Wetland frost foundation

If it were me and depending on soils, I would use concrete drilled piers to below frost line and then frame the boathouse on top of the drilled pier footings. Your loads are not going to be that great. You can pour the concrete in the piers using a tremmie without dewatering the hole.

Hi Digger57

I would be reluctant to attempt borings for cast-in-place piles. They'll slough immediately without casing at this site. A shallow foundation such as a monolithic slab (i.e. slab thickened at the edges and at any interior bearing locations if any) may be appropriate for a boat house. The foundation will have to be protected from frost by insulation.
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Am I missing something? I agree, in principal that it is good to take footings to below frost level. But, the soils are saturated gravels and sands - these are not considered frost susceptable soils. There may be a bit of heave since water expands on freezing but perhaps this can be tolerated depending on the foundations. But I certainly do not see the problems that would occur in silts/fine sands or clays with silty seams where I once had heaves of 2 inches. Just a thought.

Hi BigH

Good practical point you bring w.r.t. frost susceptible soils in relation to the type of building considered. The formation of ice lense segregation in clean sands and gravel shouldn't occur as would be expected in silt, varved clays, and fine silty sands. In most cases, however, I find it difficult as a geotechnical engineer to condone the implementation of a shallow foundation without frost protection, however small the facility may be. Maybe I'm overly cautious on this one. I would at least price the insulation option. Alternatively, the foundation could be heavily reinforced and allowed to 'float' seasonally. With overhead doors, etc... some functional difficuties may arise as a result of differential movements across the foundation, requiring periodic adjustments.

Sorry, but I have to disagree with BigH and agree with SirAl on this one. I live in the midwest US and underlying much of our area is glacial outwash, sand and gravels. I've see building sites frozen solid four feet down and had to have backhoes with frost teeth and picks to excavate for footings. Gravels can and do freeze to several feet in depth in their natural state.

I think it is only prudent to locate footings below frost line, particularly in areas subject to high water tables or flooding during the winter. That's my two cents.

Hmmm,

I'm not an expert on frost penetration and frost heave, but I do have some experience with it (beneath ice cream freezers.) And I have a pretty good handle on boat houses.

As I read the posts in this thread, it seems that [blue]BigH[/blue] acknowledged the sands and gravels would freeze - but didn't expect much heave to result, other than the heave that occurs as the water freezes. After all, [blue]Digger57[/blue] is dealing with gravels and sands - not silts and very silty fine sands.

Some differential movement can be tolerated by the boat house itself, and the walkway can be designed and built in a manner that is very tolerant of movements.

Sometimes we tend to avoid solutions for some site conditions because we have little or no experience with that foundation type. I've been guilty of this from time to time -

My suggestion? Give the owner the choice - after all, it's his/her money. Clearly lay out the advantages and disadvantages for every foundation system (including costs.) Then get the owner to write you with their decision.

As a side note:

[blue]Digger57[/blue] - have you given any thought to a pontoon structure? This would have very little impact on the wetlands, and could be moved if necessary. And it shouldn't get flooded out...

Focht3,

A nice enviornmentally friendly alternative solution!!