Looking for ideas: attaching an unheated structure and slab to an heated building

[71corvette]

I'm in the process of designing a new house that will be built in Southern Maine. Winters here can be pretty long and cold. Generally speaking, code requirements in this area specify setting frost walls about 4' below grade.

The house will be set on 8' foundation walls to provide a full basement. An attached garage will be set on 4' frost walls. Attached to the back of the house will be a large covered porch and gazebo as illustrated in the schematic below. The porch will be about 6' wide and the gazebo will be about 20' in diameter. The covered patio and gazebo will be unheated.

At the proposed home site the subsurface soils consist of well drained sandy gravel. The water table is quite deep, at least several feet below the house foundation.

The roof for the porch will be attached to the house, and also to the gazebo, to provide a covered walkway from the home to the gazebo. Normally, I'd simply support the roof for the porch and gazebo on columns founded on sonotubes extending below the frost line. However, a concrete slab on grade is proposed for the floor of the porch and gazebo.

As I think through the details of this configuration I have concerns regarding the durability of the concrete slab which will be adjacent to, and probably in direct contact with, the house and sonotube foundations. I've debated several options but none give me the "warm and fuzzies".

One option is to take advantage of the well drained soil on site (which is minimally frost susceptible) and cast the slab on a 1' thick bed of crushed stone and be done with it. However, there's no place to positively outlet and underdrain which makes me nervous (even though the water table is quite deep).

I've also thought of eliminating the sonotubes and designing this to act as a raft foundation. However, I don't like the idea of differential movement between the porch roof and the house.

At this point I'm open to ideas and suggestions for an effective and cost-conscious solution.

 

[oldestguy]

How clean is this "sandy gravel"? If P-200 sieve material is below 5%, likely no heave. Sure one foot of better stuff will help if there is frost susceptible material below there. But, remember with low moisture content, below freezing temps easily can go to 8 feet where local "codes' say frost depth is 4 ft. The more moisture, the shallower the depth of freezing.

 

[71corvette]

I don't have a gradation test, but could definitely get one. That would help give me a bit of piece of mind.

 

[aeoliantexan]

You definitely don't want the structure foundations to heave. Check your assumptions: - Can the depth of the water table change? Has the precipitation the past few years been below average? What is the expected capillary rise in the sandy gravel? Do the measured water contents confirm the capillary rise? Is the profile free of any frost-susceptible layers?

A perimeter drain and a pumped sump in the basement would be prudent protection for the basement if the water table can be expected to fluctuate.

Isolating the floor slabs from the structure and watching the architectural details to tolerate some floor movement may make the slab on grade feasible. Subfloor insulation is another option. The spans are small, so a structural floor over a void may be a reasonable extra cost.

Horizontal or sloped rigid insulation a foot or so below the grass can reduce risk, especially if combined with sub-floor insulation.

The first question is: How much risk are you and the owner willing to accept?