ᐅ Concrete slab - gravel base underneath or not?

Hello everyone,

In my soil report, it is recommended to place a layer of gravel under the concrete slab because the ground conditions are not ideal (waterlogged). The basement will be constructed as a waterproof concrete tub (white tank).

Now, the general contractor suggests skipping the gravel layer because, in his view, the benefits are lower than the risk that water (due to a slight slope) could gradually wash away the gravel, which could potentially lead to cracks at some point. The contractor has no financial interests in this, as it is not included in his fixed price. He also mentioned that this seems to be a kind of "belief conflict"—some experts recommend it, others do not. I understand the argument that small stones under the house can be washed out by water over the years.

My general contractor will consult again with the soil expert.

I just wanted to ask for opinions here in the forum—maybe someone has some insights.

Best regards,
Oliver

I can’t really help you directly; others will have to do that. BUT:
I wonder where the small stones under the house are supposed to go? The gravel is compacted in layers, and the area around it is backfilled and also compacted. If there is a risk that something could be washed away under your house, I would generally question whether this property should really be yours.

By the way, I have never heard of skipping the gravel base layer under a concrete slab for a single-family house. But you never stop learning.

What is the soil condition like down there? Is the soil stable enough to be properly compacted and still provide drainage without the gravel?

One00 schrieb:
By the way, I have never heard that you can skip the crushed stone base layer under a slab on grade, where a single-family house will stand. But you never stop learning.

Sure, it's possible – we did it the same way. Natural sandy soil – a blinding layer and then the concrete slab.

One00 schrieb:

By the way, I have never heard of anyone skipping the gravel base layer under a slab foundation where a single-family house will be built.

Basically, structural engineers and designers decide on the type of foundation based on the soil report results.

Lean concrete is more precise regarding cover thickness during the later concreting process. A blinding layer, like the one used in Sebastian’s construction project, is a bit more challenging to handle. Both methods are possible.

Of course, I don’t know any construction project where the gravel shifted. And if it had – the groundwork contractor would probably have had their first and last job.

Best regards, Bauexperte

My base layer was made of lean concrete – that was the first thing I insisted on.

That largely depends on the foundation and soil conditions.
In our case, strip footings were used, extending to a depth of 1.30 m (4 ft 3 in), with a lean concrete blinding layer, followed by gravel as a capillary break layer.