The Standard Penetration Test is an in-situ test that is useful in site exploration and foundation design. It produces an N-value, which represents the number of blows of a standardized sampler driven into the soil a standardized distance.

## How the Test Works

The sampler is 51 mm O.D. (outside diameter) and it is driven into the soil with a 63.5 kg weight having a free fall of 760 mm. The first 150 mm of soil is neglected. The next 300 mm of soil constitutes the test. The number of blows for that 300 mm becomes the N-value.

It can be done in the bottom of a borehole or at the surface.

In the United States, the standard test procedure is ASTM D1586.

Typical values are 0-10 for sand, 5-10 in loose clay and 10-30 in compacted clay.

## Pros and Cons

Although it is widely used, there are some important limitations that I have to mention. The SPT is a field test which should be used as a general guide only. The repeatability of the test is highly questionable and the relationship between N-value and soil density should be used with caution. That being said, it is a good qualitative indicator of the compactness of the soil, or in a comparison of subsoil stratification.

Also, it is highly affected by drilling and sampling operations. Many small issues can greatly affect the results, including inadequate cleaning of the borehole, failure to maintain the hydrostatic pressure of the borehole, variations in the driving of the hammer, etc. You get the idea.

As you can imagine, problems with drilling and sampling will generally result in low N-values.

Also, there are considerable variations in the test throughout the world. If you are not in the United States, you might want to consider everything I say in here as a guide because the procedure might be slightly different.

## Corrections

Although the field-tested N-value should be reported on the borehole logs, many corrections have been suggested that are in common use today.

- For actual energy delivered to the drill rod.
- For influence of overburden stress on N-value.
- To account for the length of the drill rod.
- To account for the absence of presence of a liner inside the split spoon sampler.
- To account for the influence of the diameter of the borehole.

### For Actual Energy Delivered to the Drill Rod

The procedure is to determine N_{60}, the N-value for an energy level of 60%, as follows:

$latex N_{60} = N\frac{ER_r}{60}$

Where ER_{r} is determined as per the following table. Please be cautious as this is just a compilation of the world’s hammer usage (taken from Seed et al., 1984, Seed & Harder et al., 1984, Skempton, 1986):

Country | Hammer | Release | ER_{r} (%) |
ER_{r}/60 |
---|---|---|---|---|

North and South America | Donut Safety Automatic |
2 turns of rope 2 turns of rope Trip |
45 55 55 to 83 |
0.75 0.92 0.92 to 1.38 |

Japan | Donut Donut |
2 turns of rope Auto-trigger |
65 78 |
1.08 1.3 |

China | Donut Automatic |
2 turns of rope Trip |
50 60 |
0.83 1.0 |

U.K. | Safety Automatic |
2 turns of rope Trip |
50 60 |
0.83 1.0 |

Italy | Donut | Trip | 65 | 1.08 |

### For Influence of Overburden Stress

As you can imagine, the soil deep in the ground might have a different N-value than at the surface, even at the same compaction, due the the overburden stress. A common correction factor for overburden stress is:

$latex C_N = 0.77log_{10}\frac{1920}{\sigma_v’}&s=2$

*Where σ _{v}‘ = effective overburden stress at the level of the N-value, in kPa.*

### To Account for the Length of the Drill Rod

Use the following table to find C_{r}:

Rod Length | Correction Factor Value | |
---|---|---|

m | ft | |

> 10 | > 33 | 1.0 |

6 – 10 | 20 – 33 | 0.95 |

4 – 6 | 13 – 20 | 0.85 |

3 – 4 | 10 – 13 | 0.70 |

### To Account for a Liner

For a standard sampler, use Cs = 1.0. For a U.S. sampler without liners, use Cs = 1.2.

### To Account for the Borehole Diameter

Use the following table to determine C_{d}:

Borehole Diameter | Correction Factor Value | |
---|---|---|

mm | inches | |

65 – 115 | 2.6 | 1.0 |

150 | 5.9 | 1.05 |

200 | 7.9 | 1.15 |

## How to Apply the Correction Factors

There is not a overall consensus of how to apply the correction factors. You should check with the design guidelines in your area, but I will provide the following procedure just so it is not left as a vacuum:

- Adjust for the energy delivered to the rod by calculating N
_{60}. - Apply the other correction factors to N
_{60}.

Zlatko Milisavljevic says

Mr Bernie,

I have a problem… I have SPT equipment, (donut with automatic trigger ) but I dont use it. 95% of my project is im dusty-clay soil (CL, CI, CH). Do you have some example of calculation with N (SPT). I need calculation for settlement foundation and, calculation of loading capacity.

I am engineer of geology (geotehnic)

With respect

Zlatko Milisavljevic, dipl eng. geol.

ravi bhushan says

Dear sir kindly tell me calculating formulae for N (SPT) VALUE

suresh Kumar says

Dear sir please tell me the calculating formulae iff any for calculate the SPT “N” Value.

Eng.Eakub Molla says

N=(Number of blows/30cm )

for every 2nd and 3rd 15cm penetration.

Jason E says

Hi Bernie,

I am working report for a SPT drilling investigation that we carried out recently for foundation. I have a query on the SPT correction. I have read in publications and some geotechnical investigation reports that the correction is applied differently.

In some, only the correction for hammer efficiency is applied (N60)=CE X N. In other corrections, you have to apply the full correction to account for overburden, rod length, borehole size, sampler lining, hammer energy ratio.

Do you need to apply all correction for all factors for any SPT number or it is a project specific? For example for liquefaction analysis as compared to foundation analysis? I am stuck as to what correction to apply for my case.

Dr. Md Shahidur Rahman says

why overburden pressure correction is not applied in clay soil?