

where

p_a – atmospheric pressure (p_a ≈ 100 kN/m²)

N₆₀ – the average value of the standard penetration number near the pile point (about 10D above to 4D below the pile point)

 

Correlations for calculating the "capacity" of a pile R

 

Meyerhof (Meyerhof, 1976):

where

R_t – total toe resistance

R_s – total shaft resistance

m – a toe coefficient: m = 400 kN/m² or 8 ksf – for driven piles, 120 kN/m² or 2.4 ksf – for bored piles

n – a shaft coefficient: n = 2 kN/m² or 0.04 ksf – for driven piles, 1 kN/m² or 0.02 ksf – for bored piles

N_t – N-index at the pile toe (taken as a pure number)

N_s – N-index average along the pile shaft (taken as a pure number)

A_t – pile toe area

A_s – unit shaft area; circumferential area

D – embedment depth

 

Decourt (Decourt, 1989, 1995):

where

R_t – total toe resistance

R_s – total shaft resistance

K – a toe coefficient per soil type and construction method as listed in Table 6

α – a shaft coefficient per soil type and construction method as listed in Table 7

N_t – N-index at the pile toe (taken as a pure number)

N_s – N-index average along the pile shaft (taken as a pure number)

A_t – pile toe area

A_s – unit shaft area; circumferential area

D – embedment depth

Table 6. Toe Coefficient K (Decourt 1989; 1995)

Soil Type	Displacement piles	Non-Displacement piles
Sand	325·103	165·103
Sandy Silt	205·103	115·103
Clayey Silt	165·103	100·103
Clay	100·103	80·103

 

Table 7. Shaft Coefficient α (Decourt 1989; 1995)

Soil Type	Displacement piles	Non-Displacement piles
Sand	1·103	0.6·103
Sandy Silt	1·103	0.5·103
Clayey Silt	1·103	1·103
Clay	1·103	1·103

 

 

Glossary

 

Standard penetration test (SPT) N-value – the number of hammer blows required to drive a drill rod with an attached soil sampler 300 mm or 12 inches into soil (or weak rock); this N-value yields information on the geotechnical engineering properties of the soil (Jackson, 2019).

 

 

Conclusion

 

Standard Penetration Test (SPT) is a simple and low-cost testing procedure widely used in geotechnical investigation. The interpreted results, with several corrections, are used to estimate the geotechnical engineering properties of the soil such as the relative density and angle of shearing resistance of cohesionless soils and also the strength of stiff cohesive soils.

For many years, the N-value of standard penetration test has been used to calculate "capacity" of piles. However, the standard penetration test (SPT) is a subjective and highly variable test. These days, N-value are usually adjusted to the N₆₀-value. Several additional adjustments have also been proposed.

The test and the N-value have substantial qualitative value for the experienced geotechnical engineer, but should be used only very cautiously for quantitative analysis. Indeed, using the N-index numerically in formulae is unsafe and imprudent unless used with correlation to prior experience from not just the same geology but also the same site (Fellenius, 2023).

 

 

References

 

Briaud, J. L., Tucker, L., Lytton, R. L., and Coyle, H. M. (1985). Behavior of Piles and Pile Groups, Report No. FHWA/RD-83/038, Federal Highway Administration, Washington, DC.

Cubrinovski, M. and Ishihara, K. (1999). Empirical Correlations between SPT N-Values and Relative Density for Sandy Soil, Soil and Foundations, Vol. 39, No. 5, pp. 61–92.

Das, B. M., Sivakugan, N. (2019). Principles of Foundation Engineering, IX^th ed., Cengage Learning Inc., Boston, MA, USA.

Decourt, L. (1989). The Standard Penetration Test. State-of-the-Art report. A.A. Balkema, Proc. of 12th International Conference on Soil Mechanics and Foundation Engineering, Rio de Janiero, Brazil, August 13-18, Vol. 4, pp. 2405-2416.

Decourt, L. (1995). Prediction of load-settlement relationships for foundations on the basis of the SPT. Proc. of the Conf. in honor of Leonardo Zeevaert, Mexico City, Oct. 28-Nov. 6, pp. 87-103.

Fellenius, B. H. (2023). Basics of foundation design, Electronic Edition, Available at: https://www.fellenius.net/.

Jackson, R. E. (2019). Earth Science for Civil and Environmental Engineers, Cambridge University Press, Cambridge, UK. https://doi.org/10.1017/9781139046336

Kulhawy, F. H. and Mayne, P. W. (1990). Manual on Estimating Soil Properties for Foundation Design, Electric Power Research Institute, Palo Alto, CA.

Liao, S. S. C. and Whitman, R. V. (1986). Overburden Correction Factors for SPT in Sand, Journal of Geotechnical Engineering, American Society of Civil Engineers, Vol. 112, No. 3, pp. 373–377.

Marcuson, W. F. III, and Bieganousky, W. A. (1977). SPT and Relative Density in Coarse Sands, Journal of Geotechnical Engineering Division, American Society of Civil Engineers, Vol. 103, No. 11, pp. 1295–1309.

Meyerhof, G. G. (1976). Bearing Capacity and Settlement of Pile Foundations, Journal of the Geotechnical Engineering Division, American Society of Civil Engineers, Vol. 102, No. GT3, pp. 197–228.

Peck, R. B., Hanson, W. E., and Thornburn, T. H. (1974). Foundation Engineering, 2nd ed., John Wiley & Sons, New York.

Robertson, P. K. and Campanella, R.G. (1983). Interpretation of cone penetrometer tests, Part I sand and Part II clay. Canadian Geotechnical Journal, (20)4, pp. 718-745.

Schmertmann, J. H. (1975). Measurement of In Situ Shear Strength, Proceedings, Specialty Conference on In Situ Measurement of Soil Properties, ASCE, Vol. 2, pp. 57–138.

Seed, H. B., Tokimatsu, K., Harder, L. F., and Chung, R. M. (1985). Influence of SPT Procedures in Soil Liquefaction Resistance Evaluations, Journal of Geotechnical Engineering, ASCE, Vol. 111, No. 12, pp. 1425–1445.

Skempton, A. W. (1986). Standard Penetration Test Procedures and the Effect in Sands of Overburden Pressure, Relative Density, Particle Size, Aging and Overconsolidation, Geotechnique, Vol. 36, No. 3, pp. 425–447.

Seed, H. B., Arango, I., and Chan, C. K. (1975). Evaluation of Soil Liquefaction Potential during Earthquakes, Report No. EERC 75-28, Earthquake Engineering Research Center, University of California, Berkeley.

Wolff, T. F. (1989). Pile Capacity Prediction Using Parameter Functions, in Predicted and Observed Axial Behavior of Piles, Results of a Pile Prediction Symposium, sponsored by the Geotechnical Engineering Division, ASCE, Evanston, IL, June, 1989, ASCE Geotechnical Special Publication No. 23, pp. 96–106.

Youd, T. L., Idriss, I. M, Andrus, R. D., Arango, I., Castro, G., Christian, J. T., Dobry, R., Finn, W. D. L., Harder, L. F., Hynes, M. E., Ishihara, K., Koester, J. P., Liao, S. S. C., Marcuson, W. F., Martin, G. R., Mitchell, J. K., Moriwaki, Y., Power, M. S., Robertson, P. K., Seed, R. B., Stokoe, K. H. (2001). Liquefaction resistance of soils: Summary report from the 1996 NCEER and 1998 NCEER/NSF Workshops on evaluation of liquefaction resistance of soils, ASCE Journal of Geotechnical and Geoenvironmental Engineering, 127(4), pp. 297–313.