Frost apron vs. founding cushion

Good evening everyone,

Unfortunately, I don’t have enough expertise regarding foundation and footing to properly assess my situation.

The soil report states the following:
- Clay
- Good load-bearing capacity
- Poor permeability
- Strong frost sensitivity

Now regarding the different foundation proposals from the soil report:

1. Frost apron AND cushion
Frost apron (with slab) goes 1 meter deep and the cushion under the slab should be at least 70 cm with frost protection material

2. Strip footings AND cushion
At least 1.2 meters (with slab) deep. Cushion under the slab only 0.2 meters.

My excavation contractor suggests a third option, which has also been approved by the structural engineer:

3. Cushion
Exclusive use of frost protection material under the slab in order to achieve a 1-meter deep foundation.
According to the structural engineer, a load plate test is required. Not for option 1 and 2. He wants this proof from 0.9 meter cushion upwards.

Now I am a bit unsure which option is truly the most sensible.
All options seem possible and technically sound.

Can you help me understand the advantages and disadvantages of each?
Of course, I talk to the tradespeople, but independent opinions from you are usually more honest :)

Thank you!

The basic situation sounds similar to mine. Our soil is clayey, relatively watertight (kf= 10^-7) and F3. In my case, the foundation was only on gravel, but the insulation extends well beyond the ground slab, so it forms the frost skirt:


Below is a normal layer of gravel, certainly not 70cm but maybe at most knee-high. Therefore, the proposals seem somewhat excessive to me.

Very similar to my situation: kf < 10(-8) m/s

Yes, I also have the impression that it is redundant.

Hm, maybe a professional has an opinion on this?

Hi,
Option 1:
For the stability of the building, a cushion of 70 cm is required. The frost skirt is used exclusively against frost and has no load-bearing properties. Using frost protection material here is not wrong, but other well-compacted materials (e.g. construction debris RC 00/32 or 00/45), if permitted, are cheaper and in my opinion better.

Option 2:
The foundation and load transfer occur through the strips. These were dimensioned by the structural engineer and must go at least 1 m deep to simultaneously ensure frost safety. The 20 cm cushion only serves as a capillary-breaking layer.
This option requires the least amount of soil to be removed.

Option 3:
Instead of a frost skirt, frost-resistant material is used up to 1 m depth. According to option 1, only 70 cm is statically necessary. Thus, from a foundation technology perspective, you are an additional 30 cm on the safe side.

In my opinion, all 3 options are feasible without any concerns. The responsible soil expert would probably also confirm this upon request. Option 3 is probably the easiest since the earthworker can do everything in one work step and does not have to dig frost skirts into the ground again afterwards.

Regards

We had a similar recommendation with a foundation cushion as an option, but I definitely wanted the strip foundation...

With the cushion, you have gravel under the house, which is probably wrapped in some geotextile. Who guarantees that the gravel will stay where it is over all those years?
When doing earthworks near the house, you always have to be extremely careful, etc.

I never understand why people want to save money on the foundation.

Saving money on the foundation is not a bad thing. If I have 3 variants that all work, then I can have calculated which one is the cheapest. There are so many factors that influence the whole thing that you can never generally say which is the best and cheapest variant for the construction measure.

It starts with the excavation. What is my soil anyway (soil surveyor). Can I leave the soil on my property or does it have to be hauled away? Is it contaminated? How much does the filling material cost (gravel/recycled material/frost protection)? How much do concrete and steel for strip foundations cost? How high is the labor effort for each variant?

Everything should be calculated and then you usually see differences in the costs.

Of course, you should not save in the wrong way. For example, if the neighbor says that he installed a maximum of knee-high gravel, that does not mean at all that this works for all other houses. Every soil is individual!

And personally, I have nothing against a foundation on a gravel cushion. Where should the material go? If it is wrapped in geotextile, it is actually only used for soft clay soils; it gives additional stability. Otherwise, it is backfilled with soil.