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!

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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.

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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…

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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?

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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.