dynamic probing values converted SPT values?

[besatur1]

Hi,

I require to convert DP super heavy values to SPT for interpretation. I have read in literature that simply a factor of 1.5 can be employed to obtain a SPT value. Is there a more accurate way? is this correct?

 

[Mccoy]

The limits of the correlations are shown in the Pagani site, by two test sites where the impact energy has been accurately meausered:

http://www.pagani-geotechnical.com/

>>> efficiency of beating device

Aside from the not reassuring translation (BEATING DEVICE!!) and more linguistical funnies, the method looks rigorous, they conclude the 1.5 multiplicative factor is good for (sandy) gravels, much less so for silty soils.
Beware, here in Italy casing is never used, too costly and unpractical. Instead, a tourque sensor may measure pipe's friction againts the borehole, if friction is not negligibile you have clays or a deviated hole and simply stop the test.

I'll tell you more:
calibrating the DPSH against the SPT blowcounts may not be the best practice, in the sense that the DPSH may be more accurate than NSPT, which is notoriously afflicted by a large error, according to correcteness of practice.
I recently compared DPSHs and mechanical CPTs in two sites with homogeneous sandy-silty soil, and I was stunned to realize that variability was much less in DPSH than in CPTs. There may be other causes for that aside from simple instrumental error (for instance, spatial averaging), but at first look the superheavy dynamic penetrometer in my case appeared more reliable than the mechanical CPT.

 

[BigH]

I suggest, too, that you do a search of the threads in the various geotechnical forums. Dynamic 'probing' vs SPT has been discussed so many times - there are so many different types of dynamic "cones" out there. I've always used a 2 inch 60deg apex cone attached to A-rods and driven by a 63.5 kg hammer (Standard SPT hammer) - Canadian Foundation Manual, I believe, okays 1.5:1 ratio in granular soils for at least the upper number of feet - can't remember. Others use smaller cones and lighter hammers. What do you mean by "DP Super Heavy.? - I've not heard this term before - please define for my knowledge base.

 

[Mccoy]

BigH,
that's a rig which is well known in Europe but, I don't know why, maybe just the name, looks like being unknown in America.
Super heavy is a qualitative scale which tops the energy-scale:

DPL = dynamic penetrometer light
DPML= dynamic penetrometer medium-light
DPM = Medium
DPH = Heavy
DPSH = Super heavy

It is surprising, but these are all used and of course the DPSH is the less prone to errors since its blow energy being the maximum, it tends to reduce the incidence of drag forces against the wall. It' used in 20 cm or 30 cm intervals, depending upon the variation. I've used it a lot in the past, colleaugues are using it now with very reasonable results in granular soils. The Pagani site I indicated will cointain many details, since they are a major manufacturer (the only one that I know which carried out rigorous studies in experimental sites).

Dynamic penetrometers have been listed by the Eurocode 7, part 2nd (in situ testing), which also contains .

Standard Procedures.

A table on standards is in the following:

http://www.pagani-geotechnical.com/

>>>> products >>> DPtest

DPSH = 63.5 kg, 0.75 m fall height, 32 mm rod diameter, 20 square centimeters, 90° cone

 

[BigH]

Thanks - the terms are not exactly as we would expect in North America - at least through 1995 when I left. It is like the BS Heavy Compaction and Light Compaction instead of Modified and Standard Proctor.