Deep Helix Anchors

[cbear1]

I am working on a project in an old alluvial flood plain with liquefiable sands 40-50' deep on top of sandstone. The soils engineer has recommended helix anchors, embedded 5 feet into dense sandstone. I don't have much experience with helix anchors this deep, but have heard that the longer they get the less effective they are in resisting horizontal offsets. Is this a feasible deisgn alternative, and what are the critical design parameters? I appreciate any advice you can give. Thanks!

 

[STVU]

What type of structure are you building? Do you have soil properties with depth, GWT, & Peak Ground Acceleration etc?

 

[born2drill]

Forget the depth, there is no way you'll be able to install a helical anchor/pile 5' into competent rock. It's just not gonna happen.

Are you looking to design anchors (I assume for temporary earth support) or are these really piles to support the new structure? What kind of loads are you dealing with?

Also, I'm not really sure what you mean by "resisting horizontal offsets." Do you mean an anchor's ability to carry load?

 

[Nuccio]

I am a helical anchor installer in NJ We have installed 1 3/4" shaft with 6" grout pull down piles to 70+ feet with 200 Kip ratings. But you definelty wont get 5' embedmnet in dense sand stone. You either will get something to grab and get the proper torque or will just bottom out and spin the pile and just get end bearing.

 

[csd72]

Sounds like a perfect site for old fashioned driven piles such as H or precast concrete. I am a real fan of helical piles but this is not an appropriate situation to use them in.

Check what the stiffness of the sands is with regards to lateral restraint as you might find that your effective length of the piles is the full 40-50'.