SPT rate of blows

[ayem]

The AFNOR standards specifiy 15 to 30 blows per minute in performimg SPT tests. could anyone explain the significance of this rate? Are there any correction procedures available If I am not able to maintain the rate of blows per minute within the specified ranges?

 

[Ron]

No correction factor that I'm aware of. An experienced driller can hit this range with no problem.

 

[dgillette]

ASTM 6066-96 calls for for 20 to 40, but that is specific to liquefaction assessment. If the material being tested is saturated sand, there could be an effect of dissipation of excess pore pressure between blows, and the standard is, well, to standardize that for consistency. If you are drilling in clay, unsaturated material, or in very coarse, clean sand, the rate probably doesn't matter much

Beware, however, that the CME automatic hammer has a stroke length that varies with the speed of the PTO that powers it, and it needs to be ~50 BPM to get a 30-inch stroke. Further complicating matters, that hammer is much more efficient than the typical safety hammer and transfers as much as 95% of the theoretical energy to the rods, vs more like 65% for the typical safety hammer.

It's never nothing; it's always something. - Emily Litella / Gilda Radner on Saturday Night Live. Or was that Rosanna Rosannadanna?

 

[moe333]

Dave, I have never heard of that issue with the CME automatic hammer. Do you know of a reference?

 

[dgillette]

Try this one. (Gov't publication; no limit on distribution.)

We've occasionally talked about slowing the hammer down to make it easier to measure penetration after each blow, which would shorten the drop to <30", which would lower the energy, which would get it closer to the typical 60%, and probably not do any harm, as long as we measure the energy and know what it is so we can adjust.

It's Friday afternoon, so cheers!
DRG

 

[dgillette]

Thinking about it more, I believe that was Rosanne Rosannadanna.

 

[moe333]

Reminds me of SCTV..."that one blowed up real good" John Candy

Cheers