The Standard Penetration Test (SPT) is a commonly used in-situ testing method for determining the geotechnical engineering properties of soils. The test involves driving a thick-walled sample tube into the ground at a controlled rate using a hammer with a mass of 63.5 kg and measuring the number of hammer blows required for the tube to penetrate each 300 mm. The resulting “Standard Penetration Resistance” (N-value) provides an indication of the soil’s density and strength. The test is widely used for site investigations and to provide data for geotechnical design and analysis.
Skempton (1957) proposed correlation equations to estimate the angle of internal friction (φ) from the Standard Penetration Test (SPT) N-value. One of the most commonly used correlation equations is the one proposed by Skempton (1957):
φ = 20 + (0.25 * N)
Internal Friction Angle:
Working on Destkop version.
This equation is commonly used for sands and gravels. However, it’s important to note that this equation is only valid for N-values greater than 4 and should not be used for silty or clay soils.
Furthermore, the correlation may vary depending on the location and soil type, so it’s always recommended to use multiple methodologies or consult a geotechnical engineer to confirm the correlation and obtain the shear and cohesion parameters.