helical piles and micropiles

[ajk1]

How would one support new steel column loads up to about 20 kips (unfactored) per column (supports only 1 storey; there are about 5 such new columns total), in an existing parking garage basement, where the soil may be weak below a stiff upper crust of clay under the existing 5" slab-on-grade?

Which would be used, helical piles or micropiles (clear storey height is 9'-6")?

What is the number of helical piles that should be used? 2 per column? 3 per column?

What is the detail for the top of the piles? A concrete cap extending some minimum distance beyond the pile, and of some minimal depth?

 

[BUGGAR]

You could probably do an internet search on State DOT websites. They have a lot of standard details for highway/bridge footing designs with similar loads. This will get you a start on what some people do.

 

[ajk1]

To Buggar - ok, thanks, I will give it a try. Any particular state? New York State Department of Transportation?

 

[bridgebuster]

NYSDOT is big on micropiles but not helical piles. There is a lot of design info on their website - I'd post the link but I can't connect to it right now. Anyway, on their site go to Geotechnical Engineering Bureau; plenty of design and construction info.

 

[BUGGAR]

Helical piles are still somewhat proprietary and your best technical info for them will be from the manufacturer.
Caltrans has good details, also (but no helicals).

 

[NS4U]

Check ibc requirements. I believe you need min. 3 piles per pile cap unless the pile caps are tied together with grade beams.

I have concerns about helical pile durability and generally only consider them for residential applications. In any sort of commercial or high end residential application, I use minipiles.

Your column loads are small. So my guess is you will be governed by min pile requirements as I mentioned above.

Obviously piling is expensive and you are not sure about soil conditions below this 'upper crust'. You should probably request a geotechnical investigation to confirm that piles are warranted. How has the sog fared? That may hint at whether shallow foundations are possible.

 

[Leftwow]

Off-topic question, but similar... how do helical piles handle moment forces?

 

[KootK]

Not especially well and better in groups.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JedClampett]

Why are you limiting yourself to helical piles and micropiles? Are those the only two systems the Geotechnical Engineer recommends? There's driven piles of every material (wood, concrete and steel), Auger Cast Piles, cast in place concrete piers, and so many propietary systems it will make your head spin.
I've only used micropiles in very tight areas. I thought they were more of a boutique type system (works well in specific applications, but not for a big project). And helical piles are propietary, good for uplift (tension), but once again not for the big projects.

 

[Teguci]

Why are you limiting yourself to helical piles and micropiles?

He noted the overhead clearance is limited.

Mini piles are also useful for deep foundations in karst topography (swiss cheese limestone) or where boulders might be encountered (moraines). The small size is easier to drill through those interferences.

 

[ajk1]

Thanks all for your help. I contacted the helical pile specialist contractor and he was very informative and helpful, and will send me typical details and specs. For my loads (about 20 kips unfactored), and given there are no moments or lateral loads, he says that 2 piles with a pile cap would suffice. The existing basement 5" slab-on-grade can provide lateral support to prevent the piles moving laterally. Again thanks all for the comments and suggestions. I think that helical piles are the right solution given the restricted headroom, the small loads, etc.