N-Value when sampler is advanced a total of 24 inches (instead of 18 inches) 8

[pelelo]

Engineers,

As you may know, some environmental agencies require to advance the sampler instead of the typical 18 inches (3 increments of 6 inches), to do it 24 inches (4 increments of 6 inches).

The typical way to compute the N-value in the 18 inches sampler, is adding up the number of blows of the second and third increments.

I am somewhat confused for the 24-inch case:

In order to compute the N-value, are we supposed to add up the number of blows of 2nd and 3rd increments? or the 3rd and 4th increments?

I checked ASTM D1586 (Standard Test Method for Standard Penetration Test) but no information was provided.

Please let me know your thoughts.

 

[MTNClimber]

It's still the 2nd and 3rd increments. It's common practice in New England to drive the sampler 24 inches. Helps with recovery when you have clean sand.

 

[oldestguy]

It's such a rough test with many things affecting it, I'd not get too excited about it.

 

[EireChch]

OG ~ I would thing counting 3rd and 4th instead if 2nd and 3rd would make a big difference due to borehole disturbance

 

[GeoEnvGuy]

Common in my area is to do the 24 inch spoon the n value is still only the second and third interval. This is typically to get more recovery for lab testing and I use the last interval to differentiate if the material is loose or compact having a n value of 10 as the material below the test interval influences the blow counts above.

 

[oldestguy]

In my experience we once used 30" spoons mainly to sample as much as possible. We even just advanced the hole using the split spoon in steps (No ASTM rules then). Usually took the N value in the second and third intervals anyhow. No problems, law suits, etc in many years with these methods. I suppose these days ask a driller if he knows what a wash boring is. Also who invented the split spoon? Look up Gordon Fletcher Raymond Concrete and some interesting stuff about the split spoon sampler.

 

[Eric C.]

"Also who invented the split spoon? "

Charles Gow and Harry Mohr?

(but I'm not a driller)

 

[BigH]

Drove a lot of 24" spoons - as others have said . . . Basically, in my view, the longer spoon is used to get more sample! It can also reduce the chance of missing a seam of other material as the spacing between samples is shortened.