ᐅ Which material and grain size are suitable for the foundation of a single-family house without a basement?

Hello everyone,

According to the soil report, we are supposed to backfill to a depth of 0.6 meters (2 feet) for the foundation, meaning the existing soil needs to be replaced. [...] Additionally, beneath the base slab, a bedding layer at least 30 cm (12 inches) thick made of well-graded gravel or comparable recycled concrete (RC) material should be installed with a compaction degree of Dpr ≥ 98%.

The achieved bearing capacity must be verified by plate load tests. On the bedding layer, EV2 values ≥ 70 MN/m² or EVd values ≥ 35 MN/m² must be demonstrated. For the preliminary static design of the foundation, the design value of the soil pressure should be limited to σR,d = 280 kN/m² (allowable = 200 kN/m²). When designing according to the modulus of subgrade reaction method, the subgrade modulus can be assumed as 15 MN/m² for sand-lime brick.

Unfortunately, the material for the backfill is not specified in detail, and the term "well-graded gravel" for the bedding layer is rather open to interpretation.

What material and aggregate size should we use? Mostly, I see references to 16/32 gravel ...

If more information from the soil report is needed, I can gladly provide it.

Thank you very much for your support!

The geologist conducting the soil survey would be my first point of contact. The second would be the civil engineer specialized in underground construction.

Hello,

Generally, for single-family houses, gravel with a grading of 00/32 or 00/45 is used. The fine grain content is necessary to ensure good compaction of the material.
For the required EV2 values, a sand mixture could theoretically also be used (this is common in northern regions where gravel is not available or is very expensive). However, I would tend to recommend the above-mentioned gradings, as this helps avoid problems later on.

Since you are installing only 60 cm (24 inches), I assume you are planning frost skirts or strip foundations to ensure frost protection. Therefore, the material does not need additional certification as frost protection material.

Natural materials such as basalt gravel or limestone can of course be used. It naturally depends on what is locally available in your region.

Alternatively, recycled material is a more cost-effective and, in my opinion, also a good option. However, this material must be approved by the responsible authority, since it is legally classified as "waste."

PS: Even though this might not belong here, I believe that material that has been thoroughly processed, subjected to various tests, and meets the required limit values should no longer be classified as waste.

Best regards

Pitiglianio schrieb:

The geologist conducting the soil survey would be my first point of contact. The second would be the civil engineer.

In my case, the first person to contact would be the structural engineer... the soil surveyor measures the current condition of the ground and provides certain recommendations for the foundation, but the structural engineer performs the detailed calculations and also determines the type of gravel needed. It’s quite important to know what will be built on it—whether a bungalow or a multi-family building with four floors, which the soil surveyor does not know.

After the soil survey, I received a structural engineering report specifying exactly what the civil engineer should do.

I would not consult the civil engineer at all. They simply apply the gravel specified by the structural engineer.

xMisterDx schrieb:

So my first point of contact in such a case would be the structural engineer... the geotechnical engineer measures the current condition of the soil and provides certain recommendations for the foundation, but the structural engineer calculates the specifics and also determines which gravel is required. It’s quite important what is going to be built on it, whether a bungalow or a four-story apartment building, which the geotechnical engineer doesn’t know.

After the soil report, I also received a structural analysis report that clearly stated what the foundation contractor should do.

I wouldn’t ask the foundation contractor at all. They just place the gravel specified by the structural engineer.

Hello,
unfortunately, that is not entirely correct.

The structural engineer is usually not familiar with soils, soil parameters, or material properties. Exceptions do occur, but this is generally the case. This is the responsibility of the geotechnical engineer, as mentioned above. They specify the foundation type and the assumed soil parameters as well as soil bearing pressures and the subgrade modulus. The geotechnical engineer is aware of the site and dimensions of the construction project (single-family house, multi-family building, hall, etc.). The structural engineer then takes these values and uses them to calculate the structural design for the building. Based on this, the required reinforcement, as well as the number, position, and width of the foundations, are determined.

The geotechnical engineer also specifies the material to be used. However, to avoid overly restricting the client, there is usually some flexibility here (≥ 98% Proctor density, EV2 of ≥ 70 MN/m²). At this stage, the foundation contractor can actually help, as they can advise which material they can obtain and at what cost.

As mentioned, the geotechnical engineer or foundation contractor handle material questions and execution, while the structural engineer later checks whether everything is suitable for the building.