Unveiling the Correlation Between SPT Values and Soil Parameters
The Standard Penetration Test (SPT) is a widely used in-situ soil testing method that provides valuable insights into the geotechnical properties of subsurface soils. By analyzing the correlation between SPT values and various soil parameters, geotechnical engineers can make informed decisions in the design and construction of infrastructure projects.
Correlation Between SPT-N Values and Soil Properties
The SPT-N value, which represents the number of blows required to drive a standard split-spoon sampler 300 mm into the soil, is used to estimate the relative density and strength of the soil. The following table shows the correlation between SPT-N values, soil packing, relative density, and friction angle:
| SPT N-value | Soil Packing | Relative Density (%) | Friction Angle (Β°) |
| < 4 | Very Loose | < 20 | < 30 |
| 4 – 10 | Loose | 20 – 40 | 30 – 35 |
| 10 – 30 | Compact | 40 – 60 | 35 – 40 |
| 30 – 50 | Dense | 60 – 80 | 40 – 45 |
| > 50 | Very Dense | > 80 | > 45 |
Correlation Between SPT-N Values and Soil Type
SPT-N values have shown favorable relationships with both silt and clay amounts, while they have shown negative correlations with gravel and sand contents. This suggests that SPT-N values can be used to estimate the soil type and gradation.
Correlation Between SPT-N Values and Pressuremeter Modulus (EPMT)
Several researchers have investigated the correlation between SPT-N values and pressuremeter modulus (EPMT) for both sand and clay soils. Kenmogne and Martin obtained a linear correlation between SPT-N, limit pressure (PL), and EPMT for clean sand, gravelly sand, sandy clay, and gravelly clays.Bozbey and Togrol proposed the following correlations for sandy and clayey soils using 182 sets of data:For sandy soils:
πΈπππ=0.4121π60+6.1614EPMT=0.4121N60+6.1614For clayey soils:
πΈπππ=0.2π60+2.5EPMT=0.2N60+2.5The correlation coefficient (R^2) for sandy soils was observed to be higher than that for clayey soils, indicating a stronger relationship between SPT-N values and EPMT in sandy soils.
Correlation Between SPT-N Values and Undrained Shear Strength (Su)
The undrained shear strength of clay soils can be estimated using the following correlation with SPT-N values:π’=0.22π60Suβ=0.22N60βwhere N_{60} is the SPT-N value corrected for an energy ratio of 60%.
Limitations and Precautions
While SPT-N values are widely used in geotechnical engineering, it is important to consider their limitations and take appropriate precautions when interpreting the results. SPT-N values should be used cautiously for quantitative analysis and should be correlated to prior experience from not just the same geology but also the same site.
Final Words on Correlation Between SPT Values and Soil Parameters
The Standard Penetration Test (SPT) is a valuable tool in the geotechnical engineer’s arsenal, providing insights into the properties of subsurface soils. By understanding the correlation between SPT-N values and soil parameters, geotechnical engineers can make informed decisions in the design and construction of infrastructure projects. At B S Geotech Pvt. Ltd., we are committed to providing our clients with high-quality geotechnical engineering services, leveraging the power of SPT and other soil testing methods to deliver successful projects. Contact us today to learn more about how we can assist you with your next civil engineering venture.