SPT Drilling Techniques 4

[pelelo]

Engineers,

For Basic SPT works, I have noticed the drilling technique varies between regions (or even between countries). I have seen crews using bentonite mud for drilling, others instead use continuous hollow flight augers and inside the augers they put the rods and spoon and perform the SPTs. I have seen others, that instead, the perform SPTs using casings.

After asking to several drillers what is the preferred or best method none of them were able to give a straight answer. The answer was "that is how i have been doing this all my life, so this is the right way". Few others drillers told me they prefer to deal with casings instead of hollow augers because casings are lighter than the augers. This same crew told me dealing with casings is faster to perform SPTs than using hollow flight augers.

Do you know if there is any advantage between any of these techniques?. All these techniques involve tools therefore money is a factor.

Please advise.

 

[GeoEnvGuy]

I believe the answer is the best method varies between geologies and purpose of the investigation. Where over consolidated clay deposits can be tested by advancing and removing solid stem augers in comparison to loose saturated sand deposits requiring casing with fluid rotary to get representative blow counts for liquefaction assessments.

For additional reading FERC has some good discussion on drilling methods as linked below, the USBR also has similar guidance, ASTM D6066 SPT for liquefaction assessments recommends open hole fluid rotary as the method to minimize disturbance.

FERC

https://www.ferc.gov/sites/default/files/2020-04/guidelines.pdf

 

[EireChch]

I dont even know if its the best method based on the geology. In my experience no one in the UK uses hollow stem augers, seems to be an American thing. In the UK its typically a cable and percussion drilling method, or rotary cored method if in harder stuff! I know sonic drilling is becoming more popular in the UK also.

I am in the middle east now and again no one uses hollow stem methods. Its all rotary drilled....

Regardless, the drilling method shouldnt affect the SPT test.

Some people argue that sonic drilling does as it can loose or densify sand. Sonic drill promoters refute this.

 

[GeoEnvGuy]

Some people argue that sonic drilling does as it can loose or densify sand. Sonic drill promoters refute this.

I have attached an image from a 2013 study and from my personal experience comparing CPT results to SPT's completed by sonic drilling there is an influence of moving the N value closer to the critical state line by densifying loose deposits and loosening dense deposits.

 

[EireChch]

Good insite GeoEnv, I dont have enough experience with sonic to comment. But as I have said, I have seen people argue the fact.

However, your information above is pretty damming. Sonic drilling is evolving very quickly so I would be interested so see if a newer rig would make any difference. Surely the sonic drillers have done these comparisons.

What was the frequency of the sonic drilling. I see people promoting 150Hz as the standard

 

[fattdad]

making the hole is the A part. Collecting the sample is the B part. ASTM provides multiple methods to drill a hole. ASTM provides one method to perform the SPT.

It does depend on geology. If you are drilling multiple feet below the water table and in clean sands, you'll need mud rotary. Augers will not work, either that or you'll get bad SPT results.

f-d

ípapß gordo ainÆt no madre flaca!

 

[BigH]

fattdad - in clean sands - which is why the Canadian pentest would be very useful (driving a cone using the same drop and hammer as for the SPT)

 

[fattdad]

I hadn't heard of the pentest, BigH! I understand the problems with clean sands; however. In all honesty, we typically see CPT and DMT to complement the SPT in the Coastal Plain. That said, we have (typically) big-enough budgets to collect the right data.

f-d

ípapß gordo ainÆt no madre flaca!