The full form of SPT is Standard Penetration Test.
Standard Penetration Test (SPT) Explained
The Standard Penetration Test (SPT) is a common in-situ dynamic penetration test used to determine the geotechnical engineering properties of subsurface soils. It is especially useful for assessing the relative density of granular soils and estimating the bearing capacity of soil for foundation design.
Key Aspects of the SPT
- Purpose: To assess the subsurface soil properties.
- Application: Foundation design, estimating soil strength.
- Method: Driving a standardized split-barrel sampler into the ground.
- Measurement: Recording the number of blows (N-value) required to drive the sampler a specific distance.
How the SPT Works
The SPT involves driving a standard split-barrel sampler into the soil using a drop hammer. The number of blows required to drive the sampler a distance of 300 mm (12 inches) after an initial seating drive of 150 mm (6 inches) is recorded as the N-value, also known as the standard penetration resistance.
Significance of the N-value
The N-value provides an indication of the soil's relative density or consistency. Higher N-values generally indicate denser or stiffer soils. The N-value is used in various empirical correlations to estimate soil parameters, such as:
- Angle of internal friction: A measure of the shear strength of the soil.
- Undrained shear strength: The shear strength of cohesive soils under undrained conditions.
- Bearing capacity: The capacity of the soil to support foundation loads.
Table: SPT Key Parameters
| Parameter | Description |
|---|---|
| Split-Barrel Sampler | A standardized sampler used to collect soil samples during the SPT. |
| Drop Hammer | A hammer of a specific weight dropped from a specific height to drive the sampler. |
| N-value | The number of blows required to drive the sampler 300 mm (12 inches) after the seating drive. |
| Seating Drive | Initial 150 mm (6 inches) of penetration before recording blows. |
Practical Insights
- Corrections: The measured N-value may need to be corrected for factors such as overburden pressure and hammer energy efficiency.
- Soil Type: SPT is most reliable in sandy and gravelly soils. It may not provide accurate results in very soft clays or soils containing cobbles or boulders.
