Unless the auger is kept full of water or other drilling fluid at all times; generally yes the blow counts will be lower than would otherwise be recoreded.
Agree with GPT - you will need to be "careful" when pulling the plug - as the plug is nearly the same size as the inside dia. Pull too fast you get suction and can't get water down to stabilize. In theory, you need to add water - in practice, we seldom did; the thing in your favour is that the first six inches is considered disturbed anyway and not counted. Unless you have a very large head difference, it will not be that critical. We used to drive 24 inch spoons, too. Even in SPT work, observations during driving, during retrieval, etc. are important for judgment purposes. Same as with normal wash boring techniques - we used to get "live" sand with high heads and not keeping casing full.
It's also critcal to use the auger fork tight to the ground before breaking the joint to remove the plug bit. If the auger falls, even a little, it can make more heave into the hollow stem. That heave can disturb the void ratio in bulb-shape that extends more then the first 6 inches. +1 on the use of water too. Mud rotary is likely the better tool.
Classify all this as "fugitive" information. The driller never documents these techniques and if you don't have a field geologist at the rig, you'll never know what went on out there, sigh . . .
This rig has no plug & the stem is not water tight, so ground water enters it to the table elevation +/-. Both the driller and the engineer on site said that they did not see evidence that the soil "ran" up the stem when they advanced the spoon for the spt, indicating that the soil was not disturbed. However the stp counts were consistant for the entire 2 feet.
My question is: Could it be possible for the entire 2 feet of the sample be disturbed, yielding low counts? Blow counts above and below were higher in similar soil.
How low are we talking? I would suggest that if you are getting blowcounts of 1 or 2, then yes, it is possible that the entire 2 feet was disturbed, especially since you are not drilling with a plug. I'm somewhat surprised that the driller didn't recognize the need for a plug - it is standard practice in my industry to use a plug when drilling saturated sands. When completed correctly, I agree with BigH - the first 6" should take care of any disturbance.
I also echo the response of BigH and Fattdad - geo observations are very important; I also train geo's to communicate with the drillers as well to get driller comments. Often, if you have a good geotechnical driller, his information will be as valuable as the quantitative downhole tests.
Hey guys, let's recognize the problem. Competition and getting maximum footage per day govern. If drillers had the permission to do all these neat things, then they would. However, with no one standing over them to do all these neat things, short cuts are common and even standard for some drillers.
The problem is so common that I'd bet a lot of drilling firms don't even own a plug and don't have capability for water inside the stem. Their standard may be the quickest way.
So, when there are indications of low blow count possibly caused by the drilling methods, well the conservative approach then governs and you may be calling for piles when spread footings will do.
Of course one could go back and do some mud drilling and see if there is a difference when it is significant as to what the foundation will be.
Oh for the good old days with Gow drilling procedures. Gordon Fletcher of Raymond has turned over in his grave a lot by now.
There is no way that sufficient water can flow through the auger joints to replace the volume of soil that's been cut by the bit. This is a driller's cop-out that he's trying to sell for the benefit of footage (as pointed out by BigH). Along the same lines (i.e., footage gains), I've also had drillers use the SPT sampler as the plug bit, thus saving a trip on the rods and (trying to) replicate the function of the plug.
Now, do you have any sense of what the local geology is for this "running sand" formation? Often a regional context is what is needed to understand whether you are really in a loose, saturated sand layer or this is the result of drilling technique.
If the driller was drilling without a plug, very surprising in sands below the water table, and was not adding water the hole during drilling; I would say that all blow counts below the water table are suspect.
I'm also surprised that he didn't sand his augers into the ground.
Oldest guy is right on - we, as geotechnical engineers, need to educate our clients (and, yes, it sounds easier than it really is) to the benefits - necessity of employing (1) experienced geotech technicians/engineers to log all boreholes and (2) to use only reputable, experienced drillers - otherwise, we are all in for a terrible scenario. I know of a job in the east coast of Canada where the drillers called the sand "sand.". What they failed to note and see was that the sand had thin layers of finer material (macroscopically and then each of these consisted of very thin layers of clay and fine sand). As a result, when the excavation for a deep sewer was made, all 'ell broke loose - caving, etc. when, had it been sand, it would not unravelled as it did. I have always been sceptical if drilling is done without proper geofirm supervision/logging.
Here is a question to some previous comments. If drilling thruogh heaving sand without a plug, won't the sand enter the auger and then end up at a lower elevation when the drilling process continues, only to be taken up during the next round of sampling? Then what happens if the driller shows sand on the log (which could be a few feet into the augers) when in reality it may be a very soft clay?
As a former helper we always drilled with a plug while hollow stem augering. It solves a lot of problems and makes drilling easier and faster. I don't see how drilling without one is faster or easier. When drilling through waterbearing sand we always filled the augers with water before pulling the plug. It slows you down somewhat, but not dramatically. Heaving sands did occur periodically, but almost always mitigated or stopped altogether with water in the augers.
I agree with the statements above that drilling through waterbearing sands (especially heaving sand) should be done with mud rotary.
I, too, like lovethecold never had drillers using hollow stem augers without the plug - his experience as a helper confirms the necessity. We used to do a lot of borings using wash bore techniques that worked quite well - drive the casing to sampling depth and use a chopping bit to clean out the casing until the return water was clear. Casing was "always" full of water and, yes, there could be some coarse sands that might not make it up but that is included in the upper 150 that you don't take into account for N. BTW - you would chose your chopping bit (the angle at which the water was discharged) depending on conditions - generally not straight down - or only to the side.
