Screw Piles Lateral Capacity

[Said the Sky]

Hello all,

would you guys specify battered piles at the ends for lateral load transfer into soil using screw piles? or would the lateral load be transfered into the grade beam that is perpedicular to the lateral load? (concrete grade beam is pushing against the soil, spanning horizontally and bending about its weak axis)

one storey building, plus 6 ft metal wind wall on roof. Roof is used for playground, main floor is slab on grade used for storage. low seismic low wind area.

thanks!

 

[Said the Sky]

another picture

 

[phamENG]

I would use battered piles. There's typically a reason piles are being used, and it's often because the soil can't be relied upon. Saw a building on piles once that had the entire site sink around it almost a foot in the first 3 years after construction. The only caveat here is if the geotech said it's okay. Maybe the soils at the surface are really good and can take the lateral load, but there's a lens of soft, compressible clay 15 feet down that would cause excessive settlement for shallow footings. I'd probably still use battered piles, though, if I could.

 

[dik]

Millennium Towers?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[phamENG]

Ha. Nothing so extravagant as that. Just a small hotel built on a filled site. Another engineer at my office had to go back and design grade beams for fences...it was wild.

 

[shardy]

Are you the engineer for the screw piles or others? We usually just give the loads to the helical/screw piles contractor and their engineer decide if they want to batter or not.

 

[Aesur]

I don't recall seeing any helical anchors with lateral capacity (perp to direction of helical) in the past, typically we have used battered piles/helicals to transfer this load to an axial load in the helical and then into the soil.

 

[shardy]

Helical piles have lateral capacity based on shaft dia and length and soil type. The helical piles engineer use L pile to calculate the lateral capacity. We've installed hundreds of helical piles inside the plant I work at that are straight. Residential screw pile may have much smaller shaft diameter though.

 

[phamENG]

yeah...residential screw piles typically have about a 1.5" diameter...sometimes they're square. Zilch in the lateral capacity department.

Those cool custom jobs where they weld helices to HSS are pretty neat, but not the norm where I practice.

 

[Said the Sky]

thanks guys I include the loads to provide to the screw pile supplier engineer to review and comment. We will pass him any geotechnical report.

also regarding the building being unheated, do I have to consider anything for the "interior" unheated space slab on grade? do I require insulation underneath the gravel on top of the undisturbed soil?

attached a detail,

 

[dik]

I've designed for both... generally small helical piles don't have much lateral capacity. I've done some with 12" dia shafts that do.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[canwesteng]

Depends where you are in the world. If insulation below the slab is enough to protect from frost head without large extensions past the foundation, it's probably just as easy to place a couple feet of non frost susceptible fill below the slab, since you need to some prep for the slab anyway. Otherwise I'd anticipate you'll need to extend the insulation well past the slab to protect from frost.

 

[Said the Sky]

canada here where frost depth is about 1.2m minimum and definitely susceptible for frost.

 

[MotorCity]

I would assume that helical piles have little to no lateral resistance (at least not enough to count on). If you were going to twist my arm to define a lateral capacity, I would defer to the manufacturer for the lateral strength since they are a proprietary item. Even then I'd very very skeptical and would probably divide the lateral capacity by a factor of safety = 12.0