Frost apron vs. founding cushion

Hello everyone, I have a related question on the topic, maybe we can save ourselves creating a new thread:
We also have a similar soil according to the report and were suggested the following three options:
1. Excavation of 50cm below foundation level, compact the base and lay fleece GRK 3, at least 0.5 m soil cushion (frost-resistant mineral mixtures) then a clean concrete layer. For frost protection, a 1m deep frost skirt.
2. Excavation of 1m below foundation level and use at least 1m soil cushion. Then the frost skirt can be omitted.
3. Use strip foundations in clay (about 1.2 m deep) for load transfer, due to the depth no frost protection is necessary anymore.

Now I wonder which option is the most reasonable. The first excavation company we contacted decided on the first option in the offer. Reuse of excavation material must be weighed after excavation begins, as two boreholes show different compositions (1x gravel and sand then clay and 1x directly after topsoil clay). According to the soil expert, the clay can still be "upgraded" with lime.

As far as I have read so far, the frost skirt is quite primitive and therefore not too expensive: It has no load-bearing function but is "only" meant to serve as insulation for the area below the base plate (insulated with XPS). For me, the question is: frost skirt or thicker cushion?

Thank you very much in advance for your feedback! If "hijacking" the thread is not desired, please let me know :)

Hello Toxickill,

without knowing the exact ground structure, I think that option 3 makes the least sense, as strip foundations are significantly more expensive than the frost skirts and still require some soil replacement (removing topsoil and refilling with material).

For the other two options, you unfortunately cannot make a general statement, as prices for materials and disposal costs vary regionally. It is important to clarify whether the excavated soil can remain on your property or must be transported away. The excavation volumes should not be underestimated (50 cm excavation on approx. 100 m² area is 50 m³ of soil, which corresponds to about 100 tons and then approx. 4 to 5 trucks). At 1 m excavation depth, this would double accordingly. A waste law analysis is required beforehand, otherwise no landfill (or similar) will accept the soil. If the material can remain, the problem basically solves itself.

In general, foundation with frost-proof bedding is simpler, as everything can be done in one work step. With frost skirts, after creating the bedding, a trench for the skirts must be made again with a mini excavator. In addition, option 2 offers the 1.0 m thick bedding additional safety for the foundation (1.0 m gravel > 0.5 m gravel).

So I would first consider:

1. What happens to the excavated soil at 0.5 m and 1.0 m excavation depth
2. What does the frost protection material cost (2 tons correspond to approx. 1 m³)
3. What do the frost skirts cost

Then compare both options. I will leave out the labor effort (option 2 requires more digging and installation and option 1 has the frost skirts).

I hope this somehow helps

Best regards

PS: In my opinion, with option 1 no frost protection material needs to be used due to the frost skirts.

A bit of time has passed since the question, but good post that summarizes the most important factors well! However, my civil engineer calculated 1.5t/m³, not 2.

Hi,
the response referred to the question from Sunday, it hasn’t been that long ago ;)

For the removal of clay soils, you should calculate at least 2 t/m³. Of course, this also depends on the water content. Dry soils are somewhat lighter, wet ones significantly heavier.

For gravel, it again depends on the base rock. For basalt, it’s rather around 2 t/m³, for recycled material it goes towards 1.5 t/m³ (but there are also some gradations there). I would generally take 2 t/m³ for gravel because it’s easier to calculate on the one hand and the stuff also needs to be installed highly compacted.

: First of all, thanks for your response as a contribution! In the meantime, the civil engineering work has begun and the civil engineer (who "only" excavates, compacts, and lays the pipes but does not pour the concrete slab) wants to use a 58cm thick soil cushion. Background: since the insulation and concrete slab require 37cm and the clean layer is about 5cm, this would mean a frost depth of 1m below the soil cushion. Around the concrete slab, he would then fill 1m wide + deep with frost protection gravel (a kind of frost skirt made of gravel). In his opinion, the static requirement of 50cm soil cushion as well as 1m to the ground surface as frost protection is then met, and this is the most cost-effective implementation. He could also do a 1m soil cushion but does not recommend it due to the costs. The soil is quite clayey and lies near a stream that probably once flowed over the property in ancient times.

Do you consider this proposal a feasible approach? I would also call the soil expert on Monday and get their opinion but would be grateful for your assessment/tips!

Hi, regarding frost resistance, the approach sounds good as long as the base slab is flush with the terrain later on.

In your first post, you mentioned something about 70 cm as a static requirement. Check the soil survey! In the end, a compaction control must be done on the gravel cushion.
An alluvial clay can become quite problematic, especially with the current weather conditions.

Moisture makes the soil soft and muddy; frost is also not good.

In general, work must be done quickly. Excavate and immediately cover with a layer of gravel. The first layer of about 20 cm should only be compacted statically (pressed down). If vibrated right away, the clay reacts and becomes soft.
The excavation pit should not be driven on, not even with a mini excavator. Only once the gravel cushion is fully constructed.

I don’t know the soil survey, so it’s best to check how the exposed, clayey foundation base looks. If it is too soft, mushy, or water-saturated, it’s better to bring in a professional and possibly add 10 cm more cushion underneath.

Good luck!