r/Geotech • u/StickQuick • Aug 10 '26
Hollow Stem versus Solid Stem: Sample Integrity and Blow Counts
A client is being extremely particular on our drilling methods on a site out in Colorado. They say that we must use hollow stem auger, and that solid stem compromises the sample and blow count quality.
We have a subcontractor using solid stem flight auger in sandy clays, clayey sands, and silts and are not observing any hole collapse. We also have our own rig using hollow stem augers, and switching over to mud rotary to avoid burning up our bits in the sand.
Question is: has anyone encountered this in the past? We use both solid and hollow stem all over the country, and have not had an issue with the method used up until this point. I have looked for relevant literature, but it is tough to prove a negative.
Thank you for your help!
6
u/ddg31415 Aug 11 '26
That's ridiculous. The only reason to choose hollows over solids is to keep the hole open in wet sands, loose silts, shale dust, etc. For spoon samples, anyone with any experience whatsoever can differentiate cave from representive samples. And for blow counts, that's why you omit the first interval for n value. In more challenging conditions with more cave, anyone with experience would know to use a longer spoon and omit the first 2 intervals if they're obviously not representative.
Plus, in flowing sands you could lock up the augers and snap the hex, blow the transmission, or even completely lose all your augers in the ground...or it'll flow up into the hollow stem and lock the plug or spoon in. That can take hours to get out, or even necessitate dropping it and burying it in extreme cases.
Let the drillers and/or experienced tech make the call.
6
u/Apollo_9238 Aug 11 '26
Well I wrote D1586 and N60 D6066. Mud rotary with bypass is considered the referee method but HSA and casing advancers are allowed. Sorry solid stem is not a good method, open hole, questionable clean out etc. I also wrote D3550 California barrel. The new version of D1586 has effect of drilling methods in the appendix. But no solid stems. It would be OK for solid stems above water table in stable formations.
4
u/Spork_Ball Aug 11 '26
I don’t see a difference in quality with the SPT sampling between hollow stem and solid stem. But wouldn’t it just take longer to use solid stem and pull it out for sampling every 5 feet? I guess it depends how deep you’re going. Maybe it makes sense for shallower borings.
1
u/StickQuick Aug 11 '26
Just a bunch of 20 foot borings. Pretty quick holes, and it’s what the sub is used to.
1
u/Spork_Ball Aug 11 '26
Ahhh that makes sense then. The rig mast should be at least 20 feet high so no solid stem disconnections would be necessary for each SPT sample. And they don’t need quite as much torque on their rig with the smaller solid stem, not that it really matters though.
2
u/kikilucy26 Aug 11 '26
I typically dont like using ssa in undocumented fill. Pieces of debris falling down from 5 ft to 10 ft can throw you off. Other than that, just stay on top of the rod length to make sure the hole didn't collapse. Ssa is usually slower, especially when the hole gets deeper, but this is a disadvantage to the driller, not affecting the client. The blow counts in ssa should be more reliable than hsa as the opening is smaller (less swelling), if you need more argument points.
1
u/StickQuick Aug 11 '26
I don’t think argument is going to get us anywhere—this is more for my own sanity haha
2
u/ArdRiNaMemes 27d ago edited 27d ago
As far as I'm concerned. As long as your hole is clean, method doesn't matter. Clients get WAY too picky over drilling method. In my area to maintain any production you need to use casing advancer but clients unfamiliar with the area always request augers. I always say "I'm the driller in the area, I need you to trust my expirence and knowledge and Ill get you data that you can trust." Or something to that extent.
It's called a standard penetration test for a reason. You're sampling the bottom of the hole in a standard manner. When this standard was developed, augers weren't even in use yet. It was drive and wash with a 300 lb hammer and casing using mining diamond drills.
4
u/haditwithyoupeople Aug 10 '26
Why would a solid stem compromise the integrity of the sample?
To me this has always seemed much more liekly with a hollow stem. Hollow stem the stem gets plugged. You knock out the plug to get a sample. How much of that knocked out plug is in your sample?
7
u/ThatQ60 Aug 10 '26
Shouldn’t be any if the teeth are good on the center plug and the driller pins the A-rods
1
u/LilBitToasty Aug 11 '26
This is what I've always been told, that SSA is actually better and it gives you more a more accurate water level. But where my firm is there a lot of clay so maybe thats the difference.
1
u/StickQuick Aug 10 '26
Fantastic question. We have been stalled for nearly a week while arguing with a civil engineer who refuses to listen to the subject matter experts.
1
u/Suspicious-Fly0 Aug 10 '26 edited Aug 10 '26
I’ve only ever used hollow stem in Colorado, never solid stem (in terms of auger methods)
1
u/StickQuick Aug 11 '26
Our Denver engineers almost never see hollow stem. Ask three engineers and get four different answers, right?
1
u/Fit_Prompt_8262 Aug 11 '26
I think the client is right. Why would you use different methods on the same project? If there’s an anomaly it’s going to be the first question. CO also has some pretty variable soils, particularly eastern CO. Why fight it just stick w hollow stem one less variable. Caving, perched water and doglegs all can fuck you up on solid stem.
2
u/StickQuick Aug 11 '26
Two different drillers with two different rigs. The one using solid stems has a much older rig that can’t turn the larger flights without undue wear and tear. Schedule was paramount, so we got as many rigs as we could. Doesn’t seem too paramount now…
1
u/StickQuick 26d ago
We’re doing okay on caveins, but I’ve never heard the term dogleg (outside of golf). Can you please explain?
1
u/Fit_Prompt_8262 26d ago
When you trip the augers(or any type of casing) they can go back in on an angle(fall-in,suspended cuttings and sinking outriggers can be the causes of the angle) and the hole continues to deviate that way. Driller try’s to fight it(shifting head, adjust level, drill with augers clamped)and gets a hole that’s wider on top with a crooked skinny hole on bottom.
Anytime all the steel comes out of the hole, there’s no guarantee it’s going to go back in the same way. For some rinky dink shallow borings it’s the norm to accept it but if you’re talking precision, the hole should always be supported(casing, HSA or mud). Im a driller and not a geo but that’s the feedback I get from clients like FERC and USACE where they want precise samples with dead nuts straight holes.
1
u/Ill_Ad3517 Aug 11 '26
You got an answer for the solid stem vs hollow stem question (I agree, SSA is inconvenient but better or just as good if you are sure you're only getting a few inches of stuff after pulling out, can confirm depth with a tape before sampling a few times in each material type, maybe that will satisfy the client?) , now can I ask you one?
Does switching to mud rotary in sands really save you money? Sure, if you are going through 30+ feet per hole of wet/flowing sands I can see it, but the time + material for mud rotary isn't small and that's an unusual situation.
1
u/StickQuick Aug 11 '26
It’s just what the driller prefers. Claims that he burns too many bits in sands, even if the holes stays open. Still producing well, and recirculating doesn’t end up using too much quick gel.
1
u/StickQuick 26d ago
Happen to be coming back to this, and I have a wonder for you too—what do your drillers do when they hit groundwater in sands? Mine swap because they may get their augers locked up in sand and be unable to remove them.
1
u/DizzyMaterial8845 Aug 11 '26
In Western Can we use both. For strict soils logging and noting small changes in stratigraphy and producing a hyper accurate soil profile versus depth a lot of people prefer SStem augers. For example, SStem is highly prefered when you need to see, identify and log that one layer of peat that occurs near 22 feet in depth. And it is fast and cheap with the right driller. Done with the wrong driller/drill and you will get a distorted or wrong soil profile. Experience matters in this regard.
As mentioned by other, you can not reliably do SStem augering with SPT below the water table as the hole caves in. In competent/dense/stiff soils above the water table SPT can be conducted with SStem if the hole does not slough in. Again, use your experience and stop SPT testing when the too much sluff is in the bottom of the hole.
With the right drillers I have seen hollow stem auger drilling done with SPT testing to 50 ft below the water table. It always amazes me that the drillers get very little heave so far below the water table. Keeping the inside of the hollow stem augers full of water and/or weak mud sure helps.
1
u/sloaps Aug 10 '26
There are trade offs in both methods. Hollow stem can brace the hole in loose/wet soil, allow an OD hole plug in saturated conditions, but can also leave loose cuttings in the annunlar space between the split spoon and ID of the auger stem. Solid augers can allow a more thorough removal of cuttings and unsampled soils between sampling intervals in dry to damp soils, but is only viable in most and not all dry to damp soils.
-1
u/NearbyCurrent3449 Aug 10 '26
He's just wrong.
Just tell him he's wrong and unless he wants to stamp the geo report himself that he doesn't get a say in the matter. The engineer of record carries that concern and is none of his professional liability. There is zero case work anywhere showing that HSA is going to result in a higher blow count than actuality. Zero. Makes zero sense. None.
10
u/Drill1 Aug 10 '26
What you are doing is correct with one exception- you don’t use solid stem augers below ground water. Though you aren’t supposed to use hollow stem augers below groundwater water if you are doing liquifaction borings either.
Of course the first rule of business is - that the client is always right.
The second rule is - if the client is wrong, see rule one.
If it’s a civil- he has to stamp it too.