helical screw piles

[rittz]

I submitted this post a couple of years ago, but I thought useful to post it again.. We did receive several thoughtful replies. But at that time many engineers had a few questions about them themselves, having used them very little and were a little skeptical. The post read as follows: “ Who can tell us the present status of industry acceptance of helical screw piles as a deep foundation system for small (or large) commercial buildings (wood or steel) as well as acceptable design procedures for vertical and lateral loads"

Thank you
j

 

[EngineeringEric]

I can expand later, But i have seen entire houses and commercial structures support by helicals. I guess they are supported on grade beams, supported by helicals... I know the Company Magnum with their Guru (Perko I believe) has done extensive research and designs for these applications. He once said that they design the foundation with helicals as pin in the vertical and as springs in lateral. I treat them as no load capacity in lateral and install battered piles for lateral loads.

Side note: He also said that helicals will get plastic deformation at their soil-concrete interface and that can act like a dampener in a seismic condition (not sure about this one!)

Anyways, I think they can successfully used for full structure support similar in fashion to any other more historical driven system. I think building codes even have special inspection procedures in relation to helical installation now.

 

[XR250]

The problem I see with helicals is that they can settle if not installed properly. The installer can be fooled by thinking he has hit proper bearing soil but has really just hit a random hard spot that spikes his pressure gage. Not a big deal on a retrofit job as they can go back and drive them deeper or adjust the brackets. On new construction helicals, that is not possible as there is no access to them.
I live in an area with a lot of plastic clays so they get installed pretty regularly on older homes.

 

[RFreund]

AB Chance (in the US) has a good technical manual for helicals.

http://www.abchance.com/resources/technical-literature/

EIT

http://www.HowToEngineer.com

 

[kylesito]

I've seen them becoming more and more widely accepted. The trick, and it's a big trick, is that you have to have the right soil conditions to use them. This is often overlooked but I am not sure why. Just like any other foundation system, if it's the right fit, then it's the right system to use.

The biggest mistake I see with their use is a contractor learning how to do them and then coming in and wanting to install them everywhere. In areas where qualified or affordable field labor is an issue, contractors can latch onto these as an affordable alternative to deep foundations. BUT, they MUST be installed in the right soil which can only be determined with extensive geotech analysis.

Great system if it's the right application.

PE, SE
Eastern United States

"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi

 

[EngineeringEric]

We have had projects were we specify the installer drive the piles a minimum of 3-4 feet beyond the point they hit their torque number and the pressure must be higher at the deeper (4') than where they first hit pressure. This ensures they are not hitting a thin compacted layer. We also get their drive logs which state the torque at each foot.

If the contractor doesn't drive far enough it is their responsibility to fix it once it fails. So i am not sure if i am too concerned with them cheating the system. Most contractors have a 20-lifetime warranty on their helicals

 

[dik]

I've used them for supporting 800,000 lb transformers plus a 3' thick cap, and they have been in service for 2 years, so far, and not a problem.

Dik

 

[dik]

Sorry I doubled the weight... that's 400,000 lbs.

Dik

 

[Althalus]

I tried using these back in '08. But my boss refused to even consider it since he'd never heard of them (they had over a 20 year history). Ever since the 2009 IBC, they have finally been codified. Now acceptance only means finding a client who sees the benefit.

For us, the cost was about the same for deep foundations and slightly higher for shallow piers. But the schedule was a big benefit, so that was enough for the client. An additional benefit was that when you have a lot of excavation for site grading, the PVR is not as much of a factor for helicals as concrete piles. So, again a big benefit.

I'm not sure what you're looking for as far as "industry acceptance". I don't know if you can get much better than having them accepted by the IBC.

 

[archeng59]

I have used them for renovations of existing buildings and additions to existing buildings where we did not have adequate clearances for a regular caisson drilling rig. Installed them in clayey soils without any problems. First addition using them was constructed about 5 years ago.

 

[dik]

They are great for granular or sandy soils.

Dik

 

[EngineeringEric]

It seems that they perform well for shrink-swell retrofit and even possibly new construction on shrink swell soil. The head has to be driven down far enough and the cap must resist uplift and support tributary gravity loads. Not ideal for retrofits but a lot nicer than alternatives.

 

[dcarr82775]

I have always been curious about their bearing capacity. They can't get much in the way of skin friction (design assumes none for a reason) so it is all end bearing at pressures significantly higher than you would be allowed for any other system.

Unless battered I wouldn't think of using them for lateral loads.

 

[a2mfk]

As far as history goes, they are one of the older pile systems (1830s) with a proven history even in usage of offshore lighthouses, some of which are are still standing. Then (from memory) I think their usage took off in the utility history as tension anchors for guy wires around 1930 or so... So they are by no means some new fad.

Ditto to everyone who said it is all about installation into the right soil strata. I think it helps to have at least one SPT to know what you have before you get into it. We have had issues with stiff clay as they get very high torque values and the contractor will want to stop, but if they terminate in active plastic clay that can make the problem even worse (we have worked with failed helical piers that act like a piston in active clays).

I think they would make great uplift anchors for high wind areas and applications with little dead load like pre-engineered steel buildings. Instead of using massive concrete blocks, you could use a small pile cap/foundation and then get the uplift and gravity load capacity from the helical.

 

[archeng59]

Helical piers cannot resist lateral loads. I have specified battered piles when necessary.

 

[RFreund]

Forgot about this book, while we're on the subject. Got some good review but I do not own it.

http://www.amazon.com/Helical-Piles...lation/dp/0470404795/ref=cm_cr_pr_product_top

EIT

http://www.HowToEngineer.com