SPT count procedure

[ONENGINEER]

Normally here is the SPT published procedure:

The sample tube is driven 150 mm into the ground and then the number of blows needed for the tube to penetrate each 150 mm (6 in) up to a depth of 450 mm (18 in) is recorded. The sum of the number of blows required for the second and third 6 in. of penetration is termed the "standard penetration resistance" or the "N-value".

The software I use asks for penetration up to 24 in. In that case what would be the SPT blow count. 2nd+3rd or 3rd+4th or...

Thank you.

 

[oldestguy]

Suppose the sampler was 30 inches long, then what? One firm I first worked for used that long a spoon so that they could take continuous samples to 15 feet without doing any casing, or augers on some sites. We used the second and third 6" summary, but may have used different if we saw reason. For the crudeness of the test, it is not anything to get too refined about. What does one do about a recovery length of much less than the depth penetrated? Again a crude method of sampling.

 

[BigH]

I've used both short and long tubes - the SPT value is the 2nd and 3rd 6" counts. The Brits like to count every three inches (75 mm) but still use the 12 inches corresponding from 6 to 18". Why is your software asking anything more than the SPT N value - or is this for developing the office log? If that, try writing zero (0) for the last 6 inches and see if the SPT N value is as per the definition. The software might just be written for 24 inches as some drillers/engineers like the longer spoons.

 

[fattdad]

when in doubt you can always read ASTM D 1586.

In summary, BigH and I do it the same way.

f-d

ípapß gordo ainÆt no madre flaca!

 

[MorganMWright]

ONENGINEER,

It would be most appropriate to use the sum of counts from the 2nd and 3rd 6-in drives (from 150mm to 450mm) for your SPT N-Value. Additional counts should be recorded and reported as they provide additional information which could prove useful in your analysis.

- MorganMWright.com

 

[Ron]

F-D and BigH...same here. The additional 6 inches means nothing with respect to the "N" value.