Mixing Floating Slabs, FPSF and Full Depth Foundations

[InEngCAN]

Hello All,

I often get requests from clients to connect their floating slab garage to their house to expand their living space. In the past I generally advised against this as mixing a floating slab foundation and a full depth foundation would result in differential movement between the two due to frost heave.

However, I am a solutions based engineer and don't just like to say no outright so I am wondering if anyone has successfully completed a project like this.

Here are some thoughts:

What if we frost protect the floating slab so it's not longer affected by frost heave?
What if I underpin the floating slab with helical piles past the frost depth?
What if the floating slab isn't in a frost susceptible soil?

-Andrew

http://www.INengineering.ca

 

[1503-44]

You should not use "slabs on grade" unless you remove the expansive soil below and replace that with granular, structural fill that allows drainage and provide full and adequate surface drainage that will prevent water from saturating the fill below. OR, if you build on soils susceptible to expansion from any cause, then you must provide some freedom of movement between that slab and existing adjacent foundations or structures and be fully prepared to accept such movement without cracks, warping, or other damages, or unsightly differential movements.

Piles to depth below frost depth could be acceptable, but then you need to provide a structural slab design, similar to a first floor, to span between piles with clearance below as necessary to allow the soil to freely expand. That implies significantly more cost.

If you protect the slab from freezing, you may still get movement due to water saturation and resulting soil expansion and contraction with the soil's changes of moisture content alone, even without freeze action. You have to know how the soil below will behave under those conditions. If it is sandy and well drained, you might be OK, but expansive clay could present severe problems.