I am charged with designing the foundation for a 320 square foot picnic shelter with 4 to 6 structural columns on a slab-on-grade floor.  Unfortunately, we completed two soil borings on site last week and the borings show organic silt the entire depth of the borings, to 35 feet.  Blow counts range from weight of hammer to 1, with water at about 2 feet below grade.  I was thinking of screw piles below each column, but without knowing how deep the bearing soils are, this seems a bit risky.  I guess the other option would be a structural floating slab.  Any other ideas?  Thanks.

Would drilled piers work given the cohesive soils and isolated columns?

I'd look at the floating mat/raft if the structure isn't too heavy; no masonry I hope.

Get a deeper boring.  If there is no idication where good soil is, how can one make a guess on a foundation type.  Helical piles are certainly a good answer, but in order to get an accurate cost from a contractor, he is going to need to know how deep to go.  Helical piers can be battered if necessary if worried about lateral loads.

Bored piers and deeper borings sound like overkill for a picnic shelter.  I agree with apsix.  Let it float, and make it flexible.

I'd let it float, but make it rigid.   I sometimes find that a rough rule-of thumb- for things like this is to tell the structural engineer to assume any square area, such as maybe 6 x 6' as not having any support, and then assign a limiting bearing value of maybe a few hundred pounds per sq. ft. at the edge of that zone.  For larger buildings I have used 10 x 10' as a recommendation and 1,000 psf, but here I think you can reduce those.

Try to avoid any grade raising as that, not the structure, will cause you grief.


If you can, remove say 3 or 4 feet from the site and beyond about 5 ft., backfill with compacted granular material as a means to help spireas out uneven loading from the slab.  Also, that minimizes problems from frost heave.   

I agree oldestguy.  I reread my post, and I was talking about the superstructure as being flexible, i.e. no masonry.  The footing/slab should be rigid.

Don't know your climate, but in cold weather organic silt can move a bunch... probably the worst thing to build on... Need to use a low bearing pressure, accommodate a large deformation and also, may have to accommodate serious frost heave... maybe a foot or so... You have your job cut out... best find a better site.

Dik

I've got to admit that I don't consider frost heave; it's just not an issue where I live.

I'm in Minnesota, so frost heaving is definitely an issue.  I ended up recommending screw piles (helical piles) below each column and also recommended completing an additional boring to determine the depth of bearing soils.  Further, I recommended that horizontal grade beams be used for lateral support if a structural slab was not to be used.  This seems like overkill for a picnic shelter, but I just don't think any other option will be adequate.

How much money is the owner willing to spend?  

Richard A. Cornelius, P.E.
WWW.amlinereast.com

This would seem to be a perfect application for a Pin Foundation:
http://pinfoundations.com/boardwalks.htm

Looks like they have experience with wetlands.