ᐅ Small foundation 1.30m x 1.10m – any experiences?

Hello,

I need a small foundation, about 1.30 x 1.10 m (4.3 x 3.6 ft).
A device (weighing approximately 400 kg (880 lbs)) will be placed on it and fixed to the ground with four bolts. That’s why the foundation is necessary.
Both the device and the foundation will be exposed to weather conditions, and the device may get completely wet.

My current plan is to excavate the area, lay about 10 cm (4 inches) of gravel, and then pour around 15 cm (6 inches) of concrete. I was planning to skip the steel reinforcement mesh.
The whole thing would be formed with wooden shuttering, using a release agent for the formwork.

Are 10 cm (4 inches) of gravel and 15 cm (6 inches) of concrete sufficient? Or is that too much for the intended use?
Do I need to compact the gravel layer? I don’t really have access to a plate compactor.
For the concrete, I was going to use a 40 kg (88 lbs) bag of ready-mix concrete screed from a hardware store.

What are your thoughts on this plan?

Best regards

I am not an expert in this field, but I would be concerned about damaging the 15cm (6 inches) thick concrete when anchoring the device (which type of anchors or screws should be used?). Especially since there is no reinforcement inside. Does the foundation only need to support the load, or does this machine also generate movement? In any case, I would compact the base a bit, even if only using a paving stone and a mallet.

The machine does not move. The foundation is simply there to allow the device to be anchored to the ground so it cannot be "knocked over."
I can’t say exactly which screws will be used because the device hasn’t arrived yet, and they will be supplied with it.

The foundation planning is not correct as it stands.

You should excavate about 60 cm (24 inches), depending on soil conditions. Possibly up to 80 cm (31 inches) deep. Add a frost protection layer, preferably using coarse lime gravel, and compact it thoroughly with a tamper or vibrating plate.

Place a construction foil on top. Then build formwork from boards. You don't need a release agent for this. A thickness of 15 cm (6 inches) is sufficient; with steel reinforcement, 10 cm (4 inches) is enough. I strongly recommend including steel reinforcement. Without it, the concrete slab will crack sooner or later—more likely sooner.

Without the foil, moisture will always rise from below. Adding some gravel underneath won’t help either; frost will push the concrete slab up and cause it to break. The substructure has to be done correctly.

As soon as you pour concrete, you have a rigid layer on top. Then you must also ensure frost-resistant foundations, otherwise, you will have cracks in the medium term. Depending on the soil, this means a depth of 40-80cm (16-31 inches). The gravel or crushed stone layer must also be compacted. Space for a vibrating plate compactor is quite tight, but you can rent a trench rammer for half a day at a low cost.

Bausparfuchs schrieb:

You should excavate about 60 cm (24 inches) deep, depending on the soil conditions. Maybe even 80 cm (31 inches) deep.
Install a frost protection layer, ideally using coarse limestone gravel, and compact it thoroughly with a tamper or a vibrating plate.

Place a vapor barrier foil on top. Then build formwork from boards. You don’t need a release agent. A thickness of 15 cm (6 inches) is sufficient; with steel reinforcement, 10 cm (4 inches) is also okay. I definitely recommend using steel reinforcement. Without it, the concrete slab will crack sooner or later—more likely sooner.

Without the foil, moisture will rise from below. A thin layer of gravel underneath won’t help either, as frost heave will push the concrete slab up and cause it to break. The subbase must be properly prepared.

Thank you very much. Phew, okay. I had hoped it would be easier. I should mention that the foundation isn’t on my property, but is leased, and it doesn’t need to last forever. It’s just about securely fastening the machine.

Do you really think I still need 60 cm (24 inches)?

With steel reinforcement, I’m worried about hitting the rebar exactly with the bolts when fastening the machine to the 15 cm (6 inches) concrete.

WilderSueden schrieb:

Once you pour the concrete, you have a rigid surface on top. Then you also need frost-resistant foundation; otherwise, you’ll get cracks in the medium term. Depending on the soil, that means 40–80 cm (16–31 inches). The gravel layer must also be well compacted. For a vibrating plate, the space is quite tight, but you can rent a trench tamper for half a day at a low cost.

Personally, cracks wouldn’t bother me. First, you can’t see them once the machine is installed, and second, it doesn’t have to last forever. As mentioned above, it’s only about securing the machine.
There is a hardware store right next to the site that offers tampers for rent.