Many structural engineers might not like diving deep into geotechnical matters, however, there are at least some important conditions to understand such as the SPT-N value and it's relationship with the soil bearing capacity.
What is the SPT-N Value?
You can also just call it the N-value of the soil. The N value expresses the strength of the ground. A higher N values means that the ground is more firm and strong. On the other hand, a low N values indicates weaker and softer soil.
Description of the SPT-N Value
(Meyeroff, 1956)
| SPT-N Value | Description | Relative Density (%) | Friction Angle (°) |
| < 4 | Very Loose | < 20 | < 30 |
| 4 to 10 | Loose | 20 to 40 | 30 to 35 |
| 10 to 30 | Compact | 40 to 60 | 35 to 40 |
| 30 to 50 | Dense | 60 to 80 | 40 to 45 |
| > 50 | Very Dense | > 80 | > 45 |
Another intuitive way to understand the N value on-site is as follows:
- 0 to 4: Can insert a rebar into the ground easily by hand.
- 4 to 10: Can dig the ground with a shovel easily.
- 10 to 30: Can easily insert a rebar into the ground using a 2 kg hammer.
- 30 to 50: When the above methods prove difficult.
- > 50: When you need a pickaxe to dig the ground.
Relationship to Soil Bearing Capacity
If you know the N-value of the soil, for granular soils like sand or gravel you can roughly approximate the soil Allowable Bearing Capacity (ABC) using the following formula:
ABC for Wet Granular Soil = N × 5 (kN/m2)
So if the soil N-value is 25 for a dry sandy soil, then the approximate bearing capacity is 250 kN/m2. If the soil is wet, then the approximate bearing capacity would be 125 kN/m2.
For cohesive soils like clay, the SPT value is less appropriate. In that case, you can use the undrained cohesive strength to approximate the allowable bearing capacity:
So if the cohesive strength of a clay soil is 150 kN/m2, the estimated allowable bearing capacity would be 300 kN/m2.
Calculation of Soil SPT-N Value
The SPT-N value of the soil is calculated based on the Soil Penetration Test (SPT). More information regarding the test can be found at the link below:
※ What is the Standard Penetration Test (SPT)? Significance. Testing Method.
Significance for Rock
Description coming soon...
Conclusion
You should note that the N value is just an empirical way to find the strength of the ground. It has some shortcomings, including its accuracy in very soft fine soils.