I want to know whats a correction factor or whts happen if hammering rate is limited to 5 blow per minute? SPT value increses significantly or reduced than standard speed of hammering?
kpaudel...will depend on the material and other factors. If dilation is occurring in the soil from the last hammer blow, the next will be affected by driving the spoon farther, but I doubt it makes much difference. Further, if harmonic motion in the drill rod conflicts with the next hammer blow, it will change the energy transfer.
Schmertmann did a study of this in about 1975 or 1976.
Dear Ron, As my concern is over silty sand below water level, so due to probable dilatancy effect speed of hammer is very sensative and high effect over SPT blow count. Is there any Possibility of SPT value greater than 100 in sandy layer?
Im very sure i have a very complicated situation. My borelog represents very loose silty sand in upper 6 m, followed by very dense coarse sand SPT value ranging from 65-100 from 6 m to 20 m depth followed by very soft clayey soil having field SPT 6, whose moisture is >70% and liquid limit >15% so im quite worried about liquefaction as my PGA is more than 300gal.
SPT's can be greater than 50, but not greater than 99. If you had blow counts of xx, 49 and 50 your SPT blow count would be 99. This would be very dense sand indeed.
As for the clay, a SPT of 6 results in a medium stiff clay according to the various references that I use. A far cry from very soft.
I've seen many sands with N-values of "100+." This in agreement with geopavetraffic. I mean I've seen sands of 50/3" or such. Consider lacustrine sands that underlayed a glacier. Consider iron cementation. It happens. Now the glacier would have overconsolidated the OP's underlying clay, so that's out.
Thank you sir, what if prefer means instal short pile upto a depth of 20 m from natural ground for very heavy load clay can bear load, as my problem is PGA of 300 gal.
I don't understand the question. I don't know what PGA of 300 gal. A 20 m pile would end on the soft clay. That doesn't make sense. I don't know what you mean by very heavy load. Are you talking a column load of 20 tons, 200 tons or more? Is this a bridge? Is this a water tank.?
OOh sorry, im talking shortly...actually an about load of 200 tons in each pile..., based on different criteria 40 m long pile of 1500 mm dia can stand my design load, (ie. friction pile) but im worried.. during earthquake this soil can bear load or not? PGA means Peak Ground Acceleration.
Kpaudel I do apologize in advance if I might sound a bit rude but honestly I cannot really understand you...looking to solve such a doubt but still using a dynosaur like the SPT and pretending to get the answer???? The very well known and demonstrated uncertainties coming from such a piece of museum are not enough for you? Do you really wanna also add those arising from pretending to know from the SPT the possibility your investigated soil would bear the load or not or eventually correlate it with CRR in case of an earthquake???? Why don't you and all other SPT old fashion lovers use something more adequate and up to standard for the 21st century? Sorry about this but I'm among those who feel the use of SPT no longer adequate to toady's geotechincal investigation and are no longer willing to trade safety for low cost investigations.
@BZinfandel - Please do not take offense (similar to the start of your discussion) as I applaud your desire to upgrade the geotechnical profession.
However, I am one of those old "dynosaurs". The SPT is a tool - as is your fancy black-box instruments that you appear to think are infallible. The engineer needs to take into account a variety of observations and tests into account, including experienced judgment, to develop a model and confirm that foundation support systems are adequate or not . . .
Obviously, you appear to work in an area that has a plethora of expensive equipment that can work on the sides of hills and travel without problems through forests, across streams and creeks and that. Congratulations. But there are locations in this world that one cannot drive a large vehicle either on tracks or wheels to a flat site in order to carry out such investigations. The SPT gives one a sample, too, to look at - to toucvh (and even smell) and to carry out laboratory testing - something that piezocones or seismic cones or dilatometers do not.
Most, if not nearly all, experienced geotechnical engineers are fully aware of the shortcomings of the SPT - and of the advantages and disadvantages of other "black-box" investigative tools that are now in, more or less, common use . . . but also realize that judgment is highly necessary with the use of any tool. Investigators should use a number of tools at their disposal including correlating a few SPT holes (and hence obtained samples) to the other types of probings. I, although a dinosaur, do in fact use to the extent possible and available other such devices - started using the piezocone back in 1982.
The nuclear density gauge came out in common useage in the late 60s - and I would presume that you are comfortable using this instrument. Do you believe without question the test results? Throw away that old time dinosaur known as the sand cone and rubber balloon! Truly, the nuclear guage is right - it's computerized, eh?
Do not throw the old timers under the bus . . . we do have value . . . Perhaps, one day, it might come back to haunt you.
Kpaudel indicated in another thread that he is located in Iran and does not have access to anything more than a crude SPT. Geotechnical engineering needs to be done in all parts of the world and they need work with what they got.
