Foundation w/ 5' clay over bedrock

[untitled1]

I am looking for some advise for a suitable foundation to use for a small cottage where there is 5 feet of clay over top of very flat limestone bedrock. It's in Canada where the frost (not permafrost) goes down to the bedrock. I'm considering some type of pile system and am concerned about frost heaving.

If I were to drive small diameter steel posts would I need to anchor them to the bedrock, or would the small amount of surface area make the lifting negligible (or is there a way to "grease" the posts to prevent this)? If needed, is there a way to get an anchor into the the bedrock without digging a man-sized trench, I'd like to disrupt the site as little as possible?

Also of concern is the lateral stability of such short posts - are the posts liable to tip over?

Alternatively, if I were to use concrete, would belling out the bottom of the pile stop any lifting? I found a link on this site that suggested such a system using plastic cone-shaped formwork -

http://www.foottube.com

Any suggestions will be appreciated.

 

[DRC1]

I would dig pits 3ft x 3ft maybe 12 ft C.C. If the clay is stiff, you should be able to dig straight down. If it soft, you may need to line. For 5ft, you stand a good shot of not linning, but if you have doubts, line the excavation with wood or steel linners. Drill & grout in dowels into the bedrock and backfill the pits w/ concrete. Tie the tops to the foundations with the fascia beam and you are good to go.

 

[GeoPaveTraffic]

With only 5 feet to bedrock, I would take all of the clay out and place the building on rock. The dowels may or may not be needed, but not much money to install a few.

 

[Riggly]

Is it a wood floor or concrete? Is is slab-on-grade? What is the consistency of the clay? How what about the topography? Is the area prone to getting water logged?

You would have to place the foundation below the frost line, so why not consider placing footing (square footing with beams, or strip footing, and elevate the foor of the cottage above the ground. Rock anchors can be quite costly, and I think they are not necessary under this condition.

Concerning rock anchors- the can be quite small ie few inches in diameter- I would assume single strands anchors. Those are placed by specialized equipment, and sophisticated testing, and experience is required. However, to do that you will have to test the rock to check the quality and strength. I think all this is not necessary for a small structure like you are proposing. Is there any other similar structure in the area? What is the normal practice? That info could save you a lot.

Good Luck!

 

[untitled1]

Thanks for your replies.

This is a small wood-frame cottage that will be raised off the ground a couple of feet and will not be heated in the winter. The soil is prone to being water-logged. The surface soil is about 6" of peat, then a putty-like clay with very little sand or gravel. The limestone bedrock appears to be tabletop flat from what I can see of a neighboring property that was stripped. Frost will go down to the bedrock and may even penetrate the porous limestone.

There have been a few different foundations tried in the area, but this will be their first winter, so even if they look fine now it's difficult to say they will work long term.

I'd really like to minimize the amount of site disruption, and would prefer to use only a small bobcat, otherwise I'll have to remove more trees than I'd like. There is a local concrete supplier that can drag a hose in and pump concrete.

If I was confident that there would be no frost heaving, my preferred solution would be concrete piers resting on the bedrock. If frost heaving is certain to happen then I'll have to anchor into the limestone, but this would require a hole big enough to climb down into, hammerdrilling, and a great deal more labor.

Would there be an advantage to making the piles a very small diameter to reduce the surface area? Or can a sonotube be "greased" to lessen the frost grip?

 

[cvg]

footings are regularly extended "through" the frost zone to a safe depth below the zone in regular soil. these foundations don't require "greasing" to prevent heave, nor do they require anchors into bedrock. I would think the same would apply to this situation without any special treatment necessary. Frost does not "grip" the foundation, but if freezing occurs below the foundation it can heave the foundation upward. I would think that sonotubes filled with concrete, resting on the bedrock would provide a suitable foundation for this structure. You might oversize your holes a bit and backfill around the sonotubes with gravel or sand that is non frost susceptible to provide additional protection against heave.

 

[connect2]

cvg,
frost/ice lenses do grip the pile/foundation and will move it vertically, do not backfill around the piles with a frost susceptible material.

 

[cvg]

connect2 - I believe that is what I recommended when I said "non frost susceptible material...

 

[connect2]

cvg,
my apologies.