Pin Foundations 1

[DenverKev]

I'm considering spec'ing this foundation system for a house:

http://www.pinfoundations.com/html/ink_dp.html

The diamond piers cost only $165 each and 25 mins. labor. The load rating is 8600 lbs per pier, with 2000lbs uplift, for the soil conditions. This is so cheap and quick, it's very tempting to forget about building a basement. Combined with a floor made from a 12" SIP, It eliminates so many of the challenges that basements and crawlspaces have. I think I can reliably freeze protect the incoming water pipe
Does anyone see any potential problems?

 

[SlideRuleEra]

Looks like a good system. Many homes in the southern US use this concept (not this proprietary product, but isolated footings supporting columns). In coastal areas basements are almost unheard of (high water table). Individual columns allow good air circulation under the house which cuts down on interior moisture problems. Be sure to install plenty of columns (keep spans relative short & provide lateral bracing otherwise the occupants may notice the house is "swaying".

 

[Focht3]

Hmmm,

Be careful! Portions of the Denver area (I'm guessing from your 'handle') have severe problems with expansive clay shales. Consult with a qualified geotechnical engineer before you start using a shallow foundation system, no matter how cheap -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[DenverKev]

I'm familiar with those areas, and I won't be building there. But the comment got me thinking... the biggest problem with these soils is cracking of any concrete within the foundation system. This is due to external pressures applied by the soil. If you're using pins, and the soil pressure increases, isn't it true that there would be zero net delta force on a pin? These soils when left undisturbed, don't actually move, they just expand and contract proportionally to the moisture content. The volumetric changes cause the soil pressures to change.

At any rate, it's a good point, and I'll check with the supplier.

 

[DBNodurf]

ooooohhhhh, delta force! awright! I saw a movie about delta force, but not about zero net delta force... but I thought they jumped without nets anyway...

D. Bruce Nothdurft, MSCE, PE, PG, M.ASCE, etc, etc,...
Principal Engineer/Geologist
Atlantic Geoscience & Engineering
Charlotte, NC

 

[Focht3]

[blue]DenverKev[/blue] opined:
These soils when left undisturbed, don't actually move, they just expand and contract proportionally to the moisture content. The volumetric changes cause the soil pressures to change.

Hmmm,


What makes you think that soils can expand and contract without actually moving?! Yes, they do exert pressure - if there is something impeding the soil's movement. But they do move. And try to imagine expansive soil movements that can be measured in feet...

Expansive soils/shales will push your little pin foundations around just like any other shallow foundation - since the swell pressures are often measured in tons per square foot.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[DenverKev]

If the pins are 15 ft. long, would you still consider it a shallow foundation? By the way, I'm no geotechnical engineer, so I do appreciate the input.

 

[Focht3]

Yup. I've got areas around here (San Antonio) where I've found vertical shrinkage cracks as deep as 22 feet. They were 1/2 inch wide, filled with a dark gray/black clay, in a yellowish light brown ("tan&quot highly plastic clay. The only reasonable explanation for vertical seams like that? Infilled shrinkage cracks.

And I continue to wonder:
If it's 1/2 inch wide at 22 feet, where's the bottom of the crack?



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[DenverKev]

Not that I would personally build anything on soil like that, but how would you build a foundation there? Helical piers?

 

[Focht3]

Helical piers? Not strong enough - the steel shafts would be pulled in two. And you couldn't get them deep enough to be useful since the undrained shear strengths get pretty high with depth (8 ksf+.) We typically use drilled and underreamed piers, with 2% - or more - reinforcing steel. And we try to keep the pier shafts as small as possible to avoid the uplift problem.

Needless to say, we don't typically design for bearing capacity...



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[PSlem]

I see the pin piles are 1", 1-1/2" and 2" pipe from 12 ga to sch 80. The drawing looks like they get installed about 40* off vertical. I wonder about bending loads in them, especially in the sort of soft soils that would necessitate piles, as the cantilever could be quite long.