SPT using 18 in vs 24 in sampling

[pelelo]

thread261-40499

Another advantage of using 24 inches sampling, compared to the 18 inches sampling, is that drillers will finish faster (it is more notorious the deeper the hole is).

Assuming there is a boring to be drilled 100 ft, continuously (very common in my area) in sands or clay, by the time they are sampling interval 28 ft - 30 ft, using the 24 inches sampling, they would have collected 15 jars.

However, if they are doing the same, using 18 inches sampling and continuously, they would have collected 20 jars.

Just 1 extra jar means more effort (therefore more time) for the crew because they have to retrieve the rods then add the split spoon, perform the blowcounts then retrieve the rods again to open the spoon. That whole cycle takes time, specially the deeper it gets. Now, multiply that by 5 (referring to this example).

This is even more notorious when they are drilling at depths, for example, 78 ft - 80 ft, continuously.

To retrieve the rods, since they usually do it in sections of 4 units (20 ft) due to safety, they will need to do it several times. And the same process to connect the split spoon again and perform the blowcounts.

So definitively, 24 inches sampling, is more productive than 18 inches.

In my career, I haven't found any specifications that requires ONLY 18 inches sampling. So so far, I will continue using 24 inches.

 

[BigH]

I've used 24 many times as well as 18 . . . but why are you sampling continuously? We would sample at 2.5 ft intervals to 15 ft, then 5 foot intervals to 50 ft and then at 10 ft intervals - of course, if we felt a change of strata, we would stop and take an intermediate samples. Interesting, though, in your area . . .

 

[pelelo]

The longest interval I have seen is every 5 feet.

I have worked in several countries so each region has its own standard I would say.

I am in the caribbean, I have seen both ways of sampling (continuously to 100 ft and every 5 ft up 100 ft deep). Definitively from the time point of view, every 5 ft intervals is way faster.

I would say it depends on the clients. Some clients have reviewed boring logs expecting an specific layer and for some reason the 5 ft interval crew did not find. So that is when they request the continuous sampling.

 

[BigH]

pelelo - it also depends on the company's "history" of knowing the subsurface conditions in the area in which they work.

 

[EireChch]

we do every 1m to 5ft, every 1.5m to 30m, then every 5m after that. Kind similar to BigH. I have never seen continuous sampling, I think its a US thing.

 

[MTNClimber]

24-inch SPTs are standard in New England. I only use continuous sampling when we hit problematic soils to accurately define it's thickness (undocumented fill, peat, ect.) otherwise we sample every 5'.

 

[Filsted64]

I find this thread very interesting, I'm facing a similar issue, I'll just be follow along - thanks!

 

[BigH]

MTNClimber et al: I agree that there are times that it is "important" to accurately define thickness - but one point to consider: In general practice does one "precisely" measure the depth of the beginning of a sampled location? We would say to start the SPT at 10 ft but it might be 9.7 or 10.3 . . . one gets in the habit of identifying the stick-up of the drill rods when drilling . . . A suggestion that if such accuracy is needed, a Canadian Pentest could be used - it is a dynamic continuously driven cone (51 mm dia; 60deg apex angle driven with the standard SPT hammer and drop. Very helpful in finding organic or soft clay seams in a sand, for instance. I tried to get a company in New Jersey to use such a cone in some of the coastal deposits but they were happy to "assume" sandy layers in organic deposits. However, when they had a problem in Atlantic City where a pile failed its pile load test and they discovered a clay seams beneath the bearing depth, they quickly started to use the Pentest to confirm that such seams did not occur beneath a pile tip.

 

[MTNClimber]

Precision/Accuracy being a relative term... If the sampler rovers 2" of peat in the tip and the driller advances the rollerbit, chews out 3 feet of material before starting the next sample, which only recovers sand... is there 2" of peat or 3'-2" (or more) of peat? You might be kicking yourself for not taking another SPT sample immediately after that first spoon showed peat in the tip.

But either way, nothing about SPT sampling is "precise". I'm lucky to get a couple of full recoveries on most of my projects. Continuously driven cones are great when they work, they just don't work on 99% of my jobs.

 

[EireChch]

This is why a CPT is a wonderful tool

 

[MTNClimber]

Until you hit a cobble or need to core bedrock.

 

[EireChch]

if you want to find organic or soft clay in seams of coastal sand the CPT is by far the best.

You can also get sonic rigs where you can core and then switch over to push a CPT. The pinnacle of investigation rigs

https://www.youtube.com/watch?v=W5H9p4g-NaE