SPT values in Sands 4

[burao]

Hi

What are the implications of using solid augers in cohesionless soils below the water table. I am getting very low counts.

Thanks

 

[GeoPaveTraffic]

If you are talking about hollow stem augers, pulling the center plug tends to create a vacume that inturn causes low blow counts. The only way to get acurate SPT's below the water table is to use mud rotary drilling.

 

[burao]

Hi

I used solid stem augers. I am wondering whether if I used hollow stem augers I would have obtained higher values.

thanks

 

[inaz]

Solid stem augers would mean that you had to pull the augers out before dropping in your rods and split spoon sampler. Is this what you did? Are you drilling with hollow stem? Using a plug or pilot bit?

 

[cdh61]

burao,

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

 

[BigH]

to cdh61. Pulling out solid stem augers may allow cave of material above the water table (and below) and also allow sloughing due to the "sucking" as it is pulled out. Hollow stems are better - if you take care and pull plug gently upward and keep water going into hole at all times - but time consuming.

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).

 

[burao]

Hi,
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

 

[BigH]

Actually a pentest (or dynamic cone) would be helpful (driving a 2inch cone on the end of A-rods with a 140 pound (65kg) hammer dropping 30inches (760mm) as per Canadian practice. See a number of other threads where we have discussed extensively dynamic cones.

 

[DRC1]

Holllow stem augers in sands work well below the water table. Usig solid augers in cohesionless soils below the water table is very likely to cause caving of the sides.
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.

 

[coneboy]

On many occasions I have seen SPT tests performed in a borehole created by solid stem augers.
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

 

[VAD]

Before you go to cone tests etc you should determine whether you need class 1 information for your design. All insitu tests require in the end some interpretation based on site geology and judgement. Did you determine the difficulty or lack thereof of getting the solid stem to the depth desired. This could provide some indication of the nature of the deposit i.e whether loose or compact.

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.

 

[Focht3]

I am not a fan of either flight auger or HSA below the water table. Having used flight auger, HSA and wash rotary extensively, wash rotary is the only way to go for most soils when drilling below the water table. Even careful pulling of the center plug - after flooding the hollow stem so that the water level is more than 2 meters above the GWT - can still result in significant disturbance of the 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?"