I'm hoping to build a tiny (9x12) cabin on high stilts (8-10' high) in a flood plane. It's heavy, black wet soil and any hole fills with water immediately. Basically it is at the water table. Is this possible with concrete piers in Sono tubes or should I use concrete pads on the surface to spread out the weight instead? I know it's possible to build in wet conditions but which is the best foundation? I'm somewhat hindered Tool-wise by being entirely off grid, so if I needs to pump water out if the hole to work I would need a hand pump.
The issue you have with the water can greatly affect the bearing capacity of the soil.
In order to build a concrete pier foundation correctly in those soil conditions, you would require an Engineer to calculate the bearing capacity of the soil and the correct sizing of the foundation. This would require a test bore (or several).
A further complication arises from the presence of all of that water. Water is unique in that when it freezes it expands rather than contracts. This expansion displaces whatever is around it.
Typically the frost depth around here is a little less than 4'.
That means that in the coldest weather, the ground freezes but never beyond 4' down.
That changes a bit if there is a large amount of water in the soil that extends below the 4' mark.
The simplest and best solution for your project would be hiring a specialized contractor that installs helical pier or push pier foundations in the area you wish to build in.
These are engineered and will come with a stamped detail for your municipal permit (which WILL be required).
Essentially, they are metal pipes that are pushed or screwed into the ground to provide a level and stable foundation for your structure.
The benefit in your case is that the installer monitors the torque or force needed to twist or push the pipe into the ground and well established calculations allow the installer to know that when a certain amount of force is required, you have reached a certain bearing capacity.
The water becomes largely irrelevant and you now know that that structure is on a solid "footing" and will not settle or collapse.
The installer will keep adding (welding) sections of pipe, going deeper and deeper as long as it takes to reach the correct bearing capacity.
You also mentioned that you are looking for the cabin to be elevated 8'-10' off the ground. This poses another problem for concrete pier foundations. For a structure such as a cabin or a deck you need the posts to bear on the foundation unlike a fence where the post would be set in the concrete to resist lateral forces (such as wind). The issue you may have with such an elevated structure is the "hinge" effect the connection from the posts to the piers allows. You would require cross bracing in both directions between the posts to mitigate the lateral forces.
The helical or push pier foundation process can be a bit more expensive but far cheaper than constructing concrete piers in sonotubes that you "thought" were deep enough only to discover major structural damage to your cabin from frost heave or one of the piers sinking into the ground and racking the structure.
You can find companies that perform this type of service throughout the province with a simple Google search.
Best of luck on your cabin.
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