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 × 2/3 (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
Significance for Foundation Design
By being able to derive an
Suppose that a single column is supported on a pad footing with a working load (1.0*Total Dead + 1.0*Live Load) of 1800 kN. The required pad footing size would be 1800 kN / 300 kPa = 6 m2, or approximately a 2.5x2.5 m pad footing.
SPT-N values can also be used more directly to perform pile design calculations. Refer to the post below.
※ How to Calculate Pile Design Capacity in Granular Soils?
Finally, the N value can be used to judge various strength related characteristics of the soil, such as vulnerability to soil liquefaction for sandy soils in high seismic regions.
Calculation of Soil SPT-N Value
The SPT-N value of the soil is calculated based on the Soil Penetration Test (SPT) which is performed to understand the resistance of the soil. 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
The SPT test is a simple and low-cost procedure, mostly applicable to granular soils. It is less suited to gravels and cohesive soils, and as you can imagine, difficult to perform in rock. The strength of rock must be assessed using different methods, with multiple different considerations including the weathering/fracturing of the rock and orientation of the bedding planes. See more information in the separate article below.
※ How to Calculate the Bearing Capacity of Rock? Explained in 2-minutes.
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 as well as it's applicability to other soils including gravels, cohesive soils, and rock.