BELLED PIER, ACP,vs NO FOUNDATION

[Lesali]

Hello All,

I have a project on tribal lands in Nevada where BLM is insisting on no soil disturbance i.e. NO foundation!!!.
This site is on a mountain top where high winds have ripped some existing equipment from their anchors. So, this time they came to an engineer for a design

The new equipment will include a metal shipping container on a platform with 8 legs and a 17'x37' sail of a solar array attached to it. I can overcome OTM from the wind forces by widening the platform base, but sliding is a problem. I already considered adding weight such that the sliding frictional resistance, µ x shelter weight > wind forces; but the amount needed for that was impractical.

I see no alternative but to recommend 4 piers/piles and hopefully BLM will cave if I keep surface disturbance to a minimum. And I have no soils report - they won't allow it! I used IBC soil values and the pole formula to determine embedment = 14' for 16" dia pier. I do not know how far to bedrock or have any soil design parameters so I imagine a test pier will be the only way to determine any capacity.

The soil surface is shale rock mixed with granular fines so I do not anticipate much cohesion. That is why I thought of using the Auger Cast Piers but if a belled pier is used I get the advantage of the weight of the shear cone above but I'm not sure if it would be possible to undercut in the soil. I was also planning on pouring on top of the soil 3'x3'x12" pedestals at the 8 leg locations to level the surface. Pier loads: T=22k,P=44k V=13k.

Any other ideas of how to deal with shear?
What is a reasonable assumed sidewall friction value for an ACP in dry cohesionless soil?
Is it possible to dry drill and catch the spoils so to keep the area around undisturbed.

The comments of the Sages would be most appreciated.

LES-C.E.
(In too deep to cut & run)

 

[jdonville]

Lesali,

Could you install helical piers/anchors at say 45 degrees from vertical to provide your lateral resistance? If installed in opposing directions you can get both push and pull.

Vertical ACPs are rather inefficient in lateral loading. Suggest you think more along the lines of a grouted tieback, possibly using a hollow-bar approach. Either way, the grout spoils are messy.

Or you can tell them to start packing in water- and sand- filled containers for ballast.

Good Luck!

JD

 

[JedClampett]

I would question what amount of weight is unreasonable. Two of the good and bad things about concrete is that it's heavy and it's cheap. Adding thickness costs nothing more than a little bit of formwork and the delivered concrete ($100 per cu yd; although if you're on a mountain the price might be higher). And a cu yd weighs 4000 lb.
While a massive foundation might seem wasteful, you're accomodating your client's requests and solving the sliding problem.
One tip; Make sure you're using the right factors for the wind forces. With ASCE 7-10, it's easy to forget you have a .6 factor in there.