How does this sound?

[Dan1234]

My mother recently bought some land in south western Ontario near Long Point and wants to build a small cottage on it. From what I have heard the ground is mostly sand dunes with a layer of soil on top (approx. 12" of soil), and we are unable to drill a well for water so ground water issues in terms of the foundation are not a problem. My question is what is the best type of foundation to use here given the following, cottage will be sitting on top of a sand dune, the cottage is going to be around 30' x 40' single story. My first thought was to simply have a floating raft foundation thickened around the edges, however Im not sure about this since up here we get severe freeze/thaw cycles so Im not sure if that will be an issue. The problem I have with a suggestion that was given to me, ie. having a 4' wall around the perimeter of the slab below the frost line is this; by building a slab with this wall around it am I not essentially creating a slab that has the potential to be in a position where it is being supported just around its edges, say if there were any diff. settlement underneath the slab. Im hoping we can just have a raft foundation that floats, made thick enough, say 8", that it can handle any ground movement which I don't think there will be too much of since were building on sand which won't retain water so frost heave will be minimal. Does this strike anybody as a really bad idea or do you think this could work? I would greatly appreciate any ideas anyone has.
Thanks,

Dan

{p.s. Should I put a vapour barrier under the slab? worried about slab curling if I do this and if Im building on a sand hill do I really need one?}

 

[dicksewerrat]

Will this be occupied, heated during the Winter?

 

[BigH]

I'm not familiar with sand dunes in Ontario - I mainly practised the 401 corridor and up north although I was involved in some Bluff work on Lake Erie shore - so the question I have is, is the dune you are talking about stable? Have there been any history of dune movement? May want to think of this. If dune is stable, I don't know why you just don't go with strip footings and the like - why do you think you need a slab on grade? You have problems with frost depth at the edges although this could be handled by the use of an insulating blanket (geofoam) either laid horizontally, say at 1.5 ft depth or so, or vertically. If horizontally, you wan the path to the slab to be greater than 4ft. Also, with a slab, founded at any depth, you are below outside level unless you put in a crawlspace and raise the floor level above the outside ground (which you would want to do).

 

[cdh61]

Dan1234,

You will need to know what is the grain size distribution of the sand. Saying sand is not enough information, because fine grained sand or silty sand with some fines (> 15% passing 0.02 mm)is a frost susceptable soil. Nevertheless, you could use several different foundation types to support the cottage, even if it will not be heated during the winter months.

1st, piles, either cast-in-place concrete or driven pipe/timber piles. In this area of Ontario driven piles should be common given the sand conditions. No foundation walls (concrete grade beams)are required, you insulate between the floor joists and leave the underside open, or put pony walls to grade (leave void space for movement and air vents to prevent a build-up of moisture).

2nd, shallow strip footing (say 0.6m deep)and grade beam. Wood floor joists supported on grade beam and square footings on interior. Insulate as above.

3rd, Strip and square footings at grade. Floor joists as above. This option is more susceptable to movement due to frost action.

4th, pad and pier. Pad could be placed below depth of frost
followed by placement of pier on pad to support cottage. Typically, I do not recommend this for cottages due to cost. But I have used this foundation for commercial buildings with no basements where hard soil conditions exist which are overlaid by soft soils.

5th, raft foundation. Not my first choose because of at grade conditions are typically not desiable for cottages and concrete is colder in fall and spring. Best to use wood floor joists and insulate. Are you going to put in a radiant heating system?

As for concrete slab with pony wall 4 ft below grade, why? This is expensive and with a concrete slab as a floor, cold on feet.

The amount of movement will depend on option choosen. And insulation around the perimeter will help to prevent frost from penitrating below footings.

Eventhough this is a cottage, you should retain a geo in the area to check soil conditions and recommend best suited foundation to support cottage. I do this all the time for people who are building a cottage, but how much $$$ is this cottage going to cost. Something else to keep in mind, how are you going to finish the interior. Note wood finishing is better than drywall for being less notisable if cottage should move a few inches. Think about what are your objectives for this cottage and local codes. Some codes or R.M.s will not allow footing at grade.

My options would be 1 or 2, maybe 3. Option 1 or 2 will not make a difference if cottage is heated in winter months when the foundation is properly designed for the soil conditions encountered.

Regards

 

[Dan1234]

Thanks for the replies,

It is not planned to have anyone staying in the cottage in the winter months. I have very limited information on the soil conditions/stability as of right now, from what I understand this is a fairly stable region and in particular the dune my mother wants to build on is covered in soil so I think it should have fairly stable slopes.

Thanks a bunch cdh61 for your suggestions. I think I might go with your 2nd suggestion having a footing below grade. I am hoping a geotechnical eng. won't be necessary as long as I oversize the footings and assume a reasonable soil bearing capacity (say around 1000 p.s.f.). Driving piles seems like a more expensive way to do it since I would probally want to get bore samples taken to find out exactly whats down there. I figure as long as Im near the surface and its properly compacted with oversized footings I should be fine. I'm hoping I can save my mom some money this way.

I plan on visiting the site in spring and digging a couple pits to find out what kind of sand we have out there. I'll probally post another message then once I find out exactly whats out there. Thanks for your help everyone.

Dan

 

[BigH]

CDH61 has a good point on frost susceptibility - one that I messed up with post and actually started another thread - see "ps" down the thread list. You have a couple of options for geotechs out of Windsor and also London - I'd suggest you see them, too. You've a lot of options given but they would be most knowledgeable in the local area.