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Concrete blinding.
Blinding, in general, serves 3 purposes. I'm not always sure about the third which is one reason I say CONCRETE blinding. Blinding can also be compacted crushed stone fill, such as Type 1.
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Blinding protects the undisturbed soil beneath that your structure will be supported by.
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Blinding supports spacers beneath your floor steel reinforcement so that concrete gets under to surround the bars fully.
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Blinding provides drainage under your structure.
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Concrete blinding.
About 50mm of concrete all over the finished excavation. They have left the edges for perimeter drainage already in.
I have circled a piece of concrete spacer (known as a Mars bar) that ensures the steel does not sink and concrete will get beneath the lowest bars.
The first rule of blinding is it should be stronger, structurally, than the soil you replaced.
Second, the soil beneath should be as strong as the best soil beneath. Sometimes, especially in chalk, you get soft spots. These need digging out and replacing with crushed stone before you blind.
Occasionally the drawing will say blind with sand. Useless. The spacers and the steel will sink into it.
Occasionally, the drawings will say a thick layer, such as 150mm, of crushed, compacted Type 1 road stone.
I have never understood why drainage beneath a basement should be beneficial. Drainage lets water in. Why would you want a lot of water under your structure.
If you have to dig extra for 150mm of stone rather than 50mm of concrete, that is more dig, more waste and more likelihood you are going deeper than you have insurance cover for.
I think concrete blinding is essential. Question blinding with any other material.
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