NONSTANDARD SPT RIG 1

[AKniazeff]

Dear colleges! Here is the situation: due to lack of space we had to use small SPT rig with weight of hammer = 70 lbs., drop=15 in., same size spoon as for a standard rig. Theoretically the energy for smaller hammer is 4 times less, so to convert its N-values to standard ones, just divide it by 4. But here comes a problem. First at all, comparing two different borings about 50 feet apart (standard and lightweight rig) I found that stratas in two borings pretty much identical, but N-values for lightweight rig after dividing by four do not make any sense and in some stratas are less than 1.
Secondly, after some research, I found that there is no direct correlation between these two N-values. There are too many other unknowns involved besides weight of hammer and its drop.
Any suggestions how can I do correlations? Thanks for any help.

 

[Ron]

Read the attached paper done by John Davidson, PhD, University of Florida. Then read the paper done by Schmertmann, Smith, and Ho in 1978....I think you'll find your relationship won't hold.

 

[BigH]

Don't have my copy at hand but Winterkorn and Fang (later only Fang) did have a chart regarding this I believe - section on investigations.

 

[molerat2210]

Foundation Engineering Handbook (Fang?) also has formulas for converting the drop height, weight, sampler diam. to SPT.

 

[BigH]

molerat2210 - I think that is what I said . . . but I could be wrong - I am getting old . . . .