SPT values in Sands
SPT values in Sands
(OP)
Hi
What are the implications of using solid augers in cohesionless soils below the water table. I am getting very low counts.
Thanks
What are the implications of using solid augers in cohesionless soils below the water table. I am getting very low counts.
Thanks





RE: SPT values in Sands
RE: SPT values in Sands
Hi
I used solid stem augers. I am wondering whether if I used hollow stem augers I would have obtained higher values.
thanks
RE: SPT values in Sands
RE: SPT values in Sands
Yes you could end up with higher values if you used hollow stem augers rather than solid stem, but not always. With solid stem augers you may not have placed the SP on the native sands but rather on slough from the side walls of the bore hole. The reliability of the data depends on what went on in the field, for example I have drilled using hollow stems and the SPT values obtained were basically garbage. The type of drilling method used needs to be assessed based on information required and soil conditions.
regards
RE: SPT values in Sands
For solid stem - to see if slough, you will need a very accurate measurement of the depth of drilling to the tip, then an accurate measurement of the spoon, backend and rods - then compare. Will need some "touch" rather than just "dropping" the rods into the hole. Also time consuming and not the "approximate" depth of borehole that most drillers use (i.e., within 4 inches, or so or the "real" depth).
RE: SPT values in Sands
Thanks for all the input. I used the SPT with soild augers. What I did is basically to use solid augers and then pulled them out, installed the rod with the spoon, drove them for the 18 inches. Since it was cohesionless soils, and under water table the blow count indicated the soil to be very loose. What I was wondering is if I had used hollow augers, installed the rod with the spoon inside the hollow augers, drove the spoon for 18 inches, what kind of N values would I have obtained. Or should have used a cone penotrometer.
Burao
RE: SPT values in Sands
RE: SPT values in Sands
You should measure the depth drilled vs. the depth acheived by the spoon prior to driving for each sample to ensure there is not a significant amout of caving.
RE: SPT values in Sands
If the drillers ream the hole really well and no gravel, cobbles or soil cuttings fall down the hole as the augers are removed then you can get ok results. If some soil falls down the hole then it will definitely alter your SPT results. If the SPT sampler is not lowered down the hole perfectly then the sampler will knock some soil loose from the side wall and you will get erroronous blow counts.
When you are attempting a SPT of this nature at a depth of say 20 feet you can never really be sure if the hole got cleaned out.
Notice I used a lot of " if's" in the preceeding comments?
Yes, this procedure is faster than setting up a mud tank and doing mud rotary, but the results can be questionable.
As BigH mentioned you might want to consider a pen test. Of course you never really know how much rod friction you have with a pen test so your data might/will be off.
Based on my experience, solid stem augers with SPT tests do not work at all below the water table.
Have you had the hammer checked for energy calibration?
If not then you could probably adjust your SPT values 20 - 30% either way to account for hammer transfer energy.
I have found that unless the site is loaded with gravel and cobbles it is always more economical to do a CPT test.
Far less "if's" and no guessing as to how much you should adjust the data with CPT data.
Hope this helps some.
Coneboy
RE: SPT values in Sands
I have seen enough N=2 to N=5 in sandy soils below the water table from SPT testing irrespective of whether solid or hollow stem was used. For friction piles this makes no difference. For spread footings, we tend to reduce the bearing capacity if the water table is within a certain distance below the footing. In this case with low blow counts there would be no need to reduce the bearing capacity as the results are already reduced. The only question would be toe capacity of piles if piles are not designed as friction piles. I would double the values of SPT from your tests and use that value to determine toe bearing.
If liquefaction is a concern then the uncertainty of what state the deposit is in might be of concern. Your cone test here may be of importance.
One of the beautiful aspects of geotechnical engineering is trying to make sense out of information that you know might be erroneous.
It is said that 90 % of one's design originates in the field by being able to observe the drilling etc.
RE: SPT values in Sands
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"